Tt criogenico aisid2

Wear 266 (2009) 297–309

Contents lists available at ScienceDirect

Wear
journal homepage: www.elsevier.com/locate/wear

Influence of varied cryotreatment on the wear behavior of AISI D2 steel
D. Das a , A.K. Dutta b , K.K. Ray c,∗
a
Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India
b
Department of Mechanical Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India
c
Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India

a r t i c l e i n f o a b s t r a c t

Article history: Exploration of the benefit of cryotreatment for achieving improvement in wear resistance of die/tool
Received 5 September 2007 steel is a topic of current research interest. A series of wear tests has been carried out on AISI D2 steel
Received in revised form 9 June 2008 samples subjected to cryotreatment at 77 K for different durations. The wear rates at different loads and
Accepted 9 July 2008
sliding velocities, morphologies of the worn-out surfaces and the characteristics of the wear debris have
Available online 17 September 2008
been systematically examined to assess the possible critical duration of cryotreatment to achieve the best
wear resistance property. The wear experiments have been supplemented by detailed microstructural
Keywords:
investigations with an emphasis to reveal the amount of retained austenite and the characteristics of
AISI D2 steel
Cryogenic treatment
the secondary carbide particles apart from hardness evaluation. The results unambiguously establish that
Sliding wear behavior ‘critical time duration’ exists for achieving the best wear resistance for AISI D2 steel through cryotreatment.
Retained austenite This has been explained by the nature of precipitation of fine carbide particles and their possible growth,
Secondary carbides which govern the wear resistance of the material. Categorization of the secondary carbides to support this
explanation is a new approach. The revelation of the wear mechanisms under different wear conditions
is an integral part of this work.
© 2008 Published by Elsevier B.V.

1. Introduction resistance by cryotreatment are yet to be clearly established. Some
investigators [5,7,9] contend that the enhancement of wear resis-
The primary objective of the heat treatment of die/tool steels tance occurs only due to transformation of R to martensite. But,
is to impart high wear resistance. One of the major problems in this phenomenon is a common feature to both the cold treat-
the conventional heat treatment through hardening and temper- ment and the cryotreatment, and thus the significant enhancement
ing of these steels is the content of retained austenite ( R ), which of wear resistance of tool steels by cryotreatment vis-à-vis cold
is soft, unstable at low temperature and transforms into brittle treatment cannot be solely attributed to the minimization of R
martensite during service. Transformation of austenite to marten- [2,6,8,10,13,17]. Several investigators [2–4,10,12,15,18] indicate that
site is associated with approximately 4% volume expansion, which the refinement of secondary carbides is the major cause for the
causes distortion of the components. Thus, either sub-zero treat- improvement in wear resistance by cryotreatment; but this opinion
ment or multiple tempering at relatively high temperature and/or lacks appropriate experimental evidences [2,10,12,15].
for longer duration is used for minimizing the amount of R content While the mechanism behind the improvement of wear resis-
in tool steels. The sub-zero treatment is popularly termed as either tance by cryotreatment is yet to get crystallized, the reported wear
‘cold treatment’ (193–213 K) or ‘cryogenic treatment’ (148–77 K) data for cryotreated tool steels also do not provide any guideline
[1–7]. In this report, deep cryogenic processing (77 K) has been for the assessment of the degree of improvement in quantitative
referred to as cryogenic treatment and has been applied in between terms. It is thus difficult to compile the existing results to achieve
conventional hardening and tempering cycles. For convenience of any unified picture. This uncertainty can be attributed to the differ-
discussion, it has been termed simply as cryotreatment in subse- ent sets of experimental conditions for comparative assessment of
quent discussion. the wear resistance of cryotreated steels, e.g., variation of applied
The benefit of cryotreatment for the enhancement of wear resis- loads, sliding velocities and types of counter faces employed for
tance of tool steels has been cited by several researchers [5–16]. the evaluation of wear rates. Some typical results reported by dif-
However, the mechanisms responsible for enhancing the wear ferent investigators [5,8,10–12,14,19] are compiled in Table 1, which
vividly illustrates this difficulty. Thus, one of the major aims of this
investigation is to address this problem through organized and sys-
∗ Corresponding author. Tel.: +91 3222 283278; fax: +91 3222 282280/255303. tematic wear experiments on differently cryotreated specimens of
E-mail address: kkrmt@metal.iitkgp.ernet.in (K.K. Ray). AISI D2 steel.

0043-1648/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.wear.2008.07.001

298 D. Das et al. / Wear 266 (2009) 297–309

Table 1
Summary of the different test parameter used to evaluate the dry sliding wear resistance of cryotreated tool/die steels

Sl. 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 [5]
A6, O1, P20, S7, etc. grinding wheel
2. D2 Block Wheel Hardened D2 steel 21 0.50–3.62 200, 400, 600 [8]
3. D2 Pin Disc WC-coated En-35 steel 49, 69, 78 1.50 900 [10]
4. M2, H13 Disc Disc Hardened 100Cr6 150 0.80 5000 [11]
5. M2, D3 Pin Disc Grinding wheel 20, 30, 50 0.18–0.60 324–1080 [12]
6. M2 Disc Ball Silicon nitride 50 0.027 200 [14]
7. M2 Pin Disc Alumina abrasive paper 10 0.11 3.22 [19]
(800 and 80 mesh)

In addition, one of the major uncertainties associated with the 2. Experimental procedure
earlier investigations related to cryotreatment of tool steels is the
duration of cryotreatment at the selected temperatures [1–4,20]. 2.1. Material
The existing literature does not provide any guideline related to
the selection of time duration for cryotreatment [3,10]. The hold- The selected steel has been obtained as a commercial hot forged
ing 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. [14], 20 h by de Silva Fe. This composition conforms to the AISI specification of D2
et al. [19], 35 h by Molinari et al. [11] and 168 h by Huang et al. [18]. steel.
Such wide variation in the selected holding time even for the same
material is due to the lack of systematic investigation related to the 2.2. Heat treatments
influence of holding time on the wear resistance of tool/die steels
by cryotreatment. Thus another major aim of this report is to unfold Specimen blanks of 24 mm × 16 mm × 85 mm dimension were
the 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 guide
AISI D2 steel were subjected to five different holding times dur- [21]. Deep cryogenic processing (C) was incorporated intermediate
ing 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 cryogenic
been assessed together with the determination of macrohardness, processing was done by uniform cooling of the samples to 77 K, and
microhardness and wear rates by standard experimental proce- holding the samples at this temperature for different time durations
dures. 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 illustrated
of the worn-out surfaces and wear debris have assisted to reveal the in Fig. 1(b). The specimens subjected to different treatments are
underlying mechanisms for the improvement in wear resistance by referred henceforth with codes as shown in Table 2, where the
cryotreatment and throw light on the wild scatter of the reported numerals in the codes represent the time of holding in hour at
improvement 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) typical
time-temperature profile of a deep cryogenic processing cycle.

D. Das et al. / Wear 266 (2009) 297–309 299

Table 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’Pert
Sample code Description of heat treatment cycles software.
Hardening Duration of deep cryogenic Tempering
processing at 77 K (h) 2.4. Hardness measurement
QT –
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 h
QCT36 36
QCT60 60
Indentations for microhardness measurement of the matrix were
QCT84 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 to
2.3. Microstructural characterization
estimate the average value of microhardness of the specimen
matrix.
Sample blanks (10 mm × 10 mm × 7 mm) for metallographic
examinations were cut using wire electro-discharge machining.
These were polished and finally lapped by diamond paste of 1 ␮m 2.5. Evaluation of wear behavior
size prior to etching with picral solution (3 gm picric acid in 100 ml
ethanol), 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 debris
microscope (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 conventionally
particles have been classified as primary carbides (PCs: size > 5 ␮m) treated samples. Dry sliding wear tests following ASTM standard
and secondary carbides (SCs: size ≤ 5 ␮m). The SCs are further sub- G99-05 [24] were carried out by using a computerized pin-on-disc
classified 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-35
ware to estimate (i) the volume fraction and size of PCs, LSCs and steel disc of 8 mm thickness and 160 mm diameter was selected as
SSCs and (ii) the population density of LSCs and SSCs. The number the rotating counter surface having Hv ∼ 1750 and Ra < 0.5 ␮m. The
of carbide particles considered for quantitative characterizations pins were machined from the suitably heat-treated specimens by
is >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 velocity
tures 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. The
PHILIPS, 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 force
standard E975-00 [22] 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 three
and (3 1 1) of R . Identification of the exact nature of the carbides consistent readings were obtained for each set of test condition to
in the heat-treated samples was difficult by XRD analysis of the estimate the average WR .
bulk specimens due to their small amount [22]. Therefore, car- The worn-out surfaces of the pins were cleaned in acetone using
bide particles were electrolytically extracted from both QT and an ultrasonic cleaner for 10 min and were subsequently examined
QCT specimens following the report of Nykiel and Hryniewicz under a SEM to identify the possible wear mechanisms. The wear
[23]. 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 tempered
martensite: black (PC, primary carbide; SC, secondary carbide).

300 D. Das et al. / Wear 266 (2009) 297–309

Fig. 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 of
3.1.1. 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 illustrates

Fig. 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 vertical
direction 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) 297–309 301

that the QCT specimens possess considerably larger numbers of in literature [27] too. The SCs in both QT and QCT specimens have
SSCs 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 with
to (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 microstructural
constituents (Fig. 5) for all of the heat-treated specimens have been
3.1.4. Amount of primary carbides
estimated in the following manner. The volume fraction of R has
The amounts of PCs are similar in QT and QCT specimens
been 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 agreement
yses on digital micrographs and the volume fraction of tempered
with the value of 7.8% as reported by Fukaura et al. [27] 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 between
age 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 by
3.1.2. Measurement of retained austenite content Wei et al. [29].
Typical representative XRD line profiles of three bulk specimens
QT, 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 QCT00
QT samples and their indistinct appearance in the XRD profiles of specimens
QCT specimens assist to compare the amounts of R in these spec- The variations in the amounts of LSCs and SSCs with holding
imens 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 holding
deep cryogenic processing in between hardening and tempering time are shown in Fig. 6; the variation of the similar microstructural
almost completely converts the R in D2 steel to martensite. This characteristics for the carbide particles in QT specimens are also
observation 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 [20] istics of SCs, LSCs and SSCs of QCT00 with respect to QT specimens
steels. 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% associated
3.1.3. 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 and
cles 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 thus
Hence, 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 both
QT and QCT specimens have been identified mainly as M7 C3 with
small 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 specimens
bide 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 cryotreatment
as (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) 297–309

Fig. 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 at
77 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 K
in cryotreatment has significant effect on the precipitation behavior
of SCs, and the specimen cryotreated for 36 h offer the optimum
size and population density of SCs for enhancing the mechanical
properties 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 K
are shown in Fig. 7. The bulk hardness of QCT samples has been
found to be at least 4.2% higher than that of QT specimens and is in
agreement 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 at
of QCT specimens with holding time at 77 K is similar to that negative holding time are for conventionally treated (QT) samples, which are not
of 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) 297–309 303

Fig. 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 49.05
increase in bulk and microhardness of the matrix occurs due to and 98.1 N are shown in Fig. 8, which exhibits both the regimes
cryotreatment. of ‘running-in’ and ‘steady-state’ wear [38,39]. Steady-state wear
has been further examined to reveal the effect of cryotreatment
3.3. 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 the
estimating the volume loss with respect to sliding distance, exam- selected combinations of test conditions. The effect of FN on Wv loss
ination 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 for
lar sliding distance has been evaluated by multiplying the recorded different FN are illustrated for QT, QCT00 and QCT36 specimens in
cumulative 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 Wv
pared to that of the pin specimens, the volume loss is considered loss per unit sliding distance [39] corresponding to the steady-state
to 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 increase

Fig. 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 conventionally
treated (QT) samples, which are not subjected to cryogenic processing cycle.

304 D. Das et al. / Wear 266 (2009) 297–309

in 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 times
as: 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 QT

Fig. 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 features
depicting delamination of carbides in QCT00 specimen.

D. Das et al. / Wear 266 (2009) 297–309 305

Fig. 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 sliding
velocity (VS ). All micrographs taken at the same magnification of 250×, but the insets at different magnifications represent the detailed features of debris of the same
micrographs. 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 specimens
teristics of the generated wear debris amongst the QCT specimens can be explained as follows. At the early stage of tempering SSCs
tested at two different FN at the constant VS = 2 m/s, have been nucleate in both QT and QCT specimens, but this phenomenon
compared 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 austenite
than that of the QT specimen (Fig. 10(e) vis-à-vis Fig. 10(a)), when to martensite at cryogenic temperature followed by prolonged
both 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 at
than that of the QT specimen; this is in good agreement with the the nearby crystal defects. These segregated regions have been
estimated WR (Fig. 9). hypothesized as the newer sites for nucleation of SSCs [10]. The
increase in the population density of SSCs in QCT specimens
4. Discussion compared to QT specimens thus gets explained on the consid-
erations of these phenomena. Further, increased number of sites
4.1. 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 between
marked 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) 297–309

Fig. 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. Insets
represent 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 wear
tation, 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 pronounced
by precipitation of carbide particles at the selected tempering tem- with increasing VS . Within the investigated range of wear test
perature. Due to lack of sufficient information on these factors, it parameters, the effect of variation of FN is more pronounced on
is 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 of
size and population density of the carbide particles in cryotreated achievable benefit by cryotreatment is thus significantly dependent
specimens. The obtained results in Fig. 6, however, unambiguously on the wear test conditions. The values of ˇ for M2 steel calculated
lead to conclude that the most favorable combination of the size from the reports of Molinari et al. [11], Barron [5] and Mohan Lal
and population density is obtained for specimens held for 36 h at et al. [12] are 1.68 (FN = 150 N and SV = 0.8 m/s), 3.03 (FN = 430 N and
77 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 wear
4.2. 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 be
imen to that of QCT00 specimen, has been considered to compare attributed to the employment of different types of experimental
the 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. [12], Collins and Dormer [13] and Yun
et al. [17]. 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 time
Fig. 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
QT
is 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 for
Fig. 14. Wear rate ratio (ˇ ) as a function of holding time for different combinations QT specimens as severe delamination wear [40] under FN = 98.1 N
QCT00
of 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 the
wear 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 wear
ment in the wear resistance of the cryotreated specimens over that mechanism is found to be operative in the QT specimens for all
of 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 in
and 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 the
4.3. 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 amount
of 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 and
men 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 operative
The results in Fig. 14 lead to infer that: (i) the magnitude of ˇ is wear mechanism for QCT00 specimens is predominantly oxidative

Table 3
Summary of wear rates and the wear rate ratio parameters

Specimens 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 1.00
QCT12 0.53 − 1.22 17.93 − 1.14 0.09 − 1.63
QCT36 0.39 − 1.67 10.06 − 2.03 0.04 − 3.59
QCT60 0.48 − 1.34 13.45 − 1.52 0.06 − 2.32
QCT84 0.62 − 1.04 15.90 − 1.28 0.08 − 1.81
QT QCT00 QCT QCT
FN : normal load; VS : sliding velocity; WR : wear rate; ˇ = WR /WR ; ˇ = WR 00 /WR .

308 D. Das et al. / Wear 266 (2009) 297–309

wear 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 significantly
QT 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) the
of 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 in
mens 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 futuristic
generation of the similar nature of wear debris as large metallic quantitative analysis of the wear debris and surface morphology of
platelets 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 in
wear resistance of QCT00 compared to QT is only 60% (Fig. 13). 5. Conclusions
The delamination wear in QT specimens and the predominantly
oxidative 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 examined
tutes plastic deformation of surface layer, crack nucleation and to probe the micro-mechanism of wear and to find out the critical
crack propagation [40]. 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 to
nucleation of cracks, and hence leads to delamination wear under infer the following:
all the investigated test conditions. Conversely, the microstructural
constituents of QCT specimens hinder plastic deformation and get (1) The wear resistance of the AISI D2 steel gets considerably
subjected 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 at
limited 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 is
4.4.2. 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 oxidative
tive’ for the other two test conditions; and the best wear resistance in nature associated with pull-out of primary carbides and/or
being 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 cryotreated
thus made with one sample having holding time <36 h and another specimens compared to the conventionally treated ones is
one 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 degree
fered minimum surface damage and exhibited finest size of the of improvement depends on the test conditions. Hardness of the
wear debris. This assessment corroborates well with the inference investigated steel samples is found to increase marginally by
drawn from Figs. 9 and 14 that the specimen cryotreated for 36 h cryotreatment in contrast to significant increase in their wear
exhibit 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 test
reach a steady-state value around 36 h (Fig. 5). Similar trend was conditions. However, the mode of wear changes from mild to
also 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 velocity
otreatment, the volume fraction of SCs remains almost constant of 2 m/s and (b) sliding velocity from 1 to 2 m/s at a constant
but 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 up
reduction of strength of the matrix of QCT specimens held at 77 K to 36 h at 77 K beyond which it shows monotonic decrease with
beyond 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 of
closely 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 improves
4.4.3. 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 best
debris 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.

D. Das et al. / Wear 266 (2009) 297–309 309

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