Genesis of Spalling in Tandem Mill Work-Rolls

Genesis of Spalling in Tandem Mill Work-Rolls (continued)

30 Journal of Failure Analysis and PreventionVolume 5(6) December 2005

Genesis of Spalling in Tandem Mill Work-Rolls:Some Observations in Microstructural Degeneration

M.S. Prasad, S.K. Dhua, C.D. Singh, and Amitava Ray

(Submitted August 12, 2005; in revised form October 16, 2005)

Macroscopic investigations of spalled 3% Cr-variety forged steel work-rolls used in the tandem millof an integrated steel plant showed steel strip welding on roll surfaces. Microstructural observationsof roll samples at regions away from strip-welded zones showed the desired uniform dispersion of fineglobular carbides in tempered martensite. Quantitative image analysis of all investigated rolls alsoshowed desirable carbide characteristics, with >4 vol.% carbides and >200,000 individual carbides/mm2. The carbide sizes ranged from 0.69 to 0.83 µm. In contrast, optical and scanning electronmicroscopy (SEM) observations of the strip-welded regions showed microstructural degenerationthought to have occurred from surface and/or subsurface damage caused by localized thermal shockand intense pressure. This possibly resulted in the formation of a rehardened and heavily retemperedzone at the strip-welded region. Cracks originated in the heavily retempered zone due to residualtensile stresses and propagated under the applied rolling stresses to produce spalling.

Keywords:

JFAPBC (2005) 6:30-38 © ASM InternationalDOI: 10.1361/154770205X76349 1547-7029 / $19.00

IntroductionForged hardened and tempered steel work-rolls

are generally used in the finishing stands ofmultistand cold rolling mills for the production offlat products. For effective performance in coldrolling operations, work-rolls should have highsurface hardness and an adequate depth of harden-ing to assure the requisite wear resistance.[1] Inaddition, work-rolls must have sufficient resistanceagainst rolling-contact fatigue and adequate tough-ness to prevent premature failures due to spalling.[2]

To achieve the desired properties, the manufactureof superior-performance rolls has primarilyconcentrated on alloy design and heat treatment.The alloy design approach is based on increasingthe chromium content of steel to achieve higherhardness, greater hardenability, and increased wearresistance and rolling-contact fatigue strength.[3]

Concurrently, heat treatment practices involvinginduction hardening and tempering have beendeveloped to achieve high surface hardness, a desir-able microstructure, and a favorable stress conditionon the roll surface. Regardless of heat treatment oralloy content, the microstructure of a hardened and

microstructural degeneration, spalling, tandem mill, work-rolls

M.S. Prasad, S.K. Dhua, C.D. Singh, and Amitava Ray, R&D Center for Iron & Steel, Steel Authority of India Limited,Doranda, Ranchi-834002, India. Contact e-mail: [email protected] and [email protected].

tempered steel work-roll usually consists of adispersion of alloy carbides in a matrix of temperedmartensite and very little retained austenite.[4] Ahardened steel roll, even after tempering, containshigh internal residual stress, which, when coupledwith the high surface hardness, often renders theroll thermally unstable, crack-sensitive, and vul-nerable to spalling.[5]

In multistand cold rolling mills, such as thetandem mill, the performance of work-rolls isdetermined by the metallurgical quality and by milloperational conditions. This is particularly impor-tant in present-day cold rolling mills where work-rolls are subjected to severe conditions, includinghigh mill speeds, high loads, and stringent require-ments for mill productivity and product quality.[6]

The severe service conditions imposed on work-rolls accentuate the propensity for prematurefailures, notably by spalling. Spalling is a type offatigue failure that involves fracturing away of largeor small metal pieces from localized areas of theroll working surface.[7] Spalled rolls are generallyconditioned by grinding for reuse, although con-tinued spalling may often result in the early

31Journal of Failure Analysis and Prevention Volume 5(6) December 2005

scrapping of rolls.[8] In the past, the causes of rollspalling have been attributed to mechanical damage,thermal damage, and hydrogen embrittlement;there is, however, no general consensus or agreementon the failure mechanism or on whether the failuresinitiate at or below the roll working surface.[1,9]

Studies have revealed that the main consumptionfactor for work-rolls in cold rolling mills is notabrasive wear but rather cracks and spalls on theroll surface. Incidentally, 60% of roll consumptionhas been attributed to accidental damage causedby strip welding or skidding.[10] Moreover, recentinvestigations of cold mill work-rolls that exhibitedvarying performances under similar mill operatingenvironments have elucidated the influence ofmicrostructure on spalling propensity and rolllife.[11,12] Spalling is a complex phenomenon thatmay be accentuated by inappropriate mill operatingpractices and/or poor roll material quality.

In order to understand the genesis of work-rollspalling that occurred during a specific rollingcampaign, extensive metallurgical investigationswere carried out on prematurely failed roll samples.The samples investigated were from 3% Cr-varietyforged steel work-rolls (nominal diameter: 585mm; barrel length: 1420 mm) used in the finishingstands of a five-stand tandem mill in an integratedsteel plant under the Steel Authority of IndiaLimited. This paper provides a microstructuralinsight into the macro- and microstructural mani-festations of spalled roll samples and elucidates thepossible causes of failure.

ExperimentalSpalled pieces were collected from the barrel

portion of six tandem mill work-rolls for micro-structural and fractographic examination. The par-ticulars of the spalled roll samples (T1 to T6) withrespect to their starting diameter, scrap diameter,and roll life (i.e., tonnage rolled in metric tonne, ort) were collected from the roll shop and are shownin Table 1.

The spalled samples were ultrasonically cleanedwith acetone to remove rolling debris and lubricantprior to examination. Subsequently, specimens werecut from the working surface as well as from theradial cross section for microstructural examinationusing light and scanning electron microscopy. For

microstructural observation in the light microscope,the spalled roll specimens were prepared byconventional metallographic procedures and etchedin 5% nital reagent (5 mL concentrated HNO3 in95 mL ethanol). Microscopic examinations werecarried out under bright-field illumination at 500and 1000× magnifications to observe the character-istics of the phases present. Additionally, micro-hardness measurements (Vickers pyramid number,or VPN) were made on the cross section of metallo-graphically prepared spalled roll specimens. Themicrohardness measurements were made usingautomatic test equipment equipped with a pneu-matically actuated diamond indenter; a load of 50gram-force was applied for the measurements.

Scanning electron microscopic observations onpolished and nital-etched spalled specimens atdifferent magnifications were used to determinemicrostructural features at the surface and sub-surface zones as well as to observe crack contours.The fracture surfaces of the spalled roll specimenswere also observed in the SEM to study topographicfeatures.

Results and DiscussionsRoll Life

The service life (Table 1) of the six spalledtandem mill work-rolls investigated ranged from873 to 34,136 t compared to an expected life of30,000 t. In general, the bulk of the rolls (T2 toT6) exhibited poor performance in the mill, asmanifest by the low life values (873 to 9697 t).The highest roll life (34,136 t) was achieved byroll T1 and was attributed to its extensive use priorto the specific campaign period associated with theabnormal incidence of spalling. The scrap diameters

Table 1 Roll Life and Spalling Data of TandemMill Work-Roll Samples

Roll Starting Diameter, Scrap Diameter, Roll Life,Sample mm mm tT1 585.40 564.75 34,136

T2 586.00 576.60 9,697T3 584.60 578.70 6,004T4 584.50 580.80 7,805T5 586.00 583.00 2,839T6 585.60 584.60 873



of rolls T2 to T6 (Table 1) ranged from 576.60 to584.60 mm; these values were very close to thecommissioning diameter of 585 mm and signifiedlow use in the mill prior to spalling-induceddegradation. The stipulated scrap diameter (i.e., thediameter corresponding to the desired hardnessdepth) for such work-rolls in normal use is 545mm; the scrap diameters of failed rolls (Table 1)are therefore indicative of inadequate use and poorservice life owing to spalling-induced failures.

Roll ChemistryThe chemical compositions (Table 2) of the six

spalled roll samples (T1 to T6) showed variationof carbon from 0.82 to 0.92 wt.%, chromium from2.94 to 3.21 wt.%, molybdenum from 0.19 to0.24 wt.%, vanadium from 0.12 to 0.16 wt.%, andnickel from 0.11 to 0.22 wt.%. These rangescorrespond to the broad compositional range for3% Cr-base forged steel work-rolls. Increasedchromium content in steel work-rolls is known toconfer higher wear resistance, ensure greaterhardenability, and facilitate the formation of M7C3-type carbides. An increase in chromium contentfrom 1.75 to 3.25 wt.% changes the lower-hardness (840 to 1100 VPN) M3C carbides tohigher-hardness (1200 to 1600 VPN) M7C3-typecarbides, which are crucial to the wear resistance ofsteels.[6] The presence of nickel in the work-rollcomposition confers hardenability; its content,however, is restricted in steels because nickelpromotes the formation of retained austenite, whichreduces hardness and is undesirable from thestandpoint of roll performance. The presence ofmolybdenum in the steel composition (0.19 to 0.24wt.%) is primarily aimed at improving thehardenability, temper resistance, and wear resistanceof the tandem mill work-rolls.

Fig. 1 Spalled roll sample T4 showing (a) working surface and(b) fracture surface

Macrostructural FeaturesThe visual appearance of the working and fracture

surfaces of all six investigated spalled roll sampleswas similar; the working surfaces of these rollsamples exhibited metal sticking, indicative of stripwelding during rolling. Figures 1(a) and (b) showthe working and fracture surfaces of a typical spalledsample and illustrate the presence of strip welding(Fig. 1a) on the working surface of the roll.Furthermore, the roll working surface showedmechanical damage marks that were probably

Table 2 Chemical Composition of Spalled Tandem Mill Work-Roll Samples

Composition, wt.%Roll Sample C Mn Si P S Cr Mo Ni Al V

T1 0.82 0.34 0.44 0.020 0.015 2.94 0.21 0.21 0.016 0.13T2 0.88 0.29 0.37 0.015 0.011 3.21 0.20 0.18 0.011 0.15T3 0.91 0.28 0.43 0.013 0.010 3.10 0.19 0.22 0.01 0.14T4 0.92 0.26 0.32 0.015 0.019 2.99 0.23 0.14 0.006 0.16T5 0.86 0.27 0.36 0.012 0.010 3.12 0.24 0.19 0.013 0.13T6 0.87 0.44 0.54 0.013 0.0111 3.05 0.22 0.11 0.006 0.12


count (201,342 to 268,110 number/mm2) andsize (0.69 to 0.83 µm) parameters were found tobe in close range and were typical of good-qualityforged steel rolls.[11,12] Because the microstructureand quantitative attributes of microstructuralconstituents in all the investigated roll samples (atregions away from strip-welded zones) were similar

generated as a result of skidding. It is apparent fromFig. 1(b) that the fracture surface shows fatiguelikecharacteristics followed by final overload fracture.

The macroscopic evidence of steel strip weldingto the working surface of the roll (Fig. 1a) and thefracture pattern on the spalled surface (Fig. 1b)are possibly indicative of a tandem mill wreck. Theinteraction of a propagating subsurface fatigue crackwith the wreck generated a shock wave. Themicrographs in Fig. 2 to 4 demonstrate the presenceof subsurface cracks, and such crack formationindicates the presence of a microstructural regionthat is susceptible to cracking. In many of thespalled roll samples, it was difficult to determinethe crack origin; however, the one spalled roll piece(Fig. 1a and b) demonstrates a case in which theorigin of the crack was near the roll surface. Afterinitiation, the crack propagated into the roll at anoblique angle and then continued to run almostparallel to the roll surface. This crack pattern istypical of rolling-contact fatigue.

Microstructural FeaturesFigure 5 shows the nital-etched

microstructure of a typical spalledtandem mill work-roll (sample T4)away from the strip-welded region.The microstructure in this regionshows a uniform dispersion of fineglobular carbides in a matrix oftempered martensite. This type ofmicrostructure is desirable for good-quality cold mill work-rolls.[13] Infact, the microstructures of theworking surface of all the investi-gated spalled rolls were similar tothat observed in sample T4. Thequantitative image analysis datapertaining to carbide content (vol-ume fraction), carbide count (num-ber/mm2), and carbide size (µm) ofthe investigated roll samples andpertaining to regions away from thestrip-welded spot are furnished inTable 3. It is evident from this tablethat carbide content in all thespalled roll samples was greater than4 vol.% and ranged from 4.32 to5.42%. Furthermore, the carbide

Fig. 2 Optical micrograph of spalled roll sample T4 at strip-welded region showing different microstructuralfeatures. 500×

Fig. 3 Scanning electron micrographs of spalled roll sample T4 showing (a) structureat some other location, (b) rehardened zone, (c) less rehardened zone, (d)heavily tempered zone, and (e) low-temperature tempered zone



to each other and normal, the possibility that theroll material was defective was ruled out in this case.

The optical microstructure (Fig. 2) as well as theSEM micrographs (Fig. 6) of the same spalled roll(sample T4), but at the strip-welded region,revealed microstructural heterogeneity and thepresence of different structural features. The

Fig. 4 SEM images showing crack initiation, propagation, and branching. (a) Region with different structural features. 500×. (b)Montage of crack initiation, propagation, and branching. 2000×

Fig. 5 Optical micrograph of nital-etched spalled roll sampleT4 away from strip-welded region showing uniformdispersion of fine globular carbides in the matrix oftempered martensite. 1000×

Table 3 Quantitative Image Analysis Data ofSpalled Tandem Mill Work-RollSamples

Roll Carbide content, Carbide count, Carbide size,Sample vol.% number/mm2 µm

T1 4.96 202,468 0.83T2 4.95 227,471 0.69T3 5.42 268,110 0.71T4 4.32 201,342 0.77T5 5.35 244,402 0.76T6 5.42 223,058 0.82

martensitic needles in the strip-welded regiondemonstrate that this area experienced rehardeningand produced a brittle zone at the near-surfaceregion (Fig. 6c). In the optical microscopic image,this zone etches white (Fig. 2) and overlies a lessrehardened zone having microstructural features(Fig. 6d) similar to that of the rehardened zone,except for the presence of primary carbides. TheSEM photographs (Fig. 6c and d) show that both


primary and spheroidized carbides at the strip-welded region are few in number. Directly belowthe less rehardened zone is a region of heavilytempered martensite with bimodal distribution ofcarbides (Fig. 6e). This bimodal distribution depictsthe presence of very fine carbides (M3C) at needleboundaries, as well as spheroidized carbides in thematrix. This region appears black in the opticalmicroscopic image (Fig. 2). Furthermore, there isthe presence of a region having microstructuralfeatures comprising low-temperature (<200 °C)tempered martensite along with spheroidized car-bides. This region represents the normal roll matrixstructure, as shown in Fig. 5 and 6(f ) at 1000 and4500× magnifications, respectively. In addition tothe previously mentioned regions, there is a weldedstrip zone showing the deformed ferrite-pearlitemicrostructure (Fig. 6b) characteristic of normalcold-rolled steel. The optical micrograph (Fig. 2)also reveals a crack as well as evidence of brittlefracture emanating from the heavilytempered region at an angle of 45°toward the working surface (outerlayers) and the inside layers of theroll, respectively.

The SEM images of these micro-structures in the radial section of thesame spalled roll sample at anotherlocation are shown in Fig. 3(a) at750× magnification. The micro-structural features of the re-hardened, less rehardened, heavilytempered, and low-temperaturetempered regions are shown in Fig.3(b-e), respectively, at 3000×magnification. This region alsocontains a crack along the interfacebetween the rehardened and lessrehardened zones. Further, Fig. 3(a)and (b) suggest that the crackemanated from the heavily re-tempered region and moved bothoutward and inward of the roll crosssection. The crack appears to origi-nate at an angle of 45° and changespath during propagation. Thepresence of the secondary cracksbranching from the main crack in

the rehardened zone as well as along the interfacebetween the rehardened and less rehardened zoneswas also apparent.

Optical (Fig. 2) and scanning (Fig. 3, 4, and 6)microscopic examinations of the spalled roll crosssection further indicate damage to the workingsurface of the roll. The damage to the roll surfaces,in a sense, creates subsurface regions having alteredmicrostructures and properties. These subsurfaceregions include a rehardened zone near the surface,a less rehardened zone next to the surface layer, aheavily retempered zone directly below the lessrehardened zone, and finally, a low-temperaturetempered zone representing the normal roll struc-ture. In fact, the rehardened and less rehardenedzones appear to have formed as a result of local-ized reaustenitizing above Accm temperature andquenching to form an untempered martensite.Investigations have proven that the temperature ofthe roll surface within a depth of 0.2 mm may

Fig. 6 Scanning electron micrographs of polished and etched section of spalled rollsample T4 showing (a) surface structure with welded metal strip, (b) weldedstrip of deformed ferrite-pearlite zone, (c) rehardened zone with freshly formedmartensitic structure, (d) less rehardened zone, (e) heavily retemperedmartensitic zone, and (f ) low-temperature tempered zone



increase to 900 °C. This inevitably results in struc-tural transformations in the thermally influencedzone and produces internal radial and tangentialstresses in the roll. These stresses are superimposedon the thermal and shearing stresses in the roll gap,and the total stresses may attain values resulting incracking and spalling.[10] Figures 1(a), 2, and 6(a)illustrate a situation where steel strip had weldedto the roll surface. In consequence, the roll materialin contact with the steel strip was reaustenitizedand quenched and therefore became harder thanthe matrix. However, the temperature below theroll surface and subsurface was not sufficient toreaustenitize the material but was higher than theoriginal roll-tempering temperature (rolls areusually tempered in the range of 100 to 260 °C).This region became heavily tempered because thetemperature range was approximately 500 to 600°C. This tempering softened (633 VPN) thematerial. The degradation of the original roll micro-structure in the surface and subsurface regions isalso corroborated by the microhardness distribution(Fig. 7) and is consistent with the occurrence ofsurface damage.

This surface damage appears to have originatedfrom localized heating, which seems to be the resultof pinching or skidding of the steel strip betweenthe work-rolls or, in more acute cases of a mill wreck,where the strip breaks and wraps around the roll.This pinching, skidding, or mill wreck created alocalized thermal shock and intense pressure, whichcaused the reaustenitizing and quenching of theroll material as well as the welding of the steel stripto the roll surface.[1] The hardness changes (Fig. 7)and the associated residual tensile stresses accom-panying localized thermal shock are sufficient toinduce cracks in the heavily retempered subsurfaceregion of the roll, and these cracks can then prop-agate both outward and inward (Fig. 4) to inducespalling.

Fractographic FeaturesThe SEM fractographs of regions pertaining to

coarse beach, fine beach, and radial marks are shownin Fig. 8(a-c), respectively, at 2000× magnification.The SEM fractograph in Fig. 8(a) is consistentwith a brittle fracture. The fracture surface in Fig.8(b) has features similar to that in Fig. 8(a) but ona smaller scale and is consistent with rolling-contact

Fig. 7 Distribution of microhardness across roll depth


fatigue or simple surface damage during the frac-ture process. In contrast, evidence from intergran-ular cracking and overload fracture is shown in Fig.8(c).

MicrohardnessFigure 7 shows the variation in cross-sectional

hardness of a typical spalled roll specimen. It isseen that a rehardened zone (970 VPN) exists veryclose to the roll surface, along with a comparatively

less rehardened zone (900 VPN) directly under it.Furthermore, the sharp decrease in hardness (633VPN) marks the presence of a heavily temperedzone just below the less rehardened zone. Finally,the hardness increases to a saturation level (837VPN), which appears to be the hardness of thelow-temperature tempered region. Interestingly,the hardness of the welded cold-rolled steel strip isapproximately 425 VPN.

Conclusions• Macroscopic observations of all investigated

work-rolls showed steel strip welding and evi-dence of mechanical damage on the working sur-faces, suggesting mill abuse and/or inappropriateoperational conditions.

• Microstructural observations of all roll samplesat regions away from strip-welded zones showeduniform dispersion of fine globular carbides intempered martensite and signified a desirablemicrostructure for such quality rolls. This obser-vation is consistent with the initial use of properlyheat treated roll material.

• Quantitative image analysis of all investigatedrolls at regions away from strip-welded zonesshowed desirable carbide characteristics, withcontents >4 vol.%, counts >200,000 number/mm2, and sizes ranging from 0.69 to 0.83 µm.This analysis corroborated good metallurgicalquality of the rolls.

• Optical and SEM observations of strip-weldedregions, however, showed microstructural degen-eration that possibly occurred from surface and/or subsurface damage caused by localized thermalshock as well as localized intense pressure thatwould be associated with pinching, skidding, ora mill wreck. This abusive use of the mill resultedin the formation of a rehardened, a less re-hardened, and a heavily retempered zone at thestrip-welded region. The cracks originated in theheavily retempered zone due to a combinationof residual and applied stresses and propagatedunder applied rolling stresses to cause prematurefailure by spalling.

AcknowledgmentsThe authors are grateful to the management of

Fig. 8 Scanning electron micrographs of spalled roll sample T4showing (a) brittle fracture surface, (b) brittle zone withvery small cleavage facets, and (c) overload fractureregion. 2000×



the R&D Center for Iron and Steel (RDCIS), SteelAuthority of India Limited, Ranchi, for theencouragement and support in pursuing this study.Thanks are expressed to the personnel of themetallography laboratory, notably U.N. Jha, C.B.Sharma, and J. Guria, at RDCIS for their help inthe investigation and to B. Khalkho for typing themanuscript in a short span of time.

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Genesis of Spalling in Tandem Mill Work-Rolls

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