Inconsistent wear behaviour of cryotreatedtool steels: role of mode and mechanism

D. Das1, A. K. Dutta2 and K. K. Ray*3

This report aims to reveal the cause of wide variation in the reported degree of improvement in

wear resistance of cryotreated tool steels. Sliding wear tests at different normal loads have been

carried out on conventional and cryotreated AISI D2 steel specimens together with SEM

examinations and EDX microanalyses of the surfaces and subsurfaces of the worn specimens

and that of the generated debris. The obtained results reveal that when the modes and

mechanisms of wear are similar for both types of specimens, mild oxidative at lower load or

severe delaminative at higher load, the improvement in wear resistance is 1?62?2 times. At the

intermediate load, the modes and mechanisms are dissimilar, and the observed improvement is

as high as 53?2 times. The reported varied degree of improvement in wear resistance by

cryotreatment has been attributed to the operating test conditions that govern the modes and

mechanisms of wear.

Keywords: Cryogenic treatment, Wear resistance, Wear mode, Wear mechanism, AISI D2 steel, Microstructure

IntroductionWear resistance of tool steels is one of the key governingfactors that determines the useful life of a tool and inturn controls the productivity of the manufacturingprocess. One of the approaches to achieve improvedwear resistance of tool steels is the use of deep cryogenictreatment.110 Deep cryogenic treatment, often simplyreferred to as cryotreatment, is applied in betweenconventional hardening and tempering treatmentsUnlike age old cold treatments (213193 K), cryotreat-ments are usually carried out between 148 K and 77 Kfor a sufficiently long time (1272 h) with controlledcooling and heating cycles.8,9 The most prevalent claimregarding the benefit of cryotreatment of tool steels isthe increment in wear resistance,112 apart from theenhancement of dimensional stability,13 hard-ness,3,5,7,8,12,14,15 fatigue resistance,14 toughness,16 bendstrength3 and reduction of residual stress.17 Theenhancement of the mechanical properties of tool steelsby cryotreatment has been attributed to the nearlycomplete transformation of retained austenite to mar-tensite,4,18 precipitation of ultrafine carbide parti-cles5,6,19 or both.8,15,20

The improvements in wear resistance (IWR), typicallyfor AISI D2 steel, by cryotreatment reported bydifferent investigators48,11,16 are compiled in Table 1.It is obvious from the data in Table 1 that the degree of

IWR varies widely from a few per cent to a few hundredper cent. The inconsistency in the reported values ofIWR, even for the same material, and the lack ofscientific explanation behind it are creating mistrustregarding the benefit of cryotreatment for tool steels andhindering its commercial exploitation. It is well estab-lished that the wear rate of a material can vary widelywhen the mode of wear changes from mild to severe withthe associated changes in the wear mechanism due to thechanges in the magnitude of normal load, slidingdistance and sliding velocity.21,22 A comparative studyof the wear rates of the cryotreated and conventionallytreated specimens with identification of the operativemode and mechanism of wear through analyses of thecharacteristics of worn surfaces, subsurfaces and weardebris is thus considered essential for resolving the issueof the reported inconsistent improvements in wearresistance of tool steels by cryotreatment. This is theinherent motivation of this investigation. An attempthas also been made to understand the wear behaviour inrelation to the concerned microstructures. The materialselected for this investigation is AISI D2 steel.

Experimental proceduresThe selected steel for this investigation is a commercialAISI D2 tool steel containing Fe1?49C0?29Mn0?42Si11?38Cr0?80Mo0?68V0?028S0?029P (wt.-%).Sample blanks of the steel were first subjected toconventional (QT) and cryogenic treatment (QCT) inseparate batches; QT consisted of hardening (Q) andsingle tempering (T), whereas QCT incorporated anadditional step of controlled deep cryogenic (C) proces-sing in between hardening and tempering. The detailsof the hardening and tempering treatments havebeen reported earlier.8 The cryogenic processing was

1Department of Metallurgy and Materials Engineering, Bengal Engineeringand Science University, Shibpur, Howrah, India2Department of Mechanical Engineering, Bengal Engineering and ScienceUniversity, Shibpur, Howrah, India3Department of Metallurgical and Materials Engineering, Indian Institute ofTechnology, Kharagpur, India

*Corresponding author, email kkmt@metal.iitkgp.ernet.in

2009 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 6 July 2008; accepted 8 August 2008DOI 10.1179/174328408X374685 Materials Science and Technology 2009 VOL 25 NO 10 1249

performed at 77 K for 12 h with controlled cooling andheating rate (

the specimens and the generated wear debris have beenexamined using SEM coupled with energy dispersive X-ray (EDX) microanalyses.

Results

Microstructure and hardnessFigure 1 depicts typical representative SEM micro-graphs of the QT and QCT specimens. The micro-structures of these specimens exhibit a non-uniformdistribution of large elongated dendritic-type PCs anduniform distribution of nearly spherical SCs on thetempered martensitic matrix. In this study, the SCs havebeen classified into two different size groups LSCs and

SSCs. The amount and the number of both LSCs andSSCs are significantly higher in the QCT specimen thanthat in the QT specimen (Fig. 1). The PCs and SCs havebeen identified as M7C3 and M23C6 (M5Cr, Fe, Mo andV) respectively by the XRD (Fig. 2) and EDX analysesof electrolytically extracted carbide particles from bothQT and QCT specimens. These results are in agreementwith the earlier reports.27

The XRD profiles of bulk specimens exhibit thecharacteristics diffraction peaks of retained austenite(cR) in the QT sample unlike that in the QCT sample asshown in Fig. 2. The presence of cR has also beenrevealed in the microstructure of the QT specimens athigher magnifications.8 The estimated amounts of thedifferent phases by XRD technique and/or imageanalyses for both the QT and QCT specimens aresummarised in Table 2. The cR content is found to be9?80?7 vol.-% in the QT specimen compared with thenegligible amount in the QCT specimen. It can beconcluded from these results that cryogenic processingimmediately after hardening does not alter the nature ofcarbides, but almost completely removes the soft cR witha consequent increase in the amount of hard phases, likeSCs and tempered martensite (Table 2).

The results in Table 2 related to the characteristics ofSSCs and LSCs for both QT and QCT specimens inferthat cryotreatment refines SCs, increases their amountand population density and decreases their meaninterparticle spacing. The possible reasons for therefinement of SCs by cryotreatment have been postu-lated earlier by Das et al.8 and Huang et al.20 In brief,transformation of austenite to martensite at lowtemperatures with associated controlled cooling of thespecimens during cryotreatment induces a high densityof crystal defects such as dislocations and twins, andthus increases lattice distortion and thermodynamicinstability of martensite. These phenomena result intothe segregation of carbon atoms to nearby defects asclusters that act as nucleating sites for the formation ofcarbide on subsequent tempering.

The results in Table 2 also indicate that cryotreatmentincreases both macrohardness (6?6%) and microhard-ness (10?9%) of D2 steel compared with those of theconventionally treated specimens. These observationsare in agreement with some earlier reports.3,7,8,14,15 Thelower macrohardness of QT specimens is naturalbecause of its higher content of soft cR and lowercontent of hard SCs (Table 2). The microhardnessvalues are influenced by the amount and the distributionof the finer SC particles. The precipitation of a higheramount of finer SCs and the formation of a largeramount of tempered martensite in the QCT specimenresult in its higher microhardness than that of QTspecimens.

Estimation of wear rateWear rate (WR) is estimated as wear volume loss (m

3)per unit sliding distance (m).28 The estimated values ofWR for all the specimens tested under different normalloads (FN) are summarised in Table 3. The results inTable 3 suggest that WR of QT specimens is alwayshigher than that for QCT specimens and this is inagreement with the earlier reports.2,5,6,811

A new parameter b, defined here as the ratio of WR ofthe QT specimen to WR of the QCT specimen at a givenFN, has been considered to quantify the degree of

a bulk specimens; b electrochemically extracted carbideparticles

2 XRD proles of QT and QCT specimens

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Materials Science and Technology 2009 VOL 25 NO 10 1251

improvement in wear resistance by cryotreatment. Theestimated magnitudes of b under different test condi-tions have also been compiled in Table 3 and anexamination of these values shows that the magnitudeof b is strongly dependent on the wear test conditionsand varies over a wide range. This dependence of themagnitude of b on the test conditions is the cause of thereported inconsistency in the enhancement of wearresistance of tool steels by cryotreatment as illustratedin Table 1.

Examinations of worn surface and wear debrisThe morphology and the characteristics of the wornsurfaces and subsurfaces and that of the generated

debris of all the specimens have been examined underSEM with associated EDX microanalyses in order toidentify the operative wear mechanisms. Typical micro-graphs and representative EDX profiles for different FNare depicted in Figs. 35. Figure 3 illustrates compactedoxide layers, groove marks, cracking and pull-out ofPCs for both QT and QCT specimens, whereas Fig. 5shows plastic deformation of the surface and subsurfacewith associated occasional subsurface cracking. Thewear debris of both the QT and the QCT specimensshow oxide granules (Fig. 3) and large metallic platelets(Fig. 5). However, Fig. 4 shows different features ofthe worn surfaces and the wear debris of QT andQCT specimens. The salient features in Figs. 35 are

Table 2 Summary of microstructural and hardness values of QT and QCT specimens

Microstructural featuresand hardness

Specimens

QT QCT

Amount (vol.-%) Retained austenite 9.800.7 NegligiblePCs 6.990.3 6.840.5SCs (5SSCszLSCs) 6.570.3 9.650.3Tempered martensite 76.64 83.51

Amount (vol.-%) SSCs 3.520.2 5.470.2LSCs 3.050.7 4.180.4

Mean diameter (mm) SSCs 0.490.01 0.360.01LSCs 2.240.05 1.640.03

Population density (6103, nos. mm2) SSCs 16112 47522LSCs 6.40.5 15.60.4

Mean interparticle spacing (mm) SSCs 13.5 6.1LSCs 71.2 37.6

Macrohardness, Hv60 (GPa) 7.440.04 7.930.04Microhardness, Hv0.05 (GPa) 9.030.06 10.010.07

PCs, primary carbides; SCs, secondary carbides; SSCs, small secondary carbides; LSCs, large secondary carbides.

Table 3 Estimated wear parameters, features of worn surfaces and wear debris, and the proposed modes andmechanisms of wear

Parameters/features

Normal load (N)

29.43 58.86 117.72

QT QCT QT QCT QT QCT

WR (61029, m3 m1) 1.0461024 0.4861024 4.9661022 9.3261024 0.49 0.31

b (5WRQT/WR

QCT) 2.16 53.21 1.58K 2.6361025 1.2961025 6.2761023 1.2661024 3.1061022 2.0961022

Worn surfaces Compacted oxide layer Plasticdeformation ofsurface andsubsurface

Compactedoxide layer

Heavy surface and subsurfaceplastic deformation

Groove marks Subsurfacecracking

Groovemarks

Subsurface cracking

Cracking and putt out of PCs Cracking andpull out of PCs

Extrusion of material

Wear debris Oxide granules Metallicplatelets

Oxidegranules

Large metallic platelets

Maximum length Maximumlength

Maximumlength

Maximum length

summarised in Table 3 and will be discussed in thefollowing section to bring forth the operative modes andmechanisms in these specimens.

Discussion

Mode and mechanism of wearOne of the simple ways to ascertain the operative modeof wear is through the estimation of the specific wearcoefficient K,22,28,29 which is expressed as

K~WRHV

FN(1)

where WR in m3 m1, FN in N and Hv (Vickers

hardness value) in N m2. The values of K for both

QT and QCT specimens at different FN values have beencalculated and presented in Table 3. The magnitudes ofK for both QT and QCT specimens are of the sameorder at FN529?43 N; a similar observation is also madewhen the magnitude of K is compared at FN5117?72 N.These observations imply that the operative modes ofwear at these loads are similar for both types ofspecimens.22

The values of K at FN5117?72 N are, however, threeorders of magnitude higher than those at FN529?43 N.The aforementioned comparison of the K valuessuggests that the modes of wear for both QT and QCTspecimens are mild and severe at lower and higher FN,respectively.22,28,29 At FN558?86 N, the value of K forthe QT specimen is higher by over an order of magnitude

a worn surface of QT; b worn surface of QCT; c typical micrograph of the region marked as 1 in a and b; d typicalmicrograph of the region marked as 2 in a and b; e wear debris of QT; f wear debris of QCT

3 Typical backscatter electron (a, b and c) and secondary electron (d) SEM micrographs of worn surfaces; and second-

ary electron (e and f) SEM micrographs of wear debris tested at FN529?43 N

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Materials Science and Technology 2009 VOL 25 NO 10 1253

than that for the QCT specimen (Table 3); so the mode ofwear is different for QT and QCT specimens at thisintermediate load.28,29 The mode of wear changes frommild to severe at a characteristic FN value, popularlytermed the T1 transition, in the dry sliding wear of steel ata constant sliding velocity.21,22 The results in Table 3suggest that the FN529?43 and 117?72 N are lower andhigher than the characteristics T1 transition values forboth QT and QCT specimens, whereas FN558?86 N ishigher than the T1 transition for the QT specimen, butlower than that for the QCT specimen.

At FN529?43 N, the worn surfaces of both QT(Fig. 3a) and QCT (Fig. 3b) specimens are almost

covered by a compact oxide layer (Fig. 3d). A repre-sentative micrograph (Fig. 3c) of the oxide depletedregions of the worn surfaces (Figs. 3a and 3b) reveals thepresence of groove marks, cracking and pull out of PCs.The wear debris of these QT and QCT specimens inFigs. 3e and 3f exhibit fine granular oxide particles.These observations suggest that the mechanism of wearat FN529?43 N for both QT and QCT specimens ispredominantly oxidative coupled with cracking and pullout of PCs and are similar in nature.30 However, thefraction of area of worn surface covered by the compactoxide layer is higher (Fig. 3) and the generated debris isfiner (Table 3) for the QCT specimens than that for the

a worn surface of QT; b worn surface of QCT; c wear debris QT, d wear debris of QCT, inset at higher magnification ofsame; e EDX profile of the area marked as 1 in wear debris of QT in c; f EDX profile of the area marked as 2 in weardebris of QCT in d

4 Typical secondary electron SEM micrographs and EDX proles of worn surfaces and wear debris tested at

FN558?86 N

Das et al. Wear behaviour in cryotreated tool steels

1254 Materials Science and Technology 2009 VOL 25 NO 10

QT specimens. These observations are in accordancewith the estimated lower value of WR of the QCT thanthat of the QT specimens.

At FN558?86 N, the morphology of the worn surfacesand the nature of the generated debris are distinctlydifferent for the QT and the QCT specimens (Fig. 4).The worn surface of the QCT specimen (Fig. 4b)exhibits oxide, whereas that for QT specimen (Fig. 4a)is metallic in nature with the presence of fracture ridges,deformation lips and subsurface cracking. Moreover,the generated debris for the QCT specimens (Fig. 4d) isfine granular particles, whereas those for the QTspecimen (Fig. 4c) are large platelets. The EDX micro-analyses identify the debris as oxides (Fig. 4f) andmetallic (Fig. 4e) for the QCT and the QT specimens

respectively. These observations suggest that the opera-tive wear mechanisms are different for the QT and theQCT specimens at FN558?86 N. These are oxidative

30

wear for the QCT specimens and delaminative31 wearfor the QT specimens. Under this test condition, the WRand K for the QT specimen compared with the QCTspecimen is higher by an order of magnitude and theestimated value of b is 53?2 times.

Examinations of the morphology of the worn surfacesand the generated debris of both QT and QCT speci-mens at FN5117?72 N reveal: (i) extrusion of subsur-faces in the sliding direction (Figs. 5a and b), (ii) a roughmetallic nature (Figs. 5c and d) and (iii) large metallicplatelets (Figs. 5e and f). These observations indicatethat the operative wear mechanism for both QT and

a macrograph of worn surface of QT; b macrograph of worn surface of QCT; c micrograph of worn surface of QT, dmicrograph of worn surface of QCT; e wear debris of QT; f wear debris of QCT

5 Typical secondary electron SEM macro- and micrographs of worn surface, and micrographs of wear debris tested at

FN5117?72 N

Das et al. Wear behaviour in cryotreated tool steels

Materials Science and Technology 2009 VOL 25 NO 10 1255

QCT specimens at FN5117?72 N is deformationinduced delaminative wear.31 However, the size ofmetallic debris, the extent of worn surface damage andthe extrusion of subsurfaces are found to be higher forthe QT specimen than the QCT specimen. Theseobservations are in good agreement with the estimatedvalues of WR and K (Table 3).

In general, the modes and the mechanisms of wear forthe investigated specimens are functions of the appliedload at constant sliding velocity. The degree ofimprovement in wear resistance (b) by cryotreatment isthus strongly dependent on the experimental conditionsthat determine whether the operative mode and mechan-ism of wear for the QT and the QCT specimens would besimilar or dissimilar.

The mechanisms associated with wear of QT andQCT specimens include several physical phenomena likesubsurface cracking, cracking and pull out of PCs,fracture ridges, and deformation lips depending on theapplied load of wear tests (Figs. 36). All thesephenomena could be linked with the inherent resistanceto cracking or fracture toughness of these materials indifferent heat-treated conditions. Reported literaturerelated to the role of cryotreatment on the resistance tocracking of tool/die steels is limited and contradictory innature. For example, Leskovsek et al.10 have reportedthat cryotreatment reduces the fracture toughness of M2steel due to the elimination of cR and refinement ofcarbide particles. Reduction in the impact toughness

values by cryotreatment has been reported for A2 steelby Zurecki12 and 4340 steel by Zhirafar et al.14 Molinariet al.3 have suggested a marginal increase in fracturetoughness with an attendant reduction in impacttoughness for M2 and H13 steels when cryogenicprocessing was applied after hardening and doubletempering. In contrast, a considerable improvement inimpact toughness has been reported for T1 and M2 steelby Yun et al.15 Recently, Rhyim et al.16 have reportedimprovement in impact toughness for D2 steel bycryotreatment.

With reference to the microstructural states (Figs. 2and 3), a reduction in cR and an increased amount offiner carbide precipitates have been attributed to reducethe fracture toughness of tool/die steels under cryo-treated conditions by Ogel and Tekin,32 Leskovseket al.,10 Zurecki12 and Zhirafar et al.14 In contrast,reduction in residual stress17 by cryotreatment shouldimprove fracture toughness. The observed larger metal-lic platelets in the wear debris of the QT specimencompared with the QCT specimen (Fig. 5e and f) andthe prominent presence of subsurface cracks beneath theworn surface of the QT specimen unlike that in the QCTspecimen (Fig. 6a and b) could be indicative of the factthat fracture toughness of D2 steel in the cryotreatedcondition is similar in magnitude, if not greater, to thatof the conventionally treated ones. Attempts are beingdirected to understand the fracture behaviour of D2steel under cryotreated and conventionally treatedconditions, and the results will constitute the contentfor some future reports.

Factors responsible for the inconsistentenhancement of wear of cryotreated steelsThe wear behaviour of materials is a complex functionof the test conditions apart from the mechanicalproperties of the test specimen and the counterbody.17

In the preceding section, the influence of the appliedloads at constant sliding velocity on the wear behaviourof QT and QCT specimens has been discussed in detailto illustrate the variation in the operative modes andmechanisms of wear for the selected steel. Although, theimprovement in wear resistance by cryotreatment hasbeen reported by several authors112 (Table 1), carefuland systematic diagnosis of the modes and mechanismsof wear under different test conditions have not beenaddressed by most of the earlier workers. This has led tothe apparent inconsistency about the benefit of cryo-treatment in relation to enhancement of wear resistance.

The relative wear rates of the QT and QCT specimensare governed by their microstructural conditions. The QTspecimens containing higher amounts of cR and loweramounts of SCs have exhibited higher wear rates underidentical conditions of testing as expected;22 the hardnessof the QT specimens are also lower than that of QCTspecimens, leading to their lower wear resistance.Additional supportive evidence for a higher wear ratefor the QT specimen could be generated by subsurfaceexaminations as illustrated in Fig. 6. Figure 6 indicatesthat (a) the thickness of the plastically deformed layer inthe QT specimen is significantly higher than that in theQCT specimen and (b) the subsurface cracks in thedeformed layer and cracking of PCs exists in the QT butnot in the QCT specimens. Thus, the role of cryotreatmentin enhancing wear resistance of AISI D2 steel is firmly

a QT; b QCT6 Typical back scatter SEM micrographs of subsurfaces

of worn specimens tested at FN558?86 N

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1256 Materials Science and Technology 2009 VOL 25 NO 10

established from the overall evidence from wear rates,surface features, subsurface deformation and cracking,and morphology and nature of wear debris. However, allthis evidence is a function of the applied test conditions,which need to be taken into account when judging thebenefit of deep cryogenic processing with respect to theimprovement of wear resistance of tool steels.

ConclusionsThe experimental results and their pertinent analysesresult in the following major conclusions:

1. Incorporation of deep cryogenic processing inbetween conventional hardening and tempering treat-ments (QCT) improves the wear resistance of D2 steelcompared with that of conventionally treated (QT)specimens. However, the degree of improvement in wearresistance (b) by cryotreatment is strongly dependent onthe applied load, which determines whether the opera-tive mode and mechanism of wear for both QT and QCTspecimens would be similar or dissimilar.

2. At the sliding velocity of 2 m s1, the operativemodes and mechanisms of wear for both the QT andQCT specimens are mild and oxidative at normal loads(FN) of 29?43 N, whereas at FN5117?72 N these aresevere and delaminative. The values of b at these testconditions are 2?2 and 1?6 times, respectively. When thewear tests have been carried out at FN558?86 N, QCTspecimens exhibited mild and oxidative wear in contrastto severe and delaminative wear illustrated by the QTspecimens; the corresponding value of b is 53?2 times.

3. Correlation of wear resistance with the correspond-ing microstructures indicates that the improvement inwear resistance by cryotreatment is due to the nearlycomplete removal of soft retained austenite with aconcurrent increase in the amount of hard secondarycarbides and tough tempered martensite.

Acknowledgement

The financial assistance received from the UniversityGrants Commission, Government of India [Grant no. F.No. 31-48/2005(SR)] to carry out a part of this researchis gratefully acknowledged.

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