298 D. Das et al. / Wear 266 (2009) 297–309Table 1Summary of the different test parameter used to evaluate the dry sliding wear resistance of cryotreated tool/die steelsSl. no. Material (AISI speciﬁcation) Shape of the Counter body Wear test parameters References sample Shape Material Normal load (N) Sliding velocity Total sliding (m/s) distance (m)1. M2, M1, T2, T1, H13, D2, A10, Pin Wheel Coarse grit alumina 430 0.48 2160  A6, O1, P20, S7, etc. grinding wheel2. D2 Block Wheel Hardened D2 steel 21 0.50–3.62 200, 400, 600 3. D2 Pin Disc WC-coated En-35 steel 49, 69, 78 1.50 900 4. M2, H13 Disc Disc Hardened 100Cr6 150 0.80 5000 5. M2, D3 Pin Disc Grinding wheel 20, 30, 50 0.18–0.60 324–1080 6. M2 Disc Ball Silicon nitride 50 0.027 200 7. M2 Pin Disc Alumina abrasive paper 10 0.11 3.22  (800 and 80 mesh) In addition, one of the major uncertainties associated with the 2. Experimental procedureearlier investigations related to cryotreatment of tool steels is theduration of cryotreatment at the selected temperatures [1–4,20]. 2.1. MaterialThe existing literature does not provide any guideline related tothe selection of time duration for cryotreatment [3,10]. The hold- The selected steel has been obtained as a commercial hot forgeding time in cryogenic processing has been varied widely by earlier bar and its chemical composition in wt.% is 1.49, C; 0.29, Mn;investigators. For example, holding time employed in the cryotreat- 0.42, Si; 11.38, Cr; 0.80, Mo; 0.68, V; 0.028, S; 0.029, P; balance-ment for AISI M2 steel is 1 h by Leskovsek et al. , 20 h by de Silva Fe. This composition conforms to the AISI speciﬁcation of D2et al. , 35 h by Molinari et al.  and 168 h by Huang et al. . steel.Such wide variation in the selected holding time even for the samematerial is due to the lack of systematic investigation related to the 2.2. Heat treatmentsinﬂuence of holding time on the wear resistance of tool/die steelsby cryotreatment. Thus another major aim of this report is to unfold Specimen blanks of 24 mm × 16 mm × 85 mm dimension werethe inﬂuence of the duration of cryotreatment on the enhancement subjected to conventional and cryotreatment in separate batches.of wear resistance at varying experimental conditions. The conventional treatment (QT) consisted of hardening (Q) and In order to fulﬁll the goals of this investigation, specimens of single tempering (T), and was done as per ASM Heat Treater’s guideAISI D2 steel were subjected to ﬁve different holding times dur- . Deep cryogenic processing (C) was incorporated intermediateing cryotreatment apart from the conventional heat treatment. between hardening (Q) and tempering (T) in cryotreatment (QCT),The characteristics of R , primary and secondary carbides have the details of each step being illustrated in Fig. 1. The cryogenicbeen assessed together with the determination of macrohardness, processing was done by uniform cooling of the samples to 77 K, andmicrohardness and wear rates by standard experimental proce- holding the samples at this temperature for different time durationsdures. The analyses of the wear behavior with respect to the (0, 12, 36, 60 and 84 h), followed by uniform heating to room tem-generated microstructures, hardness characteristics, morphology perature. A typical deep cryogenic processing cycle is illustratedof the worn-out surfaces and wear debris have assisted to reveal the in Fig. 1(b). The specimens subjected to different treatments areunderlying mechanisms for the improvement in wear resistance by referred henceforth with codes as shown in Table 2, where thecryotreatment and throw light on the wild scatter of the reported numerals in the codes represent the time of holding in hour atimprovement in wear resistance of tool/die steels by cryotreatment. 77 K.Fig. 1. (a) Schematic representation of the heat treatment schedule consisting of hardening (Q), deep cryogenic processing (C) and tempering (T) cycles, and (b) typicaltime-temperature proﬁle of a deep cryogenic processing cycle.
D. Das et al. / Wear 266 (2009) 297–309 299Table 2 in the extracted carbide particles and bulk specimens were iden-Sample codes for differently heat treated specimens tiﬁed from the XRD proﬁles with the help of PHILIPS X’PertSample code Description of heat treatment cycles software. Hardening Duration of deep cryogenic Tempering processing at 77 K (h) 2.4. Hardness measurementQT –QCT00 0 The macro- and microhardness values of the developed spec-QCT12 12 imens were determined using 60 kgf and 50 gf load, respectively. 1293 K, 0.5 h 483 K, 2 hQCT36 36QCT60 60 Indentations for microhardness measurement of the matrix wereQCT84 84 taken carefully avoiding the easily separable PCs, but this value is inﬂuenced by the characteristics of SCs. At least 10 readings are considered for estimating the average value of macro- hardness, whereas a minimum of 50 readings are taken to2.3. Microstructural characterization estimate the average value of microhardness of the specimen matrix. Sample blanks (10 mm × 10 mm × 7 mm) for metallographicexaminations were cut using wire electro-discharge machining.These were polished and ﬁnally lapped by diamond paste of 1 m 2.5. Evaluation of wear behaviorsize prior to etching with picral solution (3 gm picric acid in 100 mlethanol), and digital micrographs were recorded using both optical The study related to wear behavior of the cryotreated steels has(Axiovert 40 MAT, Carl Zeiss, Switzerland) and scanning electron been done to assess (i) wear rate, (ii) morphology of wear debrismicroscope (JSM-5510, JEOL, Japan). The microstructures exhibit and (iii) the characteristics of the worn-out surfaces of the speci-carbide particles in a matrix of tempered martensite. The carbide mens and to compare these features with those of conventionallyparticles have been classiﬁed as primary carbides (PCs: size > 5 m) treated samples. Dry sliding wear tests following ASTM standardand secondary carbides (SCs: size ≤ 5 m). The SCs are further sub- G99-05  were carried out by using a computerized pin-on-discclassiﬁed as large secondary carbides (LSCs: 1 m < size ≤5 m) wear-testing machine (TR-20LE, DUCOM, India). Cylindrical speci-and small secondary carbides (SSCs: 0.1 m ≤ size < 1 m). Image mens of 4 mm diameter and 30 mm length were used as static pins,analyses of the microstructures were done using Leica QMetals soft- whereas, tungsten carbide coated hardened and tempered En-35ware to estimate (i) the volume fraction and size of PCs, LSCs and steel disc of 8 mm thickness and 160 mm diameter was selected asSSCs and (ii) the population density of LSCs and SSCs. The number the rotating counter surface having Hv ∼ 1750 and Ra < 0.5 m. Theof carbide particles considered for quantitative characterizations pins were machined from the suitably heat-treated specimens byis >1000 in order to estimate the stereological parameters with using wire-EDM and the faces of the pins were polished to rough-signiﬁcant statistical reliability. ness, Ra < 0.1 m. The wear tests were carried out using normal X-ray diffraction (XRD) analyses of the generated microstruc- loads (FN ) of 49.05 (5 kgf) and 98.1 N (10 kgf) at sliding velocitytures were done by using an X-ray diffractometer (PW 1830, (VS ) of 2 m/s as well as under FN = 98.1 N (10 kgf) at VS = 1 m/s. ThePHILIPS, USA) with Mo K␣ radiation at 0.01 degree/min scan rate. wear-testing machine was interfaced with a computer, which con-The volume fraction of R was estimated in accordance with ASTM tinuously recorded the height loss of the pin and the friction forcestandard E975-00  considering the diffraction peaks of (1 1 0), at the pin–disc interface. The wear rate (WR ) was estimated by the(2 0 0), (2 1 1) and (3 1 0) of martensite and (1 1 1), (2 0 0), (2 2 0) volume loss method. These tests were repeated until at least threeand (3 1 1) of R . Identiﬁcation of the exact nature of the carbides consistent readings were obtained for each set of test condition toin the heat-treated samples was difﬁcult by XRD analysis of the estimate the average WR .bulk specimens due to their small amount . Therefore, car- The worn-out surfaces of the pins were cleaned in acetone usingbide particles were electrolytically extracted from both QT and an ultrasonic cleaner for 10 min and were subsequently examinedQCT specimens following the report of Nykiel and Hryniewicz under a SEM to identify the possible wear mechanisms. The wear. XRD analysis of the extracted carbide particles were done debris were collected during wear tests and were subjected to mor-in an identical manner to that for bulk specimens. The phases phological characterization under SEM.Fig. 2. Typical optical micrographs of (a) QT and (b) QCT60 specimens. The microstructures revealed by etching with picral solution exhibit carbides: white; and temperedmartensite: black (PC, primary carbide; SC, secondary carbide).
300 D. Das et al. / Wear 266 (2009) 297–309Fig. 3. Typical SEM micrographs of QT and QCT specimens exhibiting size, morphology and distribution of small secondary carbides (SSC) and large secondary carbides (LSC):(a) QT, (b) QCT12, (c) QCT36 and (d) QCT84 samples.3. Results bides (PCs) and secondary carbides (SCs) in the form of either white spherical or tiny black patches on tempered martensite matrix.3.1. Microstructures of differently heat-treated specimens Fig. 3 shows a series of typical SEM micrographs, which reveal the distinguished nature of SCs in QT and QCT specimens. Two types of3.1.1. Microstructural constituents SCs (white regions and black patches) in Fig. 2 get manifested with Fig. 2 depicts typical representative optical microstructures for almost identical grey level in Fig. 3; these carbides belong to two dif-QT and QCT specimens, which exhibit large dendritic primary car- ferent size ranges, referred to as LSC and SSC. Fig. 3 also illustratesFig. 4. X-ray diffraction line proﬁles of (a) bulk specimens and (b) electrochemically extracted carbide particles of QT, QCT00 and QCT36 samples: The set of (h k l) in verticaldirection indicates the 2Â positions of different diffraction planes of martensite, austenite, M7 C3 , Cr7 C3 and M23 C6 carbides.
D. Das et al. / Wear 266 (2009) 297–309 301that the QCT specimens possess considerably larger numbers of in literature  too. The SCs in both QT and QCT specimens haveSSCs compared to that of the QT specimen. It appears from Fig. 3(b) been identiﬁed as M23 C6 (M = Fe, Cr, Mo, V) type in agreement withto (d) that the number, size and amount of both SSCs and LSCs vary earlier reports [10,23,27]. It is interesting to note that the applica-considerably with the time of holding at 77 K. tion of deep cryogenic processing immediately after conventional The different phases in the microstructures have been identi- hardening does not alter the nature of PCs and SCs.ﬁed by XRD analyses (Fig. 4). The amounts of the microstructuralconstituents (Fig. 5) for all of the heat-treated specimens have been 3.1.4. Amount of primary carbidesestimated in the following manner. The volume fraction of R has The amounts of PCs are similar in QT and QCT specimensbeen estimated by XRD analyses, the volume fractions of PCs, LSCs, (Fig. 5(a)). The mean volume fraction of PCs for all types of sam-SSCs and SCs (LSCs + SSCs) have been determined by image anal- ples can be represented as 7.3 ± 0.4%, which is in good agreementyses on digital micrographs and the volume fraction of tempered with the value of 7.8% as reported by Fukaura et al.  for a sim-martensite has been considered as 100 minus the volume percent- ilar steel. The lengths of the major axis of the PCs range betweenage of R and that of all types of carbides. 5 and 26 m, the upper bound being in good agreement with the maximum length (28 m) of PCs in forged D2 steel as reported by3.1.2. Measurement of retained austenite content Wei et al. . Typical representative XRD line proﬁles of three bulk specimensQT, QCT36 and QCT60 are shown in Fig. 4(a). The prominent pres-ence of the (2 2 0) and (3 1 1) peaks of R in the XRD proﬁles of 3.1.5. Comparison of secondary carbides in QT and QCT00QT samples and their indistinct appearance in the XRD proﬁles of specimensQCT specimens assist to compare the amounts of R in these spec- The variations in the amounts of LSCs and SSCs with holdingimens qualitatively. The average volume fraction of R is found to time at 77 K for QCT specimens are depicted in Fig. 5(b) and the vari-be 9.8 ± 0.7% in QT specimen (Fig. 5(a)). These results suggest that ations of their mean diameter and population density with holdingdeep cryogenic processing in between hardening and tempering time are shown in Fig. 6; the variation of the similar microstructuralalmost completely converts the R in D2 steel to martensite. This characteristics for the carbide particles in QT specimens are alsoobservation is in excellent agreement with the reported results for shown in these ﬁgures. A comparative assessment of the character-cryotreated AISI D2 [10,13,26], M2 [11,12,14,17–19] and 52100  istics of SCs, LSCs and SSCs of QCT00 with respect to QT specimenssteels. assists to infer that: (i) the amounts of LSCs and SSCs increase by approximately 22.3% and 11.4%, respectively (Fig. 5(b)), and (ii) the population density of SSCs increases by almost 250% associated3.1.3. Identiﬁcation of carbide particles with the reduction in their mean diameter by approximately 34% The intensities of the XRD peaks for the different carbide parti- (Fig. 6(a)), (iii) the population density of LSCs almost doubles andcles are very weak in the line proﬁle of bulk specimens in Fig. 4(a). the mean diameter decreases by ∼23% (Fig. 6(b)). These results thusHence, the nature of these particles has been examined using the unambiguously indicate that cryogenic processing reﬁnes the SCs.ones obtained by electrolytic extraction (Fig. 4(b)). The PCs in bothQT and QCT specimens have been identiﬁed mainly as M7 C3 withsmall amount of Cr7 C3 by XRD analyses (Fig. 4(b)). The M7 C3 car- 3.1.6. Effect of holding time on microstructure of QCT specimensbide is the main eutectic carbide for AISI D2 steel [25–30]. The EDX The results in Figs. 5 and 6 related to the characteristic on SCs,microanalysis reveals the chemical composition of M7 C3 carbides LSCs and SSCs versus holding time at 77 K during cryotreatmentas (Fe28 Cr39 V2 Mo1 )C30 , which is in good agreement with the data assist to infer that:Fig. 5. Variations of amount of (a) retained austenite, primary carbides (PCs), secondary carbides (SCs) and tempered martensite phases, and (b) small secondary carbides(SSCs) and large secondary carbides (LSCs) as functions of holding time at 77 K during cryotreatment (QCT). Data at negative holding time are for conventionally treated (QT)samples, which are not subjected to cryogenic processing cycle.
302 D. Das et al. / Wear 266 (2009) 297–309Fig. 6. Variations of mean diameter and population density of (a) small secondary carbides (SSCs) and (b) large secondary carbides (LSCs) as functions of holding time at77 K during cryotreatment (QCT). Data at negative holding time refer to that for the samples, which are not subjected to cryogenic processing, i.e., for conventionally treated(QT) samples. (i) The nature of variation of the amounts of SSCs and samples are higher than their corresponding bulk hardness; this LSCs (Fig. 5(b)) is signiﬁcantly different from that of SCs phenomenon can be simply attributed to the indentation size effect (=LSCs + SSCs) as shown in Fig. 5(a). The volume fraction of [36,37]. The microhardness of QCT00 specimens increases signiﬁ- SCs appears to saturate after soaking time of 12 h, whereas the cantly (9.1%) over that of the QT specimen. The standard deviations amount of SSCs increases up to the soaking time of approxi- associated with the microhardness values of the QCT specimens mately 36 h and then decreases with further holding time at are lower than that of QT sample, which is indicative of better 77 K; however, the amount of LSCs increases continuously. microstructural homogeneity of the former due to improved uni- (ii) The variation of the population density with holding time for form distribution of SC particles by cryotreatment, as evident from both LSCs and SSCs is nearly identical in nature (Fig. 6), unlike Fig. 3. The variation of HV 0.05 with duration of cryotreatment fur- their variations of volume fraction (Fig. 5(b)). For both types of ther shows that the maximum microhardness is obtained for QCT36 carbides, the population density sharply increases up to hold- specimen. This observation is in accordance with the character- ing time of 12 h at 77 K, has minor variation between 12 and 36 h, and exhibits monotonic decrease beyond 36 h (Fig. 6).(iii) The variation of the mean diameter of both LSCs and SSCs with holding 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 holding time up to 12 h, has minor variation in between 12 to 36 h fol- lowed by monotonic increase; while the size of LSCs remains almost invariant with holding time up to 36 h followed by rapid increase (Fig. 6). These results suggest that increased holding time in cryotreatment leads to growth of both SSCs and LSCs. The above results assist to conclude that the holding time at 77 Kin cryotreatment has signiﬁcant effect on the precipitation behaviorof SCs, and the specimen cryotreated for 36 h offer the optimumsize and population density of SCs for enhancing the mechanicalproperties of D2 steel.3.2. Inﬂuence of heat treatment on hardness The variations of bulk hardness (HV 60) and microhardness(HV 0.05) of QT and QCT specimens with duration of holding at 77 Kare shown in Fig. 7. The bulk hardness of QCT samples has beenfound to be at least 4.2% higher than that of QT specimens and is inagreement with the earlier reports [9–11,13,30–35]. Fig. 7. Inﬂuence of holding time during cryotreatment at 77 K on Vickers bulk macro- The nature of the variation of microhardness of the matrix hardness and matrix microhardness of the cryotreated (QCT) specimens. Data atof QCT specimens with holding time at 77 K is similar to that negative holding time are for conventionally treated (QT) samples, which are notof bulk hardness (Fig. 7). The magnitudes of HV 0.05 for all the subjected to cryogenic processing cycle.
D. Das et al. / Wear 266 (2009) 297–309 303Fig. 8. Typical representation of cumulative wear volume loss versus sliding distance for differently treated specimens tested at sliding velocity of 2 m/s for normal load of(a) 49.05 N and (b) 98.1 N.istic of SCs (Figs. 5 and 6). The obtained results thus infer that lative wear volume (Wv ) loss versus sliding distance at FN of 49.05increase in bulk and microhardness of the matrix occurs due to and 98.1 N are shown in Fig. 8, which exhibits both the regimescryotreatment. of ‘running-in’ and ‘steady-state’ wear [38,39]. Steady-state wear has been further examined to reveal the effect of cryotreatment3.3. Effect of cryotreatment on wear rates on the tribological behavior of AISI D2 steel. The results in Fig. 8 exhibit that Wv loss for QT specimens in the steady-state regime The wear characteristics of the specimens have been assessed by is considerably higher than those of the QCT specimens at all theestimating the volume loss with respect to sliding distance, exam- selected combinations of test conditions. The effect of FN on Wv lossination of the worn surfaces and analysis of the wear debris, as is more pronounced for QCT specimens compared to QT specimens.described in Section 2.5. Cumulative wear volume loss at a particu- The variations in the Wv loss for identical sliding distances but forlar sliding distance has been evaluated by multiplying the recorded different FN are illustrated for QT, QCT00 and QCT36 specimens incumulative height loss by the area of the pin specimen. Since the Fig. 8.wear of the WC-coated disc is observed to be insigniﬁcant com- The wear rates (WR ) have been estimated from cumulative Wvpared to that of the pin specimens, the volume loss is considered loss per unit sliding distance  corresponding to the steady-stateto have occurred only due to wear of the pins. regime. The estimated values of WR for the specimens are compiled Wear tests have been carried out at different combinations of in Fig. 9(a) and (b) for the conditions of constant VS and constant FN ,FN = 49.05 and 98.1 N and VS = 1 and 2 m/s. Typical plots of cumu- respectively. The WR of all the specimens increase with the increaseFig. 9. Variation of wear rate with holding time in cryotreatment (QCT): effect of (a) sliding velocity and (b) normal load. Data at negative holding time are for conventionallytreated (QT) samples, which are not subjected to cryogenic processing cycle.
304 D. Das et al. / Wear 266 (2009) 297–309in either FN or VS (Fig. 9). The results in Fig. 9 can be summarized (vi) Wear resistance of QCT36 specimens is 13.2, 3.3 and 76.2 timesas: more 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. (i) The WR of QT specimen is signiﬁcantly higher than that of the QCT specimens. The above observations suggest that the variation in WR of QT (ii) The WR of QT specimen increases from 3.2 × 10−2 to specimens is much less compared to that of QCT specimens due to 32.8 × 10−2 mm3 /m when the FN is increased from 49.05 to the variation of either FN or VS . 98.1 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 when the VS is increased from 1 to 2 m/s (Fig. 9(b)). 3.4. Characteristics of worn surfaces and wear debris(iv) The WR of cryotreated specimens ﬁrst decreases with increase in holding time up to 36 h and then increases with further The operative wear mechanisms have been examined by analyz- increase in holding time (Fig. 9). This implies that QCT36 spec- ing the morphology of the worn-out surfaces of the pin specimens imens exhibit the highest wear resistance amongst all the and the collected wear debris generated during the steady-state specimens considered in the present investigation. wear regime under different test conditions. The salient features (v) The increase in FN from 49.05 to 98.1 N, increases WR of QCT related to the morphology of the worn-out surfaces of QT and specimens by 136.9–193.1 times, whereas doubling the VS QCT00 specimens are illustrated in Fig. 10. The nature, size, and increases WR of these specimens by 25.9–31.8 times. morphology of the wear debris generated during wear tests of QTFig. 10. SEM micrographs of typical worn surfaces generated under wear tests at 98.1 N of normal load and 1 m/s of sliding velocity: (a) overview of QT specimen, (b)sub-surface cracking, (c) deformation lip and (d) fractured ridges of QT specimen, whereas (e) overview of QCT00 specimen and (f) oxides with surface grooves and featuresdepicting delamination of carbides in QCT00 specimen.
D. Das et al. / Wear 266 (2009) 297–309 305Fig. 11. Comparison of wear debris generated in the steady-state wear regime between QT and QCT00 specimens at different combinations of normal load (FN ) and slidingvelocity (VS ). All micrographs taken at the same magniﬁcation of 250×, but the insets at different magniﬁcations represent the detailed features of debris of the samemicrographs. Insets in (b) and (f) are at 2500×, and (c) and (d) at 50× magniﬁcation.and QCT00 specimens have been compared in different parts of in the precipitation behavior of the SCs. The variation in the char-Fig. 11. The morphology of the worn-out surfaces and the charac- acteristics of carbide particles between QT and QCT specimensteristics of the generated wear debris amongst the QCT specimens can be explained as follows. At the early stage of tempering SSCstested at two different FN at the constant VS = 2 m/s, have been nucleate in both QT and QCT specimens, but this phenomenoncompared in Fig. 12. is not sufﬁcient enough to explain the difference between the Worn surface of the QCT00 specimen is considerably smoother observed results in Figs. 5 and 6. Transformation of austenitethan that of the QT specimen (Fig. 10(e) vis-à-vis Fig. 10(a)), when to martensite at cryogenic temperature followed by prolongedboth are subjected to FN = 98.1 N and VS = 1 m/s. The wear debris holding induces micro-internal stresses which results in the for-is ﬁne oxides (Fig. 11(b)) for QCT00 specimen and large metallic mation of crystal defects such as dislocations and twins [2,6,17,18].platelets (Fig. 11(a)) for QT specimen. These observations indicate While, lattice distortion and thermodynamic instability of marten-that the wear resistance of the QCT specimen is signiﬁcantly higher site at 77 K drive carbon and alloying atoms to segregate atthan that of the QT specimen; this is in good agreement with the the nearby crystal defects. These segregated regions have beenestimated WR (Fig. 9). hypothesized as the newer sites for nucleation of SSCs . The increase in the population density of SSCs in QCT specimens4. Discussion compared to QT specimens thus gets explained on the consid- erations of these phenomena. Further, increased number of sites4.1. Microstructural modulations through cryotreatment can only allow the precipitates to grow in a limited manner for the constant amount of available carbon atoms, which explains, in The results in Figs. 5 and 6 reveal that cryotreatment causes turn, the difference in the size of the carbide particles betweenmarked reduction in the amount of R and signiﬁcant alteration QT and QCT specimens. The LSCs can thus be considered as
306 D. Das et al. / Wear 266 (2009) 297–309Fig. 12. SEM micrographs of worn surfaces of cryotreated (QCT) specimens at the end of wear test, carried out at 2 m/s sliding velocity under different normal loads. Insetsrepresent the wear debris generated at the steady-state wear regime for the corresponding tests.a category of SCs having higher growth in localized environ- The magnitudes of ˇ are 22.21, 7.91 and 1.61 for the test condi-ments. tions ‘FN = 49.05 N and VS = 2 m/s’, ‘FN = 98.1 N and VS = 1 m/s’ and The variation in the amount of SSCs and LSCs amongst the QCT ‘FN = 98.1 N and VS = 2 m/s’, respectively (Fig. 13). The results indi-specimens is governed by several factors; like kinetics of precipi- cate that for constant VS , the beneﬁt of cryotreatment on weartation, initial defect generation in the martensite, mobility of the resistance is reduced drastically with increasing FN ; while for con-interstitial and substitutional elements, and dissolution followed stant FN , the improvement in wear resistance is less pronouncedby precipitation of carbide particles at the selected tempering tem- with increasing VS . Within the investigated range of wear testperature. Due to lack of sufﬁcient information on these factors, it parameters, the effect of variation of FN is more pronounced onis difﬁcult to explain the exact cause for the nature of variation in the WR than the effect of the variation of SV (Fig. 13). The extent ofsize and population density of the carbide particles in cryotreated achievable beneﬁt by cryotreatment is thus signiﬁcantly dependentspecimens. The obtained results in Fig. 6, however, unambiguously on the wear test conditions. The values of ˇ for M2 steel calculatedlead to conclude that the most favorable combination of the size from the reports of Molinari et al. , Barron  and Mohan Laland population density is obtained for specimens held for 36 h at et al.  are 1.68 (FN = 150 N and SV = 0.8 m/s), 3.03 (FN = 430 N and77 K. SV = 0.48 m/s) and 2.28 (FN = 50 N and SV = 0.366 m/s), respectively. Thus, the available wear data related to the improvement of wear4.2. Wear behavior of QCT vis-à-vis QT specimens resistance of tool/die steels by cryotreatment do not converge to provide any guideline to assess the magnitudes of the improve- A new parameter, ˇ, deﬁned as the ratio of the WR of QT spec- ment in quantitative terms [1,3,10,11,33]. This uncertainty can beimen to that of QCT00 specimen, has been considered to compare attributed to the employment of different types of experimentalthe degree of improvement in wear resistance by cryotreatment. procedures and employed test parameters (Table 1). The improve-
D. Das et al. / Wear 266 (2009) 297–309 307 always greater than one, i.e., cryotreatment with some holding time induces better wear resistance than that with no holding time and (ii) the magnitude of ˇ increases up to the holding time of 36 h and then it decreases. The increase in wear resistance of tool steels by cryotreatment with increasing holding time has been reported earlier by Mohan Lal et al. , Collins and Dormer  and Yun et al. . The present results of ˇ up to 36 h are in agreement with the earlier reports [2,12,13,17]. However, the results in Fig. 14 reveal, for the ﬁrst time, that there exists a critical holding time in the cryotreatment of D2 steel for obtaining the best combination of desired microstructure and wear property of die/tool steels. An attempt to correlate the results in Fig. 14 with those in Figs. 5 and 6 appears to establish the fact that the larger number of SCs and their ﬁner sizes are the key factors for the improvement in wear resis- tance in cryotreated specimens and in delineating the critical timeFig. 13. Wear rate ratio (ˇ) for different combinations of wear test conditions. ˇ of holding. For the convenience of the reader, the detailed data of QTis the ratio of wear rate of QT specimen (WR ) to wear rate of QCT00 specimen WR , ˇ, ˇ are compiled in Table 3. QCT00(WR ). 4.4. Revelation of the wear mechanisms The operative wear mechanisms have been examined by analyz- ing the morphology of the worn-out surfaces of the pin specimens and the collected wear debris generated during the steady-state regime of wear under different test conditions. The operative mech- anisms have been discussed in different sub-sections to illustrate the difference in the wear mechanisms between QT and QCT spec- imens, and amongst the QCT specimens followed by comments on the generalized wear behavior. 4.4.1. Wear mechanism: QT vis-à-vis QCT specimens The presence of severe subsurface cracking (Fig. 10(b)), defor- mation lips (Fig. 10(c)), and fractured ridges (Fig. 10(c)) in the worn surfaces and generation of debris in the form of large metal- lic platelets (Fig. 11(a)) establish the operative mechanism forFig. 14. Wear rate ratio (ˇ ) as a function of holding time for different combinations QT specimens as severe delamination wear  under FN = 98.1 N QCT00of wear test conditions. ˇ is the ratio of wear rate of QCT00 specimen (WR ) to QCT and VS = 1 m/s. Similar morphology of the worn surfaces and thewear rate of other QCT specimens (WR ). wear debris (Fig. 11(c) and (e)) are also encountered for QT spec- imens subjected to other test conditions. The delamination wearment in the wear resistance of the cryotreated specimens over that mechanism is found to be operative in the QT specimens for allof the conventionally treated one is attributed here to the absence combinations of the investigated wear test conditions. However,of R (Fig. 5(a)) coupled with the ﬁner distribution of higher amount the size of the metallic debris is found to increase with increase inand number of SC particles (Figs. 5(b) and 6). either VS (Fig. 11(a) vis-à-vis Fig. 11(c)) or FN (Fig. 11(e) vis-à-vis Fig. 11(c)). These observations are also in good agreement with the4.3. Inﬂuence of holding time of cryotreatment on wear rates estimated WR as shown in Fig. 9. These results lead to infer that the QT specimens undergo severe plastic deformation during wear The inﬂuence of holding time at 77 K on the improvement tests. This has been attributed to the presence of signiﬁcant amountof wear resistance of the selected steel has been revealed using of soft R in the microstructure.another parameter, ˇ , deﬁned as the ratio of WR of QCT00 speci- Worn surfaces of QCT00 specimen under FN = 98.1 N andmen to the WR of any of the QCT specimens. The variations of ˇ VS = 1 m/s in Fig. 10(e) and (f) exhibit the presence of oxides and sur-with holding time for different test conditions are shown in Fig. 14. face grooves due to pull-out of hard PC particles. Thus, the operativeThe results in Fig. 14 lead to infer that: (i) the magnitude of ˇ is wear mechanism for QCT00 specimens is predominantly oxidativeTable 3Summary of wear rates and the wear rate ratio parametersSpecimens Values of WR (10−2 mm3 /m), ˇ and ˇ FN = 98.1 N, VS = 1 m/s FN = 98.1 N, VS = 2 m/s FN = 49.05 N, VS = 2 m/s WR ˇ ␤ WR ˇ ␤ WR ˇ ␤QT 5.07 − − 32.75 − − 3.16 − −QCT00 0.64 7.91 1.00 20.39 1.61 1.00 0.15 22.21 1.00QCT12 0.53 − 1.22 17.93 − 1.14 0.09 − 1.63QCT36 0.39 − 1.67 10.06 − 2.03 0.04 − 3.59QCT60 0.48 − 1.34 13.45 − 1.52 0.06 − 2.32QCT84 0.62 − 1.04 15.90 − 1.28 0.08 − 1.81 QT QCT00 QCT QCTFN : normal load; VS : sliding velocity; WR : wear rate; ˇ = WR /WR ; ˇ = WR 00 /WR .
308 D. Das et al. / Wear 266 (2009) 297–309wear coupled with pull-out of carbides, and the mode of wear is AISI D2 steel specimens compared to conventionally treated ones.mild [41,42]. Thus operative mechanism and mode of wear for QCT The results in Figs. 9 and 13 are in good agreement with the exist-specimens is mild oxidative in contrast to severe delamination for ing general consensus that the wear resistance gets signiﬁcantlyQT specimens under similar test conditions. The results in Fig. 11(e) enhanced in die/tool steels by cryotreatment [1–5,8–15]. Further-vis-à-vis Fig. 11(f) provide similar comparison for the wear behavior more, the results of the present investigation also infer that: (i) theof QT and QCT specimens under FN = 49.05 N and VS = 2 m/s. Under degree of improvement of wear resistance is considerably depen-these test conditions, the estimated wear resistance of QCT speci- dent upon the test conditions and (ii) the highest improvement inmens is found to be at least an order of magnitude higher than that wear resistance for AISI D2 steel can be attained by cryogenic treat-of QT specimen (Figs. 9 and 13). ment with a holding time of 36 h at 77 K. The results presented in the When wear tests have been carried out under the condition preceding sub-sections are thus unique in their detailed treatment‘FN = 98.1 N and VS = 2 m/s’, delamination wear is found to be oper- to reveal the operative wear mechanisms of the cryotreated speci-ative for both QT and QCT00 specimens. This is evident from the mens. These also provide excellent guidelines for possible futuristicgeneration of the similar nature of wear debris as large metallic quantitative analysis of the wear debris and surface morphology ofplatelets in both QT and QCT00 samples, as shown in Fig. 11(c) and cryotreated specimens in order to bring forth ﬁner details related to(d). Thus, under this test condition, both QT and QCT specimens the operative mode and mechanisms under varied test conditions.experience severe mode of wear and the recorded improvement inwear resistance of QCT00 compared to QT is only 60% (Fig. 13). 5. Conclusions The delamination wear in QT specimens and the predominantlyoxidative wear in the QCT specimens can be correlated with the The wear behavior of a series of AISI D2 die steel specimens, cry-microstructural features. Delamination wear sequentially consti- otreated for different holding periods at 77 K, has been examinedtutes plastic deformation of surface layer, crack nucleation and to probe the micro-mechanism of wear and to ﬁnd out the criticalcrack propagation . The QT specimens possessing signiﬁcant duration of cryotreatment to achieve the best possible wear resis-amount of R is prone to plastic deformation which assists in easy tance. The obtained results and their pertinent discussion assist tonucleation of cracks, and hence leads to delamination wear under infer the following:all the investigated test conditions. Conversely, the microstructuralconstituents of QCT specimens hinder plastic deformation and get (1) The wear resistance of the AISI D2 steel gets considerablysubjected to oxidative wear at less severe combination of applied FN enhanced by cryotreatment, compared to that of the conven-and VS . But, at higher FN and higher VS , crack nucleation occurs with tionally treated one, irrespective of the time of holding atlimited amount of plastic deformation and leads to severe wear 77 K. The extent of improvement of wear resistance, however,[40,41]. is dependent on the wear test conditions, which control the active mechanisms and mode of wear. Severe mode of wear is4.4.2. Wear mechanism in varied QCT specimens identiﬁed as delamination of metallic particles caused by sub- The mode and mechanism of wear for all QCT specimens are surface cracking due to extensive plastic deformation, whereas‘severe delamination’ at FN = 98.1 N and VS = 2 m/s, and ‘mild oxida- mild mode of wear is characterized as predominantly oxidativetive’ for the other two test conditions; and the best wear resistance in nature associated with pull-out of primary carbides and/orbeing obtained for QCT36 specimen. Correlation of WR with the break-down of oxide layer.worn-out surfaces and wear debris amongst QCT specimens are (2) The marked improvement in wear resistance of the cryotreatedthus made with one sample having holding time <36 h and another specimens compared to the conventionally treated ones isone with holding time >36 h with respect to QCT36 specimen attributed to the near absence of retained austenite and more(Fig. 12). Comparative assessment of the results depicted in Fig. 12 homogeneous distribution of a larger number of ﬁner sec-indicates that under identical test conditions QCT36 specimens suf- ondary carbides in the former specimens. However, the degreefered minimum surface damage and exhibited ﬁnest size of the of improvement depends on the test conditions. Hardness of thewear debris. This assessment corroborates well with the inference investigated steel samples is found to increase marginally bydrawn from Figs. 9 and 14 that the specimen cryotreated for 36 h cryotreatment in contrast to signiﬁcant increase in their wearexhibit the highest wear resistance. The increase in wear resistance resistance.of QCT specimens with holding time up to 36 h is attributed to the (3) The mode of wear and the operative wear mechanism are iden-increased amount of SCs in the microstructure which appear to tical for all of the cryotreated specimens at the selected testreach a steady-state value around 36 h (Fig. 5). Similar trend was conditions. However, the mode of wear changes from mild toalso observed for the variation of macrohardness of QCT specimens severe in the cryotreated specimens due to increase in (a) nor-with holding time at 77 K (Fig. 7). Beyond 36 h of holding in cry- mal load from 49.05 to 98.10 N at a constant sliding velocityotreatment, the volume fraction of SCs remains almost constant of 2 m/s and (b) sliding velocity from 1 to 2 m/s at a constantbut the size of the SCs, particularly that for SSCs increases with normal load of 98.10 N.concurrent reduction of their population density (Fig. 6). These (4) Within the investigated range, the wear resistance of the cry-microstructural changes are considered to be responsible for the otreated specimens increases with increasing holding time upreduction of strength of the matrix of QCT specimens held at 77 K to 36 h at 77 K beyond which it shows monotonic decrease withbeyond 36 h as evident from the results in Fig. 7. Therefore, the further increase in holding time. The variation in wear resis-obtained results from the wear tests in this investigation indicate tance corroborates excellently with the changes of amount,that the wear resistance of differently cryotreated specimens is size, population density and morphological characteristics ofclosely related to the developed microstructures as well as the the secondary carbides as a function of holding time during cry-resultant hardness and microhardness values. otreatment. Thus, unlike the popular postulation that increase in holding time during cryotreatment monotonically improves4.4.3. Comments on the generalized wear behavior wear properties, the present results exhibit a critical value of Analyses of the morphology of worn-out surfaces and wear holding time (36 h at 77 K for AISI D2 steel) for obtaining the bestdebris in the preceding sub-sections strongly support the infer- combination for the desired microstructures and wear proper-ence of signiﬁcant improvement in wear resistance of cryotreated ties of die steels.
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