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nfluence of varied cryotreatment on the wear behavior of AISI D2 steel
. Dasa, A.K. Duttab, K.K. Rayc,∗
Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, IndiaDepartment of Mechanical Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, IndiaDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India
r t i c l e i n f o
rticle history:eceived 5 September 2007eceived in revised form 9 June 2008ccepted 9 July 2008vailable online 17 September 2008
eywords:
a b s t r a c t
Exploration of the benefit of cryotreatment for achieving improvement in wear resistance of die/toolsteel is a topic of current research interest. A series of wear tests has been carried out on AISI D2 steelsamples subjected to cryotreatment at 77 K for different durations. The wear rates at different loads andsliding velocities, morphologies of the worn-out surfaces and the characteristics of the wear debris havebeen systematically examined to assess the possible critical duration of cryotreatment to achieve the bestwear resistance property. The wear experiments have been supplemented by detailed microstructural
investigations with an emphasis to reveal the amount of retained austenite and the characteristics ofthe secondary carbide particles apart from hardness evaluation. The results unambiguously establish that‘critical time duration’ exists for achieving the best wear resistance for AISI D2 steel through cryotreatment.This has been explained by the nature of precipitation of fine carbide particles and their possible growth,which govern the wear resistance of the material. Categorization of the secondary carbides to support thisexplanation is a new approach. The revelation of the wear mechanisms under different wear conditions
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. Introduction
The primary objective of the heat treatment of die/tool steelss to impart high wear resistance. One of the major problems inhe conventional heat treatment through hardening and temper-ng of these steels is the content of retained austenite (�R), whichs soft, unstable at low temperature and transforms into brittle
artensite during service. Transformation of austenite to marten-ite is associated with approximately 4% volume expansion, whichauses distortion of the components. Thus, either sub-zero treat-ent or multiple tempering at relatively high temperature and/or
or longer duration is used for minimizing the amount of �R contentn tool steels. The sub-zero treatment is popularly termed as eithercold treatment’ (193–213 K) or ‘cryogenic treatment’ (148–77 K)1–7]. In this report, deep cryogenic processing (77 K) has beeneferred to as cryogenic treatment and has been applied in betweenonventional hardening and tempering cycles. For convenience ofiscussion, it has been termed simply as cryotreatment in subse-
uent discussion.
The benefit of cryotreatment for the enhancement of wear resis-ance of tool steels has been cited by several researchers [5–16].owever, the mechanisms responsible for enhancing the wear
esistance by cryotreatment are yet to be clearly established. Somenvestigators [5,7,9] contend that the enhancement of wear resis-ance occurs only due to transformation of �R to martensite. But,his phenomenon is a common feature to both the cold treat-
ent and the cryotreatment, and thus the significant enhancementf wear resistance of tool steels by cryotreatment vis-à-vis coldreatment cannot be solely attributed to the minimization of �R2,6,8,10,13,17]. Several investigators [2–4,10,12,15,18] indicate thathe refinement of secondary carbides is the major cause for themprovement in wear resistance by cryotreatment; but this opinionacks appropriate experimental evidences [2,10,12,15].
While the mechanism behind the improvement of wear resis-ance by cryotreatment is yet to get crystallized, the reported wearata for cryotreated tool steels also do not provide any guidelineor the assessment of the degree of improvement in quantitativeerms. It is thus difficult to compile the existing results to achieveny unified picture. This uncertainty can be attributed to the differ-nt sets of experimental conditions for comparative assessment ofhe wear resistance of cryotreated steels, e.g., variation of appliedoads, sliding velocities and types of counter faces employed forhe evaluation of wear rates. Some typical results reported by dif-
erent investigators [5,8,10–12,14,19] are compiled in Table 1, whichividly illustrates this difficulty. Thus, one of the major aims of thisnvestigation is to address this problem through organized and sys-ematic wear experiments on differently cryotreated specimens ofISI D2 steel.
In addition, one of the major uncertainties associated with thearlier investigations related to cryotreatment of tool steels is theuration of cryotreatment at the selected temperatures [1–4,20].he existing literature does not provide any guideline related tohe selection of time duration for cryotreatment [3,10]. The hold-ng time in cryogenic processing has been varied widely by earliernvestigators. For example, holding time employed in the cryotreat-
ent for AISI M2 steel is 1 h by Leskovsek et al. [14], 20 h by de Silvat al. [19], 35 h by Molinari et al. [11] and 168 h by Huang et al. [18].uch wide variation in the selected holding time even for the sameaterial is due to the lack of systematic investigation related to the
nfluence of holding time on the wear resistance of tool/die steelsy cryotreatment. Thus another major aim of this report is to unfoldhe influence of the duration of cryotreatment on the enhancementf wear resistance at varying experimental conditions.
In order to fulfill the goals of this investigation, specimens ofISI D2 steel were subjected to five different holding times dur-
ng cryotreatment apart from the conventional heat treatment.he characteristics of �R, primary and secondary carbides haveeen assessed together with the determination of macrohardness,icrohardness and wear rates by standard experimental proce-
ures. The analyses of the wear behavior with respect to the
enerated microstructures, hardness characteristics, morphologyf the worn-out surfaces and wear debris have assisted to reveal thenderlying mechanisms for the improvement in wear resistance byryotreatment and throw light on the wild scatter of the reportedmprovement in wear resistance of tool/die steels by cryotreatment.
pirn7
ig. 1. (a) Schematic representation of the heat treatment schedule consisting of hardeime-temperature profile of a deep cryogenic processing cycle.
paper)
10 0.11 3.22 [19]
. Experimental procedure
.1. Material
The selected steel has been obtained as a commercial hot forgedar and its chemical composition in wt.% is 1.49, C; 0.29, Mn;.42, Si; 11.38, Cr; 0.80, Mo; 0.68, V; 0.028, S; 0.029, P; balance-e. This composition conforms to the AISI specification of D2teel.
.2. Heat treatments
Specimen blanks of 24 mm × 16 mm × 85 mm dimension wereubjected to conventional and cryotreatment in separate batches.he conventional treatment (QT) consisted of hardening (Q) andingle tempering (T), and was done as per ASM Heat Treater’s guide21]. Deep cryogenic processing (C) was incorporated intermediateetween hardening (Q) and tempering (T) in cryotreatment (QCT),he details of each step being illustrated in Fig. 1. The cryogenicrocessing was done by uniform cooling of the samples to 77 K, andolding the samples at this temperature for different time durations0, 12, 36, 60 and 84 h), followed by uniform heating to room tem-
erature. A typical deep cryogenic processing cycle is illustrated
n Fig. 1(b). The specimens subjected to different treatments areeferred henceforth with codes as shown in Table 2, where theumerals in the codes represent the time of holding in hour at7 K.
ning (Q), deep cryogenic processing (C) and tempering (T) cycles, and (b) typical
D. Das et al. / Wear 266
Table 2Sample codes for differently heat treated specimens
Sample code Description of heat treatment cycles
Hardening Duration of deep cryogenicprocessing at 77 K (h)
Tempering
QT
1293 K, 0.5 h
–
483 K, 2 h
QCT00 0QCT12 12QQQ
2
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tPTs(aibbQ[i
its
2
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2
bamtGwmwstpunl(wtavce
The worn-out surfaces of the pins were cleaned in acetone using
Fm
CT36 36CT60 60CT84 84
.3. Microstructural characterization
Sample blanks (10 mm × 10 mm × 7 mm) for metallographicxaminations were cut using wire electro-discharge machining.hese were polished and finally lapped by diamond paste of 1 �mize prior to etching with picral solution (3 gm picric acid in 100 mlthanol), and digital micrographs were recorded using both opticalAxiovert 40 MAT, Carl Zeiss, Switzerland) and scanning electron
icroscope (JSM-5510, JEOL, Japan). The microstructures exhibitarbide particles in a matrix of tempered martensite. The carbidearticles have been classified as primary carbides (PCs: size > 5 �m)nd secondary carbides (SCs: size ≤ 5 �m). The SCs are further sub-lassified as large secondary carbides (LSCs: 1 �m < size ≤5 �m)nd small secondary carbides (SSCs: 0.1 �m ≤ size < 1 �m). Imagenalyses of the microstructures were done using Leica QMetals soft-are to estimate (i) the volume fraction and size of PCs, LSCs and
SCs and (ii) the population density of LSCs and SSCs. The numberf carbide particles considered for quantitative characterizationss >1000 in order to estimate the stereological parameters withignificant statistical reliability.
X-ray diffraction (XRD) analyses of the generated microstruc-ures were done by using an X-ray diffractometer (PW 1830,HILIPS, USA) with Mo K� radiation at 0.01 degree/min scan rate.he volume fraction of �R was estimated in accordance with ASTMtandard E975-00 [22] considering the diffraction peaks of (1 1 0),2 0 0), (2 1 1) and (3 1 0) of martensite and (1 1 1), (2 0 0), (2 2 0)nd (3 1 1) of �R. Identification of the exact nature of the carbidesn the heat-treated samples was difficult by XRD analysis of theulk specimens due to their small amount [22]. Therefore, car-
ide particles were electrolytically extracted from both QT andCT specimens following the report of Nykiel and Hryniewicz
23]. XRD analysis of the extracted carbide particles were donen an identical manner to that for bulk specimens. The phases
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ig. 2. Typical optical micrographs of (a) QT and (b) QCT60 specimens. The microstructurartensite: black (PC, primary carbide; SC, secondary carbide).
(2009) 297–309 299
n the extracted carbide particles and bulk specimens were iden-ified from the XRD profiles with the help of PHILIPS X’Pertoftware.
.4. Hardness measurement
The macro- and microhardness values of the developed spec-mens were determined using 60 kgf and 50 gf load, respectively.ndentations for microhardness measurement of the matrix wereaken carefully avoiding the easily separable PCs, but this values influenced by the characteristics of SCs. At least 10 readingsre considered for estimating the average value of macro-ardness, whereas a minimum of 50 readings are taken tostimate the average value of microhardness of the specimenatrix.
.5. Evaluation of wear behavior
The study related to wear behavior of the cryotreated steels haseen done to assess (i) wear rate, (ii) morphology of wear debrisnd (iii) the characteristics of the worn-out surfaces of the speci-ens and to compare these features with those of conventionally
reated samples. Dry sliding wear tests following ASTM standard99-05 [24] were carried out by using a computerized pin-on-discear-testing machine (TR-20LE, DUCOM, India). Cylindrical speci-ens of 4 mm diameter and 30 mm length were used as static pins,hereas, tungsten carbide coated hardened and tempered En-35
teel disc of 8 mm thickness and 160 mm diameter was selected ashe rotating counter surface having Hv ∼ 1750 and Ra < 0.5 �m. Theins were machined from the suitably heat-treated specimens bysing wire-EDM and the faces of the pins were polished to rough-ess, Ra < 0.1 �m. The wear tests were carried out using normal
oads (FN) of 49.05 (5 kgf) and 98.1 N (10 kgf) at sliding velocityVS) of 2 m/s as well as under FN = 98.1 N (10 kgf) at VS = 1 m/s. Theear-testing machine was interfaced with a computer, which con-
inuously recorded the height loss of the pin and the friction forcet the pin–disc interface. The wear rate (WR) was estimated by theolume loss method. These tests were repeated until at least threeonsistent readings were obtained for each set of test condition tostimate the average WR.
n ultrasonic cleaner for 10 min and were subsequently examinednder a SEM to identify the possible wear mechanisms. The wearebris were collected during wear tests and were subjected to mor-hological characterization under SEM.
es revealed by etching with picral solution exhibit carbides: white; and tempered
300 D. Das et al. / Wear 266 (2009) 297–309
F gy and(
3
3
3
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ig. 3. Typical SEM micrographs of QT and QCT specimens exhibiting size, morpholoa) QT, (b) QCT12, (c) QCT36 and (d) QCT84 samples.
. Results
.1. Microstructures of differently heat-treated specimens
.1.1. Microstructural constituentsFig. 2 depicts typical representative optical microstructures for
T and QCT specimens, which exhibit large dendritic primary car-
FdSaf
ig. 4. X-ray diffraction line profiles of (a) bulk specimens and (b) electrochemically extrairection indicates the 2� positions of different diffraction planes of martensite, austenite
distribution of small secondary carbides (SSC) and large secondary carbides (LSC):
ides (PCs) and secondary carbides (SCs) in the form of either whitepherical or tiny black patches on tempered martensite matrix.
ig. 3 shows a series of typical SEM micrographs, which reveal theistinguished nature of SCs in QT and QCT specimens. Two types ofCs (white regions and black patches) in Fig. 2 get manifested withlmost identical grey level in Fig. 3; these carbides belong to two dif-erent size ranges, referred to as LSC and SSC. Fig. 3 also illustrates
cted carbide particles of QT, QCT00 and QCT36 samples: The set of (h k l) in vertical, M7C3, Cr7C3 and M23C6 carbides.
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D. Das et al. / Wea
hat the QCT specimens possess considerably larger numbers ofSCs compared to that of the QT specimen. It appears from Fig. 3(b)o (d) that the number, size and amount of both SSCs and LSCs varyonsiderably with the time of holding at 77 K.
The different phases in the microstructures have been identi-ed by XRD analyses (Fig. 4). The amounts of the microstructuralonstituents (Fig. 5) for all of the heat-treated specimens have beenstimated in the following manner. The volume fraction of �R haseen estimated by XRD analyses, the volume fractions of PCs, LSCs,SCs and SCs (LSCs + SSCs) have been determined by image anal-ses on digital micrographs and the volume fraction of temperedartensite has been considered as 100 minus the volume percent-
ge of �R and that of all types of carbides.
.1.2. Measurement of retained austenite contentTypical representative XRD line profiles of three bulk specimens
T, QCT36 and QCT60 are shown in Fig. 4(a). The prominent pres-nce of the (2 2 0) and (3 1 1) peaks of �R in the XRD profiles ofT samples and their indistinct appearance in the XRD profiles ofCT specimens assist to compare the amounts of �R in these spec-
mens qualitatively. The average volume fraction of �R is found toe 9.8 ± 0.7% in QT specimen (Fig. 5(a)). These results suggest thateep cryogenic processing in between hardening and temperinglmost completely converts the �R in D2 steel to martensite. Thisbservation is in excellent agreement with the reported results forryotreated AISI D2 [10,13,26], M2 [11,12,14,17–19] and 52100 [20]teels.
.1.3. Identification of carbide particlesThe intensities of the XRD peaks for the different carbide parti-
les are very weak in the line profile of bulk specimens in Fig. 4(a).ence, the nature of these particles has been examined using thenes obtained by electrolytic extraction (Fig. 4(b)). The PCs in both
T and QCT specimens have been identified mainly as M7C3 with
mall amount of Cr7C3 by XRD analyses (Fig. 4(b)). The M7C3 car-ide is the main eutectic carbide for AISI D2 steel [25–30]. The EDXicroanalysis reveals the chemical composition of M7C3 carbides
s (Fe28 Cr39V2Mo1)C30, which is in good agreement with the data
3
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ig. 5. Variations of amount of (a) retained austenite, primary carbides (PCs), secondarySSCs) and large secondary carbides (LSCs) as functions of holding time at 77 K during cryoamples, which are not subjected to cryogenic processing cycle.
(2009) 297–309 301
n literature [27] too. The SCs in both QT and QCT specimens haveeen identified as M23C6 (M = Fe, Cr, Mo, V) type in agreement witharlier reports [10,23,27]. It is interesting to note that the applica-ion of deep cryogenic processing immediately after conventionalardening does not alter the nature of PCs and SCs.
.1.4. Amount of primary carbidesThe amounts of PCs are similar in QT and QCT specimens
Fig. 5(a)). The mean volume fraction of PCs for all types of sam-les can be represented as 7.3 ± 0.4%, which is in good agreementith the value of 7.8% as reported by Fukaura et al. [27] for a sim-
lar steel. The lengths of the major axis of the PCs range betweenand 26 �m, the upper bound being in good agreement with theaximum length (28 �m) of PCs in forged D2 steel as reported byei et al. [29].
.1.5. Comparison of secondary carbides in QT and QCT00pecimens
The variations in the amounts of LSCs and SSCs with holdingime at 77 K for QCT specimens are depicted in Fig. 5(b) and the vari-tions of their mean diameter and population density with holdingime are shown in Fig. 6; the variation of the similar microstructuralharacteristics for the carbide particles in QT specimens are alsohown in these figures. A comparative assessment of the character-stics of SCs, LSCs and SSCs of QCT00 with respect to QT specimensssists to infer that: (i) the amounts of LSCs and SSCs increase bypproximately 22.3% and 11.4%, respectively (Fig. 5(b)), and (ii) theopulation density of SSCs increases by almost 250% associatedith the reduction in their mean diameter by approximately 34%
Fig. 6(a)), (iii) the population density of LSCs almost doubles andhe mean diameter decreases by ∼23% (Fig. 6(b)). These results thusnambiguously indicate that cryogenic processing refines the SCs.
.1.6. Effect of holding time on microstructure of QCT specimensThe results in Figs. 5 and 6 related to the characteristic on SCs,
SCs and SSCs versus holding time at 77 K during cryotreatmentssist to infer that:
carbides (SCs) and tempered martensite phases, and (b) small secondary carbidestreatment (QCT). Data at negative holding time are for conventionally treated (QT)
302 D. Das et al. / Wear 266 (2009) 297–309
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sp[caamform distribution of SC particles by cryotreatment, as evident fromFig. 3. The variation of HV0.05 with duration of cryotreatment fur-ther shows that the maximum microhardness is obtained for QCT36specimen. This observation is in accordance with the character-
ig. 6. Variations of mean diameter and population density of (a) small secondary7 K during cryotreatment (QCT). Data at negative holding time refer to that for theQT) samples.
(i) The nature of variation of the amounts of SSCs andLSCs (Fig. 5(b)) is significantly different from that of SCs(=LSCs + SSCs) as shown in Fig. 5(a). The volume fraction ofSCs appears to saturate after soaking time of 12 h, whereas theamount of SSCs increases up to the soaking time of approxi-mately 36 h and then decreases with further holding time at77 K; however, the amount of LSCs increases continuously.
(ii) The variation of the population density with holding time forboth LSCs and SSCs is nearly identical in nature (Fig. 6), unliketheir variations of volume fraction (Fig. 5(b)). For both types ofcarbides, the population density sharply increases up to hold-ing time of 12 h at 77 K, has minor variation between 12 and36 h, and exhibits monotonic decrease beyond 36 h (Fig. 6).
iii) The variation of the mean diameter of both LSCs and SSCs withholding time in cryotreatment follows a reverse trend as com-pared to the variation of population density with holding time(Fig. 6). The size of SSCs increases continuously with holdingtime up to 12 h, has minor variation in between 12 to 36 h fol-lowed by monotonic increase; while the size of LSCs remainsalmost invariant with holding time up to 36 h followed by rapidincrease (Fig. 6). These results suggest that increased holdingtime in cryotreatment leads to growth of both SSCs and LSCs.
The above results assist to conclude that the holding time at 77 Kn cryotreatment has significant effect on the precipitation behaviorf SCs, and the specimen cryotreated for 36 h offer the optimumize and population density of SCs for enhancing the mechanicalroperties of D2 steel.
.2. Influence of heat treatment on hardness
The variations of bulk hardness (HV60) and microhardnessHV0.05) of QT and QCT specimens with duration of holding at 77 Kre shown in Fig. 7. The bulk hardness of QCT samples has been
ound to be at least 4.2% higher than that of QT specimens and is ingreement with the earlier reports [9–11,13,30–35].
The nature of the variation of microhardness of the matrixf QCT specimens with holding time at 77 K is similar to thatf bulk hardness (Fig. 7). The magnitudes of HV0.05 for all the
Fhns
des (SSCs) and (b) large secondary carbides (LSCs) as functions of holding time atles, which are not subjected to cryogenic processing, i.e., for conventionally treated
amples are higher than their corresponding bulk hardness; thishenomenon can be simply attributed to the indentation size effect36,37]. The microhardness of QCT00 specimens increases signifi-antly (9.1%) over that of the QT specimen. The standard deviationsssociated with the microhardness values of the QCT specimensre lower than that of QT sample, which is indicative of bettericrostructural homogeneity of the former due to improved uni-
ig. 7. Influence of holding time during cryotreatment at 77 K on Vickers bulk macro-ardness and matrix microhardness of the cryotreated (QCT) specimens. Data ategative holding time are for conventionally treated (QT) samples, which are notubjected to cryogenic processing cycle.
D. Das et al. / Wear 266 (2009) 297–309 303
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ig. 8. Typical representation of cumulative wear volume loss versus sliding distana) 49.05 N and (b) 98.1 N.
stic of SCs (Figs. 5 and 6). The obtained results thus infer thatncrease in bulk and microhardness of the matrix occurs due toryotreatment.
.3. Effect of cryotreatment on wear rates
The wear characteristics of the specimens have been assessed bystimating the volume loss with respect to sliding distance, exam-nation of the worn surfaces and analysis of the wear debris, asescribed in Section 2.5. Cumulative wear volume loss at a particu-
ar sliding distance has been evaluated by multiplying the recordedumulative height loss by the area of the pin specimen. Since the
ear of the WC-coated disc is observed to be insignificant com-ared to that of the pin specimens, the volume loss is consideredo have occurred only due to wear of the pins.
Wear tests have been carried out at different combinations ofN = 49.05 and 98.1 N and VS = 1 and 2 m/s. Typical plots of cumu-
lrir
ig. 9. Variation of wear rate with holding time in cryotreatment (QCT): effect of (a) slidinreated (QT) samples, which are not subjected to cryogenic processing cycle.
differently treated specimens tested at sliding velocity of 2 m/s for normal load of
ative wear volume (Wv) loss versus sliding distance at FN of 49.05nd 98.1 N are shown in Fig. 8, which exhibits both the regimesf ‘running-in’ and ‘steady-state’ wear [38,39]. Steady-state wearas been further examined to reveal the effect of cryotreatmentn the tribological behavior of AISI D2 steel. The results in Fig. 8xhibit that Wv loss for QT specimens in the steady-state regimes considerably higher than those of the QCT specimens at all theelected combinations of test conditions. The effect of FN on Wv losss more pronounced for QCT specimens compared to QT specimens.he variations in the Wv loss for identical sliding distances but forifferent FN are illustrated for QT, QCT00 and QCT36 specimens inig. 8.
The wear rates (WR) have been estimated from cumulative Wv
oss per unit sliding distance [39] corresponding to the steady-stateegime. The estimated values of WR for the specimens are compiledn Fig. 9(a) and (b) for the conditions of constant VS and constant FN,espectively. The WR of all the specimens increase with the increase
g velocity and (b) normal load. Data at negative holding time are for conventionally
3 r 266
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n either FN or VS (Fig. 9). The results in Fig. 9 can be summarizeds:
(i) The WR of QT specimen is significantly higher than that of theQCT specimens.
(ii) The WR of QT specimen increases from 3.2 × 10−2 to32.8 × 10−2 mm3/m when the FN is increased from 49.05 to98.1 N (Fig. 9(a)) at constant VS of 2 m/s.
iii) The WR of QT specimens (at 98.1 N) increases 6.5 times whenthe VS is increased from 1 to 2 m/s (Fig. 9(b)).
iv) The WR of cryotreated specimens first decreases with increasein holding time up to 36 h and then increases with further
increase in holding time (Fig. 9). This implies that QCT36 spec-imens exhibit the highest wear resistance amongst all thespecimens considered in the present investigation.
(v) The increase in FN from 49.05 to 98.1 N, increases WR of QCTspecimens by 136.9–193.1 times, whereas doubling the VSincreases WR of these specimens by 25.9–31.8 times.
iawrQm
ig. 10. SEM micrographs of typical worn surfaces generated under wear tests at 98.1ub-surface cracking, (c) deformation lip and (d) fractured ridges of QT specimen, whereaepicting delamination of carbides in QCT00 specimen.
(2009) 297–309
vi) Wear resistance of QCT36 specimens is 13.2, 3.3 and 76.2 timesmore than QT specimen for the combinations of FN and VS as‘98.1 N and 1 m/s’, ‘98.1 N and 2 m/s’, and ‘49.05 N and 2 m/s’,respectively.
The above observations suggest that the variation in WR of QTpecimens is much less compared to that of QCT specimens due tohe variation of either FN or VS.
.4. Characteristics of worn surfaces and wear debris
The operative wear mechanisms have been examined by analyz-ng the morphology of the worn-out surfaces of the pin specimens
nd the collected wear debris generated during the steady-stateear regime under different test conditions. The salient features
elated to the morphology of the worn-out surfaces of QT andCT00 specimens are illustrated in Fig. 10. The nature, size, andorphology of the wear debris generated during wear tests of QT
N of normal load and 1 m/s of sliding velocity: (a) overview of QT specimen, (b)s (e) overview of QCT00 specimen and (f) oxides with surface grooves and features
D. Das et al. / Wear 266 (2009) 297–309 305
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ig. 11. Comparison of wear debris generated in the steady-state wear regime betwelocity (VS). All micrographs taken at the same magnification of 250×, but the iicrographs. Insets in (b) and (f) are at 2500×, and (c) and (d) at 50× magnification
nd QCT00 specimens have been compared in different parts ofig. 11. The morphology of the worn-out surfaces and the charac-eristics of the generated wear debris amongst the QCT specimensested at two different FN at the constant VS = 2 m/s, have beenompared in Fig. 12.
Worn surface of the QCT00 specimen is considerably smootherhan that of the QT specimen (Fig. 10(e) vis-à-vis Fig. 10(a)), whenoth are subjected to FN = 98.1 N and VS = 1 m/s. The wear debris
s fine oxides (Fig. 11(b)) for QCT00 specimen and large metalliclatelets (Fig. 11(a)) for QT specimen. These observations indicatehat the wear resistance of the QCT specimen is significantly higherhan that of the QT specimen; this is in good agreement with thestimated WR (Fig. 9).
. Discussion
.1. Microstructural modulations through cryotreatment
The results in Figs. 5 and 6 reveal that cryotreatment causesarked reduction in the amount of �R and significant alteration
ecttQ
T and QCT00 specimens at different combinations of normal load (FN) and slidingat different magnifications represent the detailed features of debris of the same
n the precipitation behavior of the SCs. The variation in the char-cteristics of carbide particles between QT and QCT specimensan be explained as follows. At the early stage of tempering SSCsucleate in both QT and QCT specimens, but this phenomenon
s not sufficient enough to explain the difference between thebserved results in Figs. 5 and 6. Transformation of austeniteo martensite at cryogenic temperature followed by prolongedolding induces micro-internal stresses which results in the for-ation of crystal defects such as dislocations and twins [2,6,17,18].hile, lattice distortion and thermodynamic instability of marten-
ite at 77 K drive carbon and alloying atoms to segregate athe nearby crystal defects. These segregated regions have beenypothesized as the newer sites for nucleation of SSCs [10]. The
ncrease in the population density of SSCs in QCT specimensompared to QT specimens thus gets explained on the consid-
rations of these phenomena. Further, increased number of sitesan only allow the precipitates to grow in a limited manner forhe constant amount of available carbon atoms, which explains, inurn, the difference in the size of the carbide particles betweenT and QCT specimens. The LSCs can thus be considered as
306 D. Das et al. / Wear 266 (2009) 297–309
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ig. 12. SEM micrographs of worn surfaces of cryotreated (QCT) specimens at the eepresent the wear debris generated at the steady-state wear regime for the corresp
category of SCs having higher growth in localized environ-ents.The variation in the amount of SSCs and LSCs amongst the QCT
pecimens is governed by several factors; like kinetics of precipi-ation, initial defect generation in the martensite, mobility of thenterstitial and substitutional elements, and dissolution followedy precipitation of carbide particles at the selected tempering tem-erature. Due to lack of sufficient information on these factors, it
s difficult to explain the exact cause for the nature of variation inize and population density of the carbide particles in cryotreatedpecimens. The obtained results in Fig. 6, however, unambiguouslyead to conclude that the most favorable combination of the sizend population density is obtained for specimens held for 36 h at7 K.
.2. Wear behavior of QCT vis-à-vis QT specimens
A new parameter, ˇ, defined as the ratio of the WR of QT spec-men to that of QCT00 specimen, has been considered to comparehe degree of improvement in wear resistance by cryotreatment.
rpmap
wear test, carried out at 2 m/s sliding velocity under different normal loads. Insetsg tests.
he magnitudes of ˇ are 22.21, 7.91 and 1.61 for the test condi-ions ‘FN = 49.05 N and VS = 2 m/s’, ‘FN = 98.1 N and VS = 1 m/s’ andFN = 98.1 N and VS = 2 m/s’, respectively (Fig. 13). The results indi-ate that for constant VS, the benefit of cryotreatment on wearesistance is reduced drastically with increasing FN; while for con-tant FN, the improvement in wear resistance is less pronouncedith increasing VS. Within the investigated range of wear testarameters, the effect of variation of FN is more pronounced onhe WR than the effect of the variation of SV (Fig. 13). The extent ofchievable benefit by cryotreatment is thus significantly dependentn the wear test conditions. The values of ˇ for M2 steel calculatedrom the reports of Molinari et al. [11], Barron [5] and Mohan Lalt al. [12] are 1.68 (FN = 150 N and SV = 0.8 m/s), 3.03 (FN = 430 N andV = 0.48 m/s) and 2.28 (FN = 50 N and SV = 0.366 m/s), respectively.hus, the available wear data related to the improvement of wear
esistance of tool/die steels by cryotreatment do not converge torovide any guideline to assess the magnitudes of the improve-ent in quantitative terms [1,3,10,11,33]. This uncertainty can be
ttributed to the employment of different types of experimentalrocedures and employed test parameters (Table 1). The improve-
D. Das et al. / Wear 266
Fig. 13. Wear rate ratio (ˇ) for different combinations of wear test conditions. ˇis the ratio of wear rate of QT specimen (WQT
R ) to wear rate of QCT00 specimen
(WQCT00R ).
Fo
w
mooa
4
oamwT
ai(abeewrtoaafitoW
4
iaratit
4
mwlQawimcteFett
TS
S
QQQQQQ
F
ig. 14. Wear rate ratio (ˇ′) as a function of holding time for different combinationsf wear test conditions. ˇ′ is the ratio of wear rate of QCT00 specimen (WQCT00
R ) to
ear rate of other QCT specimens (WQCTR ).
ent in the wear resistance of the cryotreated specimens over thatf the conventionally treated one is attributed here to the absencef �R (Fig. 5(a)) coupled with the finer distribution of higher amountnd number of SC particles (Figs. 5(b) and 6).
.3. Influence of holding time of cryotreatment on wear rates
The influence of holding time at 77 K on the improvement
f wear resistance of the selected steel has been revealed usingnother parameter, ˇ′, defined as the ratio of WR of QCT00 speci-en to the WR of any of the QCT specimens. The variations of ˇ′
ith holding time for different test conditions are shown in Fig. 14.he results in Fig. 14 lead to infer that: (i) the magnitude of ˇ′ is
o
Vfw
able 3ummary of wear rates and the wear rate ratio parameters
lways greater than one, i.e., cryotreatment with some holding timenduces better wear resistance than that with no holding time andii) the magnitude of ˇ′ increases up to the holding time of 36 hnd then it decreases. The increase in wear resistance of tool steelsy cryotreatment with increasing holding time has been reportedarlier by Mohan Lal et al. [12], Collins and Dormer [13] and Yunt al. [17]. The present results of ˇ′ up to 36 h are in agreementith the earlier reports [2,12,13,17]. However, the results in Fig. 14
eveal, for the first time, that there exists a critical holding time inhe cryotreatment of D2 steel for obtaining the best combinationf desired microstructure and wear property of die/tool steels. Anttempt to correlate the results in Fig. 14 with those in Figs. 5 and 6ppears to establish the fact that the larger number of SCs and theirner sizes are the key factors for the improvement in wear resis-ance in cryotreated specimens and in delineating the critical timef holding. For the convenience of the reader, the detailed data of
R, ˇ, ˇ′ are compiled in Table 3.
.4. Revelation of the wear mechanisms
The operative wear mechanisms have been examined by analyz-ng the morphology of the worn-out surfaces of the pin specimensnd the collected wear debris generated during the steady-stateegime of wear under different test conditions. The operative mech-nisms have been discussed in different sub-sections to illustratehe difference in the wear mechanisms between QT and QCT spec-mens, and amongst the QCT specimens followed by comments onhe generalized wear behavior.
.4.1. Wear mechanism: QT vis-à-vis QCT specimensThe presence of severe subsurface cracking (Fig. 10(b)), defor-
ation lips (Fig. 10(c)), and fractured ridges (Fig. 10(c)) in theorn surfaces and generation of debris in the form of large metal-
ic platelets (Fig. 11(a)) establish the operative mechanism forT specimens as severe delamination wear [40] under FN = 98.1 Nnd VS = 1 m/s. Similar morphology of the worn surfaces and theear debris (Fig. 11(c) and (e)) are also encountered for QT spec-
mens subjected to other test conditions. The delamination wearechanism is found to be operative in the QT specimens for all
ombinations of the investigated wear test conditions. However,he size of the metallic debris is found to increase with increase inither VS (Fig. 11(a) vis-à-vis Fig. 11(c)) or FN (Fig. 11(e) vis-à-visig. 11(c)). These observations are also in good agreement with thestimated WR as shown in Fig. 9. These results lead to infer thathe QT specimens undergo severe plastic deformation during wearests. This has been attributed to the presence of significant amount
f soft �R in the microstructure.
Worn surfaces of QCT00 specimen under FN = 98.1 N andS = 1 m/s in Fig. 10(e) and (f) exhibit the presence of oxides and sur-
ace grooves due to pull-out of hard PC particles. Thus, the operativeear mechanism for QCT00 specimens is predominantly oxidative
ear coupled with pull-out of carbides, and the mode of wear isild [41,42]. Thus operative mechanism and mode of wear for QCT
pecimens is mild oxidative in contrast to severe delamination forT specimens under similar test conditions. The results in Fig. 11(e)is-à-vis Fig. 11(f) provide similar comparison for the wear behaviorf QT and QCT specimens under FN = 49.05 N and VS = 2 m/s. Underhese test conditions, the estimated wear resistance of QCT speci-
ens is found to be at least an order of magnitude higher than thatf QT specimen (Figs. 9 and 13).
When wear tests have been carried out under the conditionFN = 98.1 N and VS = 2 m/s’, delamination wear is found to be oper-tive for both QT and QCT00 specimens. This is evident from theeneration of the similar nature of wear debris as large metalliclatelets in both QT and QCT00 samples, as shown in Fig. 11(c) andd). Thus, under this test condition, both QT and QCT specimensxperience severe mode of wear and the recorded improvement inear resistance of QCT00 compared to QT is only 60% (Fig. 13).
The delamination wear in QT specimens and the predominantlyxidative wear in the QCT specimens can be correlated with theicrostructural features. Delamination wear sequentially consti-
utes plastic deformation of surface layer, crack nucleation andrack propagation [40]. The QT specimens possessing significantmount of �R is prone to plastic deformation which assists in easyucleation of cracks, and hence leads to delamination wear underll the investigated test conditions. Conversely, the microstructuralonstituents of QCT specimens hinder plastic deformation and getubjected to oxidative wear at less severe combination of applied FNnd VS. But, at higher FN and higher VS, crack nucleation occurs withimited amount of plastic deformation and leads to severe wear40,41].
.4.2. Wear mechanism in varied QCT specimensThe mode and mechanism of wear for all QCT specimens are
severe delamination’ at FN = 98.1 N and VS = 2 m/s, and ‘mild oxida-ive’ for the other two test conditions; and the best wear resistanceeing obtained for QCT36 specimen. Correlation of WR with theorn-out surfaces and wear debris amongst QCT specimens are
hus made with one sample having holding time <36 h and anotherne with holding time >36 h with respect to QCT36 specimenFig. 12). Comparative assessment of the results depicted in Fig. 12ndicates that under identical test conditions QCT36 specimens suf-ered minimum surface damage and exhibited finest size of theear debris. This assessment corroborates well with the inferencerawn from Figs. 9 and 14 that the specimen cryotreated for 36 hxhibit the highest wear resistance. The increase in wear resistancef QCT specimens with holding time up to 36 h is attributed to thencreased amount of SCs in the microstructure which appear toeach a steady-state value around 36 h (Fig. 5). Similar trend waslso observed for the variation of macrohardness of QCT specimensith holding time at 77 K (Fig. 7). Beyond 36 h of holding in cry-
treatment, the volume fraction of SCs remains almost constantut the size of the SCs, particularly that for SSCs increases withoncurrent reduction of their population density (Fig. 6). Theseicrostructural changes are considered to be responsible for the
eduction of strength of the matrix of QCT specimens held at 77 Keyond 36 h as evident from the results in Fig. 7. Therefore, thebtained results from the wear tests in this investigation indicatehat the wear resistance of differently cryotreated specimens islosely related to the developed microstructures as well as theesultant hardness and microhardness values.
.4.3. Comments on the generalized wear behaviorAnalyses of the morphology of worn-out surfaces and wear
ebris in the preceding sub-sections strongly support the infer-nce of significant improvement in wear resistance of cryotreated
(2009) 297–309
ISI D2 steel specimens compared to conventionally treated ones.he results in Figs. 9 and 13 are in good agreement with the exist-ng general consensus that the wear resistance gets significantlynhanced in die/tool steels by cryotreatment [1–5,8–15]. Further-ore, the results of the present investigation also infer that: (i) the
egree of improvement of wear resistance is considerably depen-ent upon the test conditions and (ii) the highest improvement inear resistance for AISI D2 steel can be attained by cryogenic treat-ent with a holding time of 36 h at 77 K. The results presented in the
receding sub-sections are thus unique in their detailed treatmento reveal the operative wear mechanisms of the cryotreated speci-
ens. These also provide excellent guidelines for possible futuristicuantitative analysis of the wear debris and surface morphology ofryotreated specimens in order to bring forth finer details related tohe operative mode and mechanisms under varied test conditions.
. Conclusions
The wear behavior of a series of AISI D2 die steel specimens, cry-treated for different holding periods at 77 K, has been examinedo probe the micro-mechanism of wear and to find out the criticaluration of cryotreatment to achieve the best possible wear resis-ance. The obtained results and their pertinent discussion assist tonfer the following:
1) The wear resistance of the AISI D2 steel gets considerablyenhanced by cryotreatment, compared to that of the conven-tionally treated one, irrespective of the time of holding at77 K. The extent of improvement of wear resistance, however,is dependent on the wear test conditions, which control theactive mechanisms and mode of wear. Severe mode of wear isidentified as delamination of metallic particles caused by sub-surface cracking due to extensive plastic deformation, whereasmild mode of wear is characterized as predominantly oxidativein nature associated with pull-out of primary carbides and/orbreak-down of oxide layer.
2) The marked improvement in wear resistance of the cryotreatedspecimens compared to the conventionally treated ones isattributed to the near absence of retained austenite and morehomogeneous distribution of a larger number of finer sec-ondary carbides in the former specimens. However, the degreeof improvement depends on the test conditions. Hardness of theinvestigated steel samples is found to increase marginally bycryotreatment in contrast to significant increase in their wearresistance.
3) The mode of wear and the operative wear mechanism are iden-tical for all of the cryotreated specimens at the selected testconditions. However, the mode of wear changes from mild tosevere in the cryotreated specimens due to increase in (a) nor-mal load from 49.05 to 98.10 N at a constant sliding velocityof 2 m/s and (b) sliding velocity from 1 to 2 m/s at a constantnormal load of 98.10 N.
4) Within the investigated range, the wear resistance of the cry-otreated specimens increases with increasing holding time upto 36 h at 77 K beyond which it shows monotonic decrease withfurther increase in holding time. The variation in wear resis-tance corroborates excellently with the changes of amount,size, population density and morphological characteristics ofthe secondary carbides as a function of holding time during cry-otreatment. Thus, unlike the popular postulation that increase
in holding time during cryotreatment monotonically improveswear properties, the present results exhibit a critical value ofholding time (36 h at 77 K for AISI D2 steel) for obtaining the bestcombination for the desired microstructures and wear proper-ties of die steels.
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D. Das et al. / Wea
cknowledgementsThe financial assistance received from the University Grants
ommission, Government of India [Grant no. F. No. 31-48/2005(SR)]o carry out a part of this research is gratefully acknowledged.
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