Classification and Quantification of Microstructures in Steels
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Transcript of Classification and Quantification of Microstructures in Steels
Materials perspectiveClassi cation and quanti cation ofmicrostructures in steelsG Thewlis
The International Institute of Welding (IIW) microstructure classi cation scheme for ferrous weld metals has beeninvestigated as a basis for the quanti cation of complex microstructures in steels The aim has been to cover the fullrange of microstructures observed in plain carbon and low alloy steel products as well as ferritic weld metals andparent plate heat affected zones The mechanisms of formation of the principal structures and the characteristicferrite morphologies produced in the reconstructive and displacive transformation regimes of ferrous materials havebeen brie y reviewed The classi cation and terminology used for intragranular as well as austenite grain boundarymicrostructural constituents have been considered and also the way in which transformation products are orientatedin space Problems encountered in relating microstructural constituents to principal structures have been discussedin detail and solutions proposed The microstructure classi cation and terminology used in the IIW scheme havebeen built upon and new terminology incorporated into a table providing descriptions of the principal structures andsub-category components A new classi cation scheme has been de ned in the form of ow charts with guidelines foridentifying the principal structures Evaluation exercises have been carried out with the new scheme These haveshown that a reasonable degree of consistency may be obtained between operators in identifying primary ferritepearlite martensite and the transformation products constituting ferrite sideplate and acicular ferrite structuresnotably Widmanstatten ferrite and bainite A means is thus provided of obtaining database information fordeveloping microstructure ndash property relationships or generating data for calibrating physical models which havethe principal structures as their output MST5675
Keywords Steel microstructures Low alloy steels Ferrite Bainite Martensite Metallography Microstructure classi cationPhase transformation products
The author is with Corus Research Development and Technology Swinden Technology Centre Moorgate RotherhamS60 3AR UK (grahamthewliscorusgroupcom) Manuscript received 17 October 2002 accepted 22 September 2003 2004 IoM Communications Ltd Published by Maney for the Institute of Materials Minerals and Mining
Introduction
With the advances in computer power in recent years therehas been increasing interest in the development of modelsto predict the microstructure of steel products particularlythose processed by thermomechanical treatments1 ndash 4 orfabricated by welding5 ndash 1 1 The driving force for much ofthe modelling work has been the need for computer basedsystems to control and optimise microstructure and mecha-nical properties Linear regression analysis on large data-bases of information has proved a useful tool in generatingcompositionndash structurendash propertyrelationshipsMore recentlyneural network techniques9 have enabled prediction of situ-ations too complex for simple analytical models or multipleregression techniques Regression and neural networkmodels often nd use as online models for process controlHowever they are restricted to well de ned data over limitedranges of composition and process parameters Such con-straints can to a large extent be circumvented by thedevelopment of physical models based on fundamentalmetallurgical principles The major advantage of physicalmodels is their general applicability They can be used asdesign tools for a wide variety of new materials and pro-ducts Classical nucleation and growth theory may forexample be used to predict the microstructure of steelsprocessed to a given austenite grain size and cooled atdifferent rates through the austenite to ferrite transforma-tion temperature range4 6 ndash 8 The transformation sequenceduring cooling and the phase proportions of allotrio-morphic ferrite pearlite Widmanstatten ferrite bainite andmartensite may be the outputs The latter are the principal
structures in the reconstructive (diffusion controlled withslow rates of reaction) and displacive (shear dominated withrapid rates of reaction) transformation regimes of contin-uous cooling transformation (CCT) diagrams1 2 1 3
While the development of regression and neural networkmodels requires good quality database information thedevelopment of sophisticated physical models for micro-structure prediction in steels has led to a need for accuratecalibration data However the microstructures observed insteel products are complex A variety of reaction productsmay form at austenite grain boundary sites in thermo-mechanically processed or heat treated steels In the fusionzone of welds the simultaneous and competitive formationof a variety of phases from both austenite grain boundaryand intragranular sites may occur while in the parent plateheat affected zone (HAZ) steep thermal gradients may giverise to a wide range of transformationproducts A scheme isthus required for classifying and quantifying complex steelmicrostructures
Classifying and quantifying the microstructures of steelshas long been a contentious issue1 4 ndash 1 8 Depending on theplane of observation constituents that are part of the sameprincipal structure may appear morphologically differentgiving rise to sub-category components Furthermoresome structures may have similar morphological or genericfeatures but be mechanistically different A scheme foridentifying the various ferrite morphologies in isothermallytransformed steels was rst used by Dube et al1 7 and laterextended by Aaronson1 8 However the effect of continuouscooling was to render the distinguishing morphologicalfeatures much less distinct Allotriomorphic ferrite mor-phologies were readily identi ed and also various sideplate
DOI 101179026708304225010325 Materials Science and Technology February 2004 Vol 20 143
morphologies (often classed as bainite) Widmanstattenferrite was dif cult to place but was regarded as a genericallysimilar structure to bainite Intragranularcomponent phasessuch as acicular ferrite posed a much greater degree ofdif culty Much effort was made by the welding fraternity inthe 1980s to develop an overallmicrostructurequanti cationscheme for weld metals incorporating both prior austenitegrain boundary and intragranular nucleated constituentsand addressing stereological effects ie the way constituentsare orientated in space1 9 20 A scheme was devised whichbecame recognised as the International Institute of Welding(IIW) classi cation1 9 Most of the constituents de ned in theIIW scheme were relatively easily identi ed Furthermorethe scheme could just as readily be applied to steels whereaustenite grain boundary transformations dominate as toweld metals where intragranulartransformationsare the ruleHowever identi cation of the actual transformation pro-ducts constituting component structures such as ferritesideplate and acicular ferrite has proved dif cult Anelliand Di Nunzio2 1 recently devised a scheme providingguidance on identifying transformation products associatedwith sideplate structures which has had some success butstereological effects and intragranular constituents were nottreated in depth
The objective in the current work has therefore been toinvestigate the IIW microstructure classi cation scheme as abasis for quanti cation of complex microstructures in steelsThe overall aim has been to develop a scheme that althoughrequiring a basic knowledge as to the mechanism of for-mation of the principal structures will be relatively easy touse given optical microscopy standard specimen polishingand etching techniques and appropriate guidance Theapproach has been to review microstructural constituentsin the IIW scheme in the context of the development ofprincipal structures found in the reconstructive and dis-placive transformation regimes of steels Detailed intragra-nular as well as austenite grain boundary transformationproducts have been considered and also stereological effectsProblems relating microstructural constituents to principalstructures in the IIW scheme have been investigatedtogether with possible solutions so that a new quanti cationscheme may be developed with a much broader applicationrange The intention has been to cover microstructuresobserved in carbon (up to abount 08) and low alloy (up toapproximately 5) steels as well as weld metals (up to010C and 5 alloy) and weld HAZs
Classi cation of microstructures andterminology
In this section the mechanisms of formation of the principalstructures and the characteristic ferrite morphologies pro-duced in the reconstructive and displacive transformationregimes of ferrous materials are brie y reviewed Theclassi cation and terminology used in the IIW scheme aredescribed together with that of Dube et al1 7 to provide alink with the early work on classi cation of prior austenitegrain boundary ferrite morphologies Terminology used inrecent work by the present author and co-workers2 2 is alsoincluded to provide a contemporary view of complex intra-granular transformations including those generating themicrostructure commonly known as acicular ferrite
RECONSTRUCTIVE TRANSFORMATIONREGIMEIn the high temperature reconstructive transformationregime a change from the austenite to ferrite crystal struc-ture occurs by a reconstructionprocess involving movementof atoms across the ca transformationinterfaceThe principal
phases are ferrite and pearlite Reactions tend to bediffusion controlled with slow rates
FerriteIn low hardenability materials the rst phase usuallyforming on prior austenite grain boundaries during coolingbelow the Ae3 temperature is classically referred to asallotriomorphic ferrite as shown schematically in Fig 1The ferrite nuclei have a Kurdjumovndash Sachs (K ndash S) orien-tation relationship with one austenite grain and grow intothe adjacent austenite grain with which they should normallyhave a random orientation relationship2 3 At some lowertemperature ferrite may begin to nucleate on inclusionsinside the austenite grains2 2 2 4 and this is termed idiomor-phic ferrite (see Fig 1) The indications are that ferriteidiomorphs do not have a xed orientation relationshipwith the matrix grains into which they grow2 5
Growth at reconstructive transformation temperaturestends to be controlled by substitutional element diffusionaway from the ca interface at low undercooling and carbondiffusion at high undercooling Various growth modesare recognised in order of decreasing transformationtemperature2 6
(i) local equilibrium with bulk partition of substitu-tion alloying elements (PLE)
(ii) local equilibrium with negligible partition of sub-stitutional alloying elements (NPLE)
(iii) paraequilibrium where only the interstitial carbonatoms diffuse
The diffusion rate of carbon in austenite may be manyorders of magnitude greater than that of substitutionalatoms at reconstructive transformation temperatures Trueequilibrium segregation during phase transformations atmigrating interfaces is therefore unlikely to be achieved withregard to all components Growth under diffusion controlwith local equilibrium at the interface is then envisagedTwo phases may differ either signi cantly (PLE) ornegligibly (NPLE) in terms of substitutional alloy contentElement concentration or depletion spikes are invoked tosatisfy the thermodynamic constraints In many cases asthe transformation temperature is decreased the relativerates at which elements are able to diffuse negate theassumption of local equilibrium since the interface com-position spike would be only several atomic layers thick Insuch cases the concept of paraequilibrium is applied iethere is no redistribution of iron or substitution atoms at theinterface between the phases and only the interstitial carbonatoms diffuse The different growth modes described abovemay result in signi cant changes in ferrite growth mor-phology from equiax grains towards a plate shape (seebelow)
Dube et al1 7 refer to prior austenite grain boundaryallotriomorphic ferrite as GBF The IIW classi cationscheme refers to the rst phase forming at reconstructivetransformation temperatures as primary ferrite termed PF
Prior austenite grain boundary primary ferrite allotrio-morphs are termed PF(G) in the IIW classi cation scheme
1 Allotriomorphic and idiomorphic primary ferrite
144 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
and are usually observed in the form of polygonal grains orveins as shown schematically in Fig 1 Reference is madein the IIW scheme to polygonal ferrite grains in the intra-granular regions (see Fig 1) of a size approximately threetimes greater than those of the surrounding ferrite laths orgrains These ferrite grains in reality may be cross-sectionsof ferrite allotriomorphs that have grown from prioraustenite grain boundaries beneath the plane of observationand have a wide range of sizes They are termed PF(I) in theIIW scheme The present author and co-workers2 2 havereferred to the different forms of prior austenite grainboundary primary ferrite as GB(PF) so that a distinctionmay be made with idiomorphic primary ferrite as describedbelow
In weld metals stable particle dispersed steels and somemicroalloyed steels ferrite may nucleate not only at theaustenite grain boundaries but also on particles insidethe austenite grains2 2 2 7 (see Fig 2) The author and co-workers2 2 have termed these intragranular ferrite idio-morphs I(PF) Depending on the temperature in thereconstructive regime the intragranular ferrite morpholo-gies2 2 may take the form of blocks loops ellipses rosepetals or wedges The IIW classi cation scheme does nothave a terminology for these primary ferrite idiomorphs
PearliteClassically pearlite transformation may occur at austenitegrain boundariesor an inhomogeneitysuch as an inclusion2 3
Ferrite or cementite nucleation may initiate the pearlitetransformation depending on whether the steel is hypo- orhyper-eutectoid in composition Growth of a pearlite noduleinto an austenite grain proceeds with the formation ofalternate ferrite and cementite plates or lamellae Both thecementite and ferrite possess unique crystallographic orien-tations within the pearlite nodule2 3 Edgewise growth of theplates may occur and also branching of the cementitelamellae The rate controlling process in the growth ofpearlite is the diffusion of carbon As the transformationtemperature is lowered the driving force for the reaction isincreased but the diffusivityof carbon is decreased so that thepearlite interlamellar spacing is decreased
At high transformationtemperatures pearlite is generallyobserved as nodules of alternate ferrite and cementitelamellae that may be quite coarse and degenerate Whenviewed in cross-section the lamellae may appear as aferrite ndash carbide aggregate As the transformation tempera-ture is lowered the lamellae become increasingly neuntil the structure becomes irresolvable under the light
microscope (see Fig 3) The pearlite may then have a lightetching response Alternatively the lamellae may becomesubjected to distortion and bending appearing as a darketching ferritendash carbide aggregate or barely resolvablesomewhat non-lamellar pearlite often described in oldernomenclature as primary troostite2 8 2 9
In the IIW scheme FC(P) is used to describe lamellarpearlite degenerate or coarse pearlite and ne colonyor irresolvable pearlite The term FC is used to describeferrite ndash carbide aggregate At reconstructive transforma-tion temperatures large islands of pearlite or ferrite ndashcarbide aggregate may be interspersed with prior austenitegrain boundary primary ferrite PF(G) A similar situationmay occur with idiomorphic primary ferrite I(PF) (seeFig 4)2 7 In some cases pearlite may be present as micro-phase (see below)
DISPLACIVE TRANSFORMATION REGIMEIn the low temperature displacive transformation regime achange from the austenite to ferrite crystal lattice occurs byan invariant plane strain shape change with a large shearcomponent Diffusion of interstitial carbon atoms mayaccompany the shear transformation For a purely dis-placive transformation there is no movement of atomsacross the ca interface Reactions in the displacive trans-formation regime tend to be rapid The principal phases areWidmanstatten ferrite bainite and martensite
1 intragranular ferrite idiomorphs 2 grain boundary ferriteallotriomorphs
2 Morphologies of ferrite at prior austenite grain bound-ary and intragranular sites in 006C 146Mn sub-merged arc weld metal continuously cooled icedbrine quenched from 670degC22
1 alternate ferritecementite lamellae 2 regne ferrite plusmn carbideaggregate 3 irresolvable pearlite
3 Resolvable and irresolvable pearlite in 083C050Mn as rolled rod
1 idiomorphic ferrite 2 ferrite plusmn carbide aggregate 3 irresolva-ble pearlite
4 Intragranular primary ferrite and pearlite in as cast013C 20Mn cerium sulphide particle dispersedsteel27
Thewlis Classiregcation and quantiregcation of microstructures in steels 145
Materials Science and Technology February 2004 Vol 20
WidmanstaEgrave tten ferriteA classic feature of Widmanstatten ferrite formation is thatit may occur at relatively low undercooling2 3 The growthmechanism is thought to involve the simultaneous forma-tion of pairs of mutually accommodating plates so that lessdriving force is required for transformation than withbainite or martensite3 0 The ferrite plates grow rapidly witha high aspect ratio (~10 1) resulting in parallel arraysWidmanstatten ferrite is not the result of a purely displacivetransformation but forms by a paraequilibrium mechan-ism3 0 3 1 involving the rapid diffusion of interstitial carbonatoms across the advancing interface into the remainingaustenite during the shear transformation At the relativelylow undercooling required for Widmanstatten ferrite for-mation microphases of retained austenite martensite orferritecarbide aggregate (pearlite) may be formed betweenthe growing ferrite plates
Widmanstatten ferrite can easily be confused with bainiteDube et al1 7 describe both prior austenite grain boundaryWidmanstatten ferrite and bainite as ferrite sideplate FS butreference is also made to intragranular plates IP The IIWclassi cation scheme refers to all forms of Widmanstattenferrite and bainite as ferrite with second phase FS althougha distinction may be made in the terminology whenWidmanstatten ferrite can be positively identi ed egFS(SP)
Characteristically primary Widmanstatten ferrite platesgrow directly from a prior austenitegrain boundarywhereassecondary Widmanstatten ferrite plates grow from allo-trimorphic ferrite at the grain boundaries as shown sche-matically in Fig 5 Primary Widmanstatten ferrite platesmay also grow from inclusions while secondary Widman-statten ferrite plates grow from intragranular idiomorphicferrite2 2 3 2
Widmanstatten ferrite that grows from prior austenitegrain boundary sites is usually seen as colonies of coarsesideplates with aligned microphase (see Fig 6) which aretermed FS(A) in the IIW scheme The individual plateswithin an array are separated by low angle boundaries thatare dif cult to resolve under the light microscope althoughcareful specimen polishing and etching may reveal themDepending on the plane of observation the microphasesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are islands of microphase in a matrix of ferritewithin the prior austenite grains (see Fig 6) The Widman-statten ferrite is then classi ed as FS(NA) The presentauthor and co-workers2 2 have referred to the differentforms of prior austenite grain boundary Widmanstattenferrite as GB(WF) so that a distinction may be made withintragranular Widmanstatten ferrite as described below
In the intragranular regions of weld metals and insome steels2 2 2 7 3 2 multiple large plates (aspect ratiogt4 1) of Widmanstatten ferrite with aligned microphase
may be observed that grow from inclusions (primaryWidmanstatten ferrite) or from idiomorphic ferrite(secondary Widmanstatten ferrite) as shown in Fig 7The IIW classi cation scheme does not have a terminologyfor these plates However they have been designatedintragranular ferrite sideplates FS(I) in recent work bythe present author3 2 In many cases individual plates maybe observed that have grown relatively unimpeded fromintragranular inclusions (see Fig 8) These plates do nothave aligned microphase and may be interspersed withbainite or martensite2 2 2 7 3 2 The inclusions from which theplates grow may not be viewed since they may be under theplane of observationThese plates have been designated IFPby the present author3 2 who summed FS(I) and IFP to givea total quantity of intragranular Widmanstatten ferritereferred to as I(WF) Where there is a high density ofinclusions multiple hard impingements of individualWidmanstatten ferrite plates growing from inclusions2 2 3 2
may produce a ne interlocking structure (see schematicdiagram Fig 5) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
BainiteBainite is generally recognised as forming at temperatureswhere diffusion controlled transformationsare sluggish andhas features in common with low temperature martensitic
5 Primary and secondary Widmanstatten ferrite
1 idiomorphic ferrite 2 prior austenite grain boundary Widman-staEgrave tten ferrite with aligned microphase 3 prior austenite grainboundary WidmanstaEgrave tten ferrite with non-aligned microphase
6 Interlocking colonies of Widmanstatten ferrite in 005C135Mn HSLA steel submerged arc weld HAZ
7 Intragranular Widmanstatten ferrite sideplates in asdeposited 008C 287Mn 035Mo 00027B0019Ti submerged arc weld metal32 arrow indicatesmultiple plates of Widmanstatten ferrite with alignedmicrophase nucleated on large intragranular inclusions
146 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformations2 6 It grows as individual plates or sub-unitsto form parallel arrays or sheaves The growth of each sub-unit is accompanied by an invariant plane strain shapechange with a large shear component There is noredistribution of iron or substitutional solute atoms at thetransformation interface Classically bainite has been cate-gorised into two component structures notably upper andlower bainite depending on the transformation tempera-ture Carbon partitions into the residual austenite in upperbainite and precipitates as cementite between the bainiticferrite plates In lower bainite the ferrite becomes super-saturated with carbon and some carbide precipitationoccurs within the ferrite sub-units as well as between them
The exact growth mechanism of bainite is still the subjectof much debate3 3 ndash 3 5 A paraequilibrium mechanism inupper bainite involving a shear transformation accompa-nied by the rapid diffusion of interstitial carbon atomsacross the ca interface would mean that bainitic growth wasin part similar to Widmanstatten ferrite However a purelydisplacive transformation would require no redistributionof atoms across the ca interface A temperature curve To
may be identi ed on the Fe ndash C phase diagram de ningthermodynamically where austenite and ferrite of the samecomposition have identical free energy2 6 3 3 At the To
temperature there is no driving force for transformationThe To curve has a negative slope with carbon concentra-tion lying between the Ae 1 and Ae 3 lines of the Fe ndash C phasediagram In a steel with a carbon concentration lower thanthat de ned by the To curve bainitic ferrite plates maybegin to grow without diffusion at an appropriate holdtemperature then partition excess carbon into the residualaustenite Further diffusionless growth of plates may takeplace from the carbon enriched austenite and the processcontinues until such transformation becomes thermodyna-mically impossible at the To curve This is termed theincomplete reaction phenomenon Continuous undercool-ing of the steel below To will cause the bainite reaction to bemaintained Carbide precipitation occurs when the trans-formation conditions are kinetically favourable For apurely displacive transformation therefore a rapid redis-tribution of carbon atoms is envisaged after the diffusion-less growth of bainitic ferrite sub-units2 6
Bainite can easily be confused with Widmanstatten ferriteas noted above Both structures are referred to as ferritewith second phase FS in the IIW classi cation schemealthougha distinctionmay be made in the terminologywherebainite can be clearly identi ed eg FS(B) A further dis-tinction may be made where upper and lower bainite can bepositively identi ed eg FS(UB) and FS(LB) respectively
Characteristically bainite may grow directly from a prioraustenite grain boundary2 6 or an intragranular inclusion3 6
as shown schematically in Fig 9 Sympathetic nucleation ofbainite plates from existing sheaves is a common feature
Bainite that grows from prior austenite grain boundariesis commonly observed in the form of interlocking sheaves ofvery ne plates with aligned cementite particles (seeFig 10) which are designated FS(A) in the IIW schemeIn upper bainite FS(UB) carbide particles are observedbetween the plates while in lower bainite FS(LB) thecarbides are within as well as between the plates and thestructure tends to have a darker etching response Theindividual plates within a sheaf are separated by low angleboundaries that are virtually irresolvable under the lightmicroscope The sheaves are shown in the process of growthin Fig 11 Extensive sympathetic nucleation is evidentDepending on the plane of observation cementite particlesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are carbide particles in a matrix of ferrite within theprior austenite grains (see Fig 10) The bainite is thenclassi ed as FS(NA) The present author and co-workers2 2
have referred to the different forms of prior austenite grainboundary bainitic ferrite as GB(B) so that a distinction maybe made with intragranular bainite as described below
In some steels and weld metals2 6 3 2 3 6 bainite sheaves maybe seen to grow from intragranular inclusions (see Fig 12)Individual ne plates of bainitic ferrite may also beobserved that grow relatively unimpeded from intragranu-lar inclusions (see Fig 13) The latter plates do not havealigned carbide particles and may be dif cult to distinguishfrom Widmanstatten ferrite plates IFP (see above) Theinclusions from which the plates grow may not be observed
1 idiomorphic ferrite 2 individual plate of WidmanstaEgrave tten fer-rite nucleated on large intragranular inclusions
8 Growth of intragranular Widmanstatten ferrite platesin 006C 137Mn 017Mo 00028B 0027Tisubmerged arc weld metal continuously cooledhelium quenched from 620degC22
9 Bainite sheaves and sub-units
1 lower bainite with carbide particles between as wellas within subunits 2 upper bainite with aligned carbide3 bainitic ferrite with non-aligned carbide
10 Interlocking sheaves of upper and lower bainite in017C 10Mn steel laser weld HAZ
Thewlis Classiregcation and quantiregcation of microstructures in steels 147
Materials Science and Technology February 2004 Vol 20
since they are under the plane of observation The IIWclassi cation scheme does not have a terminology for thedifferent forms of intragranular bainite but the author andco-workers2 2 have termed them I(B) Where there is a highdensity of inclusions multiple hard impingements ofindividual bainitic plates growing from the inclusions may
result in a very ne interlocking structure2 6 3 2 (see schematicdiagram Fig 9) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
MartensiteMartensite is classically an extremely rapid diffusionlesstransformation where carbon is retained in solution3 7 Asthe austenite lattice changes from fcc to the required mar-tensite bcc or bct lattice strain energy considerationsdominate and the martensite is constrained to be in the formof thin plates
In low carbon steels (less than ~02C) lath martensitewith a bcc crystal structure is the commonly occurringform3 7 and is designated M or M(L) in the IIW scheme Themartensite units are formed in the shape of laths thatare grouped into larger sheaves or packets (see Fig 14)The sub-structure consists of a high density of dislocationsarranged in cells each martensite lath is composed of manydislocation cells As the steel carbon content increases signi- cantly above about 02C plate martensite tends to formwith either a bct or bcc crystal structure3 7 The martensiteunits form as individual lenticular plates (see Fig 15) with asubstructure consisting of very ne twins This form ofmartensite is termed twinned martensite in the IIW schemeand is designated M or M(T) Martensite whether in platesor lath form is generally irresolvable under the light micro-scope and tends to have a slow etching response
12 Growth of bainite sheaves from intragranular inclu-sions in 038C 139Mn 0039S 009V0013N steel isothermally transformed 38 s at450degC arrow indicates multiple laths of bainite withcarbide particles between as well as within subunits
11 Growth of bainite sheaves and (arrowed) sympatheticnucleation of laths in 038C 139Mn 0039S009V steel isothermally transformed 45 s at 400degC
13 Growth of intragranular bainite plates in 038C139Mn 0039S 009V 0013N steel isother-mally transformed 38 s at 500degC arrows indicateindividual plates of bainitic ferrite nucleated on smallintragranular inclusions
14 Lath martensite in 013C laser weld metal arrowindicates martensite laths with highly dislocated sub-structure
15 Plate or twin martensite in 027C laser weld metalarrow indicates lenticular martensite with twinnedsubstructure
148 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
Acicular ferriteConventionally2 6 acicular ferrite is recognised as an intra-granular nucleated morphology of ferrite in which there aremultiple impingements between grains The acicular ferritenucleates on inclusions inside the prior austenite grainsduring the cda transformation Provided there is a highdensity of inclusions a ne interlocking structure (generallylt5 mm) can be produced
In the IIW scheme acicular ferrite is designated AF Fora long time acicular ferrite was thought to be a singletransformation product Early work3 8 suggested that itwas intragranularly nucleated Widmanstatten ferrite Laterresearch2 6 provided evidence for intragranularly nucleatedbainite However recent research by the author and co-workers2 2 has demonstrated that the nature of acicularferrite may be as shown schematically in Fig 16 Differentreaction products may nucleate on intragranular inclusionsat reconstructive and displacive transformation tempera-
tures during continuous cooling depending on the naturesize and amount of inclusions (see Figs 2 and 17) Acicularferrite results from multiple hard impingements of thedifferent transformation products The sequence oftransformations is consistent with the theoretical activationenergy barrier to nucleation of the different sites Acicularferrite development may thus be de ned in terms of con-ventional steel transformation products and CCT diagramsincorporating both intragranular and grain boundarytransformations
Under continuous cooling transformation conditions
AF~I(PF)zI(WF)zI(B)
This leads to acicular ferrite that may have a variety offorms depending on steel composition cooling rate andinclusion characteristics Acicular ferrite may consist ofmixtures of different intragranular transformationproducts(see Fig 18)2 2 3 2 Alternatively Widmanstatten acicularferrite or bainitic acicular ferrite may form per se2 6 3 8
However if reactions are completed at purely reconstruc-tive transformation temperatures it may be preferable touse the term idiomorphic primary ferrite instead of acicularferrite to describe the microstructure since intragranularprimary ferrite is likely to be coarse and non-acicular inmorphology (see Fig 4)
Acicular ferrite is usually observed as a ne interlockingferrite structure interspersed with microphases (see Fig 18)The shape of the ferrite plates may not appear to be needle-like as the use of the term lsquoacicularrsquo would imply This isbecause the different ferrite morphologies cannot grow veryfar before mutual hard impingement It is evident fromFig 18 that the degree of re nement of the acicular ferrite isdependent on the nature of the transformation productsinherent in its formation
16 Nature of acicular ferrite
a
b
a idiomorphic ferrite (arrowed) nucleated on large inclusionsb WidmanstaEgrave tten ferrite plates (arrowed) nucleated on smallinclusions
17 Acicular ferrite development in 006C 137Mn017Mo 00028B 0027Ti submerged arc weldmetal continuously cooled iced brine quenched from615degC22
a
b
a intragranular primary ferriteplusmn WidmanstaEgrave tten ferrite in C plusmn Mnweld metal22 b intragranular WidmanstaEgrave tten ferrite plusmn bainitein Ti plusmn Mo plusmn B alloyed weld metal32
18 Forms of acicular ferrite
Thewlis Classiregcation and quantiregcation of microstructures in steels 149
Materials Science and Technology February 2004 Vol 20
MicrophasesThe different ferrite growth modes of the principal struc-tures described above result in carbon enrichment of theremaining austenite leading to associated second phases ofretained austenite martensite bainite or ferrite ndash carbideaggregate (pearlite) depending on the degree of carbonenrichment of the austenite and the prevailing coolingconditions The second phases associated with Widman-statten ferrite and acicular ferrite are generally quite small(2 ndash 5 mm) and are termed microphases
IIW classi cation scheme problem areasand solutions
The objective in the present work was to investigate the IIWmicrostructure classi cation scheme for weld metals as abasis for quantifying the full range of microstructures foundin plain carbon and low alloy steels as well as ferritic weldmetals and parent plate heat affected zones A means maythus be provided of obtaining database information fordeveloping microstructurendash property relationships or gen-erating data for calibrating physical models that have theprincipal structures primary ferrite pearlite Widmanstat-ten ferrite bainite and martensite as output
It is clear from the above review that while the IIWscheme provides a sound structure for quantifying complexmicrostructures in steels the classi cation of constituentssuch as ferrite sideplate and acicular ferrite is incompatiblewith the principal structures found in the reconstructiveanddisplacive transformation regimes of ferrous materialsKnowledge of the actual transformation products consti-tuting ferrite sideplate and acicular ferrite structures isrequired Classi cation is also needed of idiomorphic ferriteand ferrite sideplate structures growing relatively unim-peded from intragranular inclusions
Problems that may be encountered in relating sub-category microstructural components to principal struc-tures at prior austenite grain boundary and intragranularsites are discussed below together with possible solutionsThe ways in which transformationproducts associated withferrite sideplate and acicular ferrite structures may beidenti ed will be addressed The use of optical microscopywith specimens polished to a 025 mm nish and etched in2 nital is assumed as standard However instances will begiven where different instruments and techniques may beneeded to solve problems Where possible the effects ofsteel composition and heat treatment will be highlightedbut detailed examples are outside the scope of the presentpaper
PRIMARY FERRITEIn low alloy weld metals care has to be taken in identifyingprimary ferrite due to stereological effects Ferrite allo-triomorphs growing from prior austenite grain boundariesbeneath the plane of observation may appear as polygonalferrite grains in the intragranular regions (see Fig 1) Ifthese ferrite allotriomorphs are of a size approximatelythree times greater than those of surrounding acicularferrite laths or grains it is likely that they are the constituentPF(I) described in the IIW scheme It is unlikely that suchlarge grains are idiomorphic ferrite I(PF) nucleated oninclusions as referenced in the literature2 2 since the lattertend to nucleate at lower temperatures with relatively littletime for growth (see Fig 2)
PEARLITEProblems may arise in classifying pearlite when it is presentalong with displacive transformation products
Lamellar pearlite FC(P) in the IIW classi cationscheme may be confused with martensite if the ferritecementite plates are irresolvable under the light microscopeA distinguishing feature is the generally rapid etchingresponse and lower hardness of the pearlite
The dark etching non-lamellar pearlite known as ferrite ndashcarbide aggregate FC in the IIW classi cation scheme maysometimes be confused with bainite The nodular appear-ance of pearlite as opposed to the sheaf appearance ofbainite may provide a distinguishing feature The carboncontent of the steel may also give an indication as to howmuch pearlite may be expected high volume fractionsshould not be present in low carbon steels Ultimatelyhowever knowledge of the thermal history and transforma-tion conditions of the steel may be needed to provide a checkon classi cation (see below) The reconstructive pearlitetransformation should take place slowly at high tempera-tures and over a wide temperature range A displacivetransformation to bainite should take place rapidly at lowertemperatures and over a relatively small temperature range
It is notable that in bainitic steels prolonged holding at agiven temperature may result in the incomplete reactionphenomenon (see above) Continued isothermal treatmentcan result in pearlite formation from the remaining carbonenriched austenite2 6
Dif culties in identi cation of pearlite may be com-poundedbya eutectoid transformationthathasbeen noted incontinuously cooled plain carbon steel (011C 05Mn)This involves ferrite growing in conjunction with repeatednucleation of alloy carbides on the moving ca interphaseboundary3 9 The reaction has been termed interphase pre-cipitation of cementite Dark etching equiaxed ferrite grainscontaining a ne dispersion of carbides are observed underthe light microscope while under the transmission electronmicroscope the cementite is seen in sheets
FERRITE SIDEPLATEBainite and Widmanstatten ferrite may be present insigni cant amounts in heat treated steels and the coarsegrained HAZ of welds but they are dif cult to classifyindividually so that both structures have been generallyreferred to as ferrite sideplate
WidmanstaEgrave tten ferriteClassi cation of Widmanstatten ferrite can prove dif cultbecause of its similarity to upper bainite but certainguidelines may be followed to avoid confusion
The free energy requirement or driving force would beexpected to be lower for Widmanstatten ferrite formationthan for the upper bainite transformation since the formeris thought to grow by the mutual accommodation of platesand the latter by sub-units (see above) All else being equaltherefore Widmanstatten ferrite may be expected to occurat higher temperatures than upper bainite and exhibit agenerally coarser structure with a lower dislocation densityFurthermorethe microphasesbetween Widmanstatten ferritelaths may be expected to be a mixture of pearlite bainitemartensite or retained austenite whereas the nature ofbainite formation (see above) means that cementite particlesmay generally be observed between the bainitic ferriteplates2 6 Microphases may be revealed by the use of dif-ferent chemical etchants (see below)
The identi cation of secondary Widmanstatten ferritewith aligned microphase FS(A) in the IIW scheme isrelatively easy since it grows from existing allotriomorphicferrite but care has to be taken in distinguishing theboundary between the two structures Identi cation ofprimary Widmanstatten ferrite is signi cantly more dif -cult it grows directly from prior austenite grain boundariesand may be more easily confused with upper bainite Theuse of colour etching methods4 0 4 1 in conjunction with
150 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
morphologies (often classed as bainite) Widmanstattenferrite was dif cult to place but was regarded as a genericallysimilar structure to bainite Intragranularcomponent phasessuch as acicular ferrite posed a much greater degree ofdif culty Much effort was made by the welding fraternity inthe 1980s to develop an overallmicrostructurequanti cationscheme for weld metals incorporating both prior austenitegrain boundary and intragranular nucleated constituentsand addressing stereological effects ie the way constituentsare orientated in space1 9 20 A scheme was devised whichbecame recognised as the International Institute of Welding(IIW) classi cation1 9 Most of the constituents de ned in theIIW scheme were relatively easily identi ed Furthermorethe scheme could just as readily be applied to steels whereaustenite grain boundary transformations dominate as toweld metals where intragranulartransformationsare the ruleHowever identi cation of the actual transformation pro-ducts constituting component structures such as ferritesideplate and acicular ferrite has proved dif cult Anelliand Di Nunzio2 1 recently devised a scheme providingguidance on identifying transformation products associatedwith sideplate structures which has had some success butstereological effects and intragranular constituents were nottreated in depth
The objective in the current work has therefore been toinvestigate the IIW microstructure classi cation scheme as abasis for quanti cation of complex microstructures in steelsThe overall aim has been to develop a scheme that althoughrequiring a basic knowledge as to the mechanism of for-mation of the principal structures will be relatively easy touse given optical microscopy standard specimen polishingand etching techniques and appropriate guidance Theapproach has been to review microstructural constituentsin the IIW scheme in the context of the development ofprincipal structures found in the reconstructive and dis-placive transformation regimes of steels Detailed intragra-nular as well as austenite grain boundary transformationproducts have been considered and also stereological effectsProblems relating microstructural constituents to principalstructures in the IIW scheme have been investigatedtogether with possible solutions so that a new quanti cationscheme may be developed with a much broader applicationrange The intention has been to cover microstructuresobserved in carbon (up to abount 08) and low alloy (up toapproximately 5) steels as well as weld metals (up to010C and 5 alloy) and weld HAZs
Classi cation of microstructures andterminology
In this section the mechanisms of formation of the principalstructures and the characteristic ferrite morphologies pro-duced in the reconstructive and displacive transformationregimes of ferrous materials are brie y reviewed Theclassi cation and terminology used in the IIW scheme aredescribed together with that of Dube et al1 7 to provide alink with the early work on classi cation of prior austenitegrain boundary ferrite morphologies Terminology used inrecent work by the present author and co-workers2 2 is alsoincluded to provide a contemporary view of complex intra-granular transformations including those generating themicrostructure commonly known as acicular ferrite
RECONSTRUCTIVE TRANSFORMATIONREGIMEIn the high temperature reconstructive transformationregime a change from the austenite to ferrite crystal struc-ture occurs by a reconstructionprocess involving movementof atoms across the ca transformationinterfaceThe principal
phases are ferrite and pearlite Reactions tend to bediffusion controlled with slow rates
FerriteIn low hardenability materials the rst phase usuallyforming on prior austenite grain boundaries during coolingbelow the Ae3 temperature is classically referred to asallotriomorphic ferrite as shown schematically in Fig 1The ferrite nuclei have a Kurdjumovndash Sachs (K ndash S) orien-tation relationship with one austenite grain and grow intothe adjacent austenite grain with which they should normallyhave a random orientation relationship2 3 At some lowertemperature ferrite may begin to nucleate on inclusionsinside the austenite grains2 2 2 4 and this is termed idiomor-phic ferrite (see Fig 1) The indications are that ferriteidiomorphs do not have a xed orientation relationshipwith the matrix grains into which they grow2 5
Growth at reconstructive transformation temperaturestends to be controlled by substitutional element diffusionaway from the ca interface at low undercooling and carbondiffusion at high undercooling Various growth modesare recognised in order of decreasing transformationtemperature2 6
(i) local equilibrium with bulk partition of substitu-tion alloying elements (PLE)
(ii) local equilibrium with negligible partition of sub-stitutional alloying elements (NPLE)
(iii) paraequilibrium where only the interstitial carbonatoms diffuse
The diffusion rate of carbon in austenite may be manyorders of magnitude greater than that of substitutionalatoms at reconstructive transformation temperatures Trueequilibrium segregation during phase transformations atmigrating interfaces is therefore unlikely to be achieved withregard to all components Growth under diffusion controlwith local equilibrium at the interface is then envisagedTwo phases may differ either signi cantly (PLE) ornegligibly (NPLE) in terms of substitutional alloy contentElement concentration or depletion spikes are invoked tosatisfy the thermodynamic constraints In many cases asthe transformation temperature is decreased the relativerates at which elements are able to diffuse negate theassumption of local equilibrium since the interface com-position spike would be only several atomic layers thick Insuch cases the concept of paraequilibrium is applied iethere is no redistribution of iron or substitution atoms at theinterface between the phases and only the interstitial carbonatoms diffuse The different growth modes described abovemay result in signi cant changes in ferrite growth mor-phology from equiax grains towards a plate shape (seebelow)
Dube et al1 7 refer to prior austenite grain boundaryallotriomorphic ferrite as GBF The IIW classi cationscheme refers to the rst phase forming at reconstructivetransformation temperatures as primary ferrite termed PF
Prior austenite grain boundary primary ferrite allotrio-morphs are termed PF(G) in the IIW classi cation scheme
1 Allotriomorphic and idiomorphic primary ferrite
144 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
and are usually observed in the form of polygonal grains orveins as shown schematically in Fig 1 Reference is madein the IIW scheme to polygonal ferrite grains in the intra-granular regions (see Fig 1) of a size approximately threetimes greater than those of the surrounding ferrite laths orgrains These ferrite grains in reality may be cross-sectionsof ferrite allotriomorphs that have grown from prioraustenite grain boundaries beneath the plane of observationand have a wide range of sizes They are termed PF(I) in theIIW scheme The present author and co-workers2 2 havereferred to the different forms of prior austenite grainboundary primary ferrite as GB(PF) so that a distinctionmay be made with idiomorphic primary ferrite as describedbelow
In weld metals stable particle dispersed steels and somemicroalloyed steels ferrite may nucleate not only at theaustenite grain boundaries but also on particles insidethe austenite grains2 2 2 7 (see Fig 2) The author and co-workers2 2 have termed these intragranular ferrite idio-morphs I(PF) Depending on the temperature in thereconstructive regime the intragranular ferrite morpholo-gies2 2 may take the form of blocks loops ellipses rosepetals or wedges The IIW classi cation scheme does nothave a terminology for these primary ferrite idiomorphs
PearliteClassically pearlite transformation may occur at austenitegrain boundariesor an inhomogeneitysuch as an inclusion2 3
Ferrite or cementite nucleation may initiate the pearlitetransformation depending on whether the steel is hypo- orhyper-eutectoid in composition Growth of a pearlite noduleinto an austenite grain proceeds with the formation ofalternate ferrite and cementite plates or lamellae Both thecementite and ferrite possess unique crystallographic orien-tations within the pearlite nodule2 3 Edgewise growth of theplates may occur and also branching of the cementitelamellae The rate controlling process in the growth ofpearlite is the diffusion of carbon As the transformationtemperature is lowered the driving force for the reaction isincreased but the diffusivityof carbon is decreased so that thepearlite interlamellar spacing is decreased
At high transformationtemperatures pearlite is generallyobserved as nodules of alternate ferrite and cementitelamellae that may be quite coarse and degenerate Whenviewed in cross-section the lamellae may appear as aferrite ndash carbide aggregate As the transformation tempera-ture is lowered the lamellae become increasingly neuntil the structure becomes irresolvable under the light
microscope (see Fig 3) The pearlite may then have a lightetching response Alternatively the lamellae may becomesubjected to distortion and bending appearing as a darketching ferritendash carbide aggregate or barely resolvablesomewhat non-lamellar pearlite often described in oldernomenclature as primary troostite2 8 2 9
In the IIW scheme FC(P) is used to describe lamellarpearlite degenerate or coarse pearlite and ne colonyor irresolvable pearlite The term FC is used to describeferrite ndash carbide aggregate At reconstructive transforma-tion temperatures large islands of pearlite or ferrite ndashcarbide aggregate may be interspersed with prior austenitegrain boundary primary ferrite PF(G) A similar situationmay occur with idiomorphic primary ferrite I(PF) (seeFig 4)2 7 In some cases pearlite may be present as micro-phase (see below)
DISPLACIVE TRANSFORMATION REGIMEIn the low temperature displacive transformation regime achange from the austenite to ferrite crystal lattice occurs byan invariant plane strain shape change with a large shearcomponent Diffusion of interstitial carbon atoms mayaccompany the shear transformation For a purely dis-placive transformation there is no movement of atomsacross the ca interface Reactions in the displacive trans-formation regime tend to be rapid The principal phases areWidmanstatten ferrite bainite and martensite
1 intragranular ferrite idiomorphs 2 grain boundary ferriteallotriomorphs
2 Morphologies of ferrite at prior austenite grain bound-ary and intragranular sites in 006C 146Mn sub-merged arc weld metal continuously cooled icedbrine quenched from 670degC22
1 alternate ferritecementite lamellae 2 regne ferrite plusmn carbideaggregate 3 irresolvable pearlite
3 Resolvable and irresolvable pearlite in 083C050Mn as rolled rod
1 idiomorphic ferrite 2 ferrite plusmn carbide aggregate 3 irresolva-ble pearlite
4 Intragranular primary ferrite and pearlite in as cast013C 20Mn cerium sulphide particle dispersedsteel27
Thewlis Classiregcation and quantiregcation of microstructures in steels 145
Materials Science and Technology February 2004 Vol 20
WidmanstaEgrave tten ferriteA classic feature of Widmanstatten ferrite formation is thatit may occur at relatively low undercooling2 3 The growthmechanism is thought to involve the simultaneous forma-tion of pairs of mutually accommodating plates so that lessdriving force is required for transformation than withbainite or martensite3 0 The ferrite plates grow rapidly witha high aspect ratio (~10 1) resulting in parallel arraysWidmanstatten ferrite is not the result of a purely displacivetransformation but forms by a paraequilibrium mechan-ism3 0 3 1 involving the rapid diffusion of interstitial carbonatoms across the advancing interface into the remainingaustenite during the shear transformation At the relativelylow undercooling required for Widmanstatten ferrite for-mation microphases of retained austenite martensite orferritecarbide aggregate (pearlite) may be formed betweenthe growing ferrite plates
Widmanstatten ferrite can easily be confused with bainiteDube et al1 7 describe both prior austenite grain boundaryWidmanstatten ferrite and bainite as ferrite sideplate FS butreference is also made to intragranular plates IP The IIWclassi cation scheme refers to all forms of Widmanstattenferrite and bainite as ferrite with second phase FS althougha distinction may be made in the terminology whenWidmanstatten ferrite can be positively identi ed egFS(SP)
Characteristically primary Widmanstatten ferrite platesgrow directly from a prior austenitegrain boundarywhereassecondary Widmanstatten ferrite plates grow from allo-trimorphic ferrite at the grain boundaries as shown sche-matically in Fig 5 Primary Widmanstatten ferrite platesmay also grow from inclusions while secondary Widman-statten ferrite plates grow from intragranular idiomorphicferrite2 2 3 2
Widmanstatten ferrite that grows from prior austenitegrain boundary sites is usually seen as colonies of coarsesideplates with aligned microphase (see Fig 6) which aretermed FS(A) in the IIW scheme The individual plateswithin an array are separated by low angle boundaries thatare dif cult to resolve under the light microscope althoughcareful specimen polishing and etching may reveal themDepending on the plane of observation the microphasesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are islands of microphase in a matrix of ferritewithin the prior austenite grains (see Fig 6) The Widman-statten ferrite is then classi ed as FS(NA) The presentauthor and co-workers2 2 have referred to the differentforms of prior austenite grain boundary Widmanstattenferrite as GB(WF) so that a distinction may be made withintragranular Widmanstatten ferrite as described below
In the intragranular regions of weld metals and insome steels2 2 2 7 3 2 multiple large plates (aspect ratiogt4 1) of Widmanstatten ferrite with aligned microphase
may be observed that grow from inclusions (primaryWidmanstatten ferrite) or from idiomorphic ferrite(secondary Widmanstatten ferrite) as shown in Fig 7The IIW classi cation scheme does not have a terminologyfor these plates However they have been designatedintragranular ferrite sideplates FS(I) in recent work bythe present author3 2 In many cases individual plates maybe observed that have grown relatively unimpeded fromintragranular inclusions (see Fig 8) These plates do nothave aligned microphase and may be interspersed withbainite or martensite2 2 2 7 3 2 The inclusions from which theplates grow may not be viewed since they may be under theplane of observationThese plates have been designated IFPby the present author3 2 who summed FS(I) and IFP to givea total quantity of intragranular Widmanstatten ferritereferred to as I(WF) Where there is a high density ofinclusions multiple hard impingements of individualWidmanstatten ferrite plates growing from inclusions2 2 3 2
may produce a ne interlocking structure (see schematicdiagram Fig 5) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
BainiteBainite is generally recognised as forming at temperatureswhere diffusion controlled transformationsare sluggish andhas features in common with low temperature martensitic
5 Primary and secondary Widmanstatten ferrite
1 idiomorphic ferrite 2 prior austenite grain boundary Widman-staEgrave tten ferrite with aligned microphase 3 prior austenite grainboundary WidmanstaEgrave tten ferrite with non-aligned microphase
6 Interlocking colonies of Widmanstatten ferrite in 005C135Mn HSLA steel submerged arc weld HAZ
7 Intragranular Widmanstatten ferrite sideplates in asdeposited 008C 287Mn 035Mo 00027B0019Ti submerged arc weld metal32 arrow indicatesmultiple plates of Widmanstatten ferrite with alignedmicrophase nucleated on large intragranular inclusions
146 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformations2 6 It grows as individual plates or sub-unitsto form parallel arrays or sheaves The growth of each sub-unit is accompanied by an invariant plane strain shapechange with a large shear component There is noredistribution of iron or substitutional solute atoms at thetransformation interface Classically bainite has been cate-gorised into two component structures notably upper andlower bainite depending on the transformation tempera-ture Carbon partitions into the residual austenite in upperbainite and precipitates as cementite between the bainiticferrite plates In lower bainite the ferrite becomes super-saturated with carbon and some carbide precipitationoccurs within the ferrite sub-units as well as between them
The exact growth mechanism of bainite is still the subjectof much debate3 3 ndash 3 5 A paraequilibrium mechanism inupper bainite involving a shear transformation accompa-nied by the rapid diffusion of interstitial carbon atomsacross the ca interface would mean that bainitic growth wasin part similar to Widmanstatten ferrite However a purelydisplacive transformation would require no redistributionof atoms across the ca interface A temperature curve To
may be identi ed on the Fe ndash C phase diagram de ningthermodynamically where austenite and ferrite of the samecomposition have identical free energy2 6 3 3 At the To
temperature there is no driving force for transformationThe To curve has a negative slope with carbon concentra-tion lying between the Ae 1 and Ae 3 lines of the Fe ndash C phasediagram In a steel with a carbon concentration lower thanthat de ned by the To curve bainitic ferrite plates maybegin to grow without diffusion at an appropriate holdtemperature then partition excess carbon into the residualaustenite Further diffusionless growth of plates may takeplace from the carbon enriched austenite and the processcontinues until such transformation becomes thermodyna-mically impossible at the To curve This is termed theincomplete reaction phenomenon Continuous undercool-ing of the steel below To will cause the bainite reaction to bemaintained Carbide precipitation occurs when the trans-formation conditions are kinetically favourable For apurely displacive transformation therefore a rapid redis-tribution of carbon atoms is envisaged after the diffusion-less growth of bainitic ferrite sub-units2 6
Bainite can easily be confused with Widmanstatten ferriteas noted above Both structures are referred to as ferritewith second phase FS in the IIW classi cation schemealthougha distinctionmay be made in the terminologywherebainite can be clearly identi ed eg FS(B) A further dis-tinction may be made where upper and lower bainite can bepositively identi ed eg FS(UB) and FS(LB) respectively
Characteristically bainite may grow directly from a prioraustenite grain boundary2 6 or an intragranular inclusion3 6
as shown schematically in Fig 9 Sympathetic nucleation ofbainite plates from existing sheaves is a common feature
Bainite that grows from prior austenite grain boundariesis commonly observed in the form of interlocking sheaves ofvery ne plates with aligned cementite particles (seeFig 10) which are designated FS(A) in the IIW schemeIn upper bainite FS(UB) carbide particles are observedbetween the plates while in lower bainite FS(LB) thecarbides are within as well as between the plates and thestructure tends to have a darker etching response Theindividual plates within a sheaf are separated by low angleboundaries that are virtually irresolvable under the lightmicroscope The sheaves are shown in the process of growthin Fig 11 Extensive sympathetic nucleation is evidentDepending on the plane of observation cementite particlesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are carbide particles in a matrix of ferrite within theprior austenite grains (see Fig 10) The bainite is thenclassi ed as FS(NA) The present author and co-workers2 2
have referred to the different forms of prior austenite grainboundary bainitic ferrite as GB(B) so that a distinction maybe made with intragranular bainite as described below
In some steels and weld metals2 6 3 2 3 6 bainite sheaves maybe seen to grow from intragranular inclusions (see Fig 12)Individual ne plates of bainitic ferrite may also beobserved that grow relatively unimpeded from intragranu-lar inclusions (see Fig 13) The latter plates do not havealigned carbide particles and may be dif cult to distinguishfrom Widmanstatten ferrite plates IFP (see above) Theinclusions from which the plates grow may not be observed
1 idiomorphic ferrite 2 individual plate of WidmanstaEgrave tten fer-rite nucleated on large intragranular inclusions
8 Growth of intragranular Widmanstatten ferrite platesin 006C 137Mn 017Mo 00028B 0027Tisubmerged arc weld metal continuously cooledhelium quenched from 620degC22
9 Bainite sheaves and sub-units
1 lower bainite with carbide particles between as wellas within subunits 2 upper bainite with aligned carbide3 bainitic ferrite with non-aligned carbide
10 Interlocking sheaves of upper and lower bainite in017C 10Mn steel laser weld HAZ
Thewlis Classiregcation and quantiregcation of microstructures in steels 147
Materials Science and Technology February 2004 Vol 20
since they are under the plane of observation The IIWclassi cation scheme does not have a terminology for thedifferent forms of intragranular bainite but the author andco-workers2 2 have termed them I(B) Where there is a highdensity of inclusions multiple hard impingements ofindividual bainitic plates growing from the inclusions may
result in a very ne interlocking structure2 6 3 2 (see schematicdiagram Fig 9) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
MartensiteMartensite is classically an extremely rapid diffusionlesstransformation where carbon is retained in solution3 7 Asthe austenite lattice changes from fcc to the required mar-tensite bcc or bct lattice strain energy considerationsdominate and the martensite is constrained to be in the formof thin plates
In low carbon steels (less than ~02C) lath martensitewith a bcc crystal structure is the commonly occurringform3 7 and is designated M or M(L) in the IIW scheme Themartensite units are formed in the shape of laths thatare grouped into larger sheaves or packets (see Fig 14)The sub-structure consists of a high density of dislocationsarranged in cells each martensite lath is composed of manydislocation cells As the steel carbon content increases signi- cantly above about 02C plate martensite tends to formwith either a bct or bcc crystal structure3 7 The martensiteunits form as individual lenticular plates (see Fig 15) with asubstructure consisting of very ne twins This form ofmartensite is termed twinned martensite in the IIW schemeand is designated M or M(T) Martensite whether in platesor lath form is generally irresolvable under the light micro-scope and tends to have a slow etching response
12 Growth of bainite sheaves from intragranular inclu-sions in 038C 139Mn 0039S 009V0013N steel isothermally transformed 38 s at450degC arrow indicates multiple laths of bainite withcarbide particles between as well as within subunits
11 Growth of bainite sheaves and (arrowed) sympatheticnucleation of laths in 038C 139Mn 0039S009V steel isothermally transformed 45 s at 400degC
13 Growth of intragranular bainite plates in 038C139Mn 0039S 009V 0013N steel isother-mally transformed 38 s at 500degC arrows indicateindividual plates of bainitic ferrite nucleated on smallintragranular inclusions
14 Lath martensite in 013C laser weld metal arrowindicates martensite laths with highly dislocated sub-structure
15 Plate or twin martensite in 027C laser weld metalarrow indicates lenticular martensite with twinnedsubstructure
148 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
Acicular ferriteConventionally2 6 acicular ferrite is recognised as an intra-granular nucleated morphology of ferrite in which there aremultiple impingements between grains The acicular ferritenucleates on inclusions inside the prior austenite grainsduring the cda transformation Provided there is a highdensity of inclusions a ne interlocking structure (generallylt5 mm) can be produced
In the IIW scheme acicular ferrite is designated AF Fora long time acicular ferrite was thought to be a singletransformation product Early work3 8 suggested that itwas intragranularly nucleated Widmanstatten ferrite Laterresearch2 6 provided evidence for intragranularly nucleatedbainite However recent research by the author and co-workers2 2 has demonstrated that the nature of acicularferrite may be as shown schematically in Fig 16 Differentreaction products may nucleate on intragranular inclusionsat reconstructive and displacive transformation tempera-
tures during continuous cooling depending on the naturesize and amount of inclusions (see Figs 2 and 17) Acicularferrite results from multiple hard impingements of thedifferent transformation products The sequence oftransformations is consistent with the theoretical activationenergy barrier to nucleation of the different sites Acicularferrite development may thus be de ned in terms of con-ventional steel transformation products and CCT diagramsincorporating both intragranular and grain boundarytransformations
Under continuous cooling transformation conditions
AF~I(PF)zI(WF)zI(B)
This leads to acicular ferrite that may have a variety offorms depending on steel composition cooling rate andinclusion characteristics Acicular ferrite may consist ofmixtures of different intragranular transformationproducts(see Fig 18)2 2 3 2 Alternatively Widmanstatten acicularferrite or bainitic acicular ferrite may form per se2 6 3 8
However if reactions are completed at purely reconstruc-tive transformation temperatures it may be preferable touse the term idiomorphic primary ferrite instead of acicularferrite to describe the microstructure since intragranularprimary ferrite is likely to be coarse and non-acicular inmorphology (see Fig 4)
Acicular ferrite is usually observed as a ne interlockingferrite structure interspersed with microphases (see Fig 18)The shape of the ferrite plates may not appear to be needle-like as the use of the term lsquoacicularrsquo would imply This isbecause the different ferrite morphologies cannot grow veryfar before mutual hard impingement It is evident fromFig 18 that the degree of re nement of the acicular ferrite isdependent on the nature of the transformation productsinherent in its formation
16 Nature of acicular ferrite
a
b
a idiomorphic ferrite (arrowed) nucleated on large inclusionsb WidmanstaEgrave tten ferrite plates (arrowed) nucleated on smallinclusions
17 Acicular ferrite development in 006C 137Mn017Mo 00028B 0027Ti submerged arc weldmetal continuously cooled iced brine quenched from615degC22
a
b
a intragranular primary ferriteplusmn WidmanstaEgrave tten ferrite in C plusmn Mnweld metal22 b intragranular WidmanstaEgrave tten ferrite plusmn bainitein Ti plusmn Mo plusmn B alloyed weld metal32
18 Forms of acicular ferrite
Thewlis Classiregcation and quantiregcation of microstructures in steels 149
Materials Science and Technology February 2004 Vol 20
MicrophasesThe different ferrite growth modes of the principal struc-tures described above result in carbon enrichment of theremaining austenite leading to associated second phases ofretained austenite martensite bainite or ferrite ndash carbideaggregate (pearlite) depending on the degree of carbonenrichment of the austenite and the prevailing coolingconditions The second phases associated with Widman-statten ferrite and acicular ferrite are generally quite small(2 ndash 5 mm) and are termed microphases
IIW classi cation scheme problem areasand solutions
The objective in the present work was to investigate the IIWmicrostructure classi cation scheme for weld metals as abasis for quantifying the full range of microstructures foundin plain carbon and low alloy steels as well as ferritic weldmetals and parent plate heat affected zones A means maythus be provided of obtaining database information fordeveloping microstructurendash property relationships or gen-erating data for calibrating physical models that have theprincipal structures primary ferrite pearlite Widmanstat-ten ferrite bainite and martensite as output
It is clear from the above review that while the IIWscheme provides a sound structure for quantifying complexmicrostructures in steels the classi cation of constituentssuch as ferrite sideplate and acicular ferrite is incompatiblewith the principal structures found in the reconstructiveanddisplacive transformation regimes of ferrous materialsKnowledge of the actual transformation products consti-tuting ferrite sideplate and acicular ferrite structures isrequired Classi cation is also needed of idiomorphic ferriteand ferrite sideplate structures growing relatively unim-peded from intragranular inclusions
Problems that may be encountered in relating sub-category microstructural components to principal struc-tures at prior austenite grain boundary and intragranularsites are discussed below together with possible solutionsThe ways in which transformationproducts associated withferrite sideplate and acicular ferrite structures may beidenti ed will be addressed The use of optical microscopywith specimens polished to a 025 mm nish and etched in2 nital is assumed as standard However instances will begiven where different instruments and techniques may beneeded to solve problems Where possible the effects ofsteel composition and heat treatment will be highlightedbut detailed examples are outside the scope of the presentpaper
PRIMARY FERRITEIn low alloy weld metals care has to be taken in identifyingprimary ferrite due to stereological effects Ferrite allo-triomorphs growing from prior austenite grain boundariesbeneath the plane of observation may appear as polygonalferrite grains in the intragranular regions (see Fig 1) Ifthese ferrite allotriomorphs are of a size approximatelythree times greater than those of surrounding acicularferrite laths or grains it is likely that they are the constituentPF(I) described in the IIW scheme It is unlikely that suchlarge grains are idiomorphic ferrite I(PF) nucleated oninclusions as referenced in the literature2 2 since the lattertend to nucleate at lower temperatures with relatively littletime for growth (see Fig 2)
PEARLITEProblems may arise in classifying pearlite when it is presentalong with displacive transformation products
Lamellar pearlite FC(P) in the IIW classi cationscheme may be confused with martensite if the ferritecementite plates are irresolvable under the light microscopeA distinguishing feature is the generally rapid etchingresponse and lower hardness of the pearlite
The dark etching non-lamellar pearlite known as ferrite ndashcarbide aggregate FC in the IIW classi cation scheme maysometimes be confused with bainite The nodular appear-ance of pearlite as opposed to the sheaf appearance ofbainite may provide a distinguishing feature The carboncontent of the steel may also give an indication as to howmuch pearlite may be expected high volume fractionsshould not be present in low carbon steels Ultimatelyhowever knowledge of the thermal history and transforma-tion conditions of the steel may be needed to provide a checkon classi cation (see below) The reconstructive pearlitetransformation should take place slowly at high tempera-tures and over a wide temperature range A displacivetransformation to bainite should take place rapidly at lowertemperatures and over a relatively small temperature range
It is notable that in bainitic steels prolonged holding at agiven temperature may result in the incomplete reactionphenomenon (see above) Continued isothermal treatmentcan result in pearlite formation from the remaining carbonenriched austenite2 6
Dif culties in identi cation of pearlite may be com-poundedbya eutectoid transformationthathasbeen noted incontinuously cooled plain carbon steel (011C 05Mn)This involves ferrite growing in conjunction with repeatednucleation of alloy carbides on the moving ca interphaseboundary3 9 The reaction has been termed interphase pre-cipitation of cementite Dark etching equiaxed ferrite grainscontaining a ne dispersion of carbides are observed underthe light microscope while under the transmission electronmicroscope the cementite is seen in sheets
FERRITE SIDEPLATEBainite and Widmanstatten ferrite may be present insigni cant amounts in heat treated steels and the coarsegrained HAZ of welds but they are dif cult to classifyindividually so that both structures have been generallyreferred to as ferrite sideplate
WidmanstaEgrave tten ferriteClassi cation of Widmanstatten ferrite can prove dif cultbecause of its similarity to upper bainite but certainguidelines may be followed to avoid confusion
The free energy requirement or driving force would beexpected to be lower for Widmanstatten ferrite formationthan for the upper bainite transformation since the formeris thought to grow by the mutual accommodation of platesand the latter by sub-units (see above) All else being equaltherefore Widmanstatten ferrite may be expected to occurat higher temperatures than upper bainite and exhibit agenerally coarser structure with a lower dislocation densityFurthermorethe microphasesbetween Widmanstatten ferritelaths may be expected to be a mixture of pearlite bainitemartensite or retained austenite whereas the nature ofbainite formation (see above) means that cementite particlesmay generally be observed between the bainitic ferriteplates2 6 Microphases may be revealed by the use of dif-ferent chemical etchants (see below)
The identi cation of secondary Widmanstatten ferritewith aligned microphase FS(A) in the IIW scheme isrelatively easy since it grows from existing allotriomorphicferrite but care has to be taken in distinguishing theboundary between the two structures Identi cation ofprimary Widmanstatten ferrite is signi cantly more dif -cult it grows directly from prior austenite grain boundariesand may be more easily confused with upper bainite Theuse of colour etching methods4 0 4 1 in conjunction with
150 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
and are usually observed in the form of polygonal grains orveins as shown schematically in Fig 1 Reference is madein the IIW scheme to polygonal ferrite grains in the intra-granular regions (see Fig 1) of a size approximately threetimes greater than those of the surrounding ferrite laths orgrains These ferrite grains in reality may be cross-sectionsof ferrite allotriomorphs that have grown from prioraustenite grain boundaries beneath the plane of observationand have a wide range of sizes They are termed PF(I) in theIIW scheme The present author and co-workers2 2 havereferred to the different forms of prior austenite grainboundary primary ferrite as GB(PF) so that a distinctionmay be made with idiomorphic primary ferrite as describedbelow
In weld metals stable particle dispersed steels and somemicroalloyed steels ferrite may nucleate not only at theaustenite grain boundaries but also on particles insidethe austenite grains2 2 2 7 (see Fig 2) The author and co-workers2 2 have termed these intragranular ferrite idio-morphs I(PF) Depending on the temperature in thereconstructive regime the intragranular ferrite morpholo-gies2 2 may take the form of blocks loops ellipses rosepetals or wedges The IIW classi cation scheme does nothave a terminology for these primary ferrite idiomorphs
PearliteClassically pearlite transformation may occur at austenitegrain boundariesor an inhomogeneitysuch as an inclusion2 3
Ferrite or cementite nucleation may initiate the pearlitetransformation depending on whether the steel is hypo- orhyper-eutectoid in composition Growth of a pearlite noduleinto an austenite grain proceeds with the formation ofalternate ferrite and cementite plates or lamellae Both thecementite and ferrite possess unique crystallographic orien-tations within the pearlite nodule2 3 Edgewise growth of theplates may occur and also branching of the cementitelamellae The rate controlling process in the growth ofpearlite is the diffusion of carbon As the transformationtemperature is lowered the driving force for the reaction isincreased but the diffusivityof carbon is decreased so that thepearlite interlamellar spacing is decreased
At high transformationtemperatures pearlite is generallyobserved as nodules of alternate ferrite and cementitelamellae that may be quite coarse and degenerate Whenviewed in cross-section the lamellae may appear as aferrite ndash carbide aggregate As the transformation tempera-ture is lowered the lamellae become increasingly neuntil the structure becomes irresolvable under the light
microscope (see Fig 3) The pearlite may then have a lightetching response Alternatively the lamellae may becomesubjected to distortion and bending appearing as a darketching ferritendash carbide aggregate or barely resolvablesomewhat non-lamellar pearlite often described in oldernomenclature as primary troostite2 8 2 9
In the IIW scheme FC(P) is used to describe lamellarpearlite degenerate or coarse pearlite and ne colonyor irresolvable pearlite The term FC is used to describeferrite ndash carbide aggregate At reconstructive transforma-tion temperatures large islands of pearlite or ferrite ndashcarbide aggregate may be interspersed with prior austenitegrain boundary primary ferrite PF(G) A similar situationmay occur with idiomorphic primary ferrite I(PF) (seeFig 4)2 7 In some cases pearlite may be present as micro-phase (see below)
DISPLACIVE TRANSFORMATION REGIMEIn the low temperature displacive transformation regime achange from the austenite to ferrite crystal lattice occurs byan invariant plane strain shape change with a large shearcomponent Diffusion of interstitial carbon atoms mayaccompany the shear transformation For a purely dis-placive transformation there is no movement of atomsacross the ca interface Reactions in the displacive trans-formation regime tend to be rapid The principal phases areWidmanstatten ferrite bainite and martensite
1 intragranular ferrite idiomorphs 2 grain boundary ferriteallotriomorphs
2 Morphologies of ferrite at prior austenite grain bound-ary and intragranular sites in 006C 146Mn sub-merged arc weld metal continuously cooled icedbrine quenched from 670degC22
1 alternate ferritecementite lamellae 2 regne ferrite plusmn carbideaggregate 3 irresolvable pearlite
3 Resolvable and irresolvable pearlite in 083C050Mn as rolled rod
1 idiomorphic ferrite 2 ferrite plusmn carbide aggregate 3 irresolva-ble pearlite
4 Intragranular primary ferrite and pearlite in as cast013C 20Mn cerium sulphide particle dispersedsteel27
Thewlis Classiregcation and quantiregcation of microstructures in steels 145
Materials Science and Technology February 2004 Vol 20
WidmanstaEgrave tten ferriteA classic feature of Widmanstatten ferrite formation is thatit may occur at relatively low undercooling2 3 The growthmechanism is thought to involve the simultaneous forma-tion of pairs of mutually accommodating plates so that lessdriving force is required for transformation than withbainite or martensite3 0 The ferrite plates grow rapidly witha high aspect ratio (~10 1) resulting in parallel arraysWidmanstatten ferrite is not the result of a purely displacivetransformation but forms by a paraequilibrium mechan-ism3 0 3 1 involving the rapid diffusion of interstitial carbonatoms across the advancing interface into the remainingaustenite during the shear transformation At the relativelylow undercooling required for Widmanstatten ferrite for-mation microphases of retained austenite martensite orferritecarbide aggregate (pearlite) may be formed betweenthe growing ferrite plates
Widmanstatten ferrite can easily be confused with bainiteDube et al1 7 describe both prior austenite grain boundaryWidmanstatten ferrite and bainite as ferrite sideplate FS butreference is also made to intragranular plates IP The IIWclassi cation scheme refers to all forms of Widmanstattenferrite and bainite as ferrite with second phase FS althougha distinction may be made in the terminology whenWidmanstatten ferrite can be positively identi ed egFS(SP)
Characteristically primary Widmanstatten ferrite platesgrow directly from a prior austenitegrain boundarywhereassecondary Widmanstatten ferrite plates grow from allo-trimorphic ferrite at the grain boundaries as shown sche-matically in Fig 5 Primary Widmanstatten ferrite platesmay also grow from inclusions while secondary Widman-statten ferrite plates grow from intragranular idiomorphicferrite2 2 3 2
Widmanstatten ferrite that grows from prior austenitegrain boundary sites is usually seen as colonies of coarsesideplates with aligned microphase (see Fig 6) which aretermed FS(A) in the IIW scheme The individual plateswithin an array are separated by low angle boundaries thatare dif cult to resolve under the light microscope althoughcareful specimen polishing and etching may reveal themDepending on the plane of observation the microphasesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are islands of microphase in a matrix of ferritewithin the prior austenite grains (see Fig 6) The Widman-statten ferrite is then classi ed as FS(NA) The presentauthor and co-workers2 2 have referred to the differentforms of prior austenite grain boundary Widmanstattenferrite as GB(WF) so that a distinction may be made withintragranular Widmanstatten ferrite as described below
In the intragranular regions of weld metals and insome steels2 2 2 7 3 2 multiple large plates (aspect ratiogt4 1) of Widmanstatten ferrite with aligned microphase
may be observed that grow from inclusions (primaryWidmanstatten ferrite) or from idiomorphic ferrite(secondary Widmanstatten ferrite) as shown in Fig 7The IIW classi cation scheme does not have a terminologyfor these plates However they have been designatedintragranular ferrite sideplates FS(I) in recent work bythe present author3 2 In many cases individual plates maybe observed that have grown relatively unimpeded fromintragranular inclusions (see Fig 8) These plates do nothave aligned microphase and may be interspersed withbainite or martensite2 2 2 7 3 2 The inclusions from which theplates grow may not be viewed since they may be under theplane of observationThese plates have been designated IFPby the present author3 2 who summed FS(I) and IFP to givea total quantity of intragranular Widmanstatten ferritereferred to as I(WF) Where there is a high density ofinclusions multiple hard impingements of individualWidmanstatten ferrite plates growing from inclusions2 2 3 2
may produce a ne interlocking structure (see schematicdiagram Fig 5) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
BainiteBainite is generally recognised as forming at temperatureswhere diffusion controlled transformationsare sluggish andhas features in common with low temperature martensitic
5 Primary and secondary Widmanstatten ferrite
1 idiomorphic ferrite 2 prior austenite grain boundary Widman-staEgrave tten ferrite with aligned microphase 3 prior austenite grainboundary WidmanstaEgrave tten ferrite with non-aligned microphase
6 Interlocking colonies of Widmanstatten ferrite in 005C135Mn HSLA steel submerged arc weld HAZ
7 Intragranular Widmanstatten ferrite sideplates in asdeposited 008C 287Mn 035Mo 00027B0019Ti submerged arc weld metal32 arrow indicatesmultiple plates of Widmanstatten ferrite with alignedmicrophase nucleated on large intragranular inclusions
146 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformations2 6 It grows as individual plates or sub-unitsto form parallel arrays or sheaves The growth of each sub-unit is accompanied by an invariant plane strain shapechange with a large shear component There is noredistribution of iron or substitutional solute atoms at thetransformation interface Classically bainite has been cate-gorised into two component structures notably upper andlower bainite depending on the transformation tempera-ture Carbon partitions into the residual austenite in upperbainite and precipitates as cementite between the bainiticferrite plates In lower bainite the ferrite becomes super-saturated with carbon and some carbide precipitationoccurs within the ferrite sub-units as well as between them
The exact growth mechanism of bainite is still the subjectof much debate3 3 ndash 3 5 A paraequilibrium mechanism inupper bainite involving a shear transformation accompa-nied by the rapid diffusion of interstitial carbon atomsacross the ca interface would mean that bainitic growth wasin part similar to Widmanstatten ferrite However a purelydisplacive transformation would require no redistributionof atoms across the ca interface A temperature curve To
may be identi ed on the Fe ndash C phase diagram de ningthermodynamically where austenite and ferrite of the samecomposition have identical free energy2 6 3 3 At the To
temperature there is no driving force for transformationThe To curve has a negative slope with carbon concentra-tion lying between the Ae 1 and Ae 3 lines of the Fe ndash C phasediagram In a steel with a carbon concentration lower thanthat de ned by the To curve bainitic ferrite plates maybegin to grow without diffusion at an appropriate holdtemperature then partition excess carbon into the residualaustenite Further diffusionless growth of plates may takeplace from the carbon enriched austenite and the processcontinues until such transformation becomes thermodyna-mically impossible at the To curve This is termed theincomplete reaction phenomenon Continuous undercool-ing of the steel below To will cause the bainite reaction to bemaintained Carbide precipitation occurs when the trans-formation conditions are kinetically favourable For apurely displacive transformation therefore a rapid redis-tribution of carbon atoms is envisaged after the diffusion-less growth of bainitic ferrite sub-units2 6
Bainite can easily be confused with Widmanstatten ferriteas noted above Both structures are referred to as ferritewith second phase FS in the IIW classi cation schemealthougha distinctionmay be made in the terminologywherebainite can be clearly identi ed eg FS(B) A further dis-tinction may be made where upper and lower bainite can bepositively identi ed eg FS(UB) and FS(LB) respectively
Characteristically bainite may grow directly from a prioraustenite grain boundary2 6 or an intragranular inclusion3 6
as shown schematically in Fig 9 Sympathetic nucleation ofbainite plates from existing sheaves is a common feature
Bainite that grows from prior austenite grain boundariesis commonly observed in the form of interlocking sheaves ofvery ne plates with aligned cementite particles (seeFig 10) which are designated FS(A) in the IIW schemeIn upper bainite FS(UB) carbide particles are observedbetween the plates while in lower bainite FS(LB) thecarbides are within as well as between the plates and thestructure tends to have a darker etching response Theindividual plates within a sheaf are separated by low angleboundaries that are virtually irresolvable under the lightmicroscope The sheaves are shown in the process of growthin Fig 11 Extensive sympathetic nucleation is evidentDepending on the plane of observation cementite particlesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are carbide particles in a matrix of ferrite within theprior austenite grains (see Fig 10) The bainite is thenclassi ed as FS(NA) The present author and co-workers2 2
have referred to the different forms of prior austenite grainboundary bainitic ferrite as GB(B) so that a distinction maybe made with intragranular bainite as described below
In some steels and weld metals2 6 3 2 3 6 bainite sheaves maybe seen to grow from intragranular inclusions (see Fig 12)Individual ne plates of bainitic ferrite may also beobserved that grow relatively unimpeded from intragranu-lar inclusions (see Fig 13) The latter plates do not havealigned carbide particles and may be dif cult to distinguishfrom Widmanstatten ferrite plates IFP (see above) Theinclusions from which the plates grow may not be observed
1 idiomorphic ferrite 2 individual plate of WidmanstaEgrave tten fer-rite nucleated on large intragranular inclusions
8 Growth of intragranular Widmanstatten ferrite platesin 006C 137Mn 017Mo 00028B 0027Tisubmerged arc weld metal continuously cooledhelium quenched from 620degC22
9 Bainite sheaves and sub-units
1 lower bainite with carbide particles between as wellas within subunits 2 upper bainite with aligned carbide3 bainitic ferrite with non-aligned carbide
10 Interlocking sheaves of upper and lower bainite in017C 10Mn steel laser weld HAZ
Thewlis Classiregcation and quantiregcation of microstructures in steels 147
Materials Science and Technology February 2004 Vol 20
since they are under the plane of observation The IIWclassi cation scheme does not have a terminology for thedifferent forms of intragranular bainite but the author andco-workers2 2 have termed them I(B) Where there is a highdensity of inclusions multiple hard impingements ofindividual bainitic plates growing from the inclusions may
result in a very ne interlocking structure2 6 3 2 (see schematicdiagram Fig 9) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
MartensiteMartensite is classically an extremely rapid diffusionlesstransformation where carbon is retained in solution3 7 Asthe austenite lattice changes from fcc to the required mar-tensite bcc or bct lattice strain energy considerationsdominate and the martensite is constrained to be in the formof thin plates
In low carbon steels (less than ~02C) lath martensitewith a bcc crystal structure is the commonly occurringform3 7 and is designated M or M(L) in the IIW scheme Themartensite units are formed in the shape of laths thatare grouped into larger sheaves or packets (see Fig 14)The sub-structure consists of a high density of dislocationsarranged in cells each martensite lath is composed of manydislocation cells As the steel carbon content increases signi- cantly above about 02C plate martensite tends to formwith either a bct or bcc crystal structure3 7 The martensiteunits form as individual lenticular plates (see Fig 15) with asubstructure consisting of very ne twins This form ofmartensite is termed twinned martensite in the IIW schemeand is designated M or M(T) Martensite whether in platesor lath form is generally irresolvable under the light micro-scope and tends to have a slow etching response
12 Growth of bainite sheaves from intragranular inclu-sions in 038C 139Mn 0039S 009V0013N steel isothermally transformed 38 s at450degC arrow indicates multiple laths of bainite withcarbide particles between as well as within subunits
11 Growth of bainite sheaves and (arrowed) sympatheticnucleation of laths in 038C 139Mn 0039S009V steel isothermally transformed 45 s at 400degC
13 Growth of intragranular bainite plates in 038C139Mn 0039S 009V 0013N steel isother-mally transformed 38 s at 500degC arrows indicateindividual plates of bainitic ferrite nucleated on smallintragranular inclusions
14 Lath martensite in 013C laser weld metal arrowindicates martensite laths with highly dislocated sub-structure
15 Plate or twin martensite in 027C laser weld metalarrow indicates lenticular martensite with twinnedsubstructure
148 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
Acicular ferriteConventionally2 6 acicular ferrite is recognised as an intra-granular nucleated morphology of ferrite in which there aremultiple impingements between grains The acicular ferritenucleates on inclusions inside the prior austenite grainsduring the cda transformation Provided there is a highdensity of inclusions a ne interlocking structure (generallylt5 mm) can be produced
In the IIW scheme acicular ferrite is designated AF Fora long time acicular ferrite was thought to be a singletransformation product Early work3 8 suggested that itwas intragranularly nucleated Widmanstatten ferrite Laterresearch2 6 provided evidence for intragranularly nucleatedbainite However recent research by the author and co-workers2 2 has demonstrated that the nature of acicularferrite may be as shown schematically in Fig 16 Differentreaction products may nucleate on intragranular inclusionsat reconstructive and displacive transformation tempera-
tures during continuous cooling depending on the naturesize and amount of inclusions (see Figs 2 and 17) Acicularferrite results from multiple hard impingements of thedifferent transformation products The sequence oftransformations is consistent with the theoretical activationenergy barrier to nucleation of the different sites Acicularferrite development may thus be de ned in terms of con-ventional steel transformation products and CCT diagramsincorporating both intragranular and grain boundarytransformations
Under continuous cooling transformation conditions
AF~I(PF)zI(WF)zI(B)
This leads to acicular ferrite that may have a variety offorms depending on steel composition cooling rate andinclusion characteristics Acicular ferrite may consist ofmixtures of different intragranular transformationproducts(see Fig 18)2 2 3 2 Alternatively Widmanstatten acicularferrite or bainitic acicular ferrite may form per se2 6 3 8
However if reactions are completed at purely reconstruc-tive transformation temperatures it may be preferable touse the term idiomorphic primary ferrite instead of acicularferrite to describe the microstructure since intragranularprimary ferrite is likely to be coarse and non-acicular inmorphology (see Fig 4)
Acicular ferrite is usually observed as a ne interlockingferrite structure interspersed with microphases (see Fig 18)The shape of the ferrite plates may not appear to be needle-like as the use of the term lsquoacicularrsquo would imply This isbecause the different ferrite morphologies cannot grow veryfar before mutual hard impingement It is evident fromFig 18 that the degree of re nement of the acicular ferrite isdependent on the nature of the transformation productsinherent in its formation
16 Nature of acicular ferrite
a
b
a idiomorphic ferrite (arrowed) nucleated on large inclusionsb WidmanstaEgrave tten ferrite plates (arrowed) nucleated on smallinclusions
17 Acicular ferrite development in 006C 137Mn017Mo 00028B 0027Ti submerged arc weldmetal continuously cooled iced brine quenched from615degC22
a
b
a intragranular primary ferriteplusmn WidmanstaEgrave tten ferrite in C plusmn Mnweld metal22 b intragranular WidmanstaEgrave tten ferrite plusmn bainitein Ti plusmn Mo plusmn B alloyed weld metal32
18 Forms of acicular ferrite
Thewlis Classiregcation and quantiregcation of microstructures in steels 149
Materials Science and Technology February 2004 Vol 20
MicrophasesThe different ferrite growth modes of the principal struc-tures described above result in carbon enrichment of theremaining austenite leading to associated second phases ofretained austenite martensite bainite or ferrite ndash carbideaggregate (pearlite) depending on the degree of carbonenrichment of the austenite and the prevailing coolingconditions The second phases associated with Widman-statten ferrite and acicular ferrite are generally quite small(2 ndash 5 mm) and are termed microphases
IIW classi cation scheme problem areasand solutions
The objective in the present work was to investigate the IIWmicrostructure classi cation scheme for weld metals as abasis for quantifying the full range of microstructures foundin plain carbon and low alloy steels as well as ferritic weldmetals and parent plate heat affected zones A means maythus be provided of obtaining database information fordeveloping microstructurendash property relationships or gen-erating data for calibrating physical models that have theprincipal structures primary ferrite pearlite Widmanstat-ten ferrite bainite and martensite as output
It is clear from the above review that while the IIWscheme provides a sound structure for quantifying complexmicrostructures in steels the classi cation of constituentssuch as ferrite sideplate and acicular ferrite is incompatiblewith the principal structures found in the reconstructiveanddisplacive transformation regimes of ferrous materialsKnowledge of the actual transformation products consti-tuting ferrite sideplate and acicular ferrite structures isrequired Classi cation is also needed of idiomorphic ferriteand ferrite sideplate structures growing relatively unim-peded from intragranular inclusions
Problems that may be encountered in relating sub-category microstructural components to principal struc-tures at prior austenite grain boundary and intragranularsites are discussed below together with possible solutionsThe ways in which transformationproducts associated withferrite sideplate and acicular ferrite structures may beidenti ed will be addressed The use of optical microscopywith specimens polished to a 025 mm nish and etched in2 nital is assumed as standard However instances will begiven where different instruments and techniques may beneeded to solve problems Where possible the effects ofsteel composition and heat treatment will be highlightedbut detailed examples are outside the scope of the presentpaper
PRIMARY FERRITEIn low alloy weld metals care has to be taken in identifyingprimary ferrite due to stereological effects Ferrite allo-triomorphs growing from prior austenite grain boundariesbeneath the plane of observation may appear as polygonalferrite grains in the intragranular regions (see Fig 1) Ifthese ferrite allotriomorphs are of a size approximatelythree times greater than those of surrounding acicularferrite laths or grains it is likely that they are the constituentPF(I) described in the IIW scheme It is unlikely that suchlarge grains are idiomorphic ferrite I(PF) nucleated oninclusions as referenced in the literature2 2 since the lattertend to nucleate at lower temperatures with relatively littletime for growth (see Fig 2)
PEARLITEProblems may arise in classifying pearlite when it is presentalong with displacive transformation products
Lamellar pearlite FC(P) in the IIW classi cationscheme may be confused with martensite if the ferritecementite plates are irresolvable under the light microscopeA distinguishing feature is the generally rapid etchingresponse and lower hardness of the pearlite
The dark etching non-lamellar pearlite known as ferrite ndashcarbide aggregate FC in the IIW classi cation scheme maysometimes be confused with bainite The nodular appear-ance of pearlite as opposed to the sheaf appearance ofbainite may provide a distinguishing feature The carboncontent of the steel may also give an indication as to howmuch pearlite may be expected high volume fractionsshould not be present in low carbon steels Ultimatelyhowever knowledge of the thermal history and transforma-tion conditions of the steel may be needed to provide a checkon classi cation (see below) The reconstructive pearlitetransformation should take place slowly at high tempera-tures and over a wide temperature range A displacivetransformation to bainite should take place rapidly at lowertemperatures and over a relatively small temperature range
It is notable that in bainitic steels prolonged holding at agiven temperature may result in the incomplete reactionphenomenon (see above) Continued isothermal treatmentcan result in pearlite formation from the remaining carbonenriched austenite2 6
Dif culties in identi cation of pearlite may be com-poundedbya eutectoid transformationthathasbeen noted incontinuously cooled plain carbon steel (011C 05Mn)This involves ferrite growing in conjunction with repeatednucleation of alloy carbides on the moving ca interphaseboundary3 9 The reaction has been termed interphase pre-cipitation of cementite Dark etching equiaxed ferrite grainscontaining a ne dispersion of carbides are observed underthe light microscope while under the transmission electronmicroscope the cementite is seen in sheets
FERRITE SIDEPLATEBainite and Widmanstatten ferrite may be present insigni cant amounts in heat treated steels and the coarsegrained HAZ of welds but they are dif cult to classifyindividually so that both structures have been generallyreferred to as ferrite sideplate
WidmanstaEgrave tten ferriteClassi cation of Widmanstatten ferrite can prove dif cultbecause of its similarity to upper bainite but certainguidelines may be followed to avoid confusion
The free energy requirement or driving force would beexpected to be lower for Widmanstatten ferrite formationthan for the upper bainite transformation since the formeris thought to grow by the mutual accommodation of platesand the latter by sub-units (see above) All else being equaltherefore Widmanstatten ferrite may be expected to occurat higher temperatures than upper bainite and exhibit agenerally coarser structure with a lower dislocation densityFurthermorethe microphasesbetween Widmanstatten ferritelaths may be expected to be a mixture of pearlite bainitemartensite or retained austenite whereas the nature ofbainite formation (see above) means that cementite particlesmay generally be observed between the bainitic ferriteplates2 6 Microphases may be revealed by the use of dif-ferent chemical etchants (see below)
The identi cation of secondary Widmanstatten ferritewith aligned microphase FS(A) in the IIW scheme isrelatively easy since it grows from existing allotriomorphicferrite but care has to be taken in distinguishing theboundary between the two structures Identi cation ofprimary Widmanstatten ferrite is signi cantly more dif -cult it grows directly from prior austenite grain boundariesand may be more easily confused with upper bainite Theuse of colour etching methods4 0 4 1 in conjunction with
150 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
WidmanstaEgrave tten ferriteA classic feature of Widmanstatten ferrite formation is thatit may occur at relatively low undercooling2 3 The growthmechanism is thought to involve the simultaneous forma-tion of pairs of mutually accommodating plates so that lessdriving force is required for transformation than withbainite or martensite3 0 The ferrite plates grow rapidly witha high aspect ratio (~10 1) resulting in parallel arraysWidmanstatten ferrite is not the result of a purely displacivetransformation but forms by a paraequilibrium mechan-ism3 0 3 1 involving the rapid diffusion of interstitial carbonatoms across the advancing interface into the remainingaustenite during the shear transformation At the relativelylow undercooling required for Widmanstatten ferrite for-mation microphases of retained austenite martensite orferritecarbide aggregate (pearlite) may be formed betweenthe growing ferrite plates
Widmanstatten ferrite can easily be confused with bainiteDube et al1 7 describe both prior austenite grain boundaryWidmanstatten ferrite and bainite as ferrite sideplate FS butreference is also made to intragranular plates IP The IIWclassi cation scheme refers to all forms of Widmanstattenferrite and bainite as ferrite with second phase FS althougha distinction may be made in the terminology whenWidmanstatten ferrite can be positively identi ed egFS(SP)
Characteristically primary Widmanstatten ferrite platesgrow directly from a prior austenitegrain boundarywhereassecondary Widmanstatten ferrite plates grow from allo-trimorphic ferrite at the grain boundaries as shown sche-matically in Fig 5 Primary Widmanstatten ferrite platesmay also grow from inclusions while secondary Widman-statten ferrite plates grow from intragranular idiomorphicferrite2 2 3 2
Widmanstatten ferrite that grows from prior austenitegrain boundary sites is usually seen as colonies of coarsesideplates with aligned microphase (see Fig 6) which aretermed FS(A) in the IIW scheme The individual plateswithin an array are separated by low angle boundaries thatare dif cult to resolve under the light microscope althoughcareful specimen polishing and etching may reveal themDepending on the plane of observation the microphasesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are islands of microphase in a matrix of ferritewithin the prior austenite grains (see Fig 6) The Widman-statten ferrite is then classi ed as FS(NA) The presentauthor and co-workers2 2 have referred to the differentforms of prior austenite grain boundary Widmanstattenferrite as GB(WF) so that a distinction may be made withintragranular Widmanstatten ferrite as described below
In the intragranular regions of weld metals and insome steels2 2 2 7 3 2 multiple large plates (aspect ratiogt4 1) of Widmanstatten ferrite with aligned microphase
may be observed that grow from inclusions (primaryWidmanstatten ferrite) or from idiomorphic ferrite(secondary Widmanstatten ferrite) as shown in Fig 7The IIW classi cation scheme does not have a terminologyfor these plates However they have been designatedintragranular ferrite sideplates FS(I) in recent work bythe present author3 2 In many cases individual plates maybe observed that have grown relatively unimpeded fromintragranular inclusions (see Fig 8) These plates do nothave aligned microphase and may be interspersed withbainite or martensite2 2 2 7 3 2 The inclusions from which theplates grow may not be viewed since they may be under theplane of observationThese plates have been designated IFPby the present author3 2 who summed FS(I) and IFP to givea total quantity of intragranular Widmanstatten ferritereferred to as I(WF) Where there is a high density ofinclusions multiple hard impingements of individualWidmanstatten ferrite plates growing from inclusions2 2 3 2
may produce a ne interlocking structure (see schematicdiagram Fig 5) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
BainiteBainite is generally recognised as forming at temperatureswhere diffusion controlled transformationsare sluggish andhas features in common with low temperature martensitic
5 Primary and secondary Widmanstatten ferrite
1 idiomorphic ferrite 2 prior austenite grain boundary Widman-staEgrave tten ferrite with aligned microphase 3 prior austenite grainboundary WidmanstaEgrave tten ferrite with non-aligned microphase
6 Interlocking colonies of Widmanstatten ferrite in 005C135Mn HSLA steel submerged arc weld HAZ
7 Intragranular Widmanstatten ferrite sideplates in asdeposited 008C 287Mn 035Mo 00027B0019Ti submerged arc weld metal32 arrow indicatesmultiple plates of Widmanstatten ferrite with alignedmicrophase nucleated on large intragranular inclusions
146 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformations2 6 It grows as individual plates or sub-unitsto form parallel arrays or sheaves The growth of each sub-unit is accompanied by an invariant plane strain shapechange with a large shear component There is noredistribution of iron or substitutional solute atoms at thetransformation interface Classically bainite has been cate-gorised into two component structures notably upper andlower bainite depending on the transformation tempera-ture Carbon partitions into the residual austenite in upperbainite and precipitates as cementite between the bainiticferrite plates In lower bainite the ferrite becomes super-saturated with carbon and some carbide precipitationoccurs within the ferrite sub-units as well as between them
The exact growth mechanism of bainite is still the subjectof much debate3 3 ndash 3 5 A paraequilibrium mechanism inupper bainite involving a shear transformation accompa-nied by the rapid diffusion of interstitial carbon atomsacross the ca interface would mean that bainitic growth wasin part similar to Widmanstatten ferrite However a purelydisplacive transformation would require no redistributionof atoms across the ca interface A temperature curve To
may be identi ed on the Fe ndash C phase diagram de ningthermodynamically where austenite and ferrite of the samecomposition have identical free energy2 6 3 3 At the To
temperature there is no driving force for transformationThe To curve has a negative slope with carbon concentra-tion lying between the Ae 1 and Ae 3 lines of the Fe ndash C phasediagram In a steel with a carbon concentration lower thanthat de ned by the To curve bainitic ferrite plates maybegin to grow without diffusion at an appropriate holdtemperature then partition excess carbon into the residualaustenite Further diffusionless growth of plates may takeplace from the carbon enriched austenite and the processcontinues until such transformation becomes thermodyna-mically impossible at the To curve This is termed theincomplete reaction phenomenon Continuous undercool-ing of the steel below To will cause the bainite reaction to bemaintained Carbide precipitation occurs when the trans-formation conditions are kinetically favourable For apurely displacive transformation therefore a rapid redis-tribution of carbon atoms is envisaged after the diffusion-less growth of bainitic ferrite sub-units2 6
Bainite can easily be confused with Widmanstatten ferriteas noted above Both structures are referred to as ferritewith second phase FS in the IIW classi cation schemealthougha distinctionmay be made in the terminologywherebainite can be clearly identi ed eg FS(B) A further dis-tinction may be made where upper and lower bainite can bepositively identi ed eg FS(UB) and FS(LB) respectively
Characteristically bainite may grow directly from a prioraustenite grain boundary2 6 or an intragranular inclusion3 6
as shown schematically in Fig 9 Sympathetic nucleation ofbainite plates from existing sheaves is a common feature
Bainite that grows from prior austenite grain boundariesis commonly observed in the form of interlocking sheaves ofvery ne plates with aligned cementite particles (seeFig 10) which are designated FS(A) in the IIW schemeIn upper bainite FS(UB) carbide particles are observedbetween the plates while in lower bainite FS(LB) thecarbides are within as well as between the plates and thestructure tends to have a darker etching response Theindividual plates within a sheaf are separated by low angleboundaries that are virtually irresolvable under the lightmicroscope The sheaves are shown in the process of growthin Fig 11 Extensive sympathetic nucleation is evidentDepending on the plane of observation cementite particlesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are carbide particles in a matrix of ferrite within theprior austenite grains (see Fig 10) The bainite is thenclassi ed as FS(NA) The present author and co-workers2 2
have referred to the different forms of prior austenite grainboundary bainitic ferrite as GB(B) so that a distinction maybe made with intragranular bainite as described below
In some steels and weld metals2 6 3 2 3 6 bainite sheaves maybe seen to grow from intragranular inclusions (see Fig 12)Individual ne plates of bainitic ferrite may also beobserved that grow relatively unimpeded from intragranu-lar inclusions (see Fig 13) The latter plates do not havealigned carbide particles and may be dif cult to distinguishfrom Widmanstatten ferrite plates IFP (see above) Theinclusions from which the plates grow may not be observed
1 idiomorphic ferrite 2 individual plate of WidmanstaEgrave tten fer-rite nucleated on large intragranular inclusions
8 Growth of intragranular Widmanstatten ferrite platesin 006C 137Mn 017Mo 00028B 0027Tisubmerged arc weld metal continuously cooledhelium quenched from 620degC22
9 Bainite sheaves and sub-units
1 lower bainite with carbide particles between as wellas within subunits 2 upper bainite with aligned carbide3 bainitic ferrite with non-aligned carbide
10 Interlocking sheaves of upper and lower bainite in017C 10Mn steel laser weld HAZ
Thewlis Classiregcation and quantiregcation of microstructures in steels 147
Materials Science and Technology February 2004 Vol 20
since they are under the plane of observation The IIWclassi cation scheme does not have a terminology for thedifferent forms of intragranular bainite but the author andco-workers2 2 have termed them I(B) Where there is a highdensity of inclusions multiple hard impingements ofindividual bainitic plates growing from the inclusions may
result in a very ne interlocking structure2 6 3 2 (see schematicdiagram Fig 9) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
MartensiteMartensite is classically an extremely rapid diffusionlesstransformation where carbon is retained in solution3 7 Asthe austenite lattice changes from fcc to the required mar-tensite bcc or bct lattice strain energy considerationsdominate and the martensite is constrained to be in the formof thin plates
In low carbon steels (less than ~02C) lath martensitewith a bcc crystal structure is the commonly occurringform3 7 and is designated M or M(L) in the IIW scheme Themartensite units are formed in the shape of laths thatare grouped into larger sheaves or packets (see Fig 14)The sub-structure consists of a high density of dislocationsarranged in cells each martensite lath is composed of manydislocation cells As the steel carbon content increases signi- cantly above about 02C plate martensite tends to formwith either a bct or bcc crystal structure3 7 The martensiteunits form as individual lenticular plates (see Fig 15) with asubstructure consisting of very ne twins This form ofmartensite is termed twinned martensite in the IIW schemeand is designated M or M(T) Martensite whether in platesor lath form is generally irresolvable under the light micro-scope and tends to have a slow etching response
12 Growth of bainite sheaves from intragranular inclu-sions in 038C 139Mn 0039S 009V0013N steel isothermally transformed 38 s at450degC arrow indicates multiple laths of bainite withcarbide particles between as well as within subunits
11 Growth of bainite sheaves and (arrowed) sympatheticnucleation of laths in 038C 139Mn 0039S009V steel isothermally transformed 45 s at 400degC
13 Growth of intragranular bainite plates in 038C139Mn 0039S 009V 0013N steel isother-mally transformed 38 s at 500degC arrows indicateindividual plates of bainitic ferrite nucleated on smallintragranular inclusions
14 Lath martensite in 013C laser weld metal arrowindicates martensite laths with highly dislocated sub-structure
15 Plate or twin martensite in 027C laser weld metalarrow indicates lenticular martensite with twinnedsubstructure
148 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
Acicular ferriteConventionally2 6 acicular ferrite is recognised as an intra-granular nucleated morphology of ferrite in which there aremultiple impingements between grains The acicular ferritenucleates on inclusions inside the prior austenite grainsduring the cda transformation Provided there is a highdensity of inclusions a ne interlocking structure (generallylt5 mm) can be produced
In the IIW scheme acicular ferrite is designated AF Fora long time acicular ferrite was thought to be a singletransformation product Early work3 8 suggested that itwas intragranularly nucleated Widmanstatten ferrite Laterresearch2 6 provided evidence for intragranularly nucleatedbainite However recent research by the author and co-workers2 2 has demonstrated that the nature of acicularferrite may be as shown schematically in Fig 16 Differentreaction products may nucleate on intragranular inclusionsat reconstructive and displacive transformation tempera-
tures during continuous cooling depending on the naturesize and amount of inclusions (see Figs 2 and 17) Acicularferrite results from multiple hard impingements of thedifferent transformation products The sequence oftransformations is consistent with the theoretical activationenergy barrier to nucleation of the different sites Acicularferrite development may thus be de ned in terms of con-ventional steel transformation products and CCT diagramsincorporating both intragranular and grain boundarytransformations
Under continuous cooling transformation conditions
AF~I(PF)zI(WF)zI(B)
This leads to acicular ferrite that may have a variety offorms depending on steel composition cooling rate andinclusion characteristics Acicular ferrite may consist ofmixtures of different intragranular transformationproducts(see Fig 18)2 2 3 2 Alternatively Widmanstatten acicularferrite or bainitic acicular ferrite may form per se2 6 3 8
However if reactions are completed at purely reconstruc-tive transformation temperatures it may be preferable touse the term idiomorphic primary ferrite instead of acicularferrite to describe the microstructure since intragranularprimary ferrite is likely to be coarse and non-acicular inmorphology (see Fig 4)
Acicular ferrite is usually observed as a ne interlockingferrite structure interspersed with microphases (see Fig 18)The shape of the ferrite plates may not appear to be needle-like as the use of the term lsquoacicularrsquo would imply This isbecause the different ferrite morphologies cannot grow veryfar before mutual hard impingement It is evident fromFig 18 that the degree of re nement of the acicular ferrite isdependent on the nature of the transformation productsinherent in its formation
16 Nature of acicular ferrite
a
b
a idiomorphic ferrite (arrowed) nucleated on large inclusionsb WidmanstaEgrave tten ferrite plates (arrowed) nucleated on smallinclusions
17 Acicular ferrite development in 006C 137Mn017Mo 00028B 0027Ti submerged arc weldmetal continuously cooled iced brine quenched from615degC22
a
b
a intragranular primary ferriteplusmn WidmanstaEgrave tten ferrite in C plusmn Mnweld metal22 b intragranular WidmanstaEgrave tten ferrite plusmn bainitein Ti plusmn Mo plusmn B alloyed weld metal32
18 Forms of acicular ferrite
Thewlis Classiregcation and quantiregcation of microstructures in steels 149
Materials Science and Technology February 2004 Vol 20
MicrophasesThe different ferrite growth modes of the principal struc-tures described above result in carbon enrichment of theremaining austenite leading to associated second phases ofretained austenite martensite bainite or ferrite ndash carbideaggregate (pearlite) depending on the degree of carbonenrichment of the austenite and the prevailing coolingconditions The second phases associated with Widman-statten ferrite and acicular ferrite are generally quite small(2 ndash 5 mm) and are termed microphases
IIW classi cation scheme problem areasand solutions
The objective in the present work was to investigate the IIWmicrostructure classi cation scheme for weld metals as abasis for quantifying the full range of microstructures foundin plain carbon and low alloy steels as well as ferritic weldmetals and parent plate heat affected zones A means maythus be provided of obtaining database information fordeveloping microstructurendash property relationships or gen-erating data for calibrating physical models that have theprincipal structures primary ferrite pearlite Widmanstat-ten ferrite bainite and martensite as output
It is clear from the above review that while the IIWscheme provides a sound structure for quantifying complexmicrostructures in steels the classi cation of constituentssuch as ferrite sideplate and acicular ferrite is incompatiblewith the principal structures found in the reconstructiveanddisplacive transformation regimes of ferrous materialsKnowledge of the actual transformation products consti-tuting ferrite sideplate and acicular ferrite structures isrequired Classi cation is also needed of idiomorphic ferriteand ferrite sideplate structures growing relatively unim-peded from intragranular inclusions
Problems that may be encountered in relating sub-category microstructural components to principal struc-tures at prior austenite grain boundary and intragranularsites are discussed below together with possible solutionsThe ways in which transformationproducts associated withferrite sideplate and acicular ferrite structures may beidenti ed will be addressed The use of optical microscopywith specimens polished to a 025 mm nish and etched in2 nital is assumed as standard However instances will begiven where different instruments and techniques may beneeded to solve problems Where possible the effects ofsteel composition and heat treatment will be highlightedbut detailed examples are outside the scope of the presentpaper
PRIMARY FERRITEIn low alloy weld metals care has to be taken in identifyingprimary ferrite due to stereological effects Ferrite allo-triomorphs growing from prior austenite grain boundariesbeneath the plane of observation may appear as polygonalferrite grains in the intragranular regions (see Fig 1) Ifthese ferrite allotriomorphs are of a size approximatelythree times greater than those of surrounding acicularferrite laths or grains it is likely that they are the constituentPF(I) described in the IIW scheme It is unlikely that suchlarge grains are idiomorphic ferrite I(PF) nucleated oninclusions as referenced in the literature2 2 since the lattertend to nucleate at lower temperatures with relatively littletime for growth (see Fig 2)
PEARLITEProblems may arise in classifying pearlite when it is presentalong with displacive transformation products
Lamellar pearlite FC(P) in the IIW classi cationscheme may be confused with martensite if the ferritecementite plates are irresolvable under the light microscopeA distinguishing feature is the generally rapid etchingresponse and lower hardness of the pearlite
The dark etching non-lamellar pearlite known as ferrite ndashcarbide aggregate FC in the IIW classi cation scheme maysometimes be confused with bainite The nodular appear-ance of pearlite as opposed to the sheaf appearance ofbainite may provide a distinguishing feature The carboncontent of the steel may also give an indication as to howmuch pearlite may be expected high volume fractionsshould not be present in low carbon steels Ultimatelyhowever knowledge of the thermal history and transforma-tion conditions of the steel may be needed to provide a checkon classi cation (see below) The reconstructive pearlitetransformation should take place slowly at high tempera-tures and over a wide temperature range A displacivetransformation to bainite should take place rapidly at lowertemperatures and over a relatively small temperature range
It is notable that in bainitic steels prolonged holding at agiven temperature may result in the incomplete reactionphenomenon (see above) Continued isothermal treatmentcan result in pearlite formation from the remaining carbonenriched austenite2 6
Dif culties in identi cation of pearlite may be com-poundedbya eutectoid transformationthathasbeen noted incontinuously cooled plain carbon steel (011C 05Mn)This involves ferrite growing in conjunction with repeatednucleation of alloy carbides on the moving ca interphaseboundary3 9 The reaction has been termed interphase pre-cipitation of cementite Dark etching equiaxed ferrite grainscontaining a ne dispersion of carbides are observed underthe light microscope while under the transmission electronmicroscope the cementite is seen in sheets
FERRITE SIDEPLATEBainite and Widmanstatten ferrite may be present insigni cant amounts in heat treated steels and the coarsegrained HAZ of welds but they are dif cult to classifyindividually so that both structures have been generallyreferred to as ferrite sideplate
WidmanstaEgrave tten ferriteClassi cation of Widmanstatten ferrite can prove dif cultbecause of its similarity to upper bainite but certainguidelines may be followed to avoid confusion
The free energy requirement or driving force would beexpected to be lower for Widmanstatten ferrite formationthan for the upper bainite transformation since the formeris thought to grow by the mutual accommodation of platesand the latter by sub-units (see above) All else being equaltherefore Widmanstatten ferrite may be expected to occurat higher temperatures than upper bainite and exhibit agenerally coarser structure with a lower dislocation densityFurthermorethe microphasesbetween Widmanstatten ferritelaths may be expected to be a mixture of pearlite bainitemartensite or retained austenite whereas the nature ofbainite formation (see above) means that cementite particlesmay generally be observed between the bainitic ferriteplates2 6 Microphases may be revealed by the use of dif-ferent chemical etchants (see below)
The identi cation of secondary Widmanstatten ferritewith aligned microphase FS(A) in the IIW scheme isrelatively easy since it grows from existing allotriomorphicferrite but care has to be taken in distinguishing theboundary between the two structures Identi cation ofprimary Widmanstatten ferrite is signi cantly more dif -cult it grows directly from prior austenite grain boundariesand may be more easily confused with upper bainite Theuse of colour etching methods4 0 4 1 in conjunction with
150 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformations2 6 It grows as individual plates or sub-unitsto form parallel arrays or sheaves The growth of each sub-unit is accompanied by an invariant plane strain shapechange with a large shear component There is noredistribution of iron or substitutional solute atoms at thetransformation interface Classically bainite has been cate-gorised into two component structures notably upper andlower bainite depending on the transformation tempera-ture Carbon partitions into the residual austenite in upperbainite and precipitates as cementite between the bainiticferrite plates In lower bainite the ferrite becomes super-saturated with carbon and some carbide precipitationoccurs within the ferrite sub-units as well as between them
The exact growth mechanism of bainite is still the subjectof much debate3 3 ndash 3 5 A paraequilibrium mechanism inupper bainite involving a shear transformation accompa-nied by the rapid diffusion of interstitial carbon atomsacross the ca interface would mean that bainitic growth wasin part similar to Widmanstatten ferrite However a purelydisplacive transformation would require no redistributionof atoms across the ca interface A temperature curve To
may be identi ed on the Fe ndash C phase diagram de ningthermodynamically where austenite and ferrite of the samecomposition have identical free energy2 6 3 3 At the To
temperature there is no driving force for transformationThe To curve has a negative slope with carbon concentra-tion lying between the Ae 1 and Ae 3 lines of the Fe ndash C phasediagram In a steel with a carbon concentration lower thanthat de ned by the To curve bainitic ferrite plates maybegin to grow without diffusion at an appropriate holdtemperature then partition excess carbon into the residualaustenite Further diffusionless growth of plates may takeplace from the carbon enriched austenite and the processcontinues until such transformation becomes thermodyna-mically impossible at the To curve This is termed theincomplete reaction phenomenon Continuous undercool-ing of the steel below To will cause the bainite reaction to bemaintained Carbide precipitation occurs when the trans-formation conditions are kinetically favourable For apurely displacive transformation therefore a rapid redis-tribution of carbon atoms is envisaged after the diffusion-less growth of bainitic ferrite sub-units2 6
Bainite can easily be confused with Widmanstatten ferriteas noted above Both structures are referred to as ferritewith second phase FS in the IIW classi cation schemealthougha distinctionmay be made in the terminologywherebainite can be clearly identi ed eg FS(B) A further dis-tinction may be made where upper and lower bainite can bepositively identi ed eg FS(UB) and FS(LB) respectively
Characteristically bainite may grow directly from a prioraustenite grain boundary2 6 or an intragranular inclusion3 6
as shown schematically in Fig 9 Sympathetic nucleation ofbainite plates from existing sheaves is a common feature
Bainite that grows from prior austenite grain boundariesis commonly observed in the form of interlocking sheaves ofvery ne plates with aligned cementite particles (seeFig 10) which are designated FS(A) in the IIW schemeIn upper bainite FS(UB) carbide particles are observedbetween the plates while in lower bainite FS(LB) thecarbides are within as well as between the plates and thestructure tends to have a darker etching response Theindividual plates within a sheaf are separated by low angleboundaries that are virtually irresolvable under the lightmicroscope The sheaves are shown in the process of growthin Fig 11 Extensive sympathetic nucleation is evidentDepending on the plane of observation cementite particlesmay appear non-aligned When viewing a cross-section offerrite laths that have grown from prior austenite grainboundaries beneath the plane of observation all that maybe seen are carbide particles in a matrix of ferrite within theprior austenite grains (see Fig 10) The bainite is thenclassi ed as FS(NA) The present author and co-workers2 2
have referred to the different forms of prior austenite grainboundary bainitic ferrite as GB(B) so that a distinction maybe made with intragranular bainite as described below
In some steels and weld metals2 6 3 2 3 6 bainite sheaves maybe seen to grow from intragranular inclusions (see Fig 12)Individual ne plates of bainitic ferrite may also beobserved that grow relatively unimpeded from intragranu-lar inclusions (see Fig 13) The latter plates do not havealigned carbide particles and may be dif cult to distinguishfrom Widmanstatten ferrite plates IFP (see above) Theinclusions from which the plates grow may not be observed
1 idiomorphic ferrite 2 individual plate of WidmanstaEgrave tten fer-rite nucleated on large intragranular inclusions
8 Growth of intragranular Widmanstatten ferrite platesin 006C 137Mn 017Mo 00028B 0027Tisubmerged arc weld metal continuously cooledhelium quenched from 620degC22
9 Bainite sheaves and sub-units
1 lower bainite with carbide particles between as wellas within subunits 2 upper bainite with aligned carbide3 bainitic ferrite with non-aligned carbide
10 Interlocking sheaves of upper and lower bainite in017C 10Mn steel laser weld HAZ
Thewlis Classiregcation and quantiregcation of microstructures in steels 147
Materials Science and Technology February 2004 Vol 20
since they are under the plane of observation The IIWclassi cation scheme does not have a terminology for thedifferent forms of intragranular bainite but the author andco-workers2 2 have termed them I(B) Where there is a highdensity of inclusions multiple hard impingements ofindividual bainitic plates growing from the inclusions may
result in a very ne interlocking structure2 6 3 2 (see schematicdiagram Fig 9) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
MartensiteMartensite is classically an extremely rapid diffusionlesstransformation where carbon is retained in solution3 7 Asthe austenite lattice changes from fcc to the required mar-tensite bcc or bct lattice strain energy considerationsdominate and the martensite is constrained to be in the formof thin plates
In low carbon steels (less than ~02C) lath martensitewith a bcc crystal structure is the commonly occurringform3 7 and is designated M or M(L) in the IIW scheme Themartensite units are formed in the shape of laths thatare grouped into larger sheaves or packets (see Fig 14)The sub-structure consists of a high density of dislocationsarranged in cells each martensite lath is composed of manydislocation cells As the steel carbon content increases signi- cantly above about 02C plate martensite tends to formwith either a bct or bcc crystal structure3 7 The martensiteunits form as individual lenticular plates (see Fig 15) with asubstructure consisting of very ne twins This form ofmartensite is termed twinned martensite in the IIW schemeand is designated M or M(T) Martensite whether in platesor lath form is generally irresolvable under the light micro-scope and tends to have a slow etching response
12 Growth of bainite sheaves from intragranular inclu-sions in 038C 139Mn 0039S 009V0013N steel isothermally transformed 38 s at450degC arrow indicates multiple laths of bainite withcarbide particles between as well as within subunits
11 Growth of bainite sheaves and (arrowed) sympatheticnucleation of laths in 038C 139Mn 0039S009V steel isothermally transformed 45 s at 400degC
13 Growth of intragranular bainite plates in 038C139Mn 0039S 009V 0013N steel isother-mally transformed 38 s at 500degC arrows indicateindividual plates of bainitic ferrite nucleated on smallintragranular inclusions
14 Lath martensite in 013C laser weld metal arrowindicates martensite laths with highly dislocated sub-structure
15 Plate or twin martensite in 027C laser weld metalarrow indicates lenticular martensite with twinnedsubstructure
148 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
Acicular ferriteConventionally2 6 acicular ferrite is recognised as an intra-granular nucleated morphology of ferrite in which there aremultiple impingements between grains The acicular ferritenucleates on inclusions inside the prior austenite grainsduring the cda transformation Provided there is a highdensity of inclusions a ne interlocking structure (generallylt5 mm) can be produced
In the IIW scheme acicular ferrite is designated AF Fora long time acicular ferrite was thought to be a singletransformation product Early work3 8 suggested that itwas intragranularly nucleated Widmanstatten ferrite Laterresearch2 6 provided evidence for intragranularly nucleatedbainite However recent research by the author and co-workers2 2 has demonstrated that the nature of acicularferrite may be as shown schematically in Fig 16 Differentreaction products may nucleate on intragranular inclusionsat reconstructive and displacive transformation tempera-
tures during continuous cooling depending on the naturesize and amount of inclusions (see Figs 2 and 17) Acicularferrite results from multiple hard impingements of thedifferent transformation products The sequence oftransformations is consistent with the theoretical activationenergy barrier to nucleation of the different sites Acicularferrite development may thus be de ned in terms of con-ventional steel transformation products and CCT diagramsincorporating both intragranular and grain boundarytransformations
Under continuous cooling transformation conditions
AF~I(PF)zI(WF)zI(B)
This leads to acicular ferrite that may have a variety offorms depending on steel composition cooling rate andinclusion characteristics Acicular ferrite may consist ofmixtures of different intragranular transformationproducts(see Fig 18)2 2 3 2 Alternatively Widmanstatten acicularferrite or bainitic acicular ferrite may form per se2 6 3 8
However if reactions are completed at purely reconstruc-tive transformation temperatures it may be preferable touse the term idiomorphic primary ferrite instead of acicularferrite to describe the microstructure since intragranularprimary ferrite is likely to be coarse and non-acicular inmorphology (see Fig 4)
Acicular ferrite is usually observed as a ne interlockingferrite structure interspersed with microphases (see Fig 18)The shape of the ferrite plates may not appear to be needle-like as the use of the term lsquoacicularrsquo would imply This isbecause the different ferrite morphologies cannot grow veryfar before mutual hard impingement It is evident fromFig 18 that the degree of re nement of the acicular ferrite isdependent on the nature of the transformation productsinherent in its formation
16 Nature of acicular ferrite
a
b
a idiomorphic ferrite (arrowed) nucleated on large inclusionsb WidmanstaEgrave tten ferrite plates (arrowed) nucleated on smallinclusions
17 Acicular ferrite development in 006C 137Mn017Mo 00028B 0027Ti submerged arc weldmetal continuously cooled iced brine quenched from615degC22
a
b
a intragranular primary ferriteplusmn WidmanstaEgrave tten ferrite in C plusmn Mnweld metal22 b intragranular WidmanstaEgrave tten ferrite plusmn bainitein Ti plusmn Mo plusmn B alloyed weld metal32
18 Forms of acicular ferrite
Thewlis Classiregcation and quantiregcation of microstructures in steels 149
Materials Science and Technology February 2004 Vol 20
MicrophasesThe different ferrite growth modes of the principal struc-tures described above result in carbon enrichment of theremaining austenite leading to associated second phases ofretained austenite martensite bainite or ferrite ndash carbideaggregate (pearlite) depending on the degree of carbonenrichment of the austenite and the prevailing coolingconditions The second phases associated with Widman-statten ferrite and acicular ferrite are generally quite small(2 ndash 5 mm) and are termed microphases
IIW classi cation scheme problem areasand solutions
The objective in the present work was to investigate the IIWmicrostructure classi cation scheme for weld metals as abasis for quantifying the full range of microstructures foundin plain carbon and low alloy steels as well as ferritic weldmetals and parent plate heat affected zones A means maythus be provided of obtaining database information fordeveloping microstructurendash property relationships or gen-erating data for calibrating physical models that have theprincipal structures primary ferrite pearlite Widmanstat-ten ferrite bainite and martensite as output
It is clear from the above review that while the IIWscheme provides a sound structure for quantifying complexmicrostructures in steels the classi cation of constituentssuch as ferrite sideplate and acicular ferrite is incompatiblewith the principal structures found in the reconstructiveanddisplacive transformation regimes of ferrous materialsKnowledge of the actual transformation products consti-tuting ferrite sideplate and acicular ferrite structures isrequired Classi cation is also needed of idiomorphic ferriteand ferrite sideplate structures growing relatively unim-peded from intragranular inclusions
Problems that may be encountered in relating sub-category microstructural components to principal struc-tures at prior austenite grain boundary and intragranularsites are discussed below together with possible solutionsThe ways in which transformationproducts associated withferrite sideplate and acicular ferrite structures may beidenti ed will be addressed The use of optical microscopywith specimens polished to a 025 mm nish and etched in2 nital is assumed as standard However instances will begiven where different instruments and techniques may beneeded to solve problems Where possible the effects ofsteel composition and heat treatment will be highlightedbut detailed examples are outside the scope of the presentpaper
PRIMARY FERRITEIn low alloy weld metals care has to be taken in identifyingprimary ferrite due to stereological effects Ferrite allo-triomorphs growing from prior austenite grain boundariesbeneath the plane of observation may appear as polygonalferrite grains in the intragranular regions (see Fig 1) Ifthese ferrite allotriomorphs are of a size approximatelythree times greater than those of surrounding acicularferrite laths or grains it is likely that they are the constituentPF(I) described in the IIW scheme It is unlikely that suchlarge grains are idiomorphic ferrite I(PF) nucleated oninclusions as referenced in the literature2 2 since the lattertend to nucleate at lower temperatures with relatively littletime for growth (see Fig 2)
PEARLITEProblems may arise in classifying pearlite when it is presentalong with displacive transformation products
Lamellar pearlite FC(P) in the IIW classi cationscheme may be confused with martensite if the ferritecementite plates are irresolvable under the light microscopeA distinguishing feature is the generally rapid etchingresponse and lower hardness of the pearlite
The dark etching non-lamellar pearlite known as ferrite ndashcarbide aggregate FC in the IIW classi cation scheme maysometimes be confused with bainite The nodular appear-ance of pearlite as opposed to the sheaf appearance ofbainite may provide a distinguishing feature The carboncontent of the steel may also give an indication as to howmuch pearlite may be expected high volume fractionsshould not be present in low carbon steels Ultimatelyhowever knowledge of the thermal history and transforma-tion conditions of the steel may be needed to provide a checkon classi cation (see below) The reconstructive pearlitetransformation should take place slowly at high tempera-tures and over a wide temperature range A displacivetransformation to bainite should take place rapidly at lowertemperatures and over a relatively small temperature range
It is notable that in bainitic steels prolonged holding at agiven temperature may result in the incomplete reactionphenomenon (see above) Continued isothermal treatmentcan result in pearlite formation from the remaining carbonenriched austenite2 6
Dif culties in identi cation of pearlite may be com-poundedbya eutectoid transformationthathasbeen noted incontinuously cooled plain carbon steel (011C 05Mn)This involves ferrite growing in conjunction with repeatednucleation of alloy carbides on the moving ca interphaseboundary3 9 The reaction has been termed interphase pre-cipitation of cementite Dark etching equiaxed ferrite grainscontaining a ne dispersion of carbides are observed underthe light microscope while under the transmission electronmicroscope the cementite is seen in sheets
FERRITE SIDEPLATEBainite and Widmanstatten ferrite may be present insigni cant amounts in heat treated steels and the coarsegrained HAZ of welds but they are dif cult to classifyindividually so that both structures have been generallyreferred to as ferrite sideplate
WidmanstaEgrave tten ferriteClassi cation of Widmanstatten ferrite can prove dif cultbecause of its similarity to upper bainite but certainguidelines may be followed to avoid confusion
The free energy requirement or driving force would beexpected to be lower for Widmanstatten ferrite formationthan for the upper bainite transformation since the formeris thought to grow by the mutual accommodation of platesand the latter by sub-units (see above) All else being equaltherefore Widmanstatten ferrite may be expected to occurat higher temperatures than upper bainite and exhibit agenerally coarser structure with a lower dislocation densityFurthermorethe microphasesbetween Widmanstatten ferritelaths may be expected to be a mixture of pearlite bainitemartensite or retained austenite whereas the nature ofbainite formation (see above) means that cementite particlesmay generally be observed between the bainitic ferriteplates2 6 Microphases may be revealed by the use of dif-ferent chemical etchants (see below)
The identi cation of secondary Widmanstatten ferritewith aligned microphase FS(A) in the IIW scheme isrelatively easy since it grows from existing allotriomorphicferrite but care has to be taken in distinguishing theboundary between the two structures Identi cation ofprimary Widmanstatten ferrite is signi cantly more dif -cult it grows directly from prior austenite grain boundariesand may be more easily confused with upper bainite Theuse of colour etching methods4 0 4 1 in conjunction with
150 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
since they are under the plane of observation The IIWclassi cation scheme does not have a terminology for thedifferent forms of intragranular bainite but the author andco-workers2 2 have termed them I(B) Where there is a highdensity of inclusions multiple hard impingements ofindividual bainitic plates growing from the inclusions may
result in a very ne interlocking structure2 6 3 2 (see schematicdiagram Fig 9) The IIW classi cation scheme refersgenerally to this type of structure as acicular ferrite AF(see below)
MartensiteMartensite is classically an extremely rapid diffusionlesstransformation where carbon is retained in solution3 7 Asthe austenite lattice changes from fcc to the required mar-tensite bcc or bct lattice strain energy considerationsdominate and the martensite is constrained to be in the formof thin plates
In low carbon steels (less than ~02C) lath martensitewith a bcc crystal structure is the commonly occurringform3 7 and is designated M or M(L) in the IIW scheme Themartensite units are formed in the shape of laths thatare grouped into larger sheaves or packets (see Fig 14)The sub-structure consists of a high density of dislocationsarranged in cells each martensite lath is composed of manydislocation cells As the steel carbon content increases signi- cantly above about 02C plate martensite tends to formwith either a bct or bcc crystal structure3 7 The martensiteunits form as individual lenticular plates (see Fig 15) with asubstructure consisting of very ne twins This form ofmartensite is termed twinned martensite in the IIW schemeand is designated M or M(T) Martensite whether in platesor lath form is generally irresolvable under the light micro-scope and tends to have a slow etching response
12 Growth of bainite sheaves from intragranular inclu-sions in 038C 139Mn 0039S 009V0013N steel isothermally transformed 38 s at450degC arrow indicates multiple laths of bainite withcarbide particles between as well as within subunits
11 Growth of bainite sheaves and (arrowed) sympatheticnucleation of laths in 038C 139Mn 0039S009V steel isothermally transformed 45 s at 400degC
13 Growth of intragranular bainite plates in 038C139Mn 0039S 009V 0013N steel isother-mally transformed 38 s at 500degC arrows indicateindividual plates of bainitic ferrite nucleated on smallintragranular inclusions
14 Lath martensite in 013C laser weld metal arrowindicates martensite laths with highly dislocated sub-structure
15 Plate or twin martensite in 027C laser weld metalarrow indicates lenticular martensite with twinnedsubstructure
148 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
Acicular ferriteConventionally2 6 acicular ferrite is recognised as an intra-granular nucleated morphology of ferrite in which there aremultiple impingements between grains The acicular ferritenucleates on inclusions inside the prior austenite grainsduring the cda transformation Provided there is a highdensity of inclusions a ne interlocking structure (generallylt5 mm) can be produced
In the IIW scheme acicular ferrite is designated AF Fora long time acicular ferrite was thought to be a singletransformation product Early work3 8 suggested that itwas intragranularly nucleated Widmanstatten ferrite Laterresearch2 6 provided evidence for intragranularly nucleatedbainite However recent research by the author and co-workers2 2 has demonstrated that the nature of acicularferrite may be as shown schematically in Fig 16 Differentreaction products may nucleate on intragranular inclusionsat reconstructive and displacive transformation tempera-
tures during continuous cooling depending on the naturesize and amount of inclusions (see Figs 2 and 17) Acicularferrite results from multiple hard impingements of thedifferent transformation products The sequence oftransformations is consistent with the theoretical activationenergy barrier to nucleation of the different sites Acicularferrite development may thus be de ned in terms of con-ventional steel transformation products and CCT diagramsincorporating both intragranular and grain boundarytransformations
Under continuous cooling transformation conditions
AF~I(PF)zI(WF)zI(B)
This leads to acicular ferrite that may have a variety offorms depending on steel composition cooling rate andinclusion characteristics Acicular ferrite may consist ofmixtures of different intragranular transformationproducts(see Fig 18)2 2 3 2 Alternatively Widmanstatten acicularferrite or bainitic acicular ferrite may form per se2 6 3 8
However if reactions are completed at purely reconstruc-tive transformation temperatures it may be preferable touse the term idiomorphic primary ferrite instead of acicularferrite to describe the microstructure since intragranularprimary ferrite is likely to be coarse and non-acicular inmorphology (see Fig 4)
Acicular ferrite is usually observed as a ne interlockingferrite structure interspersed with microphases (see Fig 18)The shape of the ferrite plates may not appear to be needle-like as the use of the term lsquoacicularrsquo would imply This isbecause the different ferrite morphologies cannot grow veryfar before mutual hard impingement It is evident fromFig 18 that the degree of re nement of the acicular ferrite isdependent on the nature of the transformation productsinherent in its formation
16 Nature of acicular ferrite
a
b
a idiomorphic ferrite (arrowed) nucleated on large inclusionsb WidmanstaEgrave tten ferrite plates (arrowed) nucleated on smallinclusions
17 Acicular ferrite development in 006C 137Mn017Mo 00028B 0027Ti submerged arc weldmetal continuously cooled iced brine quenched from615degC22
a
b
a intragranular primary ferriteplusmn WidmanstaEgrave tten ferrite in C plusmn Mnweld metal22 b intragranular WidmanstaEgrave tten ferrite plusmn bainitein Ti plusmn Mo plusmn B alloyed weld metal32
18 Forms of acicular ferrite
Thewlis Classiregcation and quantiregcation of microstructures in steels 149
Materials Science and Technology February 2004 Vol 20
MicrophasesThe different ferrite growth modes of the principal struc-tures described above result in carbon enrichment of theremaining austenite leading to associated second phases ofretained austenite martensite bainite or ferrite ndash carbideaggregate (pearlite) depending on the degree of carbonenrichment of the austenite and the prevailing coolingconditions The second phases associated with Widman-statten ferrite and acicular ferrite are generally quite small(2 ndash 5 mm) and are termed microphases
IIW classi cation scheme problem areasand solutions
The objective in the present work was to investigate the IIWmicrostructure classi cation scheme for weld metals as abasis for quantifying the full range of microstructures foundin plain carbon and low alloy steels as well as ferritic weldmetals and parent plate heat affected zones A means maythus be provided of obtaining database information fordeveloping microstructurendash property relationships or gen-erating data for calibrating physical models that have theprincipal structures primary ferrite pearlite Widmanstat-ten ferrite bainite and martensite as output
It is clear from the above review that while the IIWscheme provides a sound structure for quantifying complexmicrostructures in steels the classi cation of constituentssuch as ferrite sideplate and acicular ferrite is incompatiblewith the principal structures found in the reconstructiveanddisplacive transformation regimes of ferrous materialsKnowledge of the actual transformation products consti-tuting ferrite sideplate and acicular ferrite structures isrequired Classi cation is also needed of idiomorphic ferriteand ferrite sideplate structures growing relatively unim-peded from intragranular inclusions
Problems that may be encountered in relating sub-category microstructural components to principal struc-tures at prior austenite grain boundary and intragranularsites are discussed below together with possible solutionsThe ways in which transformationproducts associated withferrite sideplate and acicular ferrite structures may beidenti ed will be addressed The use of optical microscopywith specimens polished to a 025 mm nish and etched in2 nital is assumed as standard However instances will begiven where different instruments and techniques may beneeded to solve problems Where possible the effects ofsteel composition and heat treatment will be highlightedbut detailed examples are outside the scope of the presentpaper
PRIMARY FERRITEIn low alloy weld metals care has to be taken in identifyingprimary ferrite due to stereological effects Ferrite allo-triomorphs growing from prior austenite grain boundariesbeneath the plane of observation may appear as polygonalferrite grains in the intragranular regions (see Fig 1) Ifthese ferrite allotriomorphs are of a size approximatelythree times greater than those of surrounding acicularferrite laths or grains it is likely that they are the constituentPF(I) described in the IIW scheme It is unlikely that suchlarge grains are idiomorphic ferrite I(PF) nucleated oninclusions as referenced in the literature2 2 since the lattertend to nucleate at lower temperatures with relatively littletime for growth (see Fig 2)
PEARLITEProblems may arise in classifying pearlite when it is presentalong with displacive transformation products
Lamellar pearlite FC(P) in the IIW classi cationscheme may be confused with martensite if the ferritecementite plates are irresolvable under the light microscopeA distinguishing feature is the generally rapid etchingresponse and lower hardness of the pearlite
The dark etching non-lamellar pearlite known as ferrite ndashcarbide aggregate FC in the IIW classi cation scheme maysometimes be confused with bainite The nodular appear-ance of pearlite as opposed to the sheaf appearance ofbainite may provide a distinguishing feature The carboncontent of the steel may also give an indication as to howmuch pearlite may be expected high volume fractionsshould not be present in low carbon steels Ultimatelyhowever knowledge of the thermal history and transforma-tion conditions of the steel may be needed to provide a checkon classi cation (see below) The reconstructive pearlitetransformation should take place slowly at high tempera-tures and over a wide temperature range A displacivetransformation to bainite should take place rapidly at lowertemperatures and over a relatively small temperature range
It is notable that in bainitic steels prolonged holding at agiven temperature may result in the incomplete reactionphenomenon (see above) Continued isothermal treatmentcan result in pearlite formation from the remaining carbonenriched austenite2 6
Dif culties in identi cation of pearlite may be com-poundedbya eutectoid transformationthathasbeen noted incontinuously cooled plain carbon steel (011C 05Mn)This involves ferrite growing in conjunction with repeatednucleation of alloy carbides on the moving ca interphaseboundary3 9 The reaction has been termed interphase pre-cipitation of cementite Dark etching equiaxed ferrite grainscontaining a ne dispersion of carbides are observed underthe light microscope while under the transmission electronmicroscope the cementite is seen in sheets
FERRITE SIDEPLATEBainite and Widmanstatten ferrite may be present insigni cant amounts in heat treated steels and the coarsegrained HAZ of welds but they are dif cult to classifyindividually so that both structures have been generallyreferred to as ferrite sideplate
WidmanstaEgrave tten ferriteClassi cation of Widmanstatten ferrite can prove dif cultbecause of its similarity to upper bainite but certainguidelines may be followed to avoid confusion
The free energy requirement or driving force would beexpected to be lower for Widmanstatten ferrite formationthan for the upper bainite transformation since the formeris thought to grow by the mutual accommodation of platesand the latter by sub-units (see above) All else being equaltherefore Widmanstatten ferrite may be expected to occurat higher temperatures than upper bainite and exhibit agenerally coarser structure with a lower dislocation densityFurthermorethe microphasesbetween Widmanstatten ferritelaths may be expected to be a mixture of pearlite bainitemartensite or retained austenite whereas the nature ofbainite formation (see above) means that cementite particlesmay generally be observed between the bainitic ferriteplates2 6 Microphases may be revealed by the use of dif-ferent chemical etchants (see below)
The identi cation of secondary Widmanstatten ferritewith aligned microphase FS(A) in the IIW scheme isrelatively easy since it grows from existing allotriomorphicferrite but care has to be taken in distinguishing theboundary between the two structures Identi cation ofprimary Widmanstatten ferrite is signi cantly more dif -cult it grows directly from prior austenite grain boundariesand may be more easily confused with upper bainite Theuse of colour etching methods4 0 4 1 in conjunction with
150 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
Acicular ferriteConventionally2 6 acicular ferrite is recognised as an intra-granular nucleated morphology of ferrite in which there aremultiple impingements between grains The acicular ferritenucleates on inclusions inside the prior austenite grainsduring the cda transformation Provided there is a highdensity of inclusions a ne interlocking structure (generallylt5 mm) can be produced
In the IIW scheme acicular ferrite is designated AF Fora long time acicular ferrite was thought to be a singletransformation product Early work3 8 suggested that itwas intragranularly nucleated Widmanstatten ferrite Laterresearch2 6 provided evidence for intragranularly nucleatedbainite However recent research by the author and co-workers2 2 has demonstrated that the nature of acicularferrite may be as shown schematically in Fig 16 Differentreaction products may nucleate on intragranular inclusionsat reconstructive and displacive transformation tempera-
tures during continuous cooling depending on the naturesize and amount of inclusions (see Figs 2 and 17) Acicularferrite results from multiple hard impingements of thedifferent transformation products The sequence oftransformations is consistent with the theoretical activationenergy barrier to nucleation of the different sites Acicularferrite development may thus be de ned in terms of con-ventional steel transformation products and CCT diagramsincorporating both intragranular and grain boundarytransformations
Under continuous cooling transformation conditions
AF~I(PF)zI(WF)zI(B)
This leads to acicular ferrite that may have a variety offorms depending on steel composition cooling rate andinclusion characteristics Acicular ferrite may consist ofmixtures of different intragranular transformationproducts(see Fig 18)2 2 3 2 Alternatively Widmanstatten acicularferrite or bainitic acicular ferrite may form per se2 6 3 8
However if reactions are completed at purely reconstruc-tive transformation temperatures it may be preferable touse the term idiomorphic primary ferrite instead of acicularferrite to describe the microstructure since intragranularprimary ferrite is likely to be coarse and non-acicular inmorphology (see Fig 4)
Acicular ferrite is usually observed as a ne interlockingferrite structure interspersed with microphases (see Fig 18)The shape of the ferrite plates may not appear to be needle-like as the use of the term lsquoacicularrsquo would imply This isbecause the different ferrite morphologies cannot grow veryfar before mutual hard impingement It is evident fromFig 18 that the degree of re nement of the acicular ferrite isdependent on the nature of the transformation productsinherent in its formation
16 Nature of acicular ferrite
a
b
a idiomorphic ferrite (arrowed) nucleated on large inclusionsb WidmanstaEgrave tten ferrite plates (arrowed) nucleated on smallinclusions
17 Acicular ferrite development in 006C 137Mn017Mo 00028B 0027Ti submerged arc weldmetal continuously cooled iced brine quenched from615degC22
a
b
a intragranular primary ferriteplusmn WidmanstaEgrave tten ferrite in C plusmn Mnweld metal22 b intragranular WidmanstaEgrave tten ferrite plusmn bainitein Ti plusmn Mo plusmn B alloyed weld metal32
18 Forms of acicular ferrite
Thewlis Classiregcation and quantiregcation of microstructures in steels 149
Materials Science and Technology February 2004 Vol 20
MicrophasesThe different ferrite growth modes of the principal struc-tures described above result in carbon enrichment of theremaining austenite leading to associated second phases ofretained austenite martensite bainite or ferrite ndash carbideaggregate (pearlite) depending on the degree of carbonenrichment of the austenite and the prevailing coolingconditions The second phases associated with Widman-statten ferrite and acicular ferrite are generally quite small(2 ndash 5 mm) and are termed microphases
IIW classi cation scheme problem areasand solutions
The objective in the present work was to investigate the IIWmicrostructure classi cation scheme for weld metals as abasis for quantifying the full range of microstructures foundin plain carbon and low alloy steels as well as ferritic weldmetals and parent plate heat affected zones A means maythus be provided of obtaining database information fordeveloping microstructurendash property relationships or gen-erating data for calibrating physical models that have theprincipal structures primary ferrite pearlite Widmanstat-ten ferrite bainite and martensite as output
It is clear from the above review that while the IIWscheme provides a sound structure for quantifying complexmicrostructures in steels the classi cation of constituentssuch as ferrite sideplate and acicular ferrite is incompatiblewith the principal structures found in the reconstructiveanddisplacive transformation regimes of ferrous materialsKnowledge of the actual transformation products consti-tuting ferrite sideplate and acicular ferrite structures isrequired Classi cation is also needed of idiomorphic ferriteand ferrite sideplate structures growing relatively unim-peded from intragranular inclusions
Problems that may be encountered in relating sub-category microstructural components to principal struc-tures at prior austenite grain boundary and intragranularsites are discussed below together with possible solutionsThe ways in which transformationproducts associated withferrite sideplate and acicular ferrite structures may beidenti ed will be addressed The use of optical microscopywith specimens polished to a 025 mm nish and etched in2 nital is assumed as standard However instances will begiven where different instruments and techniques may beneeded to solve problems Where possible the effects ofsteel composition and heat treatment will be highlightedbut detailed examples are outside the scope of the presentpaper
PRIMARY FERRITEIn low alloy weld metals care has to be taken in identifyingprimary ferrite due to stereological effects Ferrite allo-triomorphs growing from prior austenite grain boundariesbeneath the plane of observation may appear as polygonalferrite grains in the intragranular regions (see Fig 1) Ifthese ferrite allotriomorphs are of a size approximatelythree times greater than those of surrounding acicularferrite laths or grains it is likely that they are the constituentPF(I) described in the IIW scheme It is unlikely that suchlarge grains are idiomorphic ferrite I(PF) nucleated oninclusions as referenced in the literature2 2 since the lattertend to nucleate at lower temperatures with relatively littletime for growth (see Fig 2)
PEARLITEProblems may arise in classifying pearlite when it is presentalong with displacive transformation products
Lamellar pearlite FC(P) in the IIW classi cationscheme may be confused with martensite if the ferritecementite plates are irresolvable under the light microscopeA distinguishing feature is the generally rapid etchingresponse and lower hardness of the pearlite
The dark etching non-lamellar pearlite known as ferrite ndashcarbide aggregate FC in the IIW classi cation scheme maysometimes be confused with bainite The nodular appear-ance of pearlite as opposed to the sheaf appearance ofbainite may provide a distinguishing feature The carboncontent of the steel may also give an indication as to howmuch pearlite may be expected high volume fractionsshould not be present in low carbon steels Ultimatelyhowever knowledge of the thermal history and transforma-tion conditions of the steel may be needed to provide a checkon classi cation (see below) The reconstructive pearlitetransformation should take place slowly at high tempera-tures and over a wide temperature range A displacivetransformation to bainite should take place rapidly at lowertemperatures and over a relatively small temperature range
It is notable that in bainitic steels prolonged holding at agiven temperature may result in the incomplete reactionphenomenon (see above) Continued isothermal treatmentcan result in pearlite formation from the remaining carbonenriched austenite2 6
Dif culties in identi cation of pearlite may be com-poundedbya eutectoid transformationthathasbeen noted incontinuously cooled plain carbon steel (011C 05Mn)This involves ferrite growing in conjunction with repeatednucleation of alloy carbides on the moving ca interphaseboundary3 9 The reaction has been termed interphase pre-cipitation of cementite Dark etching equiaxed ferrite grainscontaining a ne dispersion of carbides are observed underthe light microscope while under the transmission electronmicroscope the cementite is seen in sheets
FERRITE SIDEPLATEBainite and Widmanstatten ferrite may be present insigni cant amounts in heat treated steels and the coarsegrained HAZ of welds but they are dif cult to classifyindividually so that both structures have been generallyreferred to as ferrite sideplate
WidmanstaEgrave tten ferriteClassi cation of Widmanstatten ferrite can prove dif cultbecause of its similarity to upper bainite but certainguidelines may be followed to avoid confusion
The free energy requirement or driving force would beexpected to be lower for Widmanstatten ferrite formationthan for the upper bainite transformation since the formeris thought to grow by the mutual accommodation of platesand the latter by sub-units (see above) All else being equaltherefore Widmanstatten ferrite may be expected to occurat higher temperatures than upper bainite and exhibit agenerally coarser structure with a lower dislocation densityFurthermorethe microphasesbetween Widmanstatten ferritelaths may be expected to be a mixture of pearlite bainitemartensite or retained austenite whereas the nature ofbainite formation (see above) means that cementite particlesmay generally be observed between the bainitic ferriteplates2 6 Microphases may be revealed by the use of dif-ferent chemical etchants (see below)
The identi cation of secondary Widmanstatten ferritewith aligned microphase FS(A) in the IIW scheme isrelatively easy since it grows from existing allotriomorphicferrite but care has to be taken in distinguishing theboundary between the two structures Identi cation ofprimary Widmanstatten ferrite is signi cantly more dif -cult it grows directly from prior austenite grain boundariesand may be more easily confused with upper bainite Theuse of colour etching methods4 0 4 1 in conjunction with
150 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
MicrophasesThe different ferrite growth modes of the principal struc-tures described above result in carbon enrichment of theremaining austenite leading to associated second phases ofretained austenite martensite bainite or ferrite ndash carbideaggregate (pearlite) depending on the degree of carbonenrichment of the austenite and the prevailing coolingconditions The second phases associated with Widman-statten ferrite and acicular ferrite are generally quite small(2 ndash 5 mm) and are termed microphases
IIW classi cation scheme problem areasand solutions
The objective in the present work was to investigate the IIWmicrostructure classi cation scheme for weld metals as abasis for quantifying the full range of microstructures foundin plain carbon and low alloy steels as well as ferritic weldmetals and parent plate heat affected zones A means maythus be provided of obtaining database information fordeveloping microstructurendash property relationships or gen-erating data for calibrating physical models that have theprincipal structures primary ferrite pearlite Widmanstat-ten ferrite bainite and martensite as output
It is clear from the above review that while the IIWscheme provides a sound structure for quantifying complexmicrostructures in steels the classi cation of constituentssuch as ferrite sideplate and acicular ferrite is incompatiblewith the principal structures found in the reconstructiveanddisplacive transformation regimes of ferrous materialsKnowledge of the actual transformation products consti-tuting ferrite sideplate and acicular ferrite structures isrequired Classi cation is also needed of idiomorphic ferriteand ferrite sideplate structures growing relatively unim-peded from intragranular inclusions
Problems that may be encountered in relating sub-category microstructural components to principal struc-tures at prior austenite grain boundary and intragranularsites are discussed below together with possible solutionsThe ways in which transformationproducts associated withferrite sideplate and acicular ferrite structures may beidenti ed will be addressed The use of optical microscopywith specimens polished to a 025 mm nish and etched in2 nital is assumed as standard However instances will begiven where different instruments and techniques may beneeded to solve problems Where possible the effects ofsteel composition and heat treatment will be highlightedbut detailed examples are outside the scope of the presentpaper
PRIMARY FERRITEIn low alloy weld metals care has to be taken in identifyingprimary ferrite due to stereological effects Ferrite allo-triomorphs growing from prior austenite grain boundariesbeneath the plane of observation may appear as polygonalferrite grains in the intragranular regions (see Fig 1) Ifthese ferrite allotriomorphs are of a size approximatelythree times greater than those of surrounding acicularferrite laths or grains it is likely that they are the constituentPF(I) described in the IIW scheme It is unlikely that suchlarge grains are idiomorphic ferrite I(PF) nucleated oninclusions as referenced in the literature2 2 since the lattertend to nucleate at lower temperatures with relatively littletime for growth (see Fig 2)
PEARLITEProblems may arise in classifying pearlite when it is presentalong with displacive transformation products
Lamellar pearlite FC(P) in the IIW classi cationscheme may be confused with martensite if the ferritecementite plates are irresolvable under the light microscopeA distinguishing feature is the generally rapid etchingresponse and lower hardness of the pearlite
The dark etching non-lamellar pearlite known as ferrite ndashcarbide aggregate FC in the IIW classi cation scheme maysometimes be confused with bainite The nodular appear-ance of pearlite as opposed to the sheaf appearance ofbainite may provide a distinguishing feature The carboncontent of the steel may also give an indication as to howmuch pearlite may be expected high volume fractionsshould not be present in low carbon steels Ultimatelyhowever knowledge of the thermal history and transforma-tion conditions of the steel may be needed to provide a checkon classi cation (see below) The reconstructive pearlitetransformation should take place slowly at high tempera-tures and over a wide temperature range A displacivetransformation to bainite should take place rapidly at lowertemperatures and over a relatively small temperature range
It is notable that in bainitic steels prolonged holding at agiven temperature may result in the incomplete reactionphenomenon (see above) Continued isothermal treatmentcan result in pearlite formation from the remaining carbonenriched austenite2 6
Dif culties in identi cation of pearlite may be com-poundedbya eutectoid transformationthathasbeen noted incontinuously cooled plain carbon steel (011C 05Mn)This involves ferrite growing in conjunction with repeatednucleation of alloy carbides on the moving ca interphaseboundary3 9 The reaction has been termed interphase pre-cipitation of cementite Dark etching equiaxed ferrite grainscontaining a ne dispersion of carbides are observed underthe light microscope while under the transmission electronmicroscope the cementite is seen in sheets
FERRITE SIDEPLATEBainite and Widmanstatten ferrite may be present insigni cant amounts in heat treated steels and the coarsegrained HAZ of welds but they are dif cult to classifyindividually so that both structures have been generallyreferred to as ferrite sideplate
WidmanstaEgrave tten ferriteClassi cation of Widmanstatten ferrite can prove dif cultbecause of its similarity to upper bainite but certainguidelines may be followed to avoid confusion
The free energy requirement or driving force would beexpected to be lower for Widmanstatten ferrite formationthan for the upper bainite transformation since the formeris thought to grow by the mutual accommodation of platesand the latter by sub-units (see above) All else being equaltherefore Widmanstatten ferrite may be expected to occurat higher temperatures than upper bainite and exhibit agenerally coarser structure with a lower dislocation densityFurthermorethe microphasesbetween Widmanstatten ferritelaths may be expected to be a mixture of pearlite bainitemartensite or retained austenite whereas the nature ofbainite formation (see above) means that cementite particlesmay generally be observed between the bainitic ferriteplates2 6 Microphases may be revealed by the use of dif-ferent chemical etchants (see below)
The identi cation of secondary Widmanstatten ferritewith aligned microphase FS(A) in the IIW scheme isrelatively easy since it grows from existing allotriomorphicferrite but care has to be taken in distinguishing theboundary between the two structures Identi cation ofprimary Widmanstatten ferrite is signi cantly more dif -cult it grows directly from prior austenite grain boundariesand may be more easily confused with upper bainite Theuse of colour etching methods4 0 4 1 in conjunction with
150 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
optical microscopy may prove helpful in distinguishingWidmanstatten ferrite from bainiteThese techniquesinvolvecomplex electrochemical reactions and require carefulexperimentation but can provide a means of distinguishingvarious phases by their colouring response Nanohardnessmeasurements may also prove useful these are obtainedusing a modi ed scanning force microscope (SFM)4 2 Thenanoindentation technique allows very small regions ofgrains to be investigated and different phases to be dis-tinguished All else being equal Widmanstatten ferriteshould exhibit a lower hardness than bainite
Although Widmanstatten ferrite may be distinguishedfrom upper bainite using the above guidelines care has tobe taken with stereological effects Widmanstatten ferriteplates within a colony tend to grow in a common crystal-lographic orientation They are therefore generally sepa-rated by low angle boundaries When prior austenite grainboundary Widmanstatten ferrite is seen end-on with non-aligned microphase FS(NA) in the IIW scheme the platescan give the appearance of ferrite grains interspersed withmicrophase thereby creating confusion with regions ofintragranular acicular ferrite AF In the case of acicularferrite hard impingements of the different ferrite morpho-logies growing from inclusions results in high angleboundaries which are signi cantly more distinct than thelow angle boundaries of Widmanstatten ferrite Carefulspecimen polishing and etching may be required to dis-tinguish the two structures
In the intragranular regions of welds it may be relativelystraightforward to identify multiple plates of Widmanstat-ten ferrite with aligned microphase growing unimpededfrom large inclusions described as FS(I) in the literature3 2
Recognising single plates of Widmanstatten ferrite withoutaligned microphase designated IFP may be more dif cultbut these plates are likely to be quite coarse and grow fromlarge inclusions Formation of the latter may appear con-tradictory from a mechanistic viewpoint It is possible thatthe second plate is beneath the plane of observation (seeFig 8) Alternatively the absence of aligned microphasemay be because during plate growth carbon is rejected intothe remaining austenite which then undergoes a secondarytransformation at lower temperatures to bainite martensiteor ne acicular ferrite nucleated on small inclusions
BainiteThe effects of steel composition may compound many of theproblems associated with distinguishing Widmanstattenferrite from upper bainite described above
Low carbon content in bainitic steels can increase thetransformation temperature and result in a coarse lath sizeso that bainitic ferrite with aligned second phase FS(A) inthe IIW scheme appears similar to Widmanstatten ferriteHigh silicon content in bainitic steels (generally gt1) canretard the precipitation of carbide from austenite2 6 andresult in martensite or retained austenite microphasesbetween the bainitic ferrite laths thereby creating confusionwith Widmanstatten ferrite Granular bainite which tendsto form in continuously cooled low carbon bainitic steelsposes a similar problem2 6 This structure appears as arelatively coarse aggregate of bainitic ferrite and retainedaustenite or martensite islands the bainitic sub-units havevery thin regions of austenite between them which cannotbe resolved under the light microscope2 6 Ultimately highresolution SEM TEM or electron back-scattering diffrac-tion (EBSD) techniques4 3 4 4 may be needed to distinguishthese forms of bainite from Widmanstatten ferrite byrevealing the crystallographic sub-structure and thereby themechanism of formation but some electron metallographictechniques are time consuming and often dif cult
When trying to distinguish upper FS(UB) and lowerFS(LB) bainite in the IIW scheme stereological effects may
cause confusion Cross-sections of upper and lower bainitesheavesmay appear similar In generalhowever the carbidesare likely to be ner and the etching response darker in thelower bainite
In weld metals individual plates of bainitic ferrite I(B)growing unimpeded from intragranular inclusions may bedif cult to separate from Widmanstatten ferrite plates IFPHowever the former are likely to be signi cantly ner thanthe latter and the nucleating inclusions may be smallerColour etching methods4 0 4 1 may be helpful for identi ca-tion but ultimately electron metallographic techniques maybe required to determine the nature of the plates
MARTENSITEMartensite is often present together with bainite in the HAZof laser welds and to some extent electron beam welds thesephases also occur in high strength weld metals3 2 Most lowcarbon steels have martensite start temperatures aboveroom temperature so that at slower cooling rates carbonatoms can redistribute and precipitate ie autotemperingcan take place It is then dif cult to distinguish betweenautotempered martensite M and lower bainite FS(LB) inthe IIW scheme The carbides precipitated inside the laths inlower bainite are however likely to be coarser and someinterlath carbide should be evident (see above)
Colouretchingmethods4 0 4 1 maybe investigatedas a meansof distinguishing between bainite and martensite Com-paratively simple nanohardness measurements4 2 may alsoprove useful in separating martensite from other principalstructuresand in distinguishingthe different forms of marten-site Since carbon content generally governs the martensitichardness twinned martensite M(T) may be expected toexhibit a much higher hardness than lath martensite M(L)
ACICULAR FERRITEDistinguishingthe intragranulartransformationproducts thatcompose acicular ferrite AF in the IIW scheme is likely to bevery dif cult comparedwith identifyingthe structure itself It isrecommended therefore that for the purposes of calibratingmodels a pragmatic solution be adopted Thus measuredvolume fractions of acicular ferrite should be compared withthe sum of the intragranularconstituents I(PF)zI(WF)zI(B)predicted by modelling However care should be taken todistinguish between acicular ferrite AF where multipleimpingementoccursbetween the different intragranularferritemorphologies and the intragranular transformationproductsI(PF) I(WF) and I(B) which may grow relatively unimpededand may be identi ed in their own right
MICROPHASESMicrophases are normally revealed using a standard etchpolish technique with a 2 nital etch However problemsmay arise in distinguishing martensite and retainedaustenite which often occur together as MA phase TEMtechniques may be employed to separate the phases but aretime consuming and dif cult The proportion of austenite inthe MA phase may be determined using X-ray diffractiontechniques In some cases etching in picral can reveal thenature of the microphases Thus cementite may appearblack a light brown coloration indicates lath martensite ayellow-brown colour is likely to be twin martensite while agrey-white colour is indicative of retained austenite
New classi cation scheme
In the previous section problems in the IIW microstructureclassi cation scheme were discussed and guidelines pro-posed for identifying the principal structures associated
Thewlis Classiregcation and quantiregcation of microstructures in steels 151
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
Tab
le1
Cla
ssi
cati
onsc
hem
efo
rm
icro
stru
ctur
alco
nsti
tuen
ts
Cate
go
ryte
rmin
olo
gy
Pri
ncip
al
str
uctu
recla
ssi
regcati
on
Ov
era
llM
ain
Su
bC
om
po
nen
tst
ruct
ure
descr
ipti
on
Co
mm
en
ts
Rec
on
stru
ctiv
etr
ansf
orm
atio
ns
(dif
fusi
onco
ntro
lled
w
ith
slo
wra
tes
ofre
acti
on
)Ferr
ite
PF
PF(G
B)
PF(G
) G
rain
bo
un
dary
pri
mary
ferr
ite
All
otr
iom
orp
hic
ferr
ite
Po
lyg
on
al
ferr
ite
Ferr
ite
vein
s
Ferr
ite
vein
so
rp
oly
go
nal
gra
ins
alig
ned
wit
hp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
PF(N
A)
Po
lyg
on
al
pri
mary
ferr
ite
no
n-
ali
gn
ed
Po
lyg
on
al
ferr
ite
gra
ins
wit
hin
the
pri
or
au
ste
nit
eg
rain
so
fa
size
ap
pro
xim
ate
lyth
ree
tim
es
gre
ate
rth
an
the
su
rro
un
din
gfe
rrit
ela
ths
or
gra
ins
cro
ss-
secti
on
so
ffe
rrit
eallo
trio
mo
rph
sth
at
have
gro
wn
fro
mp
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bserv
ati
on
PF(I
)P
F(I
)Id
iom
orp
hic
ferr
ite
Ferr
ite
idio
mo
rph
sass
oci
ate
dw
ith
intr
ag
ran
ula
rn
ucle
ati
on
site
s(l
arg
eo
xid
es
ulp
hid
ein
clu
sio
ns)
inw
eld
meta
lsan
dp
art
icle
dis
pers
ed
steels
Pearl
ite
P
P
FC
(P)
Lam
ellar
pearl
ite
Deg
en
era
tep
earl
ite
Fin
eco
lon
yp
earl
ite
No
du
les
of
alt
ern
ate
ferr
itec
em
en
tite
lam
ell
ae
wh
ich
are
oft
en
dif
regcu
ltto
reso
lve
un
der
the
op
tical
mic
rosc
op
e
Th
estr
uct
ure
has
ara
pid
etc
hin
gre
spo
nse
in2
nit
al
an
da
gen
era
lly
low
hard
ness
Pearl
ite
may
be
pre
sen
tas
am
icro
ph
ase
FC
Ferr
ite
plusmncarb
ide
ag
gre
gate
Pearl
ite
lam
ell
ae
vie
wed
incro
ss-s
ecti
on
D
isto
rted
pearl
ite
lam
ellae
may
ap
pear
as
ad
ark
etc
hin
gvir
tuall
yir
reso
lvab
lefe
rrit
ec
arb
ide
ag
gre
gate
kno
wn
as
pri
mary
tro
osti
te
Dif
regcu
ltto
dis
tin
gu
ish
ferr
itec
arb
ide
ag
gre
gate
fro
mb
ain
ite
Dis
pla
cive
tran
sfo
rmat
ion
s(s
hea
rd
om
inat
ed
wit
hra
pid
rate
so
fre
acti
on)
Wid
man
staEgravett
en
ferr
ite
WF
WF
(GB
)FS
(A)
Wid
man
staEgravett
en
ferr
ite
wit
hali
gn
ed
mic
rop
hase
Wid
man
staEgravett
en
ferr
ite
sid
ep
late
s
Co
lon
ies
of
para
llel
ferr
ite
lath
s(o
rsid
ep
late
s)w
ith
mic
rop
hases
ali
gn
ed
betw
een
the
lath
sra
ng
ing
fro
mp
earl
ite
tom
art
en
site
Lath
bo
un
dari
es
are
dif
regcu
ltto
reso
lve
Pri
mary
Wid
ma
nstaEgrave
tten
ferr
ite
gro
ws
fro
mth
ep
rio
rau
sten
ite
gra
inb
ou
nd
ari
es
wh
ere
as
seco
nd
ary
Wid
man
staEgrave
tten
ferr
ite
gro
ws
fro
mall
otr
iom
orp
hic
ferr
ite
at
the
bo
un
dary
FS
(NA
) W
idm
an
staEgravett
en
ferr
ite
wit
hn
on
-alig
ned
mic
rop
hase
Ag
gre
gate
of
mic
rop
hase
isla
nd
san
dW
idm
an
staEgravett
en
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-s
ecti
on
so
fW
idm
an
staEgravett
en
ferr
ite
sid
ep
late
sth
at
gro
wfr
om
pri
or
au
sten
ite
gra
inb
ou
nd
ari
es
belo
wth
ep
lan
eo
fo
bse
rvati
on
WF
(I)
FS
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sM
ult
iple
coars
eW
idm
an
staEgrave
tten
ferr
ite
pla
tes
(asp
ect
rati
og
reate
rth
an
41
)w
ith
alig
ned
mic
rop
hase
sw
hic
hg
row
fro
min
trag
ran
ula
rin
clu
sio
ns
Pri
mary
intr
ag
ran
ula
rfe
rrit
esi
de
pla
tes
gro
wfr
om
inclu
sio
ns
wh
ere
as
seco
nd
ary
sid
ep
late
sg
row
fro
mfe
rrit
eid
iom
orp
hs
ass
oci
ate
dw
ith
incl
usio
ns
FP
(I)
Intr
ag
ran
ula
rW
idm
an
staEgrave
tten
ferr
ite
pla
tes
Ind
ivid
ual
coars
ep
late
so
fW
idm
an
staEgrave
tten
ferr
ite
that
gro
wre
lati
ve
lyu
nim
ped
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Wid
man
staEgravett
en
aci
cula
rfe
rrit
eFin
ein
terl
ocki
ng
str
uct
ure
form
ed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
Wid
man
staEgrave
tten
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Bain
ite
BB
(GB
)FS
(A)
Bain
itic
ferr
ite
wit
hali
gn
ed
carb
ide
Bain
ite
sheaves
Sh
eaves
of
para
llel
ferr
ite
lath
s(o
rsu
b-u
nit
s)w
ith
cem
en
tite
part
icle
salig
ned
betw
een
the
lath
s
Lath
bo
un
dari
es
are
gen
era
lly
irre
solv
ab
leu
nd
er
the
lig
ht
mic
rosco
pe
Sh
eaves
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
sym
path
eti
cn
ucl
ea
tio
no
fla
ths
fro
mexis
tin
gsh
eaves
isa
co
mm
on
featu
reFS
(NA
) B
ain
itic
ferr
ite
wit
hn
on
-alig
ned
carb
ide
Ag
gre
gate
of
co
ars
eca
rbid
es
an
db
ain
itic
ferr
ite
wit
hin
the
pri
or
au
sten
ite
gra
ins
cro
ss-
secti
on
so
fb
ain
ite
sh
eave
sth
at
gro
wfr
om
pri
or
au
ste
nit
eg
rain
bo
un
dari
es
(or
exis
tin
gsh
eaves)
belo
wth
ep
lan
eo
fo
bserv
ati
on
FS
(UB
) U
pp
er
Bain
ite
Carb
ide
part
icle
sare
pre
cip
itate
db
etw
een
the
bain
ite
sub
-un
its
Up
per
bain
ite
has
ah
igh
er
dis
loca
tio
nd
en
sit
yth
an
pri
mary
Wid
man
staEgravett
en
ferr
ite
Bain
ite
may
ap
pear
as
am
icro
ph
ase
betw
ee
nW
idm
an
staEgrave
tten
ferr
ite
sid
ep
late
sFS
(LB
) Lo
we
rb
ain
ite
Fin
ecem
en
tite
part
icle
sp
recip
itate
dw
ith
inas
well
as
betw
een
bain
itic
ferr
ite
pla
tes
Lo
wer
bain
ite
has
ag
en
era
lly
dark
er
etc
hin
gre
sp
on
se
than
up
per
bain
ite
Dif
regcu
ltto
dis
tin
gu
ish
low
er
bain
ite
fro
mau
tote
mp
ere
dm
art
en
sit
e
152 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
with prior austenite grain boundary and intragranular sitestaking into account stereological effects In this section theinformation gained has been used to develop a new classi- cation scheme The application and accuracy of the newscheme have been addressed and consideration given to itsevolution
DEFINITIONUsing the information gained above the traditional IIWclassi cation scheme has been modi ed and new termino-logy de ned as in Table 1 The main and sub-categories ofmicrostructural constituents of the table re ect the mechan-isms of formation of the principal structures and thecharacteristic ferrite morphologies produced in the recon-structive and displacive transformation regimes of steels
Traditionally the IIW classi cation scheme terminologyplaces the transformation product rst and the locationsecond whereas the reverse is often the case in the widerpublished literature1 7 2 2 3 2 For consistency therefore theterminology described in Table 1 follows the traditionalIIW notation Thus the constituents GB(PF) I(PF)GB(WF) I(WF) GB(B) I(B) described in the literature2 2
are replaced by PF(GB) PF(I) WF(GB) WF(I) B(GB)B(I) as main category terms in Table 1 Likewise theconstituent IFP in the literature3 2 is replaced by the sub-category constituent FP(I) in Table 1
To avoid con ict in Table 1 between the terminologyadopted for idiomorphic primary ferrite PF(I) and that forcross-sections of ferrite allotriomorphs growing from prioraustenite grain boundaries below the plane of observationthe latter terminology has been changed from PF(I) toPF(NA) ie primary ferrite not aligned with prior austenitegrain boundaries PF(NA) may be added together withPF(G) to give an overall quantity of reconstructive prioraustenite grain boundary nucleated ferrite PF(GB)
It should be noted in Table 1 that the new sub-categorycomponent terminology automatically de nes its locationeither at prior austenite grain boundaries or in intragranularregions In practice therefore an identi cation system maybe employed which directly links a sub-category componentto the principal structure eg B-FS(A) and WF-FS(A)
Flow charts that incorporate the classi cation andterminology of Table 1 but provide detailed guidance onidentifying principal structures are shown in Fig 19 Thekey to the ow charts is given in Fig 20 Separate charts areprovided for austenite grain boundary and intragranularmicrostructural componentsProgression through the chartsfrom sub-category component structures to the principalstructures is dependent on answering a number of boxedquestions on a yesno basis The questions are derived fromthe considerations made in this paper If the answer to aquestion is lsquoyesrsquo progression is made to the right of thechart towards the principal structure If the answer is lsquonorsquoa move vertically downwards is needed to obtain moreinformation before eventually progress is made to the rightagain The ow charts thus potentially provide a means ofquantifying complex steel microstructures in terms of theprincipal structures thereby enabling the generationof eitherdatabase information or data for calibration of theoreticalmodels
APPLICATIONTo assess the accuracy of the new classi cation scheme andidentify discrepancies between operators exercises werecarried out to quantify widely different microstructuresThe microstructures were obtained by thermally cyclingsteels of compositions 0051 ndash 017C 051 ndash 146Mn in adilatometer to peak temperatures of 900 ndash 1300degC andcooling at rates between 2 and 200 K s2 1 Full details of thequanti cation exercises including a complete statisticalT
able
1(C
on
tin
ued
) Cate
go
ryte
rmin
olo
gy
Pri
nci
pal
str
uct
ure
cla
ssi
regcati
on
Overa
llM
ain
Su
bC
om
po
nen
tstr
uct
ure
desc
rip
tio
nC
om
men
ts
B(I
)FS
(I)
Intr
ag
ran
ula
rb
ain
ite
sh
eaves
Sh
eaves
of
regn
eb
ain
itic
ferr
ite
pla
tes
wit
halig
ned
carb
ide
wh
ich
gro
wfr
om
intr
ag
ran
ula
rin
clu
sio
ns
FP
(I)
Intr
ag
ran
ula
rb
ain
ite
pla
tes
Ind
ivid
ual
regn
ep
late
so
fb
ain
itic
ferr
ite
that
gro
wre
lati
vely
un
imp
ed
ed
fro
min
trag
ran
ula
rin
clu
sio
ns
AF
Bain
itic
acic
ula
rfe
rrit
eV
ery
regn
ein
terl
ock
ing
stru
ctu
refo
rmed
by
mu
ltip
leim
pin
gem
en
tso
fin
div
idu
al
bain
itic
ferr
ite
pla
tes
gro
win
gfr
om
intr
ag
ran
ula
rin
clu
sio
ns
Mart
en
site
M
M
M(L
) Lath
mart
en
sit
eLo
wca
rbo
nm
art
en
sit
ew
ith
ala
thstr
uct
ure
an
dh
eavily
dis
loca
ted
su
b-s
tru
ctu
re
Lath
mart
en
site
has
aslo
wetc
hin
gre
sp
on
sein
2
nit
al
an
da
gen
era
lly
hig
hh
ard
ness
Co
lon
ies
of
mart
en
sit
em
ay
form
wit
hin
the
pri
or
au
ste
nit
eg
rain
s
Sm
aller
colo
nie
sm
ay
be
treate
das
mic
rop
hases
Mic
rop
hase
sm
ay
co
nsi
st
of
mart
en
sit
ew
ith
reta
ined
au
sten
ite
(MA
)M
(T)
Tw
inm
art
en
site
Hig
hcarb
on
mart
en
site
wit
ha
pla
testr
uctu
rean
dtw
inn
ed
su
b-s
tru
ctu
re
Re
tain
ed
IIW
term
ino
log
y
Thewlis Classiregcation and quantiregcation of microstructures in steels 153
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
a prior austenite grain boundary constituents b intragranular constituents
19 Guidelines and terminology for identi cation of principal structures
154 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
analysis are outside the scope of the present paper Howeverthe results for selected steels are summarised below
Six dilatometer sample microstructures covering a widetransformationtemperature range were photographed usingan appropriate magni cation The resulting microstructural elds are shown in Fig 21 A mesh grid inscribed on trans-parent acetate paper was overlaid in a xed position on thephotographs so that those microstructural constituentsunder or just touching the grid cross-lines could be quanti- ed Each cross-line was identi ed from the grid scale egA1 A2 A3 hellip B1 B2 B3 hellip A total of 500 points wascounted of each eld Because the grid points were xedresults from different operators could be compared and theconstituents that were most dif cult to quantify could berelatively easily identi ed
Initially a single operator was employed to point countthe volume percentages of microstructural constituents inthe six microstructural elds using the traditional IIWand the new classi cation schemes The results (Table 2)demonstrate the advantagesof the new scheme in being ableto rationalise the principal structures associated with ferritesideplate Ultimately the microstructural output is reducedto the ve principal constituents
Following the above exercise different operators wereemployed to determine the volume percentages of the prin-cipal structures in the six microstructural elds using thenew scheme per se The results are shown in the form ofhistograms in Fig 22 Most operators chose to identify themajor transformation products directly although someoperators chose to classify subcategories and thereby themajor components In all cases microphases associatedwith primary ferrite and Widmanstatten ferrite were treatedseparately while bainitic ferrite was quanti ed togetherwith the carbide Because of the xed position of the pointcounting grid the variations in phase proportions in Fig 22
are due to differences in microstructural interpretation bythe individual operators rather than point counting errorsthat would emerge between operators from random reposi-tioning of the grid in the dilatometer sample microstruc-ture When quantifying the volume fraction of secondaryWidmanstatten ferrite some discrepancy occurred betweenoperators owing to the need to distinguish the boundarybetween allotriomorphic ferrite and Widmanstatten ferrite(see Fig 22a) Further differences occurred because ofthe need to distinguish between ferrite carbide aggregate(pearlite) and bainite (see Fig 22b and c) and to someextent lower bainite and autotempered martensite (seeFigs 22d and f) These dif culties were compounded by thelow resolution of the photographic images
A signi cant improvement in the consistency betweenoperators was achieved after appropriate training whenquantifying phase proportions randomly over a relativelylarge area in actual steel samples In this case differentmagni cations could be used to reveal dif cult features Alight microscope with a Swift point counting stage wasemployed to count 500 points of various dilatometer samplemicrostructures again covering a wide transformation tem-perature range The statistical errors in point counting4 5 ndash 4 7
were determined using the formula according to Gladmanand Woodhead4 7
svf=Vf~permil(1Vf )=Pa Š1=2
where sv f is one standard deviation Pa the fraction ofcounts in the a phase and Vf the volume fraction of a phase
The phase proportions obtained by two operators on sixsteels are shown in Fig 23 The 95 con dence limits (2sv f)are superimposed The results show that the phase pro-portions obtained by the individual operators were in manycases within the statistical error de ned in the point count-ing exercise However to obtain a sensible statistical analysis
20 Key to ow charts
Thewlis Classiregcation and quantiregcation of microstructures in steels 155
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
of operator bias a larger number of operators is neededFurther work is required in the form of lsquoround robinrsquoexercises to determine the statistical uncertainty betweenoperators when quantifying different types of microstruc-ture and to provide appropriate training measures forwidespread dissemination of the scheme
The above studies were carried out without prior know-ledge of the thermal history of the specimens examinedHowever transformationbehaviour knowledge can providea useful check on results The six microstructural elds inFig 21 were largely representativeof the parent dilatometersample microstructures The corresponding dilation curves
percentage transformed versus temperature graphs andpeak rate transformation curves are shown in Fig 24 Thedilatometer data in Fig 24a show that for this particularsteel transformation began at 793degC and took place over awide temperature range nishing at 628degC As the trans-formation proceeded the rate of transformation increasedslowly to a peak at 715degC and then decreased slowlyindicative of transformation controlled by diffusion Thissupports the operator classi cation for the steel of about70 primary ferrite and 5 pearlite ie predominantlyreconstructive transformation (see Fig 22a) By contrastthe dilatometer data in Fig 24c show that for this steel
a
c
e
b
d
f
a 0051C 051Mn 1200degC 10 K s21 b 017C 052Mn 1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d 013C102Mn 1200degC 10 K s21 e 013C 102Mn 1300degC 50 K s21 f 013C 102Mn 1300degC 200 K s21
21 Microstructural elds of steels thermally cycled in dilatometer to temperatures of 1200 or 1300degC and cooled atrates between 2 and 200 K s21 (800 ndash 500degC)
156 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
transformation began at 717degC and nished at 582degC Therate of transformation increased slowly at rst indicative ofreconstructive transformationbut then rose very rapidly toa sharp peak at 646degC before falling steeply and then moreslowly in the nal stages of transformationThe sharp peakin the rate of transformationtook place after around 40 ofreaction had occurred and was indicative of the beginningof shear dominated transformation which should accountfor the remaining 60 of the transformation The highpeak rate transformation temperature is indicative ofWidmanstatten ferrite formation rather than bainite This isbroadly in agreement with the steel microstructure results inFig 22c where around 60 Widmanstatten ferrite wasquanti ed by most of the operators A similar analysis maybe carried out with respect to dilatometer data in Fig 24d Inthis steel the lower peak rate transformation temperature(603degC) and lower nishing temperature (532degC) are indica-tive of bainite as well as Widmanstatten ferrite formationThis again is re ected in the operator microstructureclassi cation in Fig 22d It is notable that in the dilatometerdata of Fig 24f almost 50 of the steel transformationoccurred at one temperature (421degC) This extremely rapidreaction rate and low transformation temperature areindicative of martensite transformation in agreement withthe operator classi cation for the steel in Fig 22f
Overall the above exercises show that a reasonabledegree of consistency may be obtained between operatorswhen using the new classi cation scheme to identify theprincipal structures (primary ferrite pearlite martensite)and the transformation products constituting ferrite side-plate structures notably Widmanstatten ferrite and bainite
EVOLUTIONThe new classi cation scheme de ned abovehas attempted toplace knowledge of the classi cation and quanti cation ofsteel microstructureson a rm contemporarybasis Howeverit is of interest to consider possible future developments
The guidelines proposed for phase recognition in the newscheme are based on the mechanisms of formation of prin-cipal structures but there are still questions to be addressedwith respect to the kinetics of reactions notably clari ca-tion of the growth mechanism of bainite Improved know-ledge in this area should result in greater accuracy indistinguishing bainite from other phases Overall a betterunderstanding is needed of the dynamics of phase trans-formations under continuous cooling transformation con-ditions where phases may form simultaneously and local uctuations in transformation conditions can make itdif cult to recognise the transition between one phase
Table 2 Volume percentages of microstructural constituents obtained by single operator point counting microstructural elds (see Fig 21) using traditional IIW and new classi cation schemes
New scheme IIW scheme
Principal structure Phase Component structure Phase
0051C 051Mn 1200degC 10 K s2 1 (Fig 21a)PF 726 PF(G) 726
PF(NA) 0P 64 FC(P) 07
FC 57WF 210 FS(A)zFS(NA) 108z102B 0M 0 M 0
017C 052Mn 1300degC 10 K s2 1 (Fig 21b)PF 197 PF(G) 197
PF(NA) 0P 547 FC(P) 24
FC 523WF 256 FS(A)zFS(NA) 177z79B 0M 0 M 0
013C 102Mn 1300degC 2 K s2 1 (Fig 21c)PF 284 PF(G) 284
PF(NA) 0P 94 FC(P) 16
FC 78WF 364 FS(A)zFS(NA)zFS(LB)zFS(I) 341z209z17z03B 206M 52 M 52
013C 102Mn 1200degC 10 K s2 1 (Fig 21d)PF 147 PF(G) 147
PF(NA) 0P 26 FC(P) 26
FC 0WF 555 FS(A)zFS(NA)zFS(LB) 302z312z167B 226M 46 M 46
013C 102Mn 1300degC 50 K s2 1 (Fig 21e)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 05 FS(A)zFS(NA)zFS(LB) 251z146z432B 824M 171 M 171
013C 102Mn 1300degC 200 K s2 1 (Fig 21f)PF 0 PF(G) 0
PF(NA) 0P 0 FC(P) 0
FC 0WF 0 FS(A)zFS(NA)zFS(LB) 0z0z03B 03M 997 M 997
Thewlis Classiregcation and quantiregcation of microstructures in steels 157
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
and another In this respect an atlas of optical micrographswith associated heat treatments and phase proportionswould be a useful accompaniment to the new classi cationscheme Scanning electron images with their greaterresolution may be employed to describe local features Itshould be noted that a compendium of weld metal micro-structures exists to accompany the traditional IIW classi- cation scheme1 9
The classi cation and quanti cation of complex steelmicrostructures by metallographic techniques is by naturelabour intensiveand it is appropriateto questionthe extent towhich computers may carry out such activities There hasbeen a signi cant amount of work done with regard to imageprocessing in recent years driven by the advances in com-puter technology4 8 The appropriate processing steps dependon the type of information required The measurement ofimages generally requires that features be well de ned byedges size or unique brightness and colour Image analysisthen attempts to nd numeric descriptive parameters thatsuccinctly represent the information of importance in theimage The new classi cation scheme developed in the currentwork provides guidelines on the important features forphase recognition It may thus be possible to train an imageanalysis system to recognise these features The question asto how such information can be processed and analysed bycomputeris a matter for furtherresearchHowever continuedrapid advances in computer power and image resolution maymake this type of activity tractable in the not too distant future
Summary and conclusions
The InternationalInstitute of Welding (IIW) microstructureclassi cation scheme for weld metals has been investigated
as a basis for quantifying the full range of microstructuresfound in plain carbon and low alloy steels as well as ferriticweld metals and parent plate heat affected zones Thefollowing conclusions have been drawn
1 The IIW scheme provides a sound structure for quanti-fying complex microstructures in steels but the classi ca-tion of constituents such as ferrite sideplate and acicularferrite is incompatible with the principal structures found inthe reconstructive and displacive transformation regimes ofsteels There is no classi cation in the IIW scheme ofidiomorphic ferrite and ferrite sideplate structures growingrelatively unimpeded from intragranular inclusions
2 There are problems in relating sub-category micro-structural constituents in the IIW scheme to principal struc-tures at prior austenite grain boundary and intragranularsites owing to stereological and morphological effects Thesehave been discussed in detail and solutions proposed Theways in which transformation products associated withferrite sideplate and acicular ferrite structures may beidenti ed have been de ned
3 A new classi cation scheme has been formulated Themicrostructure classi cation and terminology used in theIIW scheme have been built upon and new terminologyincorporated into a table providing descriptions of theprincipal structures and sub-category components Flowcharts have been devised with guidelines for identifying theprincipal structures
4 The new classi cation scheme has been used toquantify microstructures covering a wide transformationtemperature range A difference in interpretation between
22 Volume percentages of principal structures obtainedby different operators point counting microstructural elds (see Fig 21) using new classi cation schemePF~primary ferrite P~pearlite WF~Widmanstattenferrite B~bainite M~martensite
a 0051C 051Mn 1200degC 10 K s2 1 b 017C 052Mn1300degC 10 K s21 c 013C 102Mn 1300degC 2 K s21 d013C 102Mn 1200degC 10 K s21 e 013C 102Mn1300degC 50 K s2 1 f 013C 102Mn 1300degC 200 K s21
23 Volume percentages of principal structures obtainedby two different operators point counting microstruc-ture of steels under light microscope using new classi -cation scheme PF~primary ferrite P~pearlite WF~Widmanstatten ferrite B~bainite M~martensite
158 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
individual operators has been identi ed by point countingmicrographsusing a xed grid Some discrepancyoccurred inidentifying the boundarybetween allotriomorphicferrite andWidmanstatten ferrite distinguishing between ferrite ndash car-bide aggregate (pearlite) and bainite and differentiatingbetween lower bainite and autotempered martensite Withappropriate training phase proportions obtained by twoindividual operators point counting steel microstructures atrandom using the light microscopewere in many cases withinthe statistical error de ned in the point counting exercise
5 Overall a reasonable degree of consistency can beobtained between operators when using the new scheme toidentify and quantify the principal structures (primaryferrite pearlite martensite) and the actual transformationproducts constituting ferrite sideplate structures notablyWidmanstatten ferrite and bainite Further work is requiredin the form of lsquoround robinrsquo exercises to determine thestatistical uncertainty between operators when quantifyingdifferent types of microstructure and to identify appro-priate training measures for widespread dissemination ofthe scheme
6 A means has been provided of obtaining databaseinformation for developing microstructurendash property rela-tionships or generating data for calibrating physical modelsthat have the principal structures as their output
Acknowledgements
The author would like to thank Dr S V Parker Dr N AWhittaker Dr P L Harrison Dr C Wildash Dr J ButlerDr S A Butler Professor A A Howe and I W Martin ofCorus RDampT for helpful discussions and suggestions Theauthor is also grateful to Professor R C ThomsonLoughborough University and Dr D J Abson TWI forhelpful comments Thanks are nally extended to ECSCpartners at TWI (UK) CSM (Italy) CEIT (Spain) andIRSID (France) for support under ECSC steel researchprogramme 7210PR245(F50100)
References
1 b donnay j c jerman v leroy u lotter r grossterlindenand h pircher Proc Int Conf on lsquoModelling of metalrolling processesrsquo London UK December 1996 London TheInstitute of Materials
2 j k lee and h n han in lsquoThermomechanical processing ofsteelsrsquo Vol 1 245 ndash254 2000 London The Institute ofMaterials
3 a j trowsdale k randerson p f morris z husain and
24 Transformation data obtained from thermally cycled steels in Fig 21
Thewlis Classiregcation and quantiregcation of microstructures in steels 159
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20
d n crowther in lsquoThermomechanical processing of steelsrsquoVol 1 332ndash 341 2000 London The Institute of Materials
4 s v parker lsquoModelling of phase transformations in hot rolledsteelsrsquo PhD thesis University of Cambridge UK 1997
5 h k d h bhadeshia and l e svensson in lsquoMathematicalmodelling of weld phenomenarsquo 109ndash 174 1993 London TheInstitute of Materials
6 s j jones Modelling inclusion potency and simultaneoustransformation kinetics in steelsrsquo PhD thesis University ofCambridge UK 1996
7 s j jones and h k d h bhadhesia Acta Metall 1997 45(7) 2911ndash 2820
8 k ichikawa and h k d h bhadhesia in lsquoMathematicalmodelling of weld phenomena 4rsquo 302ndash 320 1998 London TheInstitute of Materials
9 d j c mackay in lsquoMathematical modelling of weld phe-nomena 3rsquo 359ndash 389 1997 London The Institute of Materials
10 r c reed lsquoThe characterisation and modelling of multipasssteel weld heat affected zonesrsquo PhD thesis University ofCambridge UK 1990
11 k e easterling in lsquoMathematical modelling of weld phe-nomenarsquo 183ndash 200 1993 London The Institute of Materials
12 m atkins lsquoAtlas of continuous cooling transformationdiagrams for engineering steelsrsquo 1977 Swinden LaboratoriesRotherham British Steel Corporation (ISBN 0 9500451 44)
13 z zhang and r a farrar lsquoAn atlas of continuous coolingtransformation diagrams applicable to low carbon low alloyweld metalsrsquo 1995 London The Institute of Materials
14 b l bramfittand j g speer Metall Trans 199021A 817ndash 82915 y ohmori h ohtsubo y c jung s okaguchi and h otani
Metall Trans 1994 25A 1981ndash 198916 u lotter and h p hougardy Prakt Metallogr 1992 29 (3)
151ndash 15717 c a dubE h i aaronson and r f mehl Rev Metall 1958
55 20118 h i aaronson lsquoDecomposition of austenite by diffusional
processesrsquo 389 1960 Philadelphia PA AIME19 lsquoCompendium of weld metal microstructures and propertiesrsquo
1985 Abington Woodhead Publishing20 lsquoClassi cation of microstructures in low carbonndash low alloy
steel weld metal and terminologyrsquo Committee of WeldingMetallurgy of Japan Welding Society IIW Doc IX ndash 1282ndash 83
21 e anelli and p e di nunzio lsquoClassi cation of microstructuresof low carbon steels preparation of a set of standardmicrographsrsquo ECSC Agreement 7210ndash EC405 (94ndash D302a)CSM Rome Italy June 1996
22 g thewlis j a whiteman and d j senogles Mater SciTechnol 1997 13 (3) 257ndash 274
23 r w k honeycombe and h k d h bhadeshia lsquoSteels ndashmicrostructure and propertiesrsquo 2nd edn 35 1995 LondonEdward Arnold
24 k m wu t yokomizo and m enomoto ISIJ Int 2002 421144ndash 1149
25 g myamoto t furuhara and t maki CAMP ISIJ 2001 141172
26 h k d h bhadeshia lsquoBainite in steelsrsquo 1st edn 1992 LondonThe Institute of Materials
27 g thewlis lsquoStable sulphide particle dispersed steelrsquo Interna-tional Patent Application 01052182 Corus UK Ltd Mar 2000
28 r m brick and a phillips lsquoStructure and properties of alloysrsquo2nd edn 334ndash 337 1949 New York McGraw-Hill
29 a g guy lsquoElements of physical metallurgyrsquo 2nd edn474ndash 476 1960 Reading MA Addison-Wesley
30 h k d h bhadeshia Acta Metall 1981 29 1117ndash 113031 j w christian lsquoMilitary transformations ndash an introductory
surveyrsquo 1 ndash 19 1965 London The Iron and Steel Institute32 g thewlis Sci Technol Weld Joining 2000 5 (6) 365ndash
37733 h k d h bhadeshia and j w christian Metall Trans A
1990 21A 767ndash 79734 h k d h bhadeshia Mater Sci Eng A 1999 A273 ndash A275
58 ndash 6635 subra suresh (ed) Scr Mater 2002 47 (3) (Viewpoint Set on
lsquoBainitersquo)36 madariaga i gutierrez and h k d h bhadeshia Metall
Trans A Sept 2001 32A 218737 g r speich and w c leslie Metall Trans 1972 3 1043ndash
105438 r a ricks p r howell and g s barritte J Mater Sci
1982 17 73239 a t davenport and p c becker Mater Trans 1971 2
296240 e beraha and b shpiglar lsquoColour metallographyrsquo 1977
Metal Park OH American Society for Metals41 w fin lsquoBasic principles for colour metallographyrsquo 1983
Beijing Beijing Industry University42 p maier a richter r g faulkner and r ries Mater
Charact 2002 48 329ndash 33943 i m watt lsquoThe principles and practice of electron microscopyrsquo
2nd edn 1997 Cambridge Cambridge University Press44 a j schwartz m kumar and b l adams lsquoElectron
backscatter diffraction in materials sciencersquo 2000 New YorkKluwerPlenum
45 f weinberg lsquoTools and techniques in physical metallurgyrsquoVol 1 272ndash 275 1970 New York Marcel Dekker
46 b pickering lsquoThe basis of quantitative metallographyrsquo 8 ndash 101976 London Metals and Metallurgy Trust for the Institute ofMetallurgical Technicians
47 t gladman and j h woodhead J Iron Steel Inst 1960 194189
48 j c russ lsquoThe image processing handbookrsquo 2nd edn 1995Boca Raton FL CRC Press
160 Thewlis Classiregcation and quantiregcation of microstructures in steels
Materials Science and Technology February 2004 Vol 20