Microstructure evolution and abrasive wear behavior of D2 steelKritika Singh, Rajesh K. Khatirkar, Sanjay G. SapatenDepartment of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology (VNIT), South Ambazari Road, Nagpur 440010,Maharashtra, Indiaa rti cle in foArticle history:Received 28 August 2014Received in revised form5 February 2015Accepted 9 February 2015Available online 17 February 2015Keywords:Two body abrasionHardnessSteelWear testingElectron microscopyabstractThe effect of heat treatment on microstructure and abrasive wear resistance of AISI D2 steel has beeninvestigatedinthepresentwork. Thestructuralcharacterizationofhardenedandmultipletemperedspecimens was carried out using scanning electron microscopy (SEM), energy dispersive spectrometer(EDS) and X-ray diffraction (XRD). Two body abrasive wear tests were carried out using silicon carbideabrasive with systematic and simultaneous variation of test parameters. The abrasive wear volume lossincreasedwithincreasingtemperingtemperatureandincreasingseverity oftest conditions, althoughtheincreasewasnotproportionate. Thewornoutsurface, weardebrisandsubsurfacedamagewereexaminedusingSEM, EDSandXRD. Theresultsof present workwererationalizedwithrespect tomicrostructure and operating wear mechanisms as inuenced by severity of wear test conditions.& 2015 Elsevier B.V. All rights reserved.1. IntroductionHigh-carbonhigh-chromium(HCHCr)steelswererstdevel-oped as a substitute for high speed (HS) steels, but were found tobe of limited use due to insufcient hot hardness and brittleness.However, thesesteelsproveduseful inapplicationswherehighwear resistance and non-deforming properties were required e.g.indiesandpunches[14]. Thehighwear resistanceinHCHCrsteels is attributedtohighvolumefractionof hardchromiumcarbides[4]. D2steel ndsapplicationsindrawingandformingdies, cold drawing punches, blanking/stamping dies and extrusiondies. Die steels are usually subjected to compressive-tensilestresses, shear stresses and hence die steels require high strengthand toughness apart from good wear resistance [5].In general, an increase in hardness of material results in an increaseinitswearresistance. Thiscorrelationistrueonlyincaseof puremetals in the annealed condition and alloys of same family [6]. Theresearch investigations in the past have focused on improvement ofthe wearresistancemainlyby alterationofmicrostructurebyusingconventionalheattreatmenttechniques. Theabrasive wear losshasbeenreportedtodecreasewithincreaseinthevolumefractionofmartensite. It has also been reported that the hardness of martensitehas a greater inuence on the abrasive wear resistance of steels thanits volume fraction [79]. The martensitic microstructure with carbidesexhibited better wear resistance as compared to bainitic or pearliticmicrostructures [10]. Theabrasivewear insteels withmultiphasemicrostructure is inuenced by morphology of the carbides, abrasiveparticlepropertiesandmaterialproperties[1119]. Torkamanietal.[20] noted that bright hardened samples of D2 steel showed highertensile strength, impact toughness and higher hardness in comparisontooilquenchedsampleswithuniformdistributionof necarbides.Tanget al. [21] concludedthat thedryslidingwearratesof heattreated D2 steel decreased with the increase in hardness in the slidingspeedrangeof0.050.50 m/s. Themodeofwearwasadhesiveforspecimens with hardness 5571 and 6271 HRc, whereas the modewas abrasive for specimens with hardness 5171 and 5871 HRc. Acomparative study of D2 and O1 tool steels in hardened and doubletemperedcondition withthesamehardness(60 HRc)revealedthattheir wear resistance was inuenced by wear mode and mechanismofmaterial removal under different test conditions [22]. D2 steelexhibited two times better wear resistance than O1 tool steel, whichwasattributedtoplatelikecarbidemorphology. Thestudydidnotreport the effect of retained austenite on wear behavior of D2 steel. Maet al. [23] noted the decrease in mass loss of D2 steel with increase inslidingspeedandappliedloadwhichwasattributedtofractureofabrasive particles under the used test conditions. The effect ofcryogenic treatment on abrasive wear resistance of D2 steel has alsobeeninvestigatedinthe past [2427]. The results revealed thatprecipitationof secondarycarbidesandthepercentageof retainedaustenite after quenching and cryogenic treatment signicantly inu-enced the wear behavior under diverse test conditions.Xu et al. [28] reported that wear resistance was optimum at 30%volumefractionofretainedausteniteinhighspeedsteels. Atlowerloads, conventionally heat treated samples exhibited better wearresistance as compared to laser melted specimens, whereas at higherloads superior wear resistance of laser melted tool steel was attributedto stress induced transformation of austenite into martensite [29]. InContents lists available at ScienceDirectjournalhomepage:www.elsevier.com/locate/wearWearhttp://dx.doi.org/10.1016/j.wear.2015.02.0190043-1648/& 2015 Elsevier B.V. All rights reserved.nCorresponding author. Tel.: 91 712 2801519; fax: 91 712 2223230.E-mail addresses: kritikasingh.me@gmail.com (K. Singh),rajesh.khatirkar@gmail.com, rajeshk@mme.vnit.ac.in (R.K. Khatirkar),sgsapate@yahoo.com (S.G. Sapate).Wear 328-329 (2015) 206216another study harmful effect of retained austenite on wear resistancewasreported[30]. Theabrasiveweardataonconventionallyheattreated D2 steel is relatively scarce. Previous studies did not report theeffect of multipletemperingtreatment, phasetransformations andretainedaustenite onabrasive wear of D2 steels. Inthe presentinvestigation, the effect of multiple tempering treatment on themicrostructure and two body abrasive wear behavior of D2 steel hasbeenstudiedby systematicandsimultaneousvariationoftestpara-meters.The detailed structuralcharacterization of D2 steelhas beencarried out using scanning electron microscopy (SEM), energy disper-sive spectrometer (EDS) and X-ray diffraction (XRD). The mechanismofmaterial removal hasbeensubstantiatedbyshortdurationabrasiveweartests. Theresultsof thepresentworkwererationalizedwithrespect to hardness, microstructure and operating wear mechanisms.2. Experimental method2.1. Material and heat treatmentThe material used in the present investigation (AISI D2 steel) wasobtained in form of a forged bar of 100 mm diameter. The chemicalcompositionof AISI D2steel was determinedbyoptical emissionspectrometer (OES) andis giveninTable1. Theforgedbar wasannealed at 1158 K for 60 min to remove the effect of prior thermo-mechanical processing history. The specimens for further heat treat-ment were cut from the annealed bar in the form of cylinder havinglength of 25 mm and diameter of 10 mm. The specimens wererstaustenitizedat1303 Kfor45 minfollowedbyoilquenching(hard-ening). Thetemperingtemperatureanddifferent temperingcycleswere selected to obtain variations in microstructure; carbide morphol-ogy and matrix, surface hardness and retained austenite content. Anattempt is made to correlate abrasive wear of multiple tempered D2steel with change in mechanism of material removal, associated withphase transformations during multiple tempering treatments,underdifferent abrasive wear test conditions.Thehardenedspecimenswerethensubjectedtomultipletem-pering treatments. The tempering temperatures selected were 523 K(designated as T523), 623 K (designated as T623), 723 K (designatedTable 1Chemical composition (weight%) of D2 steel.Elements C Mn Si Cr Mo P S V FeWt% 1.5 0.45 0.3 12 0.9 0.03 0.03 1.0 BalanceFig. 1. Experimental set-up used for two-body abrasion tests.Fig. 2. Secondary electron scanning electron microscopy (SEM) images of the abrasive papers used for the wear tests (a) 80 grit (320 m) (b) 150 grit (122 m) (c) 220 grit(92 m) and (d) 320 grit (52 m).Table 2Summary of test parameters used for abrasive wear of D2 steel in the present work.(Constant test parameters: linear velocity 0.8 ms1andspecimenrotation50 rpm).Specimen designation Abrasive wear test conditionsLoad (N) SiC particle size (m)AN 10,20,30 and 40 52AN 20 N 52, 92, 122 and 320OQ 10 5220 9230 12240 320T523 10 5220 9230 12240 320T723 10 5220 9230 12240 320T923 10 5220 9230 12240 320K. Singh et al. / Wear 328-329 (2015) 206216 207as T723), 823 K (designated as T823) and 923 K (designated as T923).TheannealedandasquenchedspecimensweredesignatedasANand OQ, respectively. The specimens were single tempered, double,triple, quadrupleandvestagestemperedattemperatures523 K,623 K, 723 K, 823 K and 923 K respectively, with one tempering cycleof 60 min each. All the heat treatments were carried out in an inertatmospherefurnacetoavoidoxidationofthespecimens. ThebulkhardnessofheattreatedspecimenswasmeasuredusingRockwellhardness tester at 150 kgf load. An average of ve readings isreported in the results.2.2. Microstructural characterizationThemicrostructural examinationwas carriedout for all theheat treated specimens of D2 steel using SEM (JEOL 6380 A), EDS Fig. 3. Effect of tempering temperature on the bulk hardness of D2 steel.Fig. 4. SEM micrographs of heat treated specimens showing size, distribution and morphology of carbides for different conditions (a) annealed at 1158 K (b) as-quenchedafter austenitizing at 1303 K (c) quenched and tempered at 523 K (single stage) (d) quenched and tempered at 623 K (two stage) (e) quenched and tempered at 723 K (threestage) (f) quenched and tempered at 823 K (four stage) (g) quenched and tempered at 923 K (ve stage). All the samples were etched with 2% Nital to reveal the matrix(dark) followed by etching with Vilella's reagent to reveal the carbides (bright).K. Singh et al. / Wear 328-329 (2015) 206216 208(Bruker XFlash) attached to SEM and XRD to study size, distribu-tion and morphology of carbides, to obtain chemical compositionofthe carbides and to determine retained austenite content. Thespecimensformicrostructural observationswerepreparedusingstandard metallographic polishing techniques followed by etchingwith 2% Nital and then with Villella's reagent for 10 s each [3132].XRDpatternsweremeasuredusingPANalytical XpertProMPDsystem with Cu-K radiations and diffracted beam monochroma-tor. The XRD patterns were then analyzed using XPert Highscoresoftwaretoobtaintypeof carbidesandretainedaustenite(RA)[3334]. Forthemeasurementof RA, {220}and{200}peaksofaustenite and {200} peak of ferrite were considered.2.3. Abrasive wear testsTwo body abrasive wear tests were carried out on pin-on-plateabrasive wear test apparatus [35] (DUCOM make, India) as shown inFig. 1. The commercially bonded silicon carbides paper withdifferent particlesizes was usedas abrasivemedia. Fig. 2(ad)shows secondary electron SEM micrographs of silicon carbide (SiC)abrasive paper with average particle size of 320 m, 122 m, 92 mand52 musedinthepresentstudy. Theshapeof SiCabrasiveparticles was predominantly angular, although ner particles exhib-ited relatively greater angularity. The specimens for abrasive weartestswereintheformofpin(cylindrical shape)withthelength25 mmanddiameterof10 mm. Thespecimensweremetallogra-phically polished prior to each wear test. The objective of simulta-neousvariation of abrasiveparticle size andload was to examineregime of wear i.e. mild, moderate or severe as reected in abrasivewear loss. The data on the effect of simultaneous variation of testparametersonabrasivewearof dieandtool steel isscarce. Theobjective was also to assess the change in mechanism of materialremoval from the matrix and the carbides associated with change intemperingtemperatureandtemperingcycles (singlestagetovestage) with respect to severity of abrasive wear test conditions. Twobody abrasive wear tests were conducted as per the experimentalconditions mentioned in Table 2. The mass loss was well distributedover the surface due to rotation of specimen, which also avoids thepossibility of intense wear of the edges. The lateral displacement ofspecimenensuresthatthespecimenalwaystraversesnewtrack.The specimens were cleanedwithethanol and weighed usingdigital micro-balance (to an accuracy of 0.1 mg) prior to and aftereachwear test. Thedifferenceintheinitial andnal weight ofthe sample was recordedto calculate abrasive wear loss. TwoFig. 5. EDS spectrum of as-quenched specimen of D2 steel.K. Singh et al. / Wear 328-329 (2015) 206216 209specimens were tested under similar abrasive wear test conditionsand accordingly the scatter is shown in the test result. The worn outsurfaces and wear debris particles were examined using SEM andXRD. Theshort durationwear tests werealsocarriedout withabrasiveparticlesizeof 122 mandat 10 Nand40 Nloadsforquenchedandtemperedat723 Ksampletogaininsightintothemechanism(s) of wear.3. Results and discussion3.1. Hardness and microstructuresFig. 3showstheeffect of temperingtemperatureonthebulkhardness of D2steel samples. Theannealedsampleexhibitedthelowesthardness(19 HRc), whereasas-quenchedsampleexhibitedthe highest hardness (65 HRc). It can be observed from Fig. 3 that anincreaseintemperingtemperaturefrom523 Kto723 Kresultedinmarginal dropof hardness(3 HRc), whereasasharpdecreaseinhardness was observed with the increase in tempering temperature to923 K (35 HRc). This sharp decrease in hardness was attributed todecomposition of martensite into ferrite and carbides [36] withincreaseintemperingtemperature. ThequenchedD2steel hasthehighest hardness due to the formation of martensite, which decreasedwithincreaseintemperingtemperatureandtime. Similartrendofdecrease in hardness with increase in tempering temperature has beenreported by Lee and Su. [37] and Leskovsek et al. [38] for AISI 4340steel and H11 tool steel respectively as well as other researchers [39].Fig. 4showsthesecondaryelectronSEMmicrographsof annealed,quenched and quenched-tempered D2 steel samples. The higherFig. 6. HighresolutionindexedX-raydiffractionpatternsof(a)as-quenchedsample(b)quenchedand523 Ktemperedsample(singlestage)(c)quenchedand723 Ktempered sample (three stage) and (d) quenched and 923 K tempered sample (ve stage).Table 4Retained austenite for as-quenched and quench and tempered specimens ofD2 steel.Condition Retained austenite(%)As-quenched at 1303 K 17.20Quenched and tempered at 523 K (single tempered) 17.00Quenched and tempered at 723 K (three stagetempered)11.82Quenched and tempered at 923 K (ve stage tempered) 6.39Table 3Chemical composition of primary and secondary carbides (wt%) as determined byenergy dispersive spectrometer (EDS).As-quenchedWeight %Fe Cr Si VPrimary carbides 88.25 11.06 0.69 o0.05Secondary carbides 51.19 44.67 0.22 o0.05Quenched and tempered at 523 KPrimary carbides 90.54 8.82 0.49 o0.05Coarse secondarycarbides93.88 6.21 0.37 o0.05Fine secondary carbides 51.83 48.03 0.07 o0.05Quenched and tempered at 923 KPrimary carbides 92.58 8.03 0.57 o0.05Coarse secondarycarbides51.05 46.84 0.29 0.11Fine secondary carbides 91.39 8.15 0.46 o0.05K. Singh et al. / Wear 328-329 (2015) 206216 210amount of alloying elements in D2 steel caused a bi-modal distributionof carbideparticlesintheannealedcondition(Fig. 4a). Thecoarseparticles were observedto be M7C3carbides whichformduringsolidication[4]. Theprecipitationof carbides inannealedsteel isdependent on the ratio of chromium to carbon. When the ratio is lessthan three, the only carbide which forms is the alloyed cementite. If theratio is greater than three, chromium carbides (both Cr7C3 and Cr23C6)are formed. InD2 steels only Cr7C3has beenreported to form(observed in the present work also) which is usually enriched by Fe[4]. Fig. 4b shows SEM micrograph of as-quenched specimen showingun-dissolved carbides as well as small secondary carbides. Thesecondary carbides in as-quenched specimen might have formed duetotheverysmall delayinquenchingthesamples. Inas-quenchedspecimen, the martensitic lathes are veryne and cannot be resolved.Fig.4cg shows SEMmicrographs of hardened andtempered speci-mens. It can be observed that with increase in tempering temperatureamount ofne secondary carbides increased along with coarsening ofcarbides. ThecarbidesasseenfromFig. 4gappeartoberelativelycoarser at the maximum tempering temperature of 923 K.Thenatureof theprimaryandsecondarycarbidesprecipitatedduring tempering was determined by using EDS. Secondary electronSEM image along with representative EDS spectrum of primary andsecondary carbideisshown inFig.5. ThechemicalcompositionofthecarbidesasdeterminedbyEDSissummarizedinTable3. Theprimary carbides (bigger Fig. 4b) were rich in Fe and lean in Cr inthe as-quenched condition. The secondary carbides (smaller Fig. 4b) which might have precipitated due to slight delay inquenching were having 50% Fe and 35% Cr. After tempering at523 K, the composition of the primary and secondary carbideschanged only marginally. Although,there was precipitation of newsecondarycarbides (other thanthat precipitatedduringdelayinquenching referred to asne secondary carbides). After temperingat 923 K, the primary and coarse secondary carbides were rich in Feand lean in Cr, the coarse secondary carbides had almost 50% Fe and50% Cr, whereas the ne secondary carbides which precipitated werevery rich in Fe. In chromium containing tool steels, two types of Crcarbides (Cr7C3 and Cr23C6) are often observed and these are usuallyenrichedwithalloyingelementslikeFe, Moetcdependingonthechemical composition of the steel. The precipitation sequence withincreasingtemperingtemperatureintoolsteelshasbeenreported[36] to be M3C-M7C3-M23C6 where M stands for metal atoms. Thetransformation of M3C (Fe3C) to M7C3 (Cr7C3) occurs by the nuclea-tion at the Fe3C (M3C)/ferrite interface [4]. The Cr carbide precipita-tion seems to occur at lower and intermediate temperingtemperatures. At higher temperingtemperatures, precipitationofironrichcarbidesindicatesthedecompositionof martensiteintoferrite and Fe3C, which is slightly enriched with Cr. A similarsequencehasbeenreportedfor astudyonEn31steel [40]. TheXRD patterns of the as-quenched, quenched-tempered 523 K,quenched-tempered 723 K andquenched-tempered 923 K D2 steelsamples are showninFig. 6. The XRDdata supplements SEMobservationsonmicrostructural changesassociatedwithmultipletempering treatment of D2 steel. The as-quenched D2 sampleshowedthepresenceof martensite(indexedasferrite, sincetheresolutionof the diffractomter was not sufcient toresolve thetetragonality of the martensite), alloyed cementite (indexed as Fe3C),Fig. 7. (a) Abrasive wear volume loss (cm3) as a function of load for annealed specimen for abrasive particle size of 52 m. (b) Abrasive wear volume loss (cm3) as a functionof abrasive particle size for annealed samples at a load of 20 N and at a velocity of 0.8 cm/s and at 50 rpm.Fig. 8. Abrasivewearvolumeloss(cm3)vs. simultaneousvariationof loadandabrasive particle size at a velocity of 0.8 m/s and rpm of 50.Fig. 9. Variation of abrasive wear volume loss (cm3) as a function of hardness (HRc)at a load of 10 N and 40 N.K. Singh et al. / Wear 328-329 (2015) 206216 211RA and M7C3 (indexed as Cr7C3). After tempering at 523 K and 723 K,theprimarychromiumcarbides(Cr7C3)persistedalongwithalloycementiteandRA, althoughtheir proportionvaried(asindicatedrelativelybythepeakintensities). Atthehighesttemperingtem-peratureof 923 K, theXRDpatternshowedveryweakvanadiumcarbide (V8C7) peak. The variation of RA with tempering temperatureis given in Table 4. There was almost no change in RA after temperingat 523 K which gradually decreased with increase in the temperingtemperature. Evenafter vestagesof temperingat923 K, theRAcontent was not zero. The diffraction peaks (martensite, indexed asferrite) are also much broader in the as-quenched D2 steel sample.The martensitic structure is always associated with large dislocationdensityinthestructure. Thebroadeningof X-raypeakprolesisrelated to size of the crystallites and/or micro-strain present in thesample. Reduction in the size of the crystallites/coherently diffractingdomains and increase in the micro-strain lead to the broadening ofthe peak proles. The presence of dislocations inthe structureinduces micro-strains, which results in the broadening of thediffraction peaks [40,41]. After tempering at 523 K, 723 K and923 K, thebroadeningreducedduetoreductioninthedislocationdensity as well as decomposition of martensite into ferrite.3.2. Two body abrasive wear testingFig. 7a shows the variation of abrasive wear volume loss withload for annealed D2 steel. The abrasive wear volume lossFig. 10. (a,b): SEM micrographs of worn out surface of quenched and tempered specimens of D2 steel at a load of 40 N. (a) tempered at 723 K (triple tempering) and (b)vestage tempered at 923 K.Fig. 11. (a,b): SEM micrographs of heat treated D2 steel after short duration abrasive wear test for quenched and tempered sample at 723 K (a) 10 N (b) 40 N. (c,d ): Sub-surface SEM micrographs of worn out samples at 40 N load (a) as-quenched, (b) quenched and tempered at 523 K (single stage).K. Singh et al. / Wear 328-329 (2015) 206216 212increased linearly and the volume loss at highest load (40 N) wasapproximatelytwotimes as comparedtothat observedat thelowest load (10 N).The abrasive wear volume loss increased withtheincreaseintheparticlesizeasshowninFig. 7b. Withtheincrease in particle size from 52 m to 122m, the increase inthe volume loss was approximately 1.6 times, whereas for increaseinparticlesizefrom122 mto320 m, theincreasewasonlymarginal. These results can be understood in terms of particle sizeeffect, whichcanbe attributedtoclogging, adhesionof weardebris tothetipof abrasiveparticlethus reducingits cuttingefciency, fracture of abrasive particles and increase in local owstress withdecreasing scale of deformationfor ner abrasiveparticlesleadingtoreducedwearloss[6,42,43]. Intheannealedcondition, the matrix is verysoft andthe increase inloadisexpectedtoresult inhigher depthof cut resultinginmaterialremoval proportional totheappliednormal load. SiCparticleshave higher hardness as compared to the Cr7C3 carbides and henceareexpectedtocauseindentationandfractureof thecarbidessince ratioof Ha/Hs( ratioof hardness of abrasive tothat ofcarbide) was greater than 1.2 [6]. Fig. 8 shows the effect ofsimultaneous variationof loadandparticlesizeof abrasiveonabrasivewearvolumeof heattreatedD2steel. Thedatapointsweretted by bestt line and regression coefcients were in therangeof0.930.95. Itcanbeobservedthatwiththeincreaseinseverityofwearconditionfrom(10 N, 52 mm)to(40 N, 320 mm)abrasivewearvolumelossincreasedforD2steel irrespectiveofthe heat treatment condition. The abrasive wear volume lossincreased more than two times for specimens tempered at 923 Kwhereasasquenchedspecimenexhibitedanincreaseof nearly1.25 times with increase in severity of test condition ( from 10 N,52 mmto40 N, 320 mm). Itcanalsobeobservedthatspecimentempered at 923 K exhibited nearly 2.4 times more abrasive wearvolume loss at a loadof 40 Nandparticle size of 320 mmascompared to oil quenched specimen whereas this increase was notmore than 1.6 times at a load of 10 N and particles size of 52 mm.As-quenched specimen exhibited relatively higher volumefractionof retainedaustenite, asshowninTable4, whichwasnearly three times as compared to specimen tempered at 923 K asaresult ofmultipletemperingtreatment, whereas nosignicantdifferencewas observedfor specimentemperedat 523 K. Therelatively lower abrasive wear loss exhibited by as-quenchedspecimen can be attributed to ner morphology of carbides,martensiticmicrostructureandstressinducedtransformationofaustenite to martensite during abrasive wear. At higher temperingtemperature the contribution of retained austenite in reduction ofabrasive wear loss associated with stress induced transformationwas relativelyinsignicant duetoits lower percentage. Fig. 9shows abrasive wear volume loss plotted vs bulk hardness of D2steel at 10 N and 40 N loads. The data points weretted by powerlawrelationship and the dependence of volume loss (V) onhardness (H) was expressed by the power law equation:V K H m1Fig. 12. SEM micrographs of wear debris of (a) and (b) as-quenched samples, (c) and (d) quenched and 523 K tempered samples (single stage), (e) and (f) quenched and923 K tempered samples (ve stage). For (a), (c) and (e) the load used was 10 N, while for (b), (d) and (f), the load used was 40 N.K. Singh et al. / Wear 328-329 (2015) 206216 213where Vvolume loss, Hbulk hardness, Kconstant and mhardnessexponent. Itwasobservedthatonlyamarginal differ-encewasobservedonthedependenceofabrasivewearvolumeloss on hardness as revealed by the hardness exponents (0.69 and0.66at aloadof 10 Nand40 N, respectively). Asexpected, thewearvolumelosswasgreaterataloadof40 Nascomparedto10 N. The increase in volume loss can be attributed to thestructural transformations that occurredduringheat treatmentand due to the greater depth of cut by coarseparticles at higherloads. These observations are consistent with previous studies byMoore [44]andUedaetal. [45]. Italsosuggeststhatthefactorsother than hardness, e.g. microstructure and morphology ofcarbides are expected to exhibit considerable inuence on abrasivewear volume loss of D2 steel. Hence SEM and XRD studies of wornout surfaces and wear debris particles were focused to substanti-ate the results of the wear tests.Fig. 10 (a and b) shows abraded surfaces for quenched andtempered samples at 723 K and 923 K respectively. As the temperingtemperatureincreased, theabrasionresistancereducedduetothecoarsening of carbides coupled with decomposition of martensite [36].With increase in tempering temperature the softer matrix was easilyplowedandcut bythe abrasive particles as comparedtospecimenstemperedatlowertemperatures[6,40,41,46]. Thematerialfromthematrixwasremovedbytheprocessof microcutting, plowinganddelamination as shown in Fig.10 (a and b), whereas indentation andgross fracture and carbide pull out was observed at higher load andcoarser particle size (40 N load and 320 m particle size). Withnerabrasive particles, the contact zone size was relatively small and onlysmallscalechippingofcarbideswasobservedresultinginrelativelylower abrasive wear volume loss under these conditions. Fig. 11 showsSEM micrographs of quenched and tempered (723 K) D2 steel sampleafter short duration abrasion test at a load of 10 Nand 40 Nrespectively. Fig. 11(a) showsmaterial removal fromthematrixbyplowing and some of thener carbides carried with plowed materialalsocanbeseenatthecenterof theimage. Fig. 11(b)showsSEMphotograph at a load of 40 N. The important mechanisms of materialremoval from the carbides were observed to be large scale chipping,edge fracture and gross fracture of carbides shown in the SEMphotographs. Asthematrixiswornoutbytheprocessofplowing/micro cutting, it offers reduced support to the carbide particles leadingto carbide pull out resulting in increased abrasive wear volume loss athigherloadandcoarserparticlesize. [47].ThisisalsoevidentfromFig. 11(candd)whichshowssub-surfacemorphology ofwornoutspecimens as-quenched and quenched-523 K tempered specimens.Fig. 12 shows the SEM micrographs of wear debris of as-quenched,quenched and tempered 523 K sample and quenched and tempered923 K sample at load of 10 N and 40 N. Fig. 12(a, c and e) shows weardebris at load of 10 N and SiC particle size of 52 mwhereas Fig. 12(b,dandf) showsweardebrisat loadof 40 Nwithparticlesizeof320 m. Fig. 13 shows the XRDpatterns of wear debris of as-quenched, quenched-523 K, quenched-723 Kandquenched-923 Ktempered specimens at 40 N load and 320 m SiC particle size. Theweardebrisparticlesconsistedof fracturedSiCabrasiveparticles,carbideparticlesremovedbytheprocessof fractureandcarbidesremoved by pull out (Mo2C, Cr7C3 and Fe3C) along with ferrite andmartensitematrixremovedbytheprocessofcuttingasshowninFig. 12.These observations were conrmed by XRD patterns of weardebris particles at a load of 40 N and 320 m SiC particle size (Fig. 13(ad)). It can also be observed from Fig. 13 that at a load of 10 N thefragmented and fractured SiC carbides were relatively ner ascompared to that at higher load of 40 N. XRD patterns in Fig. 13 alsoindicated presence of iron oxide (Fe2O3) peaks with signicantFig. 13. High resolution X-ray diffraction (XRD) pattern of wear debris of D2 steel for (a) as-quenched (b) quenched and 523 K tempered (single stage) (c) quenched and723 K tempered (three stage) and (d) quenched and 923 K tempered (ve stage) samples at a load of 40 N.K. Singh et al. / Wear 328-329 (2015) 206216 214intensityfor alltheheattreatedconditionswhichimpliedthatthewear mechanismwas oxidative. The ironoxide lmprovides alubricating lmwith lowshear strength reducing the friction betweensurfaceandabrasiveparticlessubsequentlysubsidingtheeffectsofcapping and clogging [6]. At higher load and coarser particle size, ironoxidelm is penetrated by abrasive particles with exposure to freshmetalsurface withincreasedwear loss. There wasnoformationofwhite layer for very high loads for D2 steel as reported in a previousstudy[25]. Theaboveobservations alsoindicatethat D2steel isrelatively more abrasive wear resistant under mildto moderateabrasive wear conditions. From the above it can be concluded that acombination of wear mechanisms was responsible for materialremovalandtherelativecontributionofeach(e.g. Plowing, micro-cutting, edge fracture, chipping, gross fracture of carbides, carbide pullout) was decided by structural transformations associated withtemperingtreatmentandmorphology ofcarbides. Thiscanexplaintheabrasivewearlosstrendsobtainedinthepresentinvestigationwith increase in tempering temperature and with change in abrasivewear test conditions from benign to relatively severe one. From theresults of the present investigation, it can be also pointed out that it isdifcult to pin point the effect of retained austenite (617%) in heattreated D2 steel on the improvement of abrasive wear resistance. Thisisfurthersupportedbytheobservationthatabrasivewearvolumeloss ofheattreatedD2steelshowednearly similar dependence onsurface hardness at both lower and higher loads as indicated by thehardness exponents. In fact, the contribution of change in mechanismof material removal toabrasivewear resistanceof D2steel withincreasing severity of abrasive wear test conditions was moresignicant as comparedtothe effect of retainedaustenite. It isdifculttocomparetheresultsof thepresentworkwiththoseofprevious studies due to different processing conditions, mode of wear,conguration of the test apparatus and the test conditions, as far asthe effect of retained austenite on abrasive wear resistance isconcerned [28,47].4. Conclusions1. The morphology of carbides became coarser with increasing temper-ing temperature during multiple tempering treatment of D2 steel.2. D2 steel in the as-quenched condition exhibited highest abra-sive wear resistance which was attributed to martensiticmicrostructure withner morphology of carbides.3. Thesimultaneous variationof loadandabrasiveparticlesizeincreased abrasive wear volume loss with increasing temperature.4. 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