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Sn. ;0:2 CEDOC UFES ,.'.: ..... Printed and bound by R. R.Donnelleyr .r .... r __.. n' !') UV:9.i!](,n(l ,[V.f ..... Ii )ICt' , !3I -,uo._.. ___ . ........... ____ .Hegis;fO Livro adquirido com !"ec!!.rS!lSdnFll'ofEPICTPETRO 2000 Informationcontainedinthisworkhasbeenobtainedby McGraw-Hill,Inc. ,fromsourcesbelievedtobereliable. However,neitherMcGraw-Hilinoritsauthorsguaranteethe accuracyorcompletenessof anyinformationpublishedherein, andneither McGrawHillnor itsauthorsshallbe responsible for any errors, omissions, or damages arising out of use of this infor-mation.Thi sworkispublishedwiththeunderstandingthat McGraw-HillanditsauthorsaresupplyingInfomiaiionbutare notattemptingtorenderengineeringorotherprofessionalser. vices. If such services are required,the assistance of anappropri-ate professional should be sought. !

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:!;!Fatigue beach marks Crackgrowlh (Smooth zone)-L.t t Origin/initiation zoneoffatigue Fig.4.SaSchematicrepresentation ofthefatiguecrack(three-stage)phenomenon Failure:Typesand Cbaracteristics73 """'1( { ( ( ( ( ( ( ( ,( ( ( < ( \. ( ( ( ( ( ( ( ( ( t i' ( l :i' K I:!'

( r ., ( ( material throughwhichthey pass. Theabsorption isprimarily dependent onthe energy spectrumoftheradiationandthedensityandthicknessofthematerial.Theradiation commonlyusedinradiographyare:(i)X-rays,(ii)Gammarays,and(iii)Neutrons. 104Metallurgy of Fa.i1ure Analysis Modern X-ray machines operate with tungsten, gold or platinum at 50 kVp. to400 kVp. Thesharpnessoftheimageisdeterminedbythesizeofthetarget,theratioofthe source-specimenandfilm-speCimendistances. Screensofaluminium,copperand lead areusedtoimprove the definition'. Radiographic sensitivity ismeasured in terms of the min4numpercentageofthicknessofthecomponentthatcorrespondstotheobserved changeinthephotographic densityoftheradiograph. Thesizeoftheflawisdetected byusingapenetrometer.. RadiographybyX_ray18requiresa predeterminedintensityoftheX-raybeamfrom anX-raymachinetopass through the component under examination.The beamwhich emergesoutofthecomponent,afterundergoingdifferentialabsorption,isallowedto fallonanX-rayfilm. Thefilmisenclosedina light-tight cassettepositionedclosestto thecomponent.The information contained in. the emergent beam isrecorded asa latent - imageonthe X-rayfilm.Thelatent image isconverted into a visibleimageby chemical treatmentofthefilmwitha developer.Thustheinformationispermanently recorded. Wherever permanent recording isnot required, X-ray fluoroscopy can be used for quick dispositionofparts. Themainareasofeffectiveapplicationsaregivenbelow. Prod uct / Process Castingots,shaped castings, weldments. Assemblies Effective detection of Internal discontinuities, e.g.shrinkage, hot tears,coldshut,sponginess,cavities, porosity, inclusions, segregation,cracks (which are not very tight),misruns. Corrosion pits,misalignments,cracks (not very tight) and entrapment of dense ma-terials . Thegreatest advantageOfradiography isthata perrnanent recordof examined compo-nentscanbepreservedforfuturereference.However,itisessentiallyaslowand expensivemethodofinspection.A.nothermajorlimitationisthatifthedifferential absorption is below a certain limit which cannot produce a readable density onthe film, theinformation will not record on the film. Precisely forthis reason, planer defects such astightcracks,folds,laminationsanddebondsarenotdetectedbyradiography. Radiography by gamma raysissimilar toX-rayradiography, except that radiation is obtainedbythedisintegrationofatomicnucleiwithinradioactivematter.Gamma-ray equipments consist of a gamma-ray source such asIr 192, Co60,placed in an adequate-ly shielded container. Selection of a particular isotopeforgammaradiography depends onthenatureandthicknessofthematerial.Thepropertiestobeconsideredwhile selectingthe isotope aregamma-ray spectrum, half-life, specific emission, curie strength anditscost.Ir192iscapableofpenetrating35mmofsteelandCo60iscapableof penetrating150mmofsteeloritsequivalent. Gamma-raysarenotrecommendedforradiographyof aeronauticalcomponentsex-ceptincaseswherethecomponentthicknessisbeyondtherangeof X-raypenetration, andwherepoorflawsensitivityisacceptable. Techniques of FailureAnalysis105 ( ( t ( ( ( ( ( ( ( ( \. ( ( ( ( ( ( ,'11 (,i:: (.". " (,:1:,., (,I" ( -J ( .J( .t

( K ( f Theadvantagesofgammaradiographyare(i)unlikeX-raysnoelectricalpower cooling system is required; (ii)deeper penetration because .of higher energies toX-rays;(iii)portable;and(iv)cheapercomparedtoX-rayequipment. Thedisadvantages are (i)the energy of gamma radiation cannot be controlled at (ii)sensitivity iscomparatively poor; (iii)longer exposure time;(iv)frequent renewal isotopesin casesof shorterhalf-livesand(v)specialsafetyprecautionsarerequired. UltrasonicInspection Thismethod of testing19consistsof transmitting energy ill theformof high soundwaves,bothlongitudinalandtransverse(frequency20,000Hz)into material undertest.Ultrasonicenergyisintroducedintothespecimenbymeans probe equipped with atransmitter and a receiver or by means of a separate andreceiverthrough a coupling media suchaswater,glycerine,mineral oilor The probe generates ultrasonic waves when an electrical fieldisapplied topiezoelectnC" crystalssuchasquartz,bariumtitanate,etc.Formostofthematerials,thetesting frequency of ultrasonic waves ranges between 1 and 15MHz.If the beam is interrupted byadiscontinuity suchascrack,voidetc.,theenergyisreflectedback.Thisreflected energyispickedup. Therearethreetypes20of informationdisplayfromthepulseechoinspection isawidelyusedsysteminindustry . 1.A-Scan,which isa quantitative display of intensity and elapsed data obtained at asinglepointonthetestpiece. 2.B-Scan,which isa quantitative display of elapsed time data obtained along a line "' onthesurfaceofthetestpiece. 3.C-Scan,whichisasemi-quantitativedisplayof echointensityobtainedoveran areaofthetestpiece. AcomparisonofthethreescansisshowninFig. 5.l. The firststep inultrasonic testing istocalibratetheequipment toensure linearity of indications,requiredlevelofsensitivity,resolutionandprobecharacteristics.Thisi donewiththehelpofthecalibrationblocks. Aftercalibrationthenext stepistoapply a suitable couplant on the deaned surface of the component and move the probe onsurfaceinazig-zagmanner,andtorecordthereadingsobtainedonthecathodescreenoftheequipment. Themainareasofeffective applicationsofultrasonictestingaregivenbelow: Laminationin Plate A TraceofDefect B TraceofDefect--........----TopofPlate BottomofPlate ShowsAreaofFlawas C Trace(PlanView) SeenfromAbove Fig.5.1Comparisonof A,Band C-scantechniques20 Themainadvantagesofultrasonictestingistheabilitytogetinstantaneousresults andafastinspectionrate.Itsmainlimitationsarecompletedependenceuponthe competence and integrity of the operator tointerpret the result,the difficulty in discrim-inating between different types of defects and thedifficulty in detecting defects near the surfaceowingtothedeadzoneoftheprobe. EddyCurrentInspection . Product/ProcessEffective detection of Thisisbasedontheprincipleofelectromagneticinduction.An.alternatingelectric flowingina coilcausesanalternating magneticfieldofthesamefrequencyin Billets,forgings,extrusions,plates, sheets, bars, rods,weI dings. Assemblies 106Metallurgy of Failure Analysis Detection of internal discontinuities as cracks, bursts, laminations, inclu-sions, voids,debonding. Stress corrosion cracking, corrosionpits andcracks. thevicinityofthecoil.Thisalternatingmagneticfieldinduceseddycurrentsinan electrically conducting material located near the coil.Theeddy currentsthusgenerated, producetheirownalternatingmagneticfieldwhichopposestheexistingalternating magnetic fieldof the coil. This causes a change inthe impedance of thecoil.This change Techniques of Failure Analysis107 "I ii ii '

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'1""" isrelatedtoavariationinphysicalpropertiesandthepresenceofsurfaceandsljb-surfacediscontinuitiesinthematerial. Theeddycurrenttestsystemconsistsofasourceofalternatmgcurrentofknown" frequency applied to a test coil assembly and an electronic detecting system to sense indicatevariationsintheoutputofthecoilassembly.Thesevariationsarecaused changesintheelectromagneticneldofthecoilduetodiscontinuities,andalsodueto a change inphysical properties effected by variation incomposition andtreatment. The test system may also include a phase discriminator, filter circuit and magnetic saturation devicesasrequiredforaparticularapplication. Eddy current methods are normally used forthe detection of surface and defectsonly.Thismethodiscomplementarytoultrasonictestingwhichis'usedfor detectingdefectsclosetothesurface.It isalsocomplementary' tothedye inspectionwhichcannotrevealsub-surfacedefects. Theequipmentiscalibratedwit appropriatestandard/sdependingonthepurposeofinspection. Theareasofeffectiveapplicationofeddycurrentinspectionareasfollows: Product/Process Allconducting materials Effective detection of Surface/sub-surfacecracks, voids andinclusions. Dimensionalmeasurementssuchasplat' r" ing/ coatingthickness, Determinationofphysicalproperties asconductivity andhardness. Sorting of metalmix-up, Theadvantagesofeddy current inspectionareitsversatility,highspeedofinspec- ,.', tion,non-requirementofcouplantbetweenthetestcoilandspecimen,andabilityta l detectcracksfilledwithnon-metalliccontaminants. This method cannot be used on non-conducting materials. Local variations intivity and permeability of an acceptable nature may interferewith crack deteco"'ii/,'tion.Themeasurement of metal coating thicknessisdifficult unless a substantialenceinconductivityexistsbetweenthecoatingandthesubstrate. OtherNDTTechniques ,,'.

.SCI') '"0> b"" .'"'"'!"' ...... ... . .. ---l'ig 8.65Fatiguezonesoneithersideof thefirstrivetholeex6) RemedialActionSincea number of casesof such cracking havebeenreported inthe mainrotorblades,theprocessofdrillingoperation01thetrailingedgeaswellasthe skinwererecommendedtothemodified,soastoavoidoffsetdrilling. Also,it was recommendedtoensure freedomfromsuchoffset drilling aftercomple:.... tionofrivettingoperationby suitableNDTmethodssuchastheX-ray. engineofatransportaircraft,catastrophicallybrokeintotwoalongtherootradius duringtheenginetestrun. Thebroken hub portion of the blade was subjectedtofractographicaswell asmicro-scopicexamination. These studies revealedthatthe failurewas caused due tocorrosion fatiguephenomenon.;,Further; it wasinteresting tofindthat almost uMoticea15iyminor atmospheric corrosion pits inthe critically loaded blade root radius zone had caused the developmentof numerousfatiguecracksaspointsof stressconcentrationduring long serviceofaircraft.Sincecorrosionpitsweretoosmalltobeidentifiedevenundera magnifyinglensduringperiodicinspection,thedefectscouldnotbedetecteduntil failureoccurred. Henceitwasrecommendedtoenforceperiodicinspectionof thecriticalrootradius portion at suitable magnification, preferably under the low powered stereoscopic micro-scopefordetectionofcorrosionpitsorcracks. Background Applicatio1ls Propeller blades(Fig.8.66)are machined out of an aluminium-copper alloy forgingand usedinthepistonenginesofthe'transportaircraft.Thepropellerassemblyunder investigationhad served forseveral yearsand had been periodicallyoverhauled.Dur-ingoverhaulthepropellerbladewassubjectedtoatestrunintheoverhaulshop.In thiscase,thebladeportionofthepropellerassemblywas wellprotectedagainstcor-rosion by anodising and p.ainting, but thehub portion which had only anodic protection wasexposed tothe atmosphere. The propeller is subjected to complex centrifugal forces duringflight. Ci,cumsta1lcesLeadingtoFailure 8.17CORROSIONFATIGUEFAILUREOF ALUMINIUMCOPPERALLOYPROPELLER BLADEOFPISTONENGINE During thetest run, after a period (not known)the blade portion of one propeller broke offfromthehub(Fig.8.67),resultinginconsiderabledamagetothesurroundings. However,itcould not beascertained whetherthepropeller blade had beenthoroughly checkedforanycrackpriortotheenginetestrun. SummaryandRecommendations Oneofthealuminium 5%copperalloyforgedpropeller blades belonging tothepistonSpecifications Materialspecificationofthealloyusedforthepropellerbladewas20256. 262Metallurgyof Failure Analysis ServiceFailure:CaseStudies263 ( ,J ( ( ( ( ( ( ( ( ( ( ( ( r ') ) ) ) ) 3 ) ) ) ) 1-----Selectionof Specimens Theonlyavailablehubportionofthebrokenpropellerbladewassubjectedto "', fractographic examination under a low.powered stereosc.opic microscope.A sectionacrosstheoriginofthefracturewasusedformicrostructuralanalysis. VisualE:xaminationof GeneralPhysical Features overalmosthalfofthefractureandtheotherhalfwithveryirregularandcoarse fracturelines(Fig.8.67). Thesm{)othareaofthefracturedsurfacealsoshowed several minorsegmentsoffatigueoriginsalongtheradiusperiphery. Visual examination revealed that the breakage had occurred circumferentially along th.e radiusportionof thehub, intheregionofminimum OD(Fig. 8.67).Thesurface inthe ' region of the fracture showed no apparent defect or abnormality. However, , under thestereoscopic microscope showed numerous corrosion pitsallaround the hub,":':,___ radius.On cleaningthem,deeppittings were foundinplaceswith interconnecting fine"'" " MacrofractographyExaminationofthefracturedsurfaceunderthestereoscopicmi-croscopeatx20magnification,revealeddistinctanduniformlycloselyspaced'beach' marksofatypicallyhighcyclefatiguefailureinthemajorsmoothsegment(which covered half of the section thickness, originating fromthedeep corrosion pits (Fig.8.68) atthehubradiusportion.In addition,atleastfiveotherminorzonesoffatiguewere observedtohaveoriginatedalongtheperipheryofthehubradius(Fig.8.67).These secondary fatigue cracks after an initial propagation for a short while were seen merging withthemaincrackfront.Thispropagatedasasinglemajorcrackuntilthereduced sectionthicknesscouldnolongerbearthesustained load.Allthefatigueoriginswere tracedtocorrosionpitsinthehubradius.The rough,irregularandfibrousareawith 'chevron'markingsindicatedthezoneoffinaloverloadfracture(Fig.8.67). cracks(Fig.8.68). Fig.8.67Distinctfatiguezonesatseveral placesmarkedbyarrows(x 3/4) Fig.8.68Deeppittingsandconnecting cracksex6) Metallography MicrostructuralAnalysisMicroexaminationofthesectionacrossthemainfatigue zonerevealedthecrackpathastrans granular.Transgranularmodeofcrackingalso confirmedthatthefailurewasduetofatigue.Otherwise,thealuminiumcopperalloy materialwasfoundsatisfactorilyheattreatedwithnostructuralabnormalities.How-ever,corrosionattackonthesurfaceintheradiusportionwasfoundintergranular, formingdeepfissuresup toamaximumsizeof1.5mmdiaand1 mmdepthinthe sectionexamined. Discussion TestingProcedureandResults Althoughfractographicanalysisof the hub fractureclearlyestablishedthefactthatthe suddenfailureof oneof thepropeller bladeswasduetoahighcyclefatiguephenom-enon,thecauseof fatiguewastraced tothe presenceof unnoticed corrosion pitswhich actedassevere stressraisersinthe highly rotating blade rootradiuszone. Fractograph (Fig. 8.67)revealedamultipronged attackfromtheseveral sitesofthecorrosionintru-sionintheouter peripheryoftheroot radius. Thefatiguecrackgrowthcontinuedtill' thesectionthicknessgotreducedto half itsdiameterand failedduetooverload.This causedaninstanthighvelocityfractureinarandommanner.Thisfactindicatedthat periodicinspectionofthehubwasnotmaintainedtodetectcorrosioninthecritical areasof the root radius portion during the long service of the aircraft,which is constant-lyexposed toextreme atmosphericcorrosion factorslikerain water / moisture and heat. Underthesecircumstances,thecriticallyloadedhubradiusportionwasmoresuscep-tibletodevelopmentofcorrosionpitssinceitsanodiccoatinghadlostitscorrosion resistanceduetoprolonged usage.On theother hand if the propeller blade surface had protectedbybothanodictreatmentandpaintingtheproblemofatmospheric ., corrosionwouldnothavearisen. SU1:faceExamillation Visual ExaminationVisuallythe fracturesurface showed two distinct zonesofdisin-tegration.One portion was showing a verysmooth surface with 'conch' shell markings 264Metallurgyof Failure Analysis ConclusionandRecommendations MostProbableCauseTheabovewasclearlyacaseof corrosionfatiguefailurewith Service Failure:CaseStudies26; ,I I i I i 1 I 'i : 1' ,+ J 11111t tf'

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Specifications MaterialusedforthemainwheelhubcomponentwasanAmericanalloytoMIL-M_ '-F 46062Bspecification.,0" Selectionof Specimells Therim cracked portion was cut open and subjected to fracto graphic examination under" a lowpowered stereoscopic microscope.A section was cut acrossfracture for examinationundertheopticalmicroscope. VisualExaminationof GeneralPhysical Features Examination revealeda 25cmlongcrackalongthemachinedgroove near therirnthat se1ltedthetubularring(Fig. 8.69).Thecrackwasfoundbranchingoffoneitherside: Numerous corrosion spots seen in the groove, on cleaning, were found penetrating deep, atplaces. Noothertypeofseriousdefectwasobserved. TestingProcedureandResults SurfaceExamillati011 MacrofractographyThecrackalongtherimwascarefullyopenedoutandthefrac-tured surfacecovering theentire section thicknesswas foundsmooth ina plane square" tothezoneofcrackinginthegroovesurfaceandshowing .distinct'shell'markings, typicaloffatiguefailure(Figs8.70and8.71).Origins offatigueweretracedtothe severaldeepcorrosionpitsonthegroovesurface,constitutingparallelratchettype fatiguezones(Fig.8.71)atdifferentplacesintheinitialstagesofcrackpropagation. a section of thecast wheel material in andaroundthezoneofcracking,revealednostructuralabnormalities,Thecorrosion attackwasfoundintergranularformingdeepfissureswhilefatiguecrackingwastypi-cally .',', ... .. :iH( \r li'\

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i r i\ ' (I ij; ( C' . P-\ Remedial ActionInview of therepetitive failureof anidentical nature, it was recom_ mendedtoreviewdesign measurementssoastoprovideadequateshimming between thematinglugsinordertominimisetherelativemovement. 8-20DETERIORATIONOFCASTMAGNESIUM ALLOYMAINGEARBOXHOUSINGDURING SERVICE SummaryandRecommendations An imported main gear box of one of the helicopters was found corroded during routine inspection. Visualexaminationrevealeddeep'furrow'typecorrosionpitsfilledwithheavy powderydeposits.SinceX-rayexaminationrevealednodefectintheaffectedportion ofthehousing,atextureexaminationwascarriedoutonthebrokensurfaceofthe castingwall.Thisrevealedflouridetypefluxinclusionsintheformof black patches at severalplacesafteradayofexposuretoatmosphericconditions.Theseblackpatches were also found increasing in area on longer exposure, confirming the reactive condition of thefluxpresent inthecasting.Therefore,itwasconcludedthatthematerialdeterio-rationwas a caseof internal chemicalreactionof theflouridefluxwiththemoisture of theenvironmentduringservice. It wasrecommendedtocarryouttexturetestsonthetestcouponsfordetectionof fluxinclusionsinfuturemeltbatches. Background ApP lic ations Themain gear box,usually castout of magnesium alloy,isused in helicopterstohouse acombinationof 'complexgearassembliesforthepowertransmissionsystem.Since magnesium alloy casting is sensitive toatmospheric corrosion,the housing, immediately aftermanufacture,isnormallyprotectedwithchromateandpaintcoatings.However anyleakageinthecastingbywayofcorrosionorheavyporositymayleadtolossof engineoilandinturn,severelyaffectstheflightperformance. CircumstancesLe!lditlgtoFailu re Themaingearbox housing showeddeepcorrosionpitswithpowderydepositsonits outer surface(Fig.8.75)aftera periodof service(not known),onroutinevisual inspec-tionduringoverhaul. Specifications ThematerialofthemaingearboxhousingwasaFrenchalloytoRZ5specification (Table8.17). 274Metallurgy of Failure Analysis 1 Fig.8.75MGBhousingoutersurfaceshowingfurrowsoncleaningoff powderydeposits inlocalisedzones( x112) Table 8. 17Chemical specification (Wt.%)for the French magneSium-zinc casting (RZ5) from whichthemain gear box housing was madeand actual composition of thefail ed part. Element Min.Max.Failedpart Zinc 4.05.04.6 Manganese 0.150.12 Silicon om0.007 Copper 0.030.01 Iron om0.006 Zirconium 0.700.62 Rare earths 1.501.48 Selectionof SpeCi mens Twosmallstripswerecutlengthwisefromtheintactportion(i.e.withoutcorrosion) ofthehousingwallfortextureexamination. VisualExaminatio nof GeneralPhysicalFeatures Examination,onbrushing off the powdery deposits,revealed 2-3mm deepfurrowtype pitsat places,asif eaten away by worms.These pits were mainly confinedtotheupper portionoftheoutersurfaceofthechroma tedandpaintedhousing. Selvice Failure:Case Studies275 -,.,.,,;;;.;-TestingProcedureandResults Non-destructiveEvaluatioll RadiographyRadiographicexamina,tionoftheentirehousingrevealedthe structurallysoundallover,eveninthevicinityofthedeepfurrows. SimulationTestsSuspecting,fromthenatureofcorrosionpitsanditspowdery de-' posits,thatthecorrosionwasaproductofaninternalchemicalreaction,atexture examination was carried out on the cut-oul strips after breaking them off under bending overload.Althoughimmediatelyonfracturingnoabnormalitieswerenoticed,after a dayofexposuretotheatmospherethefreshfracturestarted developing black patches. (Fig.8.76).These black patches werealsofoundincreasing illareawith exposure time . .. Thesecharacteristicsweretypicalof corrosionduetofluxinclusion.-"--Fig.8.76Blackzonesdevelopedinfracturedsufacewhenexposedtoatmosphere ,due tothepresenceoffluxinclusions(x 1) Discussion '\':".':r."" - .. "...,;_,:j , Jsurface, thatafter coming in contact with atmospheric moisture possibly through someporosity. It ispertinenttonotethatthepresenceof suchinclusionsarenoteasilydetectable byX-rayexaminationduetothedefectbeingofalighterdensity. ,,' 'f ,.'CondusionandRecommendations MostProbableCauseThebasiccauseofthematerialdeteriorationwasattributedto theflouridetypefluxinclusionspresentinthecastingduringmelting. RemedialActionItisimportanttonotethatnoflouridetypeoffluxinclusionis permittedtobe present inthebodyofthe casting,asitdeteriorates. thechemistry and soundnessofthematerial,speciallywhentheyareusedforcriticalapplications. Therefore,it wasrecommendedtocarry out the texture testontext coupons of the melt batchfordetectionofsuchfluxinclusions. beforeacceptance.. 8.21FAILUREOFTHEEXHAUSTPIPEASSEMBLY OFAPISTONENGINEDUETOTHEWELD DECAYPHENOMENON SummaryandRecommendations Oneexhaustpipeassemblybelongingtothepistonengineofatraineraircraftwas foundcrackedatthetoeoftheweldafteraperiodofservice. Fractographicexaminationofthecrackedsurfaceindicatedacoarsecrystallined (brittle)failure.Micro-examinationof a zone inthe. failedweld portion ofthepipeand thecollarrevealeddistinctintergranular corrosioncracksintheweldaswellasHAZ. Chemical analysis of the pipe and the collar (which was arc-welded together to formthe assembly)confirmedthat 18/8 stainlesssteelpipe material wasnot stabilised,whereas thatofthecollarwasstabilised.Frommicrostructuralanalysisitwasevidentthatthe intergranularcorrosionattheweldzonehadoccurredduetodepletionofchromium atthegrainboundariesintheformofchromium-carbideprecipitates.Sincethepipe materialwasasimple18/8typeausteniticstainlesssteelwithoutanyaddition of . stabilisingelement(Ti/Nb),it becamesusceptibletocorrosionattack. Based onthe above factsit was concluded that the cracking was duetoa weld decay phenomenon.It wasrecommendedtoheattreatthebatch,ifwelded,todissolvethe carbidesandretaintheminsolutionbyrapidquenching. Fromvisualexaminationofthecorrosionproductsalone,itbecomeevidentthatthe magnesiumalloyhousing had suffereda corrosionattackfromwithinthebody.More-over,formationofheavypowderydepositswasindicativeofsomeinternalchemical reaction. It isan establishedfactthatthepresenceofflouridecontainingflux(whichBackground generallyusedformeltingofmagnesiumalloyinthecastings)servesasasourcefor ... nucleationofcorrosionbyvirtueofitsreactivenaturewiththebasemetalwhenjn- .AppllcahOllS contact with moisture.On this basis, a simulation testwas carried out ontheunaffectedTheexhaustpipeisanarc-weldedassemblyoftubularpipeandcollar,madeofaus-portionofthehousingandthisconfirmedthepresenceoffluxinclusionsinthewallteniticstainlesssteel(Fig. 8.77)andisusedasanoutlet forhotgasesfromtheengine. thickness,whichgotentrappedduringthemeltingpractice.OveraperiodoflongThecollar endofthepipe assembly isattachedtotheenginebayareabyboltsandis service,itwasthesefluxinclusions,whichwerepresentincloseproximityofthelefthangingoutsidetheaircraft. 276Metallurgy of Failure Analysis Service Failure: CaseStudies277 " ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( Irl' I' itI , : 'II i '1""'1II' , I!tl Iff,>!

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I,i ll 1:1'I[ If (Ii j i ! I Ci"cutnstatlcesLeaditlgtoFailure During routine inspectionoftheaircraft,theexhaust pipeassembly was foundtohave cracked alongthetoe of thewelded joint (Fig,8.78).Thetotal period of servicethat the part hadundergonepriortofailurewasnotavailable.

Fig.8.77Thefailedexhaustpipe assemblyFig.8.78Closeupviewofthecracked1 shOWingthelocation,oftheareaattheweldtoe(x 2/3) crack( x113) Specificatiotlsj Thematerialusedforbothpipeandcollarwasanausteniticstainless(stablised)steel1 andtheirspecificationswerereportedtobeMIL-T-6737-CompG321andMILS-6721, respectively. 278Metallurgy of Failure Analysis ./ ,i -"' ,!;;ii, Selectiotlof Specitnetls Thecrackwasopenedoutandasectionwastakenacrossthefracturedportionfor micro-examinationandtheremaining portion wassubjectedtofractographicexamina-tionunderthestereoscopicmicroscopeandchemicalaralysis. VisualExaminationof GeneralPhysical Features Thepipewasfoundcracked/brokencircumferentiallyatthetoeoftheweldedjoint (A-B) except fora small portion intheweld'X'(Fig.8.79). TheODsurfaceof thetube whichwasinsertedintotheboreregionofthecollarshowedheavyoxidescales. However,it did not exhibit any perceptible deformation atthefractureregion.Physical damageslikenicks,dents,etc.,were not observedinthevicinityofthefailedregion. Fig. 8,79Thefr actured surfaceof thecracked portion(A-B) .Zone'X'isfra cturedatthe laboratoryexI) TestingProcedureandResults SurfaceExatnitlatiotl MacrofractographyExaminationofthecracked surfaceunder the stereoscopic micro-scopeatx20magnificationrevealedirregularandcrystallinedfeaturesofabrittle failure,with brownish discolourations(due to hot gases).No indication of any progres-sivetypeof failuresuchasfatiguewasdiscernibleonthefracturedsurface. MetallographyMicro-examinationofthesectionacrossthefailedregionofthepipe atx 400magnification,revealednumerousintergranularcorrosioncrackswithfewof thempropagatingextensivelyintotheheataffectedzone(Fig.8.80).However,on Service Failure:Case Studies279 examination of thesame area at x 1600magnification, chromium carbide particles observed precipitating along the grain boundaries (Fig.8.81).The welding characteristic suchasfusionandpenetrationwerefoundsatisfactory. Fig.8.80Severalintergranularcorrosioncrackswith someof thempenetrating deepex400) Fig.8.81Massivechromium carbideprecipitationalongthegrainboundariesat HAZex1600) 280Metallurgyof Failure Analysis nt,-:;:,, chemical Analysis Ont,l;w material.wasfouf\Q,tqbeof 18/8 stainless ,'1.(: -"'" : -:-.- ..,t.-r.> ;l,- _ ;;j" .."h.. !i, . lillil'li 1:1,:: i

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--,'t--Fig.8.87Severecorrosionattackonthesurfaceof thepropellerblade(xI) Specificatio1ls TherivetmaterialusedwasaFrenchalloytoAG5specification. PerformanceOf other PartsillSameor Similm'Service It isinterestingtonote that a similar rivet head failureof atail rotor blade made ofthe same material, belonging toanother helicopter,was alsoreported prior tothisincident. On investigation,thefracturesurfaceoftherivetheadbrokenpiece(theonlyportion available),whichwassoperfectlycleanedpriortoitsreceipt,pointedtowardsoneof the shearing.Assuch,results of the investigation were inconclusive asthe sample piece wastoosmalltocarryoutthemicrostructuralandchemicalanalysis.Also,theback-groundinformationwasinadequatetothrowanylightonthefailuremechanism. ':/i, Selectionof SpecimellThetworivetswhichbrokeintotwoalongtheshankradiusonslighttapping,were subjectedtofractographicstudiesunderthestereoscopicmicroscope andthescanning, electron microscope.One ofthebrokenrivet heads was sectionedand examined1' , theopticalmicroscope.VisualExaminationof GeneralPhysicalFeatures Examination of the main rotor blade revealed extensive surface corrosion. The corrosion patches(Fig.8.87)appearedtobeatmosphericinnature.Outofalltherivetsinthe 288Metallurgy o[ Failure Analysis ...---.... "'..:.. ' ""' "'{.. ."""t failed blade, 'only two were found intact, i.e. not separated like others but partly affected bycracking.Nocorrosionproductslikethoseonthebladesurfaceswerenoticedin thesetworivetsurfacesorinthecrackedzones. resting Results !-i' .-.e.if-"'=;,t;SurfaceExami1lation MacrofractographyExaminationofthebrokenrivet headfr acturesunderthestereo-scopicmicroscopeatx20magnification,revealeda brittle surfacewithlayeral features (Fig.8.88). This was suggestive of failure initiated by stress corrosion cracking.Also, the broken portions did not show any perceptible deformation in the region of cracking (on matchingboththerivet headandtheshanktogether). Fig.8.88Brittlelayeralfr act ure(x 20) ScanningElectronMicroscopyFractographyExaminationofthefractu.resur-faceunderthescanningelectronmicroscope,atx1500magnification,r evealed thecrack progagating predominantlyinan intercryst allinemode(Fig.8.89),with linearmarkingsonthefracture .Thesecharacteris ticsindicatedthatthefailure wasduetothestresscorrosionphenomenon.. Matallography Microstructural AnalysisMicro-examination of the longitudinal section takenthrough thefailedzoneofoneofthetworivetheadsrevealedthatthecrackingwasby Service Failure;Case Studies289 Fig.8.89Intercrystallinemodeof fractureunderthescanningelectron microscope(x1500) intergranular modewithmultiplesecondarycracksrunning paralleltothemaincrack (Figs.8.90'and8.91).Thesestructuralfeaturesgaveapositiveindicationofstresscor-rosion.Asseenfromthemicrostructureoftheadjoiningareaoftherivet head,cracks were found emanating fromthe severely cold worked regions (Fig.8.91)in the shoulder radiuszonebetweentheheadandtheshank. Chemical Analysis Therivetwaschemicallyanalysedandthecompositionofthematerialwasfound conformingtoAG5specification.(Table8,19) Corrosion DepositsTheresidual deposit of corrosion products on analysiswas found answeringforaluminiumoxideasreactant fromgeneralatmosphericcorrosionattack. Discussion Fractographicaswellasscanningelectronmicroscopicexaminationoftherivethead fractureclearlyestablishedthefailuremechanismbystresscorrosionphenomenon. Further,micro-examinationoftherivetheadalsoconfirmedthefailurebystress-290Metallurgy of Failure Analysis J J j f Fig.8.90Intergranular modeof fracturewithmultiplesecondarycracksrunningparallel tothemaincrack(x 200) Fig.8.91Cracksemanatingfromtheseverelycoldworkedregionof therivethead shoulderradius(x 200) corrosionwithpositiveevidencesofintergranular(parallel)cracksinthezoneofthe failure. Selvice Failure:CaseStudies291 . ~ ~ - 1'1' I!i !! iii'! :\:'; II, ' lUr 111:11"" !,!li!!;.1\;1.il,\I. .Ilni rt1'11'I:III I:!! illl!: lill!'1 !i!:"!; 111'11111:1'11'1\!.,Ii i'l l' I'It'itl ik.d , IH1' j' 1,",' 1'I J ; I; 'i,I, .' /i( I firi,,1!)I,II I!; ! I , 10 ,";)1,,j{....Having discussedtheeffectsof illloying elementsandtheir importance inthedevelop- of.and and gy ISbest understoocl througl;rsome t-yplca'h:asesbffallures In ferrousand non-ferrous products. DuetoSegregation Onerectangularshapedthinwalledpart belongingtoabatchquantityof10,castout of medium plain carbon steel (specified carbon between 0.36 -0.40%), was found cracked inthemid-surfaceofthepartimmediatelyonquenchinginwaterafterhardeningat 860C.Themost striking featurewasthattheremainingpartsofthebatchaswellas otherpreviousbatchesWerefoundunaffectedunderthesameheattreatmentcondi-tions.Sinceit wasa stray case,itwasinitially suspectedthatthe part inquestion might havebeenquenchedfromarelatively higherhardeningtemperaturecomparedtothe specifiedone. Microstructuralanalysisofthepartinthecrackedzonerevealednoeffectasis normally indicated bythepresenceof grain coarsening or brittle widmanstratten struc-ture.Chemical checkbywet analysismethod onthepart, confirmedcarboncontent as 0.38%,whichiswellwithinthespecificationrange. Thisledtorethinkingonthepart oftheinvestigiltorastowhythepartshouldhavecrackedeventhougheverything seemedtobesatisfactoryasregardschemistryandheattreatedstructure.Chemical analysis forcarbon was repeated but this time sample drillings weretaken fromthe area closetothecrackedregion. Surprisingly, carbon content was foundtobe0.42%,a little morethanthemaximumlimitofthespecifiedrange. It isawellestablishedfactthatanyplaincarbonsteelexceedingthelimitof0.40% C isprone tocracking on quenching in water for high temperature hardening treatment. Although thechemistry of the part with respecttocarbon content ingeneralwasfound wellwithinthespecificationrange,stray crackinghaddevelopedduetolocal segrega-tionofcarbon.Itisknownthatamongallotherelements,carbonismostproneto segregationatthemicro-levelduringsolidificationofcastproducts. Thiscasehistory relatestotheimportanceof proper sampling foranalysis,basedontheunderstanding oftherolethatcarbonplaysintheheattreatmentofsteel. DuetoDeCal"burisation Thisisanother caseofwrong sampling leading torejectionof a largebatchof medium carbon(0.4%)lowalloysteel rolledbars. Thesebarsmeant tobeusedasforgingstock arechemicallycheckedforconformancetospecification.Analysisindicatedamuch lowercarboncontent(i.e.0.2S%)andthiswasthebasisforoutrightrejection.Non availability of another batch and emergency forcritical items tobemade fromthesame consignment,areviewofthechemicalcheckwasdoneanditwasdiscoveredthat sample drillingswere taken superficially confined tothe surface, i.e.the surface of each rolledbar.Micro-examinationofasectionacrossthesurfacezonerevealedaO.Smm Cbemistry and Metallurgical Bebauiour32 5 ) ) ) ) ) ) 1 ) ) ) ) ) ) deepdecarburised layer.The presenceof decarburisationimpliesadepletion of carbon fromtheoriginal carbon content.Hence,thelowpercentage of carbonwasreported, as thesamplestakenforanalysisweremostlyfromtheareaofdecarburisation.Onre-checkingthecarbon content inthearea,aftermachining off thedecarburisedlayer,the carboT)percentagewasfoundtobemeetingthespecificationrequirements(i.e.0.4%). Thewrongsamplingwascarriedoutduetosheerignoranceoftheoccurrenceof decarburisation onthepart of theanalyst.Hence it isimportant thattheanalyst should beawareofthebasicsofheattreatmentandmanufacturingproblemswhichmay iriterferewithhisaccurate/properchemicalanalysis. DuetoCarbonEquiva.lent Analysisofcarbonequivalentintheproductionofgreycastironisveryimportant. Chemicalanalysisofacastironmeltduring pouring indicatedtotalcarbon,combined carbon,silicon,phosphorusandotheralloyingelementswellwithinthespecifieation range.But a test bar, cast separately fromthesame melt, yielded a lower hardness than thestipulated minimum. Microstructure was found satisfactory with respecttopearlite, ferriteandcementite. Asthereasonforlowhardnesscouldnotbeattributedeithertochemistryorto structure, onthebasisof results reported,thefocuswas shifted tocorrectness of carbon equivalentwhichdeterminesthedesiredproperties.However,oncalculationfromthe resultsobtained,carbonequivalentwasfoundtobeonthelowersideoftheactual requirementintendedforminimum hardnessor strength.If thecarbonequivalentwas determinedduringthepouringstage,thealterationofchemistrywithrespectto CE=TotalCISi+Pcouldhavebeenmadetosuitthepropertyrequirements.This emphasisesthattheroleofthechemicalanalyst ismoreonthebasicunderstanding of chemical-metallurgicalrelationshipsofcastiron(intermsofcarbonequivalenttothe microstructural changes) andtheir effect onthe strength property, even though histotal analysistalliedwiththespecificationrequirementsingeneral. DuetoAbsenceof TJa.ceAdditions Quiteoftentherepeatedfailuresofwelded18/8 stainless steelbyintergranular corro-sion(popularlyknownasweld-decay)werereportedeitherinpre-service(storage)or service condition.Onchemical check,the chemistry of the material was foundtobe18% Cr-8%Nistainlesssteelwithoutanystabilizersuchastitaniumorniobiumasminor additionwhichpreventsintergranularcorrosion.Haditbeenknowntothechemical analystthatsuchamaterialissusceptibletowelddecayphenomenonafterwelding, theycould have suggestedthe stable one and preventedtherecurrence of such failures. Oneglaringexampleisthatof18/8madeexhaustpipeswhichwereonlychemically checkedpriortouseandfailedprematurely.Withpriorknowledgeofmetallurgical behaviourduetoabsenceofminoradditions,achemistcouldhaveactedasametal-lurgist particularly when tests involved only chemical analYSis,towarn the users against theundesirableconsequences. 326Metalh17gj , ofFailure Analysis DuetoDeficietlcyof Elemetlts Wrought aluminium 4%copper alloysare used forhigh strength structural applications andinthesealloysof aluminium-coppersystemwhicharestrengthened byage-hard-ening process, copper acts asthe main strengthening element. Several caseswere report-edhavinglowtensilestrength propertyaftersolutionisingandageingof Alalloy4% Cuforgedparts. puring investigationsitwasnotedthatthebatcheswereacceptedon thebasisof spectrovacchemicalanalysisonseparatelycastsamples. Atrialheattreatmentwascarriedoutasperspecificationonthetensometerpiece madefrorrtthepart, suspecting thatthelowtensile strength valuesmight be associated with some lapse in the heat treatment process such asquench delay, inadequate ageing etc.Thevaluesobtainedwere foundlow.Thisnecessitatedtherecheckingof chemical analysisonthepart whichgavecopper content slightlylessthanorthelower limitof the specification range (i.e. 3.8% against 3.9% in therange 3.9-4.5%). Since repeated heat treatment did not improve the values,itwas thecopper content which was short of the . minimum copper limit that caused lowering of strength values,ascopper isconsidered tobethemost strengtheningelement intheage-hardening processofthealloysystem. Atthefirstinstance,theanalysis made onthecastsample seemedtobemisleading,in having reported(3.9% Cu) fulfilling the specification requirement either due to incorrect interpretationorsomeinstrumentalerror.It thusbecameevident thatthepercentage error however small mattered, particularly when it dealt with strengthening of elements. DuetoExcessof Eleme1lts We have seen cases where slight deficiency incertain types of chemical constituents can causeproblemsinachieving optimum properties.Letusnow examinethecaseswhere elements exceeding themaximum limit of the specification range could cause a problem oftotalrejectionintermsofbrittleness I crackormachiningdifficulties.Abatchof aluminium bronze parts werecast and during theknocking-off process fromthe mould, thecastingscrackedinthethinwalledsectionarea.Initiallyit wasthoughtthatthis wasdue to some inadvertant high impact during theknocking-off operation. Chemically thematerialwasreported meetingthespecificationrequirements. However,onmicro-structural analysisthestructure wasfoundbrittleduetothepresenceofnumerous'y' (gamma)constituentswhich are normally not expected inthespecified chemical range. Thisledtorechecking chemical analysis,asinthis alloy,zinc equivalent factor depencjs onthecontentof aluminium,ironandmanganese,and becomesanimportant criterian indeterminingthepresenceor extentofthegammaphase.Asperthebinarydiagram ofCu-Znsystem,withincreaseinZnexceeding40%thegammaphasestartsprecipi-tatingwhenzincequivalencereaches5%.EvenwhenZncontentiswellwithinthe range,any increase intheaddition of aluminium (more particularly)thanthespecified, increasesthezincequivalenceapproachingtherange' ofgammaformation. Onrechecking,applyingthisimportant condition,aluminium percentagewas found tobe higher(asexpectedfromthemicrostructuralanalysis)than theoriginalvalue(i.e. 3.02%against 2%) . Thisexplains why theutmost care has tobetakeninanalysing such elements,whichhavegotanadverseeffectonmaterialproperties,andwhyanalysts shouldbewellawareofthechemical-metallurgical consequencesinadvance,inorder topreventproblemsincludinghighrejections. Chemistry and Metallurgical Behaviour327 DuetoLackof Analysisof TraceElements Oneinterestingcaseisthatofaseverelyzincbasealloydiecasting.Itwas reported that the die castings numbering about 600were accepted based on the chemical checkof themajoralloyingelementsandstoredforsixmonths . Duringusagecracks werenoticed.Themicrostructureconfirmedthatthesesurfacecrackshadprogressed intergranularly (Le.intergranular corrosion cracking)fromcorrosion pits.The nature of crackingandthefactthatthey hadoccurredpriortoserviceraisedsomedoubtsasto thecorrectnessofthechemistryofthematerial,includingtraceaddition,suchasar-senic,antimony,whichareknowntopromoteintergranularcorrosioninzincbase alloys.Onanalysis,thematerialwasfoundtocontaintraceadditionsofarsenic. Thisemphasisesthefactthatalthoughthepresenceoftraceelementsisdueimproper meltingpractices(mayoccurwithouttheknowledgeofthemelter),theper-sonnel concernedwere not aware oftheeffectsof such elements and intheprocess full. analysiswasnotcontemplated,resultinginwastageofalargebatchquantityofcast-ings. 328.Metallurgy of FailureAnalysis I I I . i j 2. 3. 4. 5. 6. 7. 8. 9. 10. Il. 12. 13. 14. 15. 16. 17. 18. 19.

RaghuramA.C..andR.V.Krishnan,'FailureAnalysis':ARealTimeTest;Bangalore ICASTADProceedings,July29-August 3,1990. Louthan M.R.'Fault Tree Techniquesand Failure Analysis';MetallographyII,1978,33- 42. Shamala A.R.andAC.Raghuram,'Fault Tree TechniquesforFailureAnalysis',Trans.of IndianInstituieof Metals,Vol.35,1982,1-16. RaghuramA.C.andAR.Shamala,FailureAnalysis:PresentConceptsandFuturePerspec-tives,(unpublishedpaper),NationalAeronauticalLaboratory,Bangalore,1985. CollingsJ.A.,B.T.HagamandH.M.Bratt,TheFailureExperienceMatrix-AUseful DesignTool,Trans. of ASMEJournalof Engineering forIndustry,98,1976,1074-1079. SrinathL.S.,ConceptsinReliability,2nded.,AffiliatedEastWestPress,Pvt.Ltd,New Delhi,1974. DasAX,K.G.B.Nai r,L.Mallaiah,'SourcesofdefectsandtheireffectsinMetallic Materials,'HTSDigest,Vol.24,July-Dec.1983. DasAK.andPremBaveja,In-ProcessFaillireInvestigationinAeronautics,Bangalore, HindustanAeronauticsLimited,1988. Aluminium,Vol.1,Properties,PhysicalMetallurgyandPhaseDiagrams,MetalsPark, Ohio,AmericanSocietyforMetals,1967. MetalsHandbook,Vol.5,8thed.,'ForgingandCasting,'MetalsPark,Ohio,Ameri can SocietyforMetals,1970. DasA.K.,' BlisteringPhenomenaandtheirCaseStudiesinAluminiumAlloyWrought Products',HTSDigest,Vol26-27April-Sept,1984. MetalsHandbook,Vol.3,8thed,'Machining',MetalsPark,Ohio,AmericanSocietyfor Metals,1967. NairK.G.B.,AK.Dasetai,TroubleShootingforNonAttainlllentof Stre1lgthPropertiesin HeatTreatmentShop, ToolandAlloySteels,May1981. ChatterjeeB.,K.G.B.Nair,AXDasandG:S.B.Chari,'FaihlreIllvestigationsof Aircraft Components(PartI),Bangalore,CentralLaboratory,HAL,1980. MetalsHandbook,Vol.10,8thed.,'FailureAnalysisandPrevention',MetalsPark,Ohio, AmericanSocietyforMetals,1975. MetalsHandbook,Vol.9,8thed.,'FractographyandAtlasofFractographs',MetalsPark, Ohio,AmericanSOCietyforMetals,1974. Prasad r andH.S.Ganesha,' Treatise onNon-DestructiveTestingandEvaluation:Theoryand Practiceof SurfaceInspection,Vbl.3,BangaloreNDTCentre,CentralLaboratory,HAL, 1983. Prasad J.amd H.5.Ganesha,TreatiseonNOll-DestructiveTes ting andEvaluation:Theory and Practiceof IndustrialRadiography,Vol.1,NDTCentre,CentralLaboratory,HAL,1983. Prasad J., T.Rangachary andB.NS.Murthy,TreatiseonNonDestructiveTestingandEval-uation:TheoryandPracticeof UltrasunicTesting,Vol.2,BangaloreNDTCentre,Central Laboratory,HAL,1983. ....... ( ( ( ( ( ( ( " ( ( ( ( ( ( ( (' I: ( ( ( ( ( ( ( ~ 20. 21 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. Radhakrishnan S.,'Techniques of FailureAnalysis:Inspection and Testing,'Proceedingsof WorkshoponFracture, Fatigue and FailureAnalysis, Bangalore, National Aeronautical Labora-tory,1979. KrishnanRV.,'TechniquesofFailureAnalysis:MicroscopicExamination,Proceedingsof WorkshoponFracture,FatigueandFaihlreAnalysis,BangaloreNationalAeronauticalLabo-ratory,1979. DasA.K., KC.B.Nair andCC.K. Nair,'Identification, RatingandAcceptanceLimits of InclusioninAircraftSteel,'ToolandAlloySteels,AnnualIssue,Dec.1976. Asundi M.K, 'Failllre Mechanism, 'Proceedingsof WorkshoponFracture,FatigueandFailure Analysis,Bangalore,NationalAeronauticalLaboratory,1979. EngelLother and Klingele Hemann,'An Atlas of MetalDamage',Vienna,WolfePublish-ingLtd.,1981. Nambodhiri T.KC.,'Hydrogen Embrittlement of MetallicMaterials,' . Trans.of theIndian Instituteof Metals,Vol.37,No.6,Dec.,1984: KolachevB.A.,'HydrogenEmbrittlementofNon-ferrousMetals,IsraelProgrammefor ScientificTranslations.Jerusalem,1968. ZapffeCA.andSimsC.E.,'Trans,.AMIE,145,1941,225. Kazinczy,F.De.J.,IronandSteelIns/.(London),177,1954. Tetelman,A.S.,'FundamentalAspectsof StressCorrosionCracking,NationalAssociationof CorrosionEngineers,Houston,1967,446. PetchN.J.andP.Stables,Nature,169,1952,842. FrohmbergRP.,W.J.BernettandA.RTroiano,'Trans.ASM,47,1955,892. OrianiRA.andP.H.Josephic,ActaMetall,25,1977,979,also27(1979),997. BeachemCD.,MetallTrans.,3,1972,437. LynchS.P.,MetalsForum,2,1979,184. HirthJ.P.,MetallTrans. ,llA,1980,86l. HackJ.I.andC.RLeverent,MetallTrans. , l3A,1982,1729. WestlakeD.C. ,Trans.ASM,62,1969,1000. KoikeS.andT.Suzuki,ActaMetall,29,1981,553. CavannaughMA,T.P.Fehlner,J.A.KargolandN.F.Fiorie,Scr.Metall.,16,1982,709. SandozC,MetallTrans., 3,1972,1169. 330Mewlluigy.oj Failure Analysis ,,,,";it' ~ I I ----" Glossary AbrasionTheprocessofrubbing,grindingorwearmgawaybyfriction. AbrasiveAhardparticleorsubstance(suchasrock,sand,emery,corundum,alumi-niumoxide, siliconcarbide,etc.)usedforgrinding,lapping,polishingorblasting underpressuretowearoutthesurface. AbrasiveblastingAprocessofcleaningorfinishingwhichinvolvesimpingementof abrasivesathighvelocityagainsttheworkpiece. AbrasivewearTheremovalofmaterialfromthesurfacebyabrasiveparticlesunder pressurethroughslidingorrollingaction. AccuracyTheclosenessofapproachofameasurementtothetruevalueofthequan-titymeasured. AdhesionTheforceofattractionbetweenthemoleculesoratomsoftwophases. AdsorptionTheformationofathinmolecularfilmofasubstance(fluidorsolid)on thesurfaceofanother. Age-hardening.Hardening byageingatambientormoderatelyelevatedtemperature, usuallyafterrapidcoolingorcoldworking. AgeingAchangeinthepropertiesof ametalwhichoccursslowlyatroomtempera-tureandrapidlyatslightlyelevatedtemperatures,usuallyafterdrasticthermalor mechanicaltreatment. AlcladAcompositesheetproducedbybondingeithercorrosionresistantaluminium alloyoraluminiumofhighpuritytobasemetalofstructurallystrongeraluminium alloy. It isanodjctothecore materialandprovides protectionagainstcorrosion.The thicknessofcoatingisnormally5%ofthetotalsheetthickness. AllotropyThereversiblephenomenonbywhichcertainmetalsmayexistintwoor moredifferentformswithdifferentphysicalproperties(structure,density,etc.),but retainingthesamechemicalcomposition. AlloyAcombinationoftwoormorechemicalelementsofwhichatleastoneisa metal.Mostpure metalscanbestrengthened byasmalladditionofothermetalsor non-metals.Themoststrikingexampleissteelwhichisessentiallyanalloyofiron andcarbon. In preparing glossary the author acknowledges the assistance taken fromMetals Handbook,Desk Edition, edited by HowardE.Boyer, Timothy L.Gall, American Society forMetals, Ohio, May 1985 andDictionary ofMetallurgy,D.Birchon,RoyalNavalScientific Service,London,1965. I I I I . I t I I

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1'--' AlloyingelementAn elementwhich isintentionallyaddedtoimproveormodifythe propertiesofanotherelement. AlphaironThe bodycenteredcubicformofpureironstablebelow910 0c. AluminisingAprocessforimprovingtheoxidationresistanceofmildandJowalloy.. steelsattemperatureupto950C,byformingcoatingofaluminiumoralummium' alloyonthemetalsurfacebyhotdipping,hotsprayingordiffusion. AluminiumAsoftwhitemetalofdensity2.70g/ mlandmeltingpoint660C.Pure aluminiumischaracterisedbyitslightweight,ductility,corrosionresistanceand goodelectricalandthermalconductivity.Thecorrosionresistanceofaluminium owestotherapidformationof athinadherentlayer ofaluminiumoxide(A120 3)on thesurface. AnnealingAhightemperature heattreatmentprocessconsistingofheatingtoand holdingaboveaustenitizingtemperaturefollowedbyslowcoolinginthefurnaceto achieveprimarilythematerialinsoftconditionformachiningpurpose. AngstromAunitoflinearmeasurementequalto10-10mor0.1m. AnodicTheterm applied tothe component of an dectro-chemical cellwhich has lower electrodepotentiaLUndercorrodingconditionstheattackoccursattheanodearea. AnodiccoatingAfilmon the metal surfaceresultingfroman electrolytictreatment at theanode. AnodicprotectionReductionofthecorrosionrateofametalbymakingitspotential sufficientlyelectropositivesoastorenderitpassive.Thisiscontrolledusuallyby e.mJ. AnodisingAconversion coating processwhichartificiallythickensthelayerof alumi-niumoxideonthesurfaceofaluminiumand itsalloys.Theprocessinvolvespassing ofanelectriccurrent throughthechromicor sulphuricacidbathinwhich thearticle actsasanode,wherenascent oxygenevolvesandresultsinthickeningofoxidefilm. Theelectrical resistance .ofthefilmsets alimittothicknessoftheorder of0.001inch. Itprovidesprotectionagainstcorrosionandspeciallyabrasionresistancewhen anodisedbysulphuricacidprocess. ArcweldingAgroupofweldingprocessesthatfusemetalstogetherbytheheat generated by an electric current between an electrodeandthework piece,intheform ofanarc. Welding can bedone withorwithoutthepressure and with or withoutthe fillerrod. AsperitesAnameappliedtothehighspotsofnominallyflatsurface. AusteniteAsolid solution ofoneor :qloreelements in facecentered cubiciron (F.C.C). Austenitic steelAalloy steelwhose structureisnormally austeniticat roomtempera-ture,suchas,Hadfieldmanganesesteel,stainlesssteel. AustenitisingHeatinganalloysteeltoahightemperaturezonewhereprevious structuretransformstoaustenite. 332Metallurgy of FailureAnalysis i I I I I j I BakingAlow forremovalofgasespresentinthe finishedproducts,particularlyaftercompletionofplatingprocess.It isalsoapplied forcuringorhardeningthesurfacecoatingsuchas,paints. Banded -;,Astructure ofalternating . .parallelbandsofdifferentcomposition' (phases),mostlyalignedinthedirectionof hotworking BasemetalThemetalpresentinmaximumproportioninanalloy. BeachmarksProgressiveshellorconchtypemarksonafatiguefracturesurfacein-dicatingthesuccessivelocationsoftheadvancingcrackfront. BendangleTheanglethroughwhichabendingoperationiscardedout. BendradiusTheinsideradiusofabentsection. BiaxialstressAstateof stressinwhichonlyoneoftheprincipalstressesiszero,the othertwobeingintension. BilletAsolidsemifinishedor square product producedthrough hotworking byforg-ing,extrusionorrollmg.Alsoitisreferredtowroughtstartingstockforforgingor extrusion. Bimetallic corrosionThe corrosionof two metals inelectrical contact andin exposure toan electrolyteinwhichonemetalinitiatesattackontheotherwhichislessnoble metaL BinaryalloyAnalloyhavingonlytwocomponentelements. BlastingCleaningthemetalsurfacebyimpingementwithabrasiveparticlesathigh velocity. BlisterOftenadomeshaped(raised)arearesultingeitherfromthelossofadhesion betweenacoatingandthebasismetalOrexpansionofentrappedgasesunderpres-sureduringheattreatmentofaforgedorextrudedsection. BlowholeAholeinacastingoraweldcausedbygasentrapment duringsolidifica-tion. BoreProducinga hole orcylindrical cavitywith single point or multipointtools,other thanadrilL BrinellhardnesstestingDeterminationofthehardnessofmaterialbyforcingahard steelballof specifieddiameterintoitunder astandardload.Theresult isexpressed astheBrinellHardnessNumber(BHN),whichisobtainedbydividingtheapplied loadinkilogramsbythesurfaceareaoftheresultingimpressioninsquare millimetres. BrinellingOccurrenceofevenlyspaceddentsinaracewayofarollerbearing. When bearing assemblyissubjectedtoheavy forceor impact, itcausestherollingelements toindenttheracewaysurface. Glossary333 '-( ( ( ( ( ( ( r I( lit

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t BrittlecrackpropagationA verysudden propagationofcrackwithouttheabsorption ofenergy, BrittlefractureSeparationof. twometallurgicallybondedstructuresofametallic body,practicallywithoutplasticdeformation.Brittlefractureoccursbyrapidcrack propagation. BrittlenessIt isthe property of material by which crack propagation occurswith little evidenceofductility. BurningApermanentdamage causedtoametal byexposuretoahightemperature. It causeseitherincipientmeltingorintergranularoxidation. CarbideCarbonchemicallycombined with oneormoreother alloyingelements,such as,ironcarbide(Fe3C) CaburisingAbsorptionand diffusionofcarbonintothesolidsurfaceofferrousalloys byheatingtoatemperatureusuallyaboveAC3 (thecriticaltemperaturefor transformationofalpha.ironintogammairon,i.e.austenite)incontactwith carbonaceousmateriaL CasehardeningAgenerictermforseveralprocessesapplicabletosteelthatbrings about achange inchemical compositionofthesurface layer by absorptionofcarbon, nitrogenoramixtureoftwoandsimultaneouslybydiffusionwhichcreatesa concentrationgradient. CaststructureThemetallographicstructureofacastingevidencedbyshapeand orientationofgrains. Catastrophic failureSudden failureofa component or assemblyleading sometimesto mishaps(includingdestructionofpropertyandlife). CementiteA compoundofironandcarbonformedinsteelasironcarbide.It ishard andbrittleinnature. ChevronAcharacteristicpatternofradialmarksappearingasnested'V',generally observedinabrittlefracturesurface.Alsocalledsometimesaheringbonepattern. Thepointsofthechevronscanbetracedouttothefractureorigin. ChromatetreatmentTheproductionofa .conversioncoatingoftriandhexavalent chromiumcompoundsonthemetalsurface.It providesgoodadhesionfororganic finishessuch as,paints and resinsandaffordsa good measure of corrosion resistance. CleavageSplittingofacrystalonacrystallographicplaneoflowindex. CleavagefractureAfractureinwhichmostofthegrainshavefailedbycl eavage .resulting inbright crystallinefacets,commonlyobserved in low-energy brittle failure. CoherentprecipitateAcrystallineprecipitatethatformsfromsolidsolutionwithan orientation that maintains coherency betweenthecrystal latticeoftheprecipitateand thelatticeofthematrix,usuallyaccompaniedbysomestraininbothlattices. ColdshortnessLackofductilityatambienttemperature. 334Metallurgyof Fa illl reAlialysis r-:1 J Cold shutA casting defect in the formof a discontinuity on thesurface ofa cast metal as a result of failureof two streams of liquid metal to unite.Thedefect isoften caused by lowcastingtemperatureand entrappedoxidefilmsduetoturbulenceinpouring. Thisisalsoobservedasalaponthesurfaceofaforgingthatwasclosedwithout fusionduringdeformation. Cold workIt isa plastic deformationof ametal at atemperaturelowenoughtoresult inpermanentstrainhardening. ContactfatiguePittingandsubsequentfatiguecrackingofasurfacesubjectedto alternatingstressessuchasthoseunderrollingcontactorcombinedrollingand slidingaction. CorrosionChemicalor electro-chemical reactionbetweena metalanditsenvironment usuallyresultingindeteriorationinthepropertiesand/orappearanceofthemetaL Corrosion occurs in contact with an electrolyte(rain,seawater, etc.),inwhich driving forceiselectro-chemicalactionbetweenlocalanodesandcathodesorchemical dissolution. CreepAcontinuousorsemi-continuousdeformationwithtimeunder constantstress atconstanttemperature.Thecreepstrainoccurringatdiminishingrateisshownas primarycreepandat constant rateitisa secondary creep,whicharemoreimportant whiledesigning . CrystalAsolidmadeofatoms,ionsormoleculesarrangedinrepetitivegeometric pattern. CrystallinefractureAfractureshowingbrightfacets. CupandconefractureFractureoftenresultingfromtension-testspecimenexhibits acentraldepressedflatfaceregionsurrounded by ashearOlipinoneofthemating fracturesurfaces,whiletheothersurfacelookslikeaminiaturetruncatedcone.It involvesamixedfracturemodewherethecentralportionundergoesplanestrain fractureandthesurroundingregionundergoesplanestressmode. CurrentdensityTheaverageelectriccurrentdistributionoverthesurfaceofan electrode.Theunitisusuallyamps/ft2oramps/decimetre2 DeburringRemovalofburrs,sharpedgesorfinsfromthesemi-finishedorfinished partsbyfiling,grindingorrolling. DecarburisationLossorcarbonfromthesurfacelayerofsteelduetoreactionwith chemicalsubstances.It occursduringhightemperatureheatinginairandresultsin areductionoffatiguestrength. DeepdrawingDrawingacupshapedformoflargedepth/diameter ratiofromsheet materiaL DefectAdeparture fromthespecifiedquality levelwheretheproductwillnot fulfill itsdesiredfunction. ClossarJl335 I 'I :1 j

DegassingRemovalofdissolvedgasesfromliquids,usuallypractisedonmolten metalbyaddingdeoxidisingagentsorbubblingchlorinegas. DegreasingAsurfacetreatmentwhichcleansoilorgreasefromasurface. DelayedfractureFractureoccurringaftera,timeduringwhichmetalhasupdergone astaticstress,whichitselfisoflowmagnitudetocauseinstantfailure. ', DendriteAcrystalthat hasatreelikebranchingpatternusuallyobservedincasting. DeoxidiserAmaterialwhichwhenaddedtothemoltenmetalsremovesdissolved andcombinedoxygen,suchasaluminium,silicon,etc, DepthoffusionInweldingthedistancecoveredby fusionintothebasemetal. DezincificationAformofcorrosionattacktowhich somebrassesaresusceptible, p'articularlywhenexposedtowarmseawater.Theattackhaslittleeffectonthe surfaceandresultsindeepcopperredcolouratthesurface. DiamondpyramidhardnesstesterMeasuringthehardnessofamaterialby determiningitsresistancetoindentation.Thevicker'sdiamondissquarebased pyramidof1360 includedangle,usuallyappliedunder30kgloadforsteel.The hardness number is determined asthe load/area ofjmpression in kg/mm2.Hardness isdescribedasVHN. DieAtoolusuallycontainingacavitythatimpartsshapetoJ;Ilaterialbyforgin&- die castingorasadieintowhichpowder metalsarecompressed. DiecastingCastingmoltenmetalintoametaldieto giveaccuratefinishanddimen-sionundergravityorpressure. Die forgingA forgingwhose shape isdetermined by impression in specially prepared dies. DiffractionTheformationofinterferencepatternina beamofradiation.Itisusedin X-raydiffractionofmetallatticestomeasurelatticespacinganddeterminespace group. DiffusionThespontaneousmovementofatomsormoleculestonewsiteswithma material. DirectionalsolidificationSolidificationofmoltenmetalwhichoccursinsucha mannerthatit provides continuouslythemolten metaltosolidifyingfrontaslongas necessarytofeedthecasting.Whenperfectlycontrolleditresultsineliminationof shrinkagecavities. DiscontinuityAnyinterruptionasaformofcrack,lap,seams,inclusionorporosify inthenormalphysicalstructureofapart. DislocationAlinearimperfection(linedefect)inacrystallinearrayofatoms.This maybepresentasaclosedringboundingaslippedareaofthelatticeoranchored atitsendstootherdislocation,grainboundariesorasurface.Dislocationsmovein their slip plane under theinfluenceofanapplied s,tress,permitting consecutivetrans-lationofatomsoneithersideoftheslipplane. 336Metallurgyof FailureAnalysis I t i I I tJ ,-.. ageingTwodifferentageingtreatment ,areemployedtocontrolthetypeof precipitate'formedfromasupersaturatedmatrixinordertoobtainproperties. DrossTheoxidedeposit(oftencontaminatedbyotherimpurities)onthesurfaceof m:6ltenmetaLDuctile fractureFracturecharacterised by tearingofmetalsand associatedwithgross plasticdeformation. DuctilecrackpropagationSlowcrackpropagationaCG:ompaniedwithperceptible plasticdeformation. DuraluminAgroupofaluminiumalloysbasedessentiallyupon2-4.5%copperand 0.3-1.2%eachofmanganese,silicon,ironandmagnesiumandisagehardenable. DyepenetrantThinfluidusedinnon-destructivetestingfordetectionofsurface defects. ECMAnabbreviationforelectro-chemicalmachining. EddycurrentElectriccurrentsetupinaconductorbyachangingmagneticflux. EddycurrenttestingAnelectromagneticnon-destructivetestingmethodinwhich eddycurrent flowisinducedinthetestpiece.Changesinflowcausedbyvariation intheobjectarereflectedintothecoilfordetectionandbysuitable instrument. EDMAnabbreviationforelectro-dischargemachining. Electro-chemicalmachiningTechniques6fmaterialremovalinwhichelectrolytic dissolutionisusedaloneorincombinationwithsomeothertechniques. Electro-dischargemachiningRemovalofstockfromanelectricallyconductive materialbyrapid,repetitivesparkdischargethroughadielectricfluidflowing betweentheworkpieceandashapedelectrode. ElectrolyteAsubstancecontainingionswhichcanbeusedtomigrateunderthe applicationofanelectJ,'icpotential.Aqueoussolutionsofsaltsarecommon electrolytes. Electro-motiveforceElectricalpotential. Electro-beamweldingAweldingprocessthatusesastreamofelectronsaccelerated fromaheatedfilamentbyahighvoHageandfocussedintoanarrowbeam,to provide a heat source forwelding can be used fO!,velding dissimilar material aswell asreactiveandrefractorymaterials.Unlikeotherweldingprocesses,thispi-ocess providesanarrowheataffectedzone. ElongationTheextensionofthegaugelengthofatensilespecimen,dividedbythe originalgaugelength,measuredafterthespecimenbreaks. EmbrittlementLossofductilityinametal.Theembrittlementmaybecausedbyone Glossary337 \ ( ( ( ( ( ( ( ( r \ , ( ( ( I{ il. I '( 1 f ~ ~ l ~ _ -~ ,I ~ of severalfactors.Examplesincludebrittlefracture,causticembrittlement,hydrogen embrittlement,temperbrittlement,overheating. Endurance limitTheratio of fatiguelimittoultimatetensilestrength whichisusually oftheorderof0.5forsteels.Inotherwords,itisthemaximumstressbelowwhich amaterialcansustainforinfinitenumberofstresscycles. Equiaxed grain structureA structure in which grainsexhibit similar dimensionsinall directions. EquilibriumAstateofbalance,i.eastablecondition. ErosionPhysicalremovalofamaterialfroma surfacebymechanicalmeanssuchas flowingwater,steamorgas.Whencorrosionoccurssimultaneously.Itiscalled "Erosion-Corrosion" . EtchingIt isapreferentialchemicalorelectrolyticattackonthesurfaceofametalto revealstructuraldetailsformetallographicexamination. EutecticmeltingMeltingoflocalisedmicroscopicareaswhosecomposition correspondstothatoftheeutecticinthesystem. ExfoliationAtypeofcorrosionthatattacksthegrainboundaryintergranularyand progressesapproximatelyparalleltotheouter surfaceofthemetalwithlayeral separationintheformthinparallelleaves. ExtrusionConversionofaningotorbilletintolengthsofuniformcross-sectionby forcingmetaltoflowplasticallythroughadieorifice. Face-centeredcubicA cubicmetallatticestructure in whichtheatomscomposingthe cornersofone latticeof cubes lieatthecentres of the facesofanotherlattice ofcubes andviceversa.Thestructureofaustenite,aluminium,copper, nickelbelongstoFCC categoryandischaracterisedbyductility. FacetAflat,usuallyhighlyreflectivesurfaceonacrystalorfracturedsurface. FailureAgeneraltermusedtoimplythatapartisnotcapableofcarryingoutthe intendedfunction. FatigueFailureofamaterialbyrepeatedapplicationofstresses,eachofwhichis individually insufficienttocause failure. Fatigue fracturesare progressive, beginning asminutecracksthatgrowsundertheaction .ofcyclicstresses. Fatigue lifeThenumber of cyclesof stressthat can be sustained priortofailureunder aspecifiedload. FatiguelimitThe maximum stressthatleadstofatiguefractureinaspecifiednumber stresscycles. FatiguestrengthThemaximum stressthatcanbe sustainedforaspecified number of cycleswithoutfailure. FatiguestriationSmallparallellinesinbandsobservedinelectronmicroscope fractographoffatiguefracturesurfaces. 338Metallurgyof Failure Analysis j j 1..I I I j ~ ~ ~ FerriteThebody"centeredcubic(alpha) formofiron,whichisstablebelow760 Cin pureform. FibreThecharacteristicdirectionalityofwroughtproducts. FibrousfractureAfracturewhosesurfaceischaracterisedbyadullgrayorsilky appearance. FillerInwelding,therodusedtodepositmetalinthejoint. FillifonriAppliedtofibreorthreadlikematerialsorphenomenonsuch asstandard wiresorthethreadlikecorrosionsometimesobservedonthesurfaceofpainted metals,causedbysuperficialcorrosionunderthesurfaceofcoating. FilletAradiusimpartedtoinsidemeetingsurfaces. FilletweldAweldjoiningtwosurfacesessentiallyatrightanglestoeachother. FinishgrindingThefinalgrindingoperationwithprimaryobjectiveofachieving surfacefinishanddimensionalaccuracy. FlakesDiscontinuousandshortinternalcracksinsteelduetoreleaseof hydrogen. Alsocalledsnowflakes,fisheyesandshattercracks. FlaringForminganoutwardaccurate-angleflangeonatubularpart. FlashExcessmetalthatisforcedoutinforgingthroughtheopeningbetweenthe upperandlowerdies. FlashlineThelineoflocationofflashformedaroundaforging. FlawAtermoftenusedtodescribeacracklikediscontinuity. FlowlinesTexturerevealedbydeepetchingalongitudinalsectionofamechanically hot workedcomponent,showingthedirectionof metal flow.The linesareGausedby differentialetchingofparallellayershavingslightlydifferentcomposition. FlowstressTheuniaxialtruetensilestressrequiredtocauseplasticflowatany particulartemperature. FluidityTheabilityofliquidmetaltoflowintoandfillamoldcavity. FluxInmetalrefining,amaterialusedtoremoveundesirablesubstances(sand,ash, dirt, etc.)fromthemelt,as'amoltenmixture.It isalsousedasprotectivecoatingfor certainmoltenbath. ForgingPlasticdeformationofametalbysqueezingorhammering,toproducea requiredshape.It maybeopenordieforging. ForgingstockArod/barusedtomakeforging. FormabilityTherelativeeasewithwhicha metal canbe shapedthroughdeformation. FractographyFractographyisthemicroscopicinspectionoffailuresurfacestogain informationabouttheeventsinvolvedinfracture. FracturemechanicsEssentiallyitisatechniquethatinvolvesastudyofthestresses andstrainsatthetipsofsharpcrackordefectsandrelatesthemtotheapplied Glossary339 'l x""-loading.Underelasticconditionthisstressfieldisdescribedintermsofasingle parameterknownasthestressintensityfactorK. FracturetoughnessThecriticalvalueofK,necessaryforcrackgrowthunderastatic loadiscalledKc- thefracturetoughness. . FrettingAtypeofwearoccurringbetweenthesurfai esIII contactwhensub-jectedtocyclicrelativemotion. Fretting isusuallyaccompanied by corrosion,product ofwhicharefineweardebris.Thisisknownasfrettingcorrosion. FrettingfatigueFatiguefractureinitiatingfromthefrettedzoneinthesurface. GalvaniccorrosionIt iselectro-chemicalcorrosioninwhichthecurrentisgenerated bythecorrosionprocesses.Whentwodissimilarmetalsareelectricallycoupledto-getherinthepresenceofanelectrolyte,thecurrentproducedresultsindissolution oftheanode. GammaironTheface-centeredcubicformofpureiron.Stableformfrom910Cto 14000c.Exampleisaustenite. GasholesCastingdefectsintheformofsmall,evenlydistributedcavities,usually circular,withbrightsurfaces.Causedbythereleaseofdissolvedgasesduring solidification. GasporosityFineholesor poreswithinametalthatarecausedbyentrappedgasor byevolutionofdissolvedgases,primarilyhydrogen,duringsolidification. GrainflowFibrelikelinesappearingonpolishedandetchedsurfaceorsectionof forgingsinthedirectionofworking.Propergrainflowsimprovethemechanical propertiesoftheforging. GraingrowthAnincreaseintheaveragesizeofthegrains,usuallywhenexposedto anelevatedtemperature. GrainsizeThesizeattainedbythegrainsofsteelwhenheatedtotheaustenite formationrange. GrindingRemovalofmaterialbyrubbingasurfacewithabrasivebeltorgrinding wheel. GrindingcracksShallowcracksformedonthegroundhardsurfaceasresultof generationofexcessiveheat. GrindingstressResidualstresseseitherintheformoftensileorcompressive,inthe surfacelayeroftheworkedpiece. HardenabilityTherelativeabilityofasteeltoformmartensitewhenquenchedfrom theausteniticcondition.Thepropertyisrelatedtothesizeofsectionconsideredand isnotconcernedwiththeultimatehardnessvalueobtainable.Itisameasurethat includesthedepthofhardening. HairlinecracksCracksare sometimesdevelopedwithinsteelswhichhaveundergone 340Meta1l1lrgy0/ Failure AnalV:\'is I I j 1 I I I j I i j I -""-:;.;;::. k coolingfairlyrapidlythrough,;thecritical tep:tperatures.Also,presenceofhydrogen withinthemeta:ldin' leadto 'such "("racks: '. HeataffectedzoneThezonewithintheparentmetalofaweldedjointinwhich heatinghascausedstructuralchangeswithoutmelting.. Heat checkSA patterp of parallEi! by alternate rapid heating and cooling.Thisphenomenonisgenerallyobservedinthesurfaceofthermalfatigue relatedfailure.Alsoinextrudedandforgedsurfaces. Heat treatmentHeating and cooling a metal or alloy in such a manner so astodevelop thedesiredmechanicalproperties. Herringbonepattern'SameasChevronpattern. HeterogeneityVariablecompositionorstructurewithinamaterial. HomogenizingHoldingathightemperaturetoeliminateorminimisechemical segregationbydiffusionandbreakdownascaststructureforbetterworkability. HotshortnessAlsotermedasredshortnesspropertyofcertainmaterialswhich separatealongthegrainboundarieswhensubjectedtodeformationinthenear meltingzone.Thepresenceofthelowmeltingconstituent,evenmminutequantity cancausehotshortnessinsteel. Hydrogen damageAgeneraltermusedfortheembrittlement,cracking, blistering and hydrideformationthatcanoccurinthepresenceofhydrogen. HydrogenembrittlementLossofductilityoftoughnessinametalasaresultofthe presenceofhydrogenwithinthematerial. ImpactenergyTheamountofenergyrequiredtofractureamaterial,usually measuredbyizodorcharpytest. ImpacttestAtesttodeterminetoughnessbehaviourofmaterialsunderhighratesof loadingbyasingleblowasincharpyandizodtests. ImpuritiesElementsorcompoundswhichdeterioratesthematerialproperty. InclusionTheymaybeeithernon-metallicorforeignparticles.Non-metallic inclusionsincludeoxides,sulphides,silicatesandalumina.Theyareharmfulif presentmorethanthespecifiedrequirement. IngotA solidmetal casting prepared forsubsequent hotworking, normallyrectangular insectionandtaperingslightlyfromtoptobottom, IngotismThepresenceofexcessivelylargedendritesinacasting,duetotoohigha castingtemperature. IntercrystaIIineRelatingtoeffectsoccurringbetweengrainsorcrystals,i.e.inthe grainboundaries.Thisisalsocalledasintergranular. Interference fitAfitinwhichthesizeofthemalepart isslightly greaterthanthatof thefemale,requiringforcetofitthemtogether. Glossa':),341 " " / I .jk ( ( ( ( ( 1 ( ( ( ( ( C ( ( ( -( ( ( C ( ( ( IntergranularcorrosionCorrosionthatoccurspreferentiallyatthegrainboundaries withoutaffectingtheadjacentgrainsmuch. IntermediatephaseInanalloysystemadistingusihablehomogeneousphasewhose compositiondoesnotextendtoincludeanyofthepurecomponentsofthesys tem. IntermetalliccompoundAcompoundoftwoormoremetalswhichhasdistinctive crystallographic structureanddefinitecomposition.Normallyhardandbrittle.They formtheprincipal hardeninginfluencesinthealloysystems,i.e.CuA12andMg2Si inaluminiumalloys. IntracrystallineWithinoracrossthecrystalsorgrainsofametal. JointefficiencyTheratioofthestrengthofajoint(rivetedorwelded)ascompared withthestrengthoftheparentmetal,usuallymeasuredintension. JointpenetrationTheminimumdepthtowhichaflangeweldextendsfromitsface intojoint,includingrootpenetration. LapA surface imperfection, similartoseam inrolledproducts, caused bythe presence offins or sharp corner,which on being rolledor forgedremainunweldedtothe body ofthepart.,. LatticeAgeometricalpatterninspace.Ametallatticeistheregulararrangementof theatoms. latticedefectsAnyaberrationsinanotherwiseperfectgeometricalarrayofatomsin aregularthree-dimensionalnetwork. LatticedistortionDistortionoftheregulargeometricalarrayoflatticefromanycause suchassubstitutionalimpurities,intersitials,precipitates,etc.sincelocaldist()rtion impedesdislocationmovements,latticedistortionisalwaysaccompaniedbyharden-ingandusuallyincreasesthelossofductility. Low-alloysteelTypicallycontainingmaximumalloycontentupto5%. LubricantsMaterialsusedtoreducefrictionalforcesandpreventseizurebetween metalfacessubjecttocontactpressuresandrelativemotion. LinearelasticfracturemechanicsAmethodoffractureanalysisthatcandetermine thestressrequiringtoinducefractureinstabilityinastructurehavingaflawof knownsize. MachinabilityTherelativeeasewith' whichametalmaybemachined. MachiningRemovingmaterialfromametalpart usingcuttingtool s. Macro-etchingEtchinga polishedmefal 'surfacetoobservethegrossstructuraldetails suchasgrainfibre,porosityorcrackwithunaidedeyeorunderverylowmagni-fication. MacrographAgraphicreproductionofthesurfaceofapreparedspecimenatmagni-ficationlowerthantendiameters. 342Metallllrgyof Fail1lreAnalysis i i I l i I MacrostructureThe structure of metalsasrevealedbymacroscopicexamination of the etchedsurface. MagneticparticleinspectionItisoneofnon-destructivemethodsofsurfaceinspec-tior.fordetectionofcracksandotherimperfectionsinferromagneticmaterials. Finelydividedmagneticparticleappliedtothemagnetizedpartareattractedtoand outlinethepatternofanymagneticleakagefieldscreatedbydiscontinuities. MagnificationInmicroscopy,theratioofthelengthoftheimagetothelengthofthe object(oftenreferredtoas'so many' diameters).For most practical purposesit isthe product ofthemagnificationoftheeyepieceandobjectiveusedwiththe ,drawtube totheappropriatetubelength. MartensiteThenamewasoriginallycoinedforthehardestphaseformedinsteels duringthebreakdownofausteniteorrapidcoolingorquenchinginwater.Itis characterisedbyanacicular(needlelike)structure. MasteralloyAmixtureoralloyofelements whichareaddedtoachargeofmolten metalinordertoprovideadesiredcompositionorafinegrainsize,etc. MetallographyThescienceofstudyingtheconstitutionandstructureofmetalsand alloysunderopticalandelectronmicroscopes. MetallurgyThescienceandtechnologyofmetalsandalloys. MicrographAgraphicreproductionofthesurfaceofapreparedspecimenafter etchingatamagnificationhigherthantendiameters. MicrohardnessDeterminationofhardnessofamaterialunderverylightloaden-ablingthesmall indentationtobemeasuredwitha microscope.Itsadvantageisthat itcandeterminethehardnessofdifferentphaseswithinastructure. MicroprobeAn instrument forselectiveanalysisof amicroscopicfeatureinwhichan electronbeambombardsthepointofinterest. MicrostructureThestructureofa metal asrevealed by microscopicexaminationofthe etchedsurfaceofapolishedspecimen. MoleculeThesmallest particleofa materialthatretainsthephysicalpropertiesofthe originalmaterial. NaturalageingSpontaneousageingofasupersaturatedsolidsolutionatroom temperature(seeNeckingReductionofthecross-sectionalareaofametalinalocalisedareaby strectching. NeumannbandMechanicaltwininferrite. NitridingIntroducingnitrogenintothesurfacelayerofsuitableferrousalloysat suitabletemperature(usually500C)in contact withnitrogenousmaterial(ammonia moltencyanide).It developsasurfacehardnessof750-1100VPNdependingupon compositionofsteel. Glossal),343 " NormalisingHeatingaferrousalloytoasuitabletemperatureabovethe transformationrangeandthencoolinginair.Theprocessrefinesthestructure. NotchbrittlenessSusceptibilityofamaterialtobrittlefractureatpointsofstress concentrations. OrangepeelAsurfacerougheningintheformofapebble-grainedpatternnormally occurringinametalofabnormalcoarsegrainsizewhenstressedbeyonditselastic limit. OverageingAgeingforlongertimesand/orathighertemperaturesthanrequiredto achievetheoptimumpropertiesinanage-hardeningsystem. OxidationCombination ofthe material with oxygen orloss of electrons by a substance duringachemicalreaction.Heatingametalinair,oxidationoccursintheformof oxidefilm. PassivationThechangingofachemicallyactivesurfaceofametaltoamuchless reactivestate. PearliteThelamellarcarbidecontainingconstituentpresentinannealedand normalisedsteels.' PeeningIntroducing plasticdeformationintoasurfacebyhammeringor bombarding withhardroundshot.It hardensthesurfaceandintroducescompressiveresidual stressimprovingfatiguelife. PenetrationThedepth bywhichthefusedzoneofaweldpenetratesintotheparent materialorpreviouswelddeposits. PermanentsetPlasticdeformationthatremainsonreleaseofthestressthatproduces deformation. PhaseAchemicallyhomogeneousandthermodynamicallystableconstituentofan alloysystem. PicklingImmersionofametalpartinasolutionwhichwillpreferentiallyremove oxideandmillscale.Typicalsolutionforplaincarbonandlowalloysteelsare15% hydrochloricacidatIS-50C,10%sulphuricacidat 70Cand 50%phosphoricacid atroomtemperature. Pitting.Formingsmallsharpcavitiesinametalsurfacebycorrosion. Plane-strainThestresscondition inlinearelastic.fracturemechanicsinwhichthereis azerostraininadirectionnormaltoboththeaxisofappliedtensilestressandthe directionofcrackgrowth(Le.paralleltothecrackfront,)mostnearlyachievedin loadingthickplatesalongadirectionparalleltotheplatesurface. Plane-stressThestressconditioninlinearelasticfracturemechanicsinwhichthe stressinthethicknessdirectioniszeromostnearlyachievedinloadingverythin sheetalongadirectionparalleltothesurfaceofthe sheet.Underplane-stresscondi-344MetallurgyFailll1'eAnalysis ":1' i .. "-," "-':;,,'''',,:.0 P, tion,theplane of fractureins,tab.ility is45totheaxisoftheprincipaltensile stress.,,;,,': ""', . PreciousmetalOneoftherelativelyscarceandvaluablemetals,gold,silverand platinumgroupmetals. Precipitationheat treatmentaconstit\lent precipitat'es ' from asupper @.turatedsolidsolution. ProgressivecrackAslowmovingcrack. QualityQualitycanbeexpressedinthreefashions.Onebydegreeofexcellenceby comparisonagainstastandard,theotherbyanquantitativeevaluationof characteristi1:""",tPapting"" 84f"" Peeling29 Peripheralcoarsegrainenvelope29 Phosphorus321 Pinholes19 Pitting54,82 Plainstraincondition9 PlastiCdeformation123 Porosity31,50 Probe106 Processingdeficiencies51 Propagativedefect15 Propellerblade263 Pushthroughdefect24,50 Q Quenchingcracks42,51 R Radialburst144,145 Radiography104 Rareearthmetals324 Reliabilityengineering4,10 Replication7 Residualelements321 Residualstress36,52 Resistancewelding34 Replicatechnique116 Reversebendingfatigue77 Rollingdefects20 Rotarybendingfatigue77 Rotarymetalsmearing210 S Scanningelectronmicroscope7,62,118 Seam21 Secondarycracks86,291 Secondaryprocessing15 Segregation50,143 Index353 ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (r r , C "-( (' ( ( ( ( , ( ( ( (