ArticlePDF Available

Abstract and Figures

Microstructure, hardness and wear behavior of AISI D2 steel subjected to varied sub-zero treatments have been examined with reference to conventional heat treatment. Part I of this work presents the variations of microstructure and hardness, whereas part II deals with the wear behavior. The sub-zero treatments studied are cold treatment, shallow cryogenic treatment and deep cryogenic treatment. The developed microstructures have been characterized by XRD, optical microscopy and SEM examinations coupled with EDX and image analyses. Macrohardness and microhardness of the specimens have been evaluated by Vickers indentation technique. The obtained results infer that (i) retained austenite content is reduced by cold treatment, but is almost completely eliminated by both shallow and deep cryogenic treatments, (ii) the sub-zero treatments modify the precipitation behavior of secondary carbides; lower the temperature of sub-zero treatment higher is the degree of modification, (iii) the deep cryogenic treatment refines the secondary carbides, increases their amount and population density, and leads to more uniform distribution, and (iv) bulk hardness increases marginally but apparent hardness of the matrix improves considerably by deep cryogenic treatment.
Content may be subject to copyright.
This article appeared in a journal published by Elsevier. The attached
copy is furnished to the author for internal non-commercial research
and education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling or
licensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of the
article (e.g. in Word or Tex form) to their personal website or
institutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies are
encouraged to visit:
http://www.elsevier.com/copyright
Author's personal copy
Materials Science and Engineering A 527 (2010) 2182–2193
Contents lists available at ScienceDirect
Materials Science and Engineering A
journal homepage: www.elsevier.com/locate/msea
Sub-zero treatments of AISI D2 steel: Part I. Microstructure and hardness
Debdulal Dasa, Apurba Kishore Duttab, Kalyan Kumar Rayc,
aDepartment of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India
bDepartment of Mechanical Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India
cDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, West Bengal, India
article info
Article history:
Received 30 September 2009
Received in revised form 29 October 2009
Accepted 30 October 2009
Keywords:
Sub-zero treatment
Retained austenite
Decomposition of martensite
Carbide precipitation
Hardness
Die steel
abstract
Microstructure, hardness and wear behavior of AISI D2 steel subjected to varied sub-zero treatments
have been examined with reference to conventional heat treatment. Part I of this work presents the
variations of microstructure and hardness, whereas part II deals with the wear behavior. The sub-zero
treatments studied are cold treatment, shallow cryogenic treatment and deep cryogenic treatment. The
developed microstructures have been characterized by XRD, optical microscopy and SEM examinations
coupled with EDX and image analyses. Macrohardness and microhardness of the specimens have been
evaluated by Vickers indentation technique. The obtained results infer that (i) retained austenite content
is reduced by cold treatment, but is almost completely eliminated by both shallow and deep cryogenic
treatments, (ii) the sub-zero treatments modify the precipitation behavior of secondary carbides; lower
the temperature of sub-zero treatment higher is the degree of modification, (iii) the deep cryogenic
treatment refines the secondary carbides, increases their amount and population density, and leads to
more uniform distribution, and (iv) bulk hardness increases marginally but apparent hardness of the
matrix improves considerably by deep cryogenic treatment.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Presence of high carbon and high alloying elements in tool/die
steels lower their characteristic temperatures of martensite start
and martensite finish; the latter lies well below the ambient tem-
perature for commercial tool/die steels [1]. Therefore, conventional
hardening treatment of these steels fails to convert considerable
amount of austenite into martensite often leading to unacceptable
level of retained austenite (R) in the as quenched structure of these
steels. The Ris soft and thus adversely affects the desirable proper-
ties such as hardness and wear resistance (WR) [1,2]. Moreover, R
is unstable and transforms into martensite at the service conditions
of tool/die steels. The freshly formed martensite being untempered
is very brittle and hence undesirable. Furthermore, transforma-
tion of austenite to martensite is associated with approximately
4% volume expansion [3], which leads to dimensional changes and
distortion of the components [1,2], even failure in extreme cases
[4]. Therefore, one of the major challenges in the heat treatment of
tool/die steels is to minimize the amount of Ror eliminate it.
In conventional heat treatment, the amount of Rcan be reduced
by subjecting the hardened steel specimens to multiple tempering
Corresponding author. Tel.: +91 03222 283278; fax: +91 03222 282280/255303.
E-mail addresses: kkrmt@metal.iitkgp.ernet.in,kalyankumarry@yahoo.com
(K.K. Ray).
cycles at relatively higher temperature and/or for longer duration
[1,2]. However, this process has inherent drawback as it leads to
excessive softening of matrix and coarsening of carbides resulting
into lower hardness and strength [1–3]. Alternatively, Rcontent in
tool/die steels can be reduced substantially by sub-zero treatment
of conventionally quenched components [1,2,4–10]. Unlike mul-
tiple tempering treatments, sub-zero treatment does not reduce
the strength and hardness of tool/die steels, rather marginally
improves these properties [5–12]. Sub-zero treatment of tool steels
has been recognized as early as 1920 by Scott [13] and subse-
quent research works [14–25] have shown its beneficial effects
on engineering components, specifically by enhancing dimensional
stability [14–18] and service life [9–12,19–23]. Therefore, sub-zero
treatment has gained industrial acceptance and has been an impor-
tant part of commercial heat treatment cycle of tool/die steels over
the last few decades [1,2,4–9,19]. Commercial sub-zero treatment,
till recently, has been done only by employing dry ice as cool-
ing agent and is popularly termed as cold treatment (223–193 K)
[1,4,7,11,21,26]. Research endeavors over the last two decades have
indicated that improvement of mechanical properties, particularly
WR, can be achieved substantially by further lowering the tem-
perature of sub-zero treatment by using liquid nitrogen as cooling
agent [4–8,11,22,24–27]. This type of sub-zero treatment has been
termed as cryogenic treatment or simply cryotreatment (193–77 K),
which has been further classified as shallow cryogenic treatment
(193–113 K) and deep cryogenic treatment (113–77 K) [4,6,26],
0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2009.10.070
Author's personal copy
D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193 2183
the latter being the most studied ones [4–8,27–31]. Apart from
temperature difference, deep cryogenic treatment involves con-
trolled cooling/heating cycle with sufficiently long time of holding
(12–72 h) at lowest temperature unlike cold treatment [6,12,27,32].
It is now widely accepted that improvement in WR by deep
cryogenic treatment is considerably higher than that achieved by
cold treatment [5–8,21,26,33]; while improvement in WR by cold
treatment itself is higher as compared to that by conventional heat
treatment [11,12,26,32–35]. However, following queries are yet
to be answered satisfactorily. Firstly, what is the expected degree
of improvement in WR by deep cryogenic treatment over other
treatments? And why the reported degree of improvement in WR
differs so much even for the same material [27,36]?Secondly, what
mechanism is responsible for the improvement in WR by deep
cryogenic treatment over cold treatment and shallow cryogenic
treatment? The observed difference in the amount of Rbetween
deep cryogenic treatment and cold treatment is insufficient to
explain the extent of reported variation of their wear behavior
[5,6,12,27,33]. Both shallow and deep cryogenic treatments are
expected to remove Ralmost completely,thus, any difference in
their WR must be originating from some other phenomenon than
the variation in Rcontent. The primary objective of the present
study is to answer these basic queries through a systematic inves-
tigation of microstructure, hardness and wear behavior of die steel
specimens subjected to conventional heat treatment, cold treat-
ment, shallow and deep cryogenic treatments. The steel selected for
this study is AISI D2 grade which is extensively used for manufac-
turing of commercial cold-worked dies [1,2]. The results obtained
in the above investigation and their pertinent analyses are being
presented as two companion papers (parts I and II). The first part,
this paper, presents the influence of different sub-zero treatments
on the variations of microstructure and hardness. The second part,
the companion paper [37], provides experimental results regarding
wear behavior and its correlation with the pertinent microstruc-
ture and hardness through analyses of the events occurring on the
surface and subsurface during the course of wear.
2. State-of-art of sub-zero treatments
Barron [7] has examined hardness and abrasive wear resistance
of a large variety of steels subjected to deep cryogenic treatment,
cold treatment and conventional heat treatment. He has demon-
strated that for tool steels, WR increases considerably by cold
treatment and dramatically by deep cryogenic treatment when
compared to conventional heat treatment. It has been reported
for stainless steels that sub-zero treatments improve WR com-
pared to conventional heat treatment, but the difference in the
improvement in WR for specimens subjected to deep cryogenic
treatment and cold treatment is marginal. On the other hand, for
plain carbon steels and cast irons no improvement in WR has been
reported either by cold treatment or by deep cryogenic treatment.
Barron [7,11] has attributed the improvement in WR by different
sub-zero treatments and the varying response by different class
of materials to sub-zero treatments to the reduction of retained
austenite (R) content only. However, his report does not include
measurement of Rcontent or any alternate microstructural char-
acterization. Through an independent investigation on AISI 1079
steel, Soundararajan et al. [38] have supported Baron’s postula-
tion and have argued that the most significant microstructural
change due to deep cryogenic treatment is the transition of Rto
the hard and more stable martensite, which results into improve-
ment in mechanical properties of the cryotreated steels. In contrast,
Meng et al. [33] have reported that the difference in Rcontent of
AISI D2 and 52100 steels after deep cryogenic treatment and cold
treatment is negligible though the improvement in WR by deep
cryogenic treatment is substantially higher than that obtained by
cold treatment. They have concluded that rather than the removal
of R, improvement in WR by deep cryogenic treatment over cold
treatment is due to preferential precipitation of fine -carbides
by the former treatment. The -carbide referred by Meng et al.
[33] is pre-cementite transition carbides [39,40] which is known to
improve both hardness and toughness of steel [3,24], and as a con-
sequence improve WR of steel when compared to its as-hardened
state [24,25]. However, precipitation of -carbide occurs only in
the early stage of tempering of hardened steel specimens [40–42],
like the tempering condition (453 K, 0.5 h) employed by Meng et
al. [33] for AISI D2 steel. This tempering condition practically leads
to under tempered state, since tempering temperature selected by
these researchers is well below the recommended [43] tempering
temperature of 478–813 K for this steel. For heat treatment of same
steel with [5,26,34,44–47] or without [48,49] sub-zero treatments,
the tempering temperature and the time employed by several other
investigators are much higher than that selected by Meng et al.
[33], and none of the investigations [5,26,34,44–46,48,49] have
been able to detect the presence of -carbide. This is expected
since -carbide is unstable and hence transforms to more stable
carbides like cementite or alloy carbides upon proper tempering
treatment [3,26,40–42,45]. Therefore, precipitation of -carbide,
as the plausible explanation for improvement in WR by deep cryo-
genic treatment, is of little acceptability for tool/die steels.
Collins and Dormer [34] have studied the microstructure,
hardness, impact toughness and abrasive wear rate of AISI D2
steel specimens subjected to different sub-zero treatments. These
authors have attributed the improvement in WR by deep cryogenic
treatment over cold treatment to the ‘low temperature condition-
ing of martensite’ and have argued that the lowest temperature in
cold treatment is not low enough to condition the martensite unlike
that in deep cryogenic treatment. It may be mentioned here that
low temperature conditioning of martensitic structure refers to its
sub-structural changes during sub-zero treatments, which result
in precipitation of a finer distribution of carbides on reheating or
subsequent tempering treatment [12,34]. The above conclusion by
Collins and Dormer [34] is based on the experimental measure-
ment of the population density of fine carbide particles from optical
micrographs. The accuracy of this result, however, is questionable
since the average size of finer carbides reported by other inves-
tigators [5,32,35,36,45,50,51] in the sub-zero treated specimen is
too fine to be resolved in optical microscopy. Moreover, it has been
noted by several investigators [26,52,53] that optical micrographs
of conventionally heat treated and differently sub-zero treated
specimens show no noteworthy difference.
Bensely et al. have studied hardness and WR [54], tensile prop-
erties [55], impact toughness [56], fatigue properties [57] and
distribution of residual stress and Rcontent [58] of case carbur-
ized En 353 steel subjected to conventional heat treatment, cold
treatment and deep cryogenic treatment. It may be noted here that
sub-zero treatment carried out at 193 K has been termed as shal-
low cryogenic treatment by these investigators; however, it has
been called here as cold treatment as defined earlier [4,26]. From
the experimental results, Bensely et al. [54] have reported that cold
treatment and deep cryogenic treatment improve the WR by 85%
and 372% respectively with reference to conventional heat treat-
ment. However, the magnitudes of improvement in hardness (3.5%)
[54] and impact toughness (14%) [56], and reduction in compres-
sive residual stress (46.7%) [58] at the surface have been reported
to be same for both cold treatment and deep cryogenic treatment
when compared with conventional heat treatment. Furthermore,
these authors have shown that cold treatment and deep cryogenic
treatment in comparison to conventional heat treatment reduce
tensile strength by 1.5% and 9.3% [55], respectively; in contrast,
the overall fatigue life has been reported to be increased (71%)
Author's personal copy
2184 D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193
by cold treatment but decreased (26%) by deep cryogenic treat-
ment [57]. It has been reported that the volume of Rreduces from
28% after conventional heat treatment to 22% by cold treatment
and to 14% by deep cryogenic treatment [57]. Bensely et al. [58]
have attributed the large reduction in compressive residual stress
to precipitation of higher amount of fine carbides by deep cryo-
genic treatment when deep cryogenically treated steel has been
subjected to tempering treatment. However, neither the nature
of carbides has been identified nor any quantitative measurement
on the variation of carbide content with sub-zero treatments has
been attempted by these investigators. Therefore, it is not possi-
ble to correlate the effect of different sub-zero treatments on their
microstructural features with their resultant mechanical proper-
ties, like hardness and toughness which are directly influenced by
the microstructure. Moreover, attempts to relate the effect of dif-
ferent sub-zero treatments on the reported variation of WR and
fatigue properties remain inconclusive, as it is well known that
these properties are controlled by other mechanical properties, like
WR by hardness and toughness or fatigue property by hardness,
toughness and surface residual stress. For example, it is difficult to
explain the significant reduction (97%) in the overall fatigue life by
deep cryogenic treatment in comparison to cold treatment when
the values of hardness, toughness and compressive residual stress
are exactly same for both the treatments.
Rhyim et al. [45] have studied the effects of sub-zero treatments
on the microstructure and mechanical properties of D2 steel. They
have recorded moderate improvement in WR by deep cryogenic
treatment as compared to that obtained by conventional heat treat-
ment and attributed this improvement to the increased toughness
values. In contrast, Molinari et al. [59] have attributed the improve-
ment in WR by deep cryogenic treatment over conventional heat
treatment to increased hardness as well as improved homogeneity
in hardness for AISI M2 steel and to increased toughness for AISI
H13 steel. On the contrary, Leskovsek et al. [28] have mentioned
that improvement in WR by deep cryogenic treatment is only
related to suitable combination of fracture toughness and hardness
resulting from cryotreatment. Liu et al. [60,61] have investigated
the effects of deep cryogenic treatment on the microstructure and
abrasion WR of high-chromium cast irons subjected to sub-critical
treatment. These authors have shown that deep cryogenic treat-
ment improves hardness and abrasion WR due to reduction of R
content and precipitation of higher amount of finer secondary car-
bides. Babu et al. [62] have reported that abrasive WR of M1, H13
and EN19 can be improved from 193% to 289% by cold treatment
and from 315% to 335% by deep cryogenic treatment in comparison
to conventional heat treatment depending on the type of material.
Mohan Lal et al. [8] have studied the effect of sub-zero treatment on
the WR of T1, M2 and D3 steels and concluded that WR increases
in the order of conventional heat treatment, cold treatment and
deep cryogenic treatment. However, Babu et al. [62] and Mohan
Lal et al. [8] have not provided any report on mechanical proper-
ties or the characteristics of the microstructure of sub-zero treated
specimens.
One of the main aims of sub-zero treatment is to enhance the WR
of steels. While it has been widely accepted that these treatments
improve WR of tool/die steels, there exists considerable uncertainty
over the degree of improvement in WR achievable by deep cryo-
genic treatment, since the reported improvement in WR by deep
cryogenic treatment over conventional heat treatment varies from
a few percent to a few hundred percent even for the same mate-
rial [5,27,36]. For example, the improvement in WR of AISI D2
steel by deep cryogenic treatment over conventional heat treat-
ment varies from 108% as reported by Collins and Dormer [34] to
817% as reported by Barron [7]. Details of the reported uncertainty
on the degree of improvement in WR by deep cryogenic treat-
ment for tool/die steels have been compiled earlier and the reasons
behind it have been explained by the present authors [63,64]
through in-depth analyses of the characteristics of microstructures
and systematic evaluation of wear behavior of both convention-
ally and deep cryogenically treated specimens of AISI D2 steel. It
has been demonstrated that the improvement in WR by deep cryo-
genic treatment is only a few times when the operative modes and
mechanisms (mild-oxidation or severe-delamination) of wear for
both deep cryogenically and conventionally treated specimens are
identical which depends on the applied normal load and/or sliding
velocity employed in wear tests [64]. The improvement in WR by
deep cryogenic treatment is shown to be over orders of magnitude
when the operative modes and mechanisms of wear in conven-
tionally and deep cryogenically treated specimens are dissimilar
[63,64]. In addition, it has been illustrated that with respect to
conventional heat treatment, deep cryogenic treatment induces
considerable microstructural modifications that, in turn, enhances
the transition load from mild to severe wear to higher value for
tool/die steels [64]. This phenomenon results into similar and/or
dissimilar modes and mechanisms of wear between conventionally
and deep cryogenically treated specimens with varying normal load
in wear tests [63]. It has been identified that the reported uncer-
tainty over the degree of improvement in WR primarily originates
from arbitrary selection of methods and test parameters for the
evaluation of WR, and lack of suitable characterizations of worn
surfaces, subsurfaces and generated wear debris from the speci-
mens that helps to detect the operative modes and mechanisms of
wear [63,64].
Studies related to the influence of sub-zero treatments on the
mechanical properties of steels are limited and contradictory in
nature. For AISI 4340 steel, Zhirafar et al. [30] have shown that deep
cryogenic treatment in comparison to conventional heat treatment
improves hardness (3.1%) and fatigue limit (5%) at lifetimes of 107
cycles but reduces impact toughness (28%) for specimens tem-
pered at 473 K. Whereas, Baldissera et al. [31,65] have shown that
deep cryogenic treatment enhances hardness and elastic modulus
of 18NiCrMo5 carburized steel, but degrades tensile strength and
fatigue performance; it has also been shown by these authors that
the scatter in fatigue data is reduced considerably by deep cryo-
genic treatment. Harish et al. [66] have reported improvement in
hardness (14%) and reduction in impact toughness (5%) for En31
bearing steel due to deep cryogenic treatment when compared to
that obtained by conventional heat treatment. Rhyim et al. [45] and
Wierszyłłowski et al. [46] have reported that deep cryogenic treat-
ment improves hardness but reduces impact toughness of AISI D2
steel as compared to conventional heat treatment. Yun et al. [35]
have reported that deep cryogenic treatment improves hardness
(3–7%) marginally, bending strength (20%) and impact toughness
(43%) considerably of AISI M2 steel over conventional heat treat-
ment. A review of literature partnering to the influence of sub-zero
treatments in general and deep cryogenic treatment in particular
on the mechanical properties of steels reveals that there is lit-
tle consensus regarding the effect of sub-zero treatments on the
mechanical properties of steels apart from the fact that hardness
improves marginally. For example, Yun et al. [35] and Prabhakaran
et al. [56] have claimed that sub-zero treatments improve impact
toughness of steels over conventional heat treatment, but the oppo-
site trend of results have been reported by Zhirafar et al. [30],
Zurecki [36], Rhyim et al. [45], Wierszyłłowski et al. [46], Molinari
et al. [59] and Harish et al. [66]. While, Zhirafar et al. [30] have
shown that deep cryogenic treatment improves fatigue properties
of steels than that achieved by conventional heat treatment, the
opposite has been reported by Baldissera et al. [31], Bensely et al.
[57] and Jung et al. [67].
The above discussion clearly indicates out that the influence of
sub-zero treatments on the microstructure and mechanical prop-
erties of tool/die steels is far from being understood. Therefore,
Author's personal copy
D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193 2185
Table 1
Nominal composition of the investigated AISI D2 steel.
Elements Amount (wt%)
C 1.49
Mn 0.29
Si 0.42
S 0.028
P 0.029
Cr 11.38
Mo 0.80
V 0.68
Fe Balance
in the present work a systematic investigation has been directed
to highlight the effect of different types of sub-zero treatment on
the microstructure, hardness and wear behavior and their mutual
correlations using AISI D2 steel.
3. Materials and methodology
The selected steel was received as hot forged bar and its chemical
composition as given in Table 1 conforms to the AISI specifica-
tion of D2 steel. Specimen blanks of size 24 mm ×16mm ×85 mm
were subjected to conventional heat treatment and different types
of sub-zero treatments in separate batches. The conventional
heat treatment consisted of hardening and tempering, while sub-
zero treatment involves an additional low temperature treatment
cycle intermediate between hardening and tempering treatments.
Different sub-zero treatments studied in this investigation are
cold treatment, shallow cryogenic treatment and deep cryogenic
treatment which were subjected to intermittent low tempera-
ture treatment at 198 K, 148 K and 77 K, respectively. In sub-zero
treatments, conventionally hardened specimens were cooled from
ambient temperature to the predetermined sub-zero temperature,
held there for pre-selected duration before heating back to the
ambient temperature for subsequent tempering treatment. Cool-
ing and heating for all sub-zero treatments were controlled at a
uniform rate of 0.75 K/min. For both cold treatment and shallow
cryogenic treatment, the duration of soaking at lowest temper-
ature was 5 min that was considered sufficient for temperature
homogenization of the specimens due to the slow employed cooling
rate. However, the selected duration of soaking in deep cryogenic
treatment was 36 h, based on the previous reports by the authors
[68,69]. The selected duration of 36 h refers to the optimized soak-
ing duration to maximize the WR by deep cryogenic treatment for
the chosen steel [69]. For all types of treatments, the hardening and
single tempering treatments were carried out at 1297 K for 30min
and at 483 K for 120 min, respectively, following ASM standard [43],
the details of which are reported elsewhere [68]. The temperature
control at each stage of heat treatment was ±2K.
Microstructural examinations were carried out on polished and
picral (3 g picric acid in 100 ml ethanol) etched specimens using
both optical (Axiovert 40 MAT, Carl Zeiss) and scanning electron
microscopes (SEM, JSM-5510, JEOL). The microstructures reveal
the carbide particles in tempered martensitic matrix. The carbide
particles were categorized, as primary carbides (size >5m) and
secondary carbides (size 5m). The secondary carbides were fur-
ther sub-classified as large secondary carbides (1 m <size 5m)
and small secondary carbides (0.1 msize 1m). Justifications
for the classification of carbides and the selection of their size lim-
its have been reported earlier [50]. With the help of Leica QMetals
software, image analyses were done on randomly acquired digital
micrographs to estimate (a) the volume fraction and size of primary
carbides, large and small secondary carbides and (b) the popula-
tion density and interparticle spacing of large and small secondary
carbides. Estimation of the stereological parameters of the carbide
particles were made following the report of Fukaura et al. [49]. The
number of particles considered for the above analyses was a mini-
mum of 1000 for each specimen in order to estimate the statistical
reliability associated with these measurements.
Retained austenite (R) content in the developed microstruc-
tures were measured by X-ray diffraction (XRD) analyses of the bulk
specimens made with the help of a PHILIPS PW 1830 X-ray diffrac-
tometer using Mo-K˛radiation following ASTM standard E975-00
[70]. Details of the procedures followed to estimate Rcontent
with high accuracy have been reported elsewhere [68]. Identifi-
cation of the exact nature of the carbides was found to be difficult
by XRD analyses of the bulk specimens due to their presence in
small amount [70,71]. Therefore, carbide particles were electrolyt-
ically extracted from all types of specimens following the report
of Nykiel and Hryniewicz [72]. XRD analysis of the extracted car-
bide particles were done in an identical manner to that for bulk
specimens. The phases in the extracted carbide particles and bulk
specimens were identified from the XRD profiles with the help of
PHILIPS X’Pert software. The electrolytic extraction of the particles
was carried out with the help of 5%-HCl aqueous solution of spe-
cific gravity of 1.19 g/cm3using current density of 10 mA/cm2for
duration of 18–20 h. X-ray diffraction analysis of extracted carbide
particles was done in an identical manner to that done for bulk
specimens. The phases in the extracted carbide particles and bulk
specimens were identified by analysing the XRD profiles with the
help of PHILIPS X’Pert software. In addition, size and morphology
of extracted carbide particles were studied by SEM, and their ele-
mental microanalyses were carried out by energy dispersive X-ray
(EDX) method using NORAN System Six unit attached to the SEM.
The macrohardness and microhardness values of the differently
heat treated specimens were estimated by Vickers indentation
technique at the applied load of 60 kgf (589 N) and 50 gf (0.5 N),
respectively. Though the indentations for microhardness measure-
ment of the matrix were made by carefully avoiding the easily
separable primary carbides, the measured value is expected to be
influenced by the characteristics of secondary carbides. A minimum
of fifty readings were taken to estimate the average value of micro-
hardness of the specimen matrix and at least ten readings were
considered for estimating the average value of macrohardness.
4. Results
4.1. Microstructural characteristics
Fig. 1 depicts representative optical micrographs of conven-
tional heat treated and one of the sub-zero treated specimens.
These micrographs exhibit non-uniform distribution of primary
carbides and fairly uniform distribution of secondary carbides on
tempered martensite matrix with resolved prior austenite grain
boundaries. Large, elongated dendritic-type white regions repre-
sent primary carbides, while secondary carbides appear either as
tiny black patches (marked by circle-1) or small white regions
(marked by circle-2) as shown in Fig. 1. The amount, size and distri-
bution of the primary carbides appear identical irrespective of the
type of heat treatment; this is expected because the characteristics
of the primary carbides are controlled only by the time and temper-
ature of austenitization [1,2,50], which have been kept invariant in
this study. The optical micrographs in Fig. 1 fail to bring out distinc-
tive difference between the microstructures of conventional heat
treated and sub-zero treated specimens due to limited magnifica-
tion of optical microscope. Similar observations have been reported
by Vimal et al. [52] for En31 steel and Surberg et al. [53] for D2
steel. These investigators have reported difficulty in differentiating
the microstructures of variedly sub-zero treated steel specimens
by optical microscope. However, SEM micrographs in Fig. 2 clearly
Author's personal copy
2186 D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193
Fig. 1. Representative optical micrographs of (a) conventionally heat treated and
(b) shallow cryogenically treated specimens exhibiting white regions of dendritic
primary carbides (PCs) and secondary carbides (SCs) either as tiny black patches
(marked by circle-1) or as small white regions (marked by circle-1) on tempered
martensite matrix (black) with resolved prior austenite grain boundaries.
delineate the effect of varied sub-zero treatment on the modifi-
cation of precipitation behavior of secondary carbides of AISI D2
steel. It is clear from Fig. 2 that two types of secondary carbides,
i.e., small white regions and tiny black patches in optical micro-
graphs (Fig. 1) have resulted due to their size difference. The two
types of secondary carbides are classified here as large and small
secondary carbides based on their size limits as mentioned in Sec-
tion 3 [68,69]. Comparison of the micrographs in Fig. 2 reveals
that the population density of secondary carbides, specifically small
secondary carbides, increases with decreasing temperature of sub-
zero treatment. It may be mentioned that occasional patches of
retained austenite (R) have been detected in only the micrographs
of conventionally heat treated and cold treated specimens (Fig. 2).
Fig. 3 compares the XRD profiles of bulk specimens of D2
steel subjected to different heat treatment schedules. In com-
parison to conventional heat treatment, cold treatment markedly
reduces the intensity of characteristic diffraction peaks of austenite
with concurrent increase in the characteristic diffraction peaks of
martensite (Fig. 3). The diffraction peaks of austenite are absent
in XRD profiles of both shallow and deep cryogenically treated
specimens suggesting that the amount of Rin these specimens
are below the detection limit (<2 vol.%) of XRD technique [71].
The XRD line profiles in Fig. 3 appear to suggest the presence of
M7C3and M23C6(M = Fe, Cr, Mo, V) carbides in the differently
heat treated steel samples. However, it is difficult to ensure their
presence from the XRD profiles of bulk specimens because peak
intensities of the carbide phases are low due to their small amount
apart from the fact that several characteristic peaks of carbides
are superimposed with the characteristic peaks of either austen-
ite or martensite phase. In order to resolve this, carbide particles
of all types of specimens have been electrochemically extracted
and their XRD analyses have been carried out separately. The mor-
phological examinations and semi quantitative chemical analyses
of the extracted carbides have also been carried out using SEM
coupled with EDX facilities. Fig. 4 depicts the XRD patterns of the
extracted carbide particles from conventional heat treated and typ-
ical sub-zero treated specimens along with the representative SEM
micrographs of extracted carbide particles and EDX profiles taken
from the primary and secondary carbide particles. The results in
Fig. 4a confirm the presence of M7C3,M
23C6and Cr7C3carbides in
D2 steel specimens with and without sub-zero treatment. The mor-
phological examinations of extracted carbide particles (Fig. 4b and
c) and their EDX microanalyses (Fig. 4d and e) coupled with XRD
line profiles (Fig. 4a) help to identify that the secondary and primary
carbides are primarily M23C6and M7C3with small amount of Cr7C3
carbides, respectively. These are in agreement with reported results
on D2 steel without any sub-zero treatment [48,49,73]. Therefore,
it is worth mentioning here that the sub-zero treatments do not
alter the nature of primary and secondary carbides.
The amounts of primary and secondary carbides have been esti-
mated by image analyses of large numbers of digitally acquired
micrographs, while the amount of Rhas been estimated by XRD
technique following ASTM standard E975-00 [70]. The volume frac-
tion of tempered martensite has been considered as 100 minus the
volume percentage of Rand all types of carbides. The estimated
amounts of different microstructural constituents are compiled in
Fig. 5. The amount of Rin cold treated specimen is 4.6 ±0.5 vol.% in
comparison to 9.8 ±0.7 vol.% in conventionally heat treated speci-
men, while both shallow and deep cryogenically treated specimens
are free of R. The results in Fig. 5 also assist to infer that that the
amount of primary carbides remains practically independent on
the type of heat treatments unlike the amount of secondary car-
bides (=small secondary carbides + large secondary carbides) which
increases from conventionally heat treatment to cold treatment to
shallow cryogenic treatment to deep cryogenic treatment.
The different size ranges of secondary carbide particles (small
and large secondary carbides) have been further substantiated
through typical SEM micrograph of extracted carbide particles in
Fig. 4c. The amount, size, population density and distribution of
these particles vary with the type of heat treatment schedules
(Fig. 2). The results obtained through image analyses on small and
large secondary carbides for all types of specimens are compiled in
Fig. 6. The results in Fig. 6a reveal that with respect to conventional
heat treatment, the amount of both small and large secondary car-
bides increase for all types of sub-zero treatments, however, the
extent of increase is considerable for deep cryogenic treatment
than that obtained either by cold treatment or shallow cryogenic
treatment. In addition, the increase in amount is more pronounced
for small secondary carbides than for large secondary carbides. For
example, the improvements in the volume percent of small and
large secondary carbides by deep cryogenic treatment over cold
treatment are 47% and 39%, respectively. The obtained results also
indicate that sub-zero treatments considerably enhances the mean
population density (Fig. 6c) but decreases the mean spherical diam-
eter (Fig. 6b) and mean interparticle spacing (Fig. 6d) of both small
and large secondary carbides when compared to conventional heat
treatment. These variations are found to be significant for cold
treatment and shallow cryogenic treatment, but remarkable only
Author's personal copy
D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193 2187
Fig. 2. Representative SEM micrographs of (a) conventionally heat treated, (b) cold treated, (c) shallow cryogenically treated and (d) deep cryogenically treated specimens
exhibiting small secondary carbides (SSCs) and large secondary carbides (LSCs). Retained austenite (R) present only in (a).
for deep cryogenic treatment. For example, in comparison to con-
ventional heat treatment, the improvement in population density
of small and large secondary carbides by cold treatment are 80% and
23%, respectively, whereas these are 193% and 160%, respectively,
by deep cryogenic treatment. The reduction in the mean spherical
diameters of small and large secondary carbides by deep cryo-
genic treatment over shallow cryogenic treatment is 8% and 12%,
respectively; whereas the decrease of mean interparticle spacing
of small and large secondary carbides by deep cryogenic treatment
with reference to shallow cryogenic treatment are 37% and 29%,
Fig. 3. X-ray diffraction line profiles of bulk sample of conventionally heat treated
(CHT), cold treated (CT), shallow cryogenically treated (SCT) and deep cryogenically
treated specimens. The set of – (hkl) in vertical direction indicates the 2positions of
different diffraction planes of austenite (), ferrite/martensite () along with M7C3
and M23C6carbides. The variation of intensity of (2 2 0) peak has been highlighted.
respectively. These observations clearly reveal that the sub-zero
treatment in general and deep cryogenic treatment in particular
considerably modifies the precipitation behavior of secondary car-
bides. The deep cryogenic treatment indeed refines the secondary
carbides, increases their amount and population density, and leads
to their more uniform distribution in the microstructures.
4.2. Hardness values
The estimated bulk hardness (HV60) and hardness of the matrix
(HV0.05) of differently heat treated specimens are shown in Fig. 7.
The HV60 for conventional heat treated, cold treated, shallow
and deep cryogenically treated specimens are 7.44 ±0.04 GPa,
7.63 ±0.05 GPa, 7.72 ±0.06 GPa and 8.04 ±0.03 GPa, respectively.
The HV0.05 of conventional heat treated specimen is 9.03 ±0.06 GPa
while the same for cold treated, shallow and deep cryogeni-
cally treated specimens are 9.29 ±0.07 GPa, 9.42 ±0.06 GPa and
10.06 ±0.10 GPa, respectively. In general, the magnitudes of HV0.05
for all the samples are higher than their corresponding HV60; this
phenomenon can be simply attributed to the indentation size effect
[74]. The results in Fig. 7 unambiguously infer that both bulk hard-
ness and hardness of the matrix of D2 steel specimens improve
by sub-zero treatments, but the degree of improvement in hard-
ness varies with the type of sub-zero treatments. The hardness
values are considerably higher for deep cryogenic treatment than
that for cold treatment or shallow cryogenic treatment. Moreover,
the effect of sub-zero treatments is found to be more pronounced
for improvement in hardness of the matrix compared to bulk hard-
ness of the specimens. For example, improvement in hardness by
deep cryogenic treatment over conventional heat treatment is 8.1%
for bulk hardness against 11.4% for hardness of the matrix.
5. Discussion
5.1. Variation of retained austenite content
The amount of retained austenite (R) in cold treated specimen
is 4.6 ±0.5 vol.% in comparison to 9.8 ±0.7 vol.% in convention-
Author's personal copy
2188 D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193
Fig. 4. (a) X-ray diffraction line profiles of electrolytically extracted carbide particles from conventionally heat treated (CHT) and deep cryogenically treated (DCT) specimens;
the set of – (hkl) in vertical direction indicates the 2positions of different diffraction planes of M7C3,M
23C6and Cr7C3carbides. (b) Representative SEM micrograph of extracted
carbide particles from deep cryogenically treated specimen exhibiting the size and morphology of primary carbides (PCs) and secondary carbides (SCs). (c) Close up view of
secondary carbide particles illustrating large secondary carbides (LSCs) and small secondary carbides (SSCs). (d) and (e) Representative EDX profiles of primary and secondary
carbides, respectively.
ally treated specimen; while both shallow and deep cryogenically
treated specimens are free of R(Figs. 3 and 5). These results
suggest that cold treatment considerably reduces the Ras com-
pared to conventional heat treatment; while both shallow and deep
cryogenic treatments almost completely remove it. The martensite
finish temperature for AISI D2 steel is 148 K, thus both shallow
and deep cryogenic treatments, having lowest temperatures of
148 K and 77 K respectively, almost completely transforms all R
to martensite expectedly. The present observations are in agree-
ment with the reported results for tool/die steels in deep cryogenic
treatment [5,17,18,29,34,35,44–46,51,53] and shallow cryogenic
treatment [47,53] conditions. However, the obtained variations of
Rcontent with different sub-zero treatments for AISI D2 steel
specimens are in contrast to that reported by Meng et al. [33] for the
same steel. These authors have reported that the amount of Rin D2
steel specimen is considerable (6 vol.%) even after deep cryogenic
treatment and the Rcontent have been reported to be almost iden-
tical for both cold treated and deep cryogenically treated specimens
[33].
The apparent contradiction of results related to the variation
of Rcontent with sub-zero treatments obtained in the present
study and that reported by Meng et al. [33] may originate from
two sources. Firstly, the accuracy of measurement of Rcontent
through XRD analyses by Meng et al. [33] is arguable. These authors
Author's personal copy
D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193 2189
Fig. 5. Effect of heat treatment schedules on the variation of amount of different
phases. CHT: conventionally heat treated; CT: cold treated; SCT: shallow cryogeni-
cally treated; DCT: deep cryogenically treated.
have considered only (2 1 1) peak of martensite and (3 1 1) peak
of austenite which are not in conformity to the recommendation
of ASTM standard E975-00 [70] which suggests consideration of
at least three peaks of each phases for accurate estimation of R
content. Moreover, as AISI D2 is high carbon and high alloy steel,
the characteristic peaks of austenite and martensite in XRD line pro-
files of this steel are generally superimposed with the characteristic
peaks of different carbides as mentioned in the ASTM standard [70]
and as shown in Fig. 3. For example, the (2 1 1) peak of martensite
is superimposed with the (9 1 1) peak of M23C6carbide and (7 5 0)
peak of M7C3carbide, whereas (3 11) peak of austenite is superim-
posed with the (8 4 4) peak of M23C6carbide (Fig. 3). Therefore,
for this type of steels, the volume fraction of total carbide con-
tent should first be measured by other suitable techniques prior
to estimation of Rcontent by XRD analyses; this procedure has
not been followed by Meng et al. [33]. In the present investigation,
Fig. 7. Effect of heat treatment schedules on the variation of macrohardness (HV60)
and microhardness (HV0.05) values. CHT: conventionally heat treated; CT: cold
treated; SCT: shallow cryogenically treated; DCT: deep cryogenically treated.
first the volume fraction of total carbide content has been mea-
sured by image analyses and next four intense peaks of each of
martensite and austenite have been considered for estimation of
Rcontent. Secondly, in sub-zero treatments, Meng et al. [33] had
subjected conventionally hardened steel specimens to an ageing
treatment at 333 K for 1 h before sub-zero processing followed by
tempering treatment. This type of ageing treatment is popularly
termed as ‘snap tempering’ in cryogenic treatment of materials
[75]. It is well known that ageing or snap tempering of as-hardened
Fig. 6. Image analyses results of (a) amount, (b) mean spherical diameter, (c) mean population density and (d) mean interparticle spacing of small secondary carbides
(SSCs) and large secondary carbides (LSCs) in conventionally heat treated (CHT), cold treated (CT), shallow cryogenically treated (SCT) and deep cryogenically treated (DCT)
specimens.
Author's personal copy
2190 D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193
steel specimens stabilizes austenite [14,15,17,76]. During ageing of
as-hardened steel specimens, carbon atoms initially diffuses pri-
marily from martensite to the martensite/austenite interfaces and
segregate there, thus, anchoring the normally mobile dislocations;
while at later stages the diffusion carbon atoms into austenite takes
place resulting in enrichment of carbon in austenite [76]. These
phenomena are well known to hinder the transformation of austen-
ite to martensite which is generally called as thermal stabilization
of austenite [3,76]. Therefore, ageing of as-hardened steel speci-
mens before sub-zero processing is expected to reduce the extent
of transformation of austenite to martensite due to thermal stabi-
lization of austenite which, in turn, could result in higher amount
of Reven after sub-zero treatments [3,15,18,76] as observed by
Meng et al. [33]. In contrast, substantial reduction in Rcontent
has been obtained by cold treatment in comparison to conventional
heat treatment, while shallow and deep cryogenic treatments have
been found to almost completely remove Rwhen no ageing treat-
ment has been incorporated in between conventional hardening
and sub-zero processing of D2 steel as in the present study and in
same reports by other investigators [5,26,34,44–47].
5.2. Variation of characteristics of secondary carbides
The results in Fig. 5 infer that in comparison to conventional heat
treatment, the amount of secondary carbides increases by 7%, 14%
and 53% for cold treatment, shallow and deep cryogenic treatment,
respectively. Substantial enhancement in the amount of carbides
by deep cryogenic treatment over conventional heat treatment has
been reported earlier by Huang et al. [51]. Through image analyses
of TEM micrographs of AISI M2 steel, these authors have reported
that the amount of carbides in deep cryogenic treated specimen
is 11 ±1 vol.% in comparison to only 5±1 vol.% in the convention-
ally heat treated specimen. Results related to the characteristics of
secondary carbides of differently heat treated D2 steel specimens
in Fig. 6 reveal that the sub-zero treatments refine the secondary
carbides, increase their amount and population density, and lead
to their more uniform distribution in the microstructures. These
favorable modifications of secondary carbides are found to be sig-
nificantly higher for deep cryogenic treatment than that obtained
either for cold treatment or shallow cryogenic treatment.
Investigations related to the effect of sub-zero treatments on
the characteristics of carbide precipitation are very few. Collins
and Dormer [34] have estimated the population density of carbides
with mean spherical diameter <5 m, equivalent to secondary car-
bides of the present study, in conventionally heat treated and deep
cryogenically treated specimens of D2 steel by image analyses of
optical micrographs. The population density of carbides reported
by Collins and Dormer [34], however, is approximately 5.4 times
and 13.2 times less than that obtained in the present study of the
same steel for conventionally heat treated and deep cryogenically
treated specimens, respectively. This apparent contradiction is due
to the different techniques used to resolve carbide particles, i.e.,
optical microscopy by Collins and Dormer [34] and SEM in the
present investigation. The SEM micrographs in Fig. 2 illustrates
that considerable part of small secondary carbides are too small
to be resolved in the optical micrographs. Presumably, Collins and
Dormer [34] have missed large numbers of small secondary carbide
particles in the estimation of population density of carbides leading
to lower magnitude of its value. This is supported by the fact that
the difference in population density of carbide particles between
that reported by Collins and Dormer [34] and the present study is
considerably higher for deep cryogenically treated specimens than
that for conventionally heat treated specimens. This is because car-
bide particles are shown to be much finer in the former specimens
(Fig. 2d) than that in the latter specimens (Fig. 2a). By image analy-
ses of SEM micrographs for AISI M2 steel, Alexandru et al. [77] have
studied the effect of cold treatment on the population density of
fine carbide particles having size <1 m; i.e., practically equivalent
to the small secondary carbides in the present study. Comparison
of the results in Fig. 6c infers that the enhancement of population
density of small secondary carbides by cold treatment over con-
ventional heat treatment is 80%, which is in excellent agreement
with that (83%) reported by Alexandru et al. [77].
5.3. On the modification of precipitation behavior of secondary
carbide
In general, increase in the volume percent of secondary car-
bides by sub-zero treatments is associated with either considerable
reduction or almost complete removal of Rby different sub-zero
treatments as shown in Fig. 5. The alteration in the volume per-
cent of secondary carbides occur due to lowering or removal of
Rresulting into higher amount of martensite and in turn lead-
ing to formation of more amount of tempered martensite which
assists in precipitation of higher amount of secondary carbides
during tempering treatment. For example, cold treatment substan-
tially reduces Rcontent and considerably increases the amounts
of both secondary carbides and tempered martensite with respect
to conventional heat treatment (Fig. 5). The observed difference
in the amount of secondary carbides between shallow and deep
cryogenically treated specimens, however, can not be explained by
the variation of Rcontent, since both the specimens are found
to be almost free of R(Figs. 3 and 5). The degree of precipita-
tion of secondary carbides during tempering of martensite not only
depends on the amount of martensite in as-quenched structure
but is also controlled by the state of tempering of martensite. If
the decomposition of martensite is accelerated in one as-quenched
structure than the other, the former would lead to higher amount
of secondary carbides than the latter ones, even though both the
structures initially have the same of amount of martensite and
are tempered in identical condition. Deep cryogenic treatment of
tool/die is known to enhance the decomposition of martensite dur-
ing tempering in comparison to conventional heat treatment or
other sub-zero treatments like cold treatment and shallow cryo-
genic treatment [33,44,45,78]. Pellizzari and Molinari [44] have
studied tempering behavior of conventionally and cryogenically
hardened cold-worked tool steels employing differential scanning
calorimetry (DSC) and dilatometry. These investigators have shown
that with respect to conventional hardening, cryogenic hardening
accelerates the decomposition of martensite and decreases tem-
perature range of different phase transformations occurred during
tempering. Stojko [78] has studied the effect of cold treatment and
deep cryogenic treatment on tempering behavior of AISI 52100
and 1070 steels by using DSC, dilatometry, synchrotron X-ray
diffraction, vibrating sample magnetometry (VSM), SEM, optical
microscopy and microhardness measurements. This investigator
has observed decrease in coercivity (magnetic softening) on reheat-
ing from temperatures below 193 K to room temperature and this
phenomenon has been attributed to the depletion of carbon atoms
from the solid solution of martensite either due to segregation or
clustering (/ordering) of carbon atoms. From the results of dilatom-
etry and VSM of differently hardened steel specimens on tempering
from ambient temperature to 375 K, he has reported that the auto-
tempering is completed for sub-zero hardened specimens, whereas
it is incomplete for conventionally hardened specimen. In addition,
Stojko [78] has also shown that the precipitation of first transition
carbide particles is considerably higher for cryogenically hardened
specimens as compared to the conventionally hardened ones.
Meng et al. [33] have investigated the microstructure of D2
steel specimens subjected to conventional heat treatment, cold
treatment and deep cryogenic treatment by using TEM. These
investigators have shown that microstructures of deep cryogeni-
Author's personal copy
D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193 2191
cally treated specimens consist of very fine rod shaped precipitates,
characterized as pre-cementite -carbide; however, no carbides
have been observed in the microstructures of either convention-
ally heat treated or cold treated specimens. The microstructures
of both conventional heat treated and cold treated specimens
exhibit modulated structure of carbon rich and carbon depleted
regions developed by the spinodal decomposition of martensite
[41]. Therefore, the microstructural states after identical temper-
ing treatment of differently hardened steel specimens as reported
by Meng et al. [33] suggest that the stage of tempering is at the
1st for deep cryogenically treated specimens but 0th for both cold
treated and conventionally heat treated specimens [41]. However,
within 0th stage of tempering the decomposition of martensite is at
advanced state in cold treated specimens than that in convention-
ally heat treated specimens [3,41], since the reported wavelength of
modulated structure is 5 nm for former specimens but only 1 nm for
the latter specimens [33]. Thus, careful examinations of microstruc-
tural characteristics reported by Meng et al. [33] indicate that the
effect of cold treatment on the decomposition of martensite is
marginal but the same is considerable for deep cryogenic treat-
ment, when both are compared to conventional heat treatment.
It is reasonable to consider from the above discussion that the
martensite decomposition enhances with decrease in temperature
of sub-zero treatments. In other words, the influence of shallow
cryogenic treatment on decomposition kinetics of martensite dur-
ing tempering will be higher than that for cold treatment but lesser
as compared to deep cryogenic treatment. This gets supported from
the results recently reported by Pellizzari [79]; they have shown
that deep cryogenic treatment considerably lowers the temper-
ing temperature corresponding to the secondary hardening peak
for a wide range of high speed steels. Furthermore, the present
authors have earlier shown that the time of soaking has consider-
able effect on precipitation of secondary carbides in deep cryogenic
treated D2 steel [50,68,69]. The selected duration of soaking at low-
est temperature is significantly higher for deep cryogenic treatment
in comparison to shallow cryogenic treatment or cold treatment.
Therefore, it is natural to expect that the acceleration of martensite
decomposition during tempering is much less for shallow cryogenic
treated specimens as compared to deep cryogenically treated spec-
imens. This in turn, leads to lower amount of secondary carbides
in the former specimens than in the latter ones as observed in the
present study (Figs. 2 and 6), though the amount of martensite in
the as-quenched structures is almost identical for both specimens.
The effect of different sub-zero treatments on the microstruc-
tures of the selected steel in Figs. 1–6 clearly suggest that sub-zero
treatments not only alter the amount of Rcontent but also influ-
ence the decomposition of martensite, which in turn modify the
precipitation behavior of secondary carbides during tempering. The
latter effect is increasingly pronounced in the order of cold treat-
ment, shallow cryogenic treatment and deep cryogenic treatment.
The plausible reasons related to the modification of precipita-
tion behavior of secondary carbides by sub-zero treatments in
general and deep cryogenic treatment in particular is as follows.
A high density of crystal defects such as dislocations and twins
are generated in martensite during the sub-zero cooling cycle,
since high internal stresses are developed from the transformation
of austenite to martensite as well as from the different thermal
contractions of the phases. These effects are enhanced with the
decrease in temperature of sub-zero treatments due to higher
transformation of austenite to martensite apart from the increase
in magnitude of differential thermal contraction. The martensite
becomes more supersaturated with decreasing temperature and
this phenomenon, in turn, increases its lattice distortion and ther-
modynamic instability resulting into the segregation of carbon
atoms to nearby defects forming clusters. These clusters either act
as or grow into nuclei for the formation of carbide on subsequent
warming up or during tempering. The number of such atomic clus-
ters increases with decreasing temperature of sub-zero treatments
and increasing soaking duration at the lowest temperature. Thus,
decomposition of martensite will be faster in the order of cold treat-
ment, shallow cryogenic treatment and deep cryogenic treatment
that in turn leads to higher amount of refined secondary carbides in
the same order as observed in the present study. These explanations
are in line with the earlier contentions of Das et al. [50] and Huang
et al. [51]. Experimental observation of a high density of crystal
defects in cryotreated tool steels [35,45] and the requirement of a
long holding time at cryogenic temperature for achieving consider-
able benefit from the cryotreatment process [8,12,27,34,36,53,69]
supports this proposition.
5.4. Variation of hardness
The results in Fig. 7 infer that the improvement in bulk hard-
ness by cold treatment, shallow and deep cryogenic treatments
with respect to conventional heat treatment are 2.6%, 3.8% and
8.1%, respectively. The bulk hardness values for specimens sub-
jected to different heat-treatment schedules can be directly related
to the magnitude of reduction of soft Rand improvement in the
amount of hard secondary carbides and tough tempered martensite
(Figs. 5 and 6). Increase in bulk hardness of tool/die steels by deep
cryogenic treatment has been reported earlier by several investiga-
tors [5,9–12,30–38,44–47,50–54,59,63–66]. For example, Molinari
et al. [59] have reported that deep cryogenic treatment enhances
the bulk hardness with reference to conventional heat treatment
by 5.8% and 3.0% for AISI M2 and H13 tool steels, respectively.
Baldissera and Delprete [65] have reported that bulk hardness
of 18NiCrMo5 carburized steel improves by 4.1% due to deep
cryogenic treatment as compared to conventional heat treatment.
Enhancement of bulk hardness by deep cryogenic treatment over
conventional heat treatment between 3.1% and 1.6% depending
on the tempering temperature for 4340 steel have been reported
by Zhirafar et al. [30]. However, the reported variation of hard-
ness with temperature of sub-zero treatments is contradictory in
nature. Akhbarizadeh et al. [80] have shown that improvement in
bulk hardness of D3 steel by deep cryogenic treatment is higher
than that achieved by cold treatment, and similar results have been
reported by Harish et al. [66] for En 31 steel. In contrast, Bensely
et al. [54] have shown that improvement in bulk hardness by both
cold treatment and deep cryogenic treatment over conventional
heat treatment is identical for case carburized En 353 steel. Collins
and Dormer [34] have also shown that the hardness of D2 steel
increases with decrease in temperature of sub-zero treatments
upto 193 K, but further decrease in temperature either reduces or
has no effect on hardness. On the contrary, Moore and Collins [47]
have shown that the hardness of D2 steel is higher for specimens
subjected to sub-zero treatment at 133 K (shallow cryogenic treat-
ment) than that achieved either at 223 K (cold treatment) or at 77 K
(deep cryogenic treatment). The contradictory reports on varia-
tion of bulk hardness with different sub-zero treatments of tool/die
steels are considered to originate from the variations in the param-
eters selected for sub-zero processing as well as hardening and
tempering treatments by different investigators, even for the same
steel. The influence of sub-zero processing parameters, like soak-
ing time at lowest temperature, on bulk hardness of die steel has
recently been demonstrated by the present authors [68,69]. More-
over, it is well known that bulk hardness of tool/die steels depends
on the amount, size and distribution of primary carbides, secondary
carbides, Rand tempered martensite phases [1–3]. These charac-
teristic features are significantly influenced by selection of (i) time
and temperature of austenitization, (ii) type of quenching medium
and its temperature and (iii) the number of cycles as well as time
and temperature of tempering treatment.
Author's personal copy
2192 D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193
Investigations pertaining to the influence of the above men-
tioned parameters on the microstructural constituents and
mechanical properties of steels are a few. Collins and Dormer [34]
have studied the influence of austenitizing temperature on the
microstructure and hardness of D2 steel specimens subjected to
different types of sub-zero treatments and conventional heat treat-
ment. These authors have shown that increase of austenitizing
temperature enhances the difference of bulk hardness between
conventionally treated and deep cryogenically treated D2 steel
specimens; this has been attributed to increase in the amount of
Rwith increasing austenitizing temperature. Surberg et al. [53]
have supported the contention of Collins and Dormer [34] and have
also reported that amount of Rincreases with increasing austen-
itizing temperature. Increase in the amount of Rwith increasing
austenitizing temperature for tool/die steels is well known [1–6].
Higher austenitizing temperature causes more dissolution of pri-
mary carbides that, in turn, increases the level of carbon and
alloying elements in austenite phase and thereby increases the
stability of austenite or lowers the martensite finish temperature.
Therefore, the extent of conversion of austenite to martensite by
hardening treatment would be less and this would naturally result
in higher amount of Rin the as-hardened steels.
Surberg et al. [53] have also examined the effects tempera-
ture of sub-zero treatment and number of tempering cycle on the
amount of Rin the D2 steel. For higher austenitizing tempera-
ture, these authors have shown that hardening by sub-zero cooling
to 153 K fails to convert all the austenite to martensite, though
the amount Rgets reduced considerably as compared to the con-
ventionally hardened ones. It has also been shown that sub-zero
hardened specimens require single tempering treatment to com-
pletely remove R; while to attain the same effect double tempering
treatments is required for conventionally hardened specimens [53].
Rhyim et al. [45] have investigated the influence of tempering tem-
perature on the microstructure and hardness of conventionally
treated, cold treated and deep cryogenically treated specimens of
D2 steel. These authors have concluded that both types of sub-zero
treatments have no effect on the amount of Runlike convention-
ally treated ones, when hardened specimens are tempered at higher
temperatures. It has been reported by these authors that increase
in tempering temperature monotonically reduces the hardness of
differently hardened specimens of D2 steel; however, the nature
of variation of hardness with tempering temperature is different
for conventionally and deep cryogenically hardened samples. The
above discussion, therefore, infers that the optimum heat treat-
ment parameters for conventional heat treatment and different
types of sub-zero treatments are different.
All types of sub-zero treatments improve the hardness of the
matrix of the D2 steel specimens (Fig. 7), however, the degree of
improvement of hardness of the matrix by deep cryogenic treat-
ment (11.4%) is considerably higher than that achieved either by
shallow cryogenic treatment (4.3%) or by cold treatment (2.9%)
when compared to conventional heat treatment. It is worthy to
mention here that the hardness of the matrix obtained by micro-
hardness testing is the apparent hardness, since microhardness
inherently accounts for the influence of the secondary carbides,
specifically of small secondary carbides. Investigation pertaining
to the variation of hardness of the matrix of tool/die steels sub-
jected to varied sub-zero treatments is scanty. Molinari et al. [59]
have reported that the influence of deep cryogenic treatment on
microhardness is material dependent. These authors have noted
that deep cryogenic treatment does not change microhardness but
enhances the homogeneity of hardness distribution for AISI M2
steel; whereas, significant improvement in microhardness value
(29.5%) has been reported by them for ASP 60 spline forming
tool. Zurecki [36] has shown that microhardness of A2 grade tool
steel increases by 7% due to deep cryogenic treatment over con-
ventional heat treatment, which is in good agreement with the
obtained results in this study (Fig. 7). The observed variation of
apparent hardness values of the matrix for differently heat treated
specimens is attributed to the effect of varied heat treatment sched-
ules on the estimated variations in the amounts of different phases
(Fig. 5) as well as characteristics of secondary carbide particles
(Fig. 6). The improvement in apparent hardness of the matrix (6.8%)
by deep cryogenic treatment over shallow cryogenic treatment is
due to the pronounced effect of the former treatment on enhance-
ment in the amount of secondary carbides and reduction of their
size as well as interparticle spacing.
6. Conclusions
The obtained experimental results and their pertinent analyses
assist to infer the following major conclusions:
(i) X-ray diffraction analyses reveal that retained austenite
(R) content in conventional heat treated specimens is
9.80 ±0.7 vol.% against 4.60 ±0.5 vol.% in cold treated speci-
mens, while the same is below the detection limit for both
shallow and deep cryogenically treated specimens. These
observations assist to infer that incorporation of sub-zero
treatment immediately after conventional hardening leads to
either substantial reduction or almost complete removal of R
after tempering. The extent of removal of Rdepends on the
lowest temperature of sub-zero treatment.
(ii) Sub-zero treatments accelerate the decomposition of marten-
site and modify the precipitation behavior of secondary
carbides. However, characterization of bulk specimens as well
as electrolytically extracted carbide particles reveals that sub-
zero treatments do not alter the nature of secondary carbides.
(iii) In general, sub-zero treatments refine the size of the secondary
carbides, increase their amount and population density, and
lead to their more uniform distribution in the microstructures.
These favorable modifications of secondary carbides are found
to be significantly higher in deep cryogenically treated speci-
mens than that in cold treated or shallow cryogenically treated
specimens.
(iv) Both bulk hardness and apparent hardness of the matrix of
the selected steel specimens increase with decreasing tem-
perature of sub-zero treatments; however, the improvement
in hardness values is marginal for cold treated and shal-
low cryogenically treated specimens but significant for deep
cryogenically treated specimens when compared to the con-
ventionally heat treated specimens. The effect of sub-zero
treatment is more pronounced on the manifestation of the
hardness of the matrix than that of the bulk hardness of the
heat treated specimens.
Acknowledgements
The authors would like to thank Dr. P.P. Chattopadhyay, B.E.S.U.
Shibpur, India, for stimulating discussions on carbide precipita-
tion. Authors are also grateful to Dr. A. Samanata for supplying
the material, Mr. M. Kundu and Mr. R. Mukherjee for their help in
microstructural characterizations. The financial assistance received
from the University Grants Commission, Government of India
[Grant no. F. No. 31-48/2005(SR)] to carry out a part of this research
is also gratefully acknowledged.
References
[1] G. Roberts, G. Krauss, R. Kennedy, Tool Steels, 5th ed., ASM International, Metals
Park, OH, USA, 1998.
Author's personal copy
D. Das et al. / Materials Science and Engineering A 527 (2010) 2182–2193 2193
[2] K.E. Thelning, Steel and Its Heat Treatment, 2nd ed., Butterworths, London,
1984.
[3] R.E. Reed-Hill, R. Abbaschian, Physical Metallurgy Principles, 3rd ed., PWS Pub-
lishing Company, Boston, 1992.
[4] R.G. Bowes, Heat Treat. Met. 1 (1974) 29–32.
[5] D. Das, A.K. Dutta, V. Toppo, K.K. Ray, Mater. Manuf. Process 22 (2007) 474–
480.
[6] E.A. Carlson, ASM Handbook, vol. 4, Heat Treating, 10th ed., ASM International,
Metals Park, Ohio, 1990, pp. 203–206.
[7] R.F. Barron, Cryogenics 22 (1982) 409–413.
[8] D. Mohan Lal, S. Renganarayanan, A. Kalanidhi, Cryogenics 41 (2001) 149–155.
[9] A. Gulyev, Metallurgy 12 (1937) 65–77.
[10] A.N. Popandopulo, L.T. Zhukova, Met. Sci. Heat Treat. 22 (1980) 708–710.
[11] R.F. Barron, Prog. Refrig. Sci. Technol. 1 (1973) 529–533.
[12] D.N. Collins, Heat Treat. Met. 2 (1996) 40–42.
[13] H. Scott, Scientific Papers, Bureau of Standards 16 (1920) 521–536.
[14] J.A. Mathews, Trans. Am. Soc. Steel Treat. 8 (1925) 565–583.
[15] R.L. Dowdell, O.E. Harder, Trans. Am. Soc. Steel Treat. 11 (1927) 391.
[16] G.V. Luerssen, O.V. Greene, Trans. Am. Soc. Steel Treat. 19 (1932) 501–552.
[17] P. Gordon, M. Cohen, Trans. ASM 30 (1942) 569–588.
[18] J.R. Kennedy, Trans. ASM 34 (1945) 251–309.
[19] N.W. Nordquist, Tooling and Production, July 1953, pp. 72–100.
[20] V. Morris, Machine and Tool Blue Book, Jan 1955, pp. 124–134.
[21] V. Wilson, Iron Age 207 (1971) 58.
[22] J. Taylor, Metalworking Production, May 1978, pp.73–77.
[23] P.C. Miller, Tooling and Production, vol. 45, 1980, pp. 82–86.
[24] T.P. Sweeney, Heat Treat. 18 (1986) 28–32.
[25] R.B. Reasbeck, Metallurgia 56 (1989) 178–179.
[26] D. Das, K.K. Ray, A.K. Dutta, Wear 267 (2009) 1361–1370.
[27] P.F. Stratton, Mater. Sci. Eng. A449–451 (2007) 809–812.
[28] V. Leskovsek, M. Kalin, J. Vizintin, Vacuum 80 (2006) 507–518.
[29] M. Preciado, P.M. Bravo, J.M. Alegre, J. Mater. Process. Tech. 176 (2006)
41–44.
[30] S. Zhirafar, A. Rezaeian, M. Pugh, J. Mater. Proc. Tech. 186 (2007) 298–303.
[31] P. Baldissera, Mater. Des. 30 (2009) 3636–3642.
[32] D. Das, A.K. Dutta, K.K. Ray, Wear 267 (2009) 1371–1380.
[33] F. Meng, K. Tagashira, R. Azuma, H. Sohma, ISIJ Int. 34 (1994) 205–210.
[34] D.N. Collins, J. Dormer, Heat Treat. Met. 3 (1997) 71–74.
[35] D. Yun, L. Xiaoping, X. Hongshen, Heat Treat. Met. 3 (1998) 55–59.
[36] Z. Zurecki, In: D. Herring, R. Hill (Eds.), Proceedings of the 23rd ASM Heat Treat-
ing Society Conference, ASM Proceedings: Heat Treating, Pennsylvania, 2006,
pp. 106–113.
[37] D. Das, A.K. Dutta, K.K. Ray, Mater. Sci. Eng. A. 527 (2010) 2194–2206.
[38] V. Soundararajan, N. Alagurmurthi, K. Palaniradja, Trans. Mater. Heat Treat. 25
(2004) 531–535.
[39] Y. Hirotsu, S. Nagakura, Acta Met. 20 (1972) 645–655.
[40] L. Cheng, C.M. Brakman, B.M. Korevaar, E.J. Mittemeijer, Metall. Trans. A19
(1988) 2415–2426.
[41] K.A. Taylor, G.B. Olson, M. Cohen, J.B. Vander Sande, Metall. Trans. A20 (1989)
2749–2765.
[42] L. Cheng, N.M. van der Pers, A. Böttger, Th.H. de Keijser, E.J. Mittemeijer, Metall.
Trans. A22 (1991) 1957–1967.
[43] P.M. Unterweiser, H.E. Boyer, J.J. Kubbs (Eds.), Hear Treater’s Guide—Standard
Practices and Procedures for Steel, 4th ed., ASM, Metal Park, Ohio, 1987, pp.
300–312.
[44] M. Pellizzari, A. Molinari, in: J. Bergstrom, G. Fredriksson, M. Johansson, O.
Kotik, F. Thuvander (Eds.), Proceedings of the 6th Int. Tooling Conf., Karlstad
University, September 2002, pp. 657–669.
[45] Y.M. Rhyim, S.H. Han, Y.S. Na, J.H. Lee, Solid State Phenom. 118 (2006) 9–14.
[46] I. Wierszyllowski, Defect Diff. Forum 258–260 (2006) 415–420.
[47] K.E. Moore, D.N. Collins, Key Eng. Mater. 86–87 (1993) 47–54.
[48] A. Tiziani, A. Molinari, Mater. Sci. Eng. A101 (1988) 125–133.
[49] K. Fukaura, Y. Yokoyama, D. Yokoi, N. Tsujii, K. Ono, Metall. Mater. Trans. A35
(2004) 1289–1300.
[50] D. Das, A.K. Dutta, K.K. Ray, Philos. Mag. 89 (2009) 55–76.
[51] J.Y. Huang, Y.T. Zhu, X.Z. Liao, I.J. Beyerlein, M.A. Bourke, T.E. Mitchell, Mater.
Sci. Eng. A339 (2003) 241–244.
[52] A.J. Vimal, A. Bensely, D. Mohan Lal, K. Srinivasan, Mater. Manuf. Process. 23
(2008) 369–376.
[53] C.H. Surberg, P. Stratton, Klaus Lingenhöle, Cryogenics 48 (2008) 42–47.
[54] A. Bensely, A. Prabhakaran, D. Mohan Lal, G. Nagarajan, Cryogenics 45 (2005)
747–754.
[55] A. Bensely, D. Senthilkumar, D. Mohan Lal, G. Nagarajan, A. Rajadurai, Mater.
Char. 58 (2007) 485–491.
[56] A. Prabhakaran, A. Bensely, G. Nagarajan, D. Mohan Lal, Proc. Int. Mech. Eng.
Conf. (2004) 1–5.
[57] A. Bensely, L. Shyamala, S. Harish, D. Mohan Lal, G. Nagarajan, K. Junik, A.
Rajadurai, Mater. Des. 30 (2009) 2955–2962.
[58] A. Bensely, S. Venkatesh, D. Mohan Lal, G. Nagarajan, A. Rajadurai, K. Junik,
Mater. Sci. Eng. A479 (2008) 229–235.
[59] A. Molinari, M. Pellizzari, S. Gialanella, G. Straffelini, K.H. Stiasny, J. Mater.
Process. Tech. 118 (2001) 350–355.
[60] H. Liu, J. Wang, H. Yang, B. Shen, Mater. Sci. Eng. A478 (2008) 324–328.
[61] H. Liu, J. Wang, B. Shen, H. Yang, S. Gao, S. Huang, Mater. Des. 28 (2007)
1059–1064.
[62] P.S. Babu, P. Rajendran, K.N. Rao, IE(I) Journal-MM 86 (2005) 64–67.
[63] D. Das, A.K. Dutta, K.K. Ray, Mater. Sci. Technol. 25 (2009) 1249–1257.
[64] D. Das, A.K. Dutta, K.K. Ray, Philos. Mag. Lett. 88 (2008) 801–811.
[65] P. Baldissera, C. Delprete, Mater. Des. 30 (2009) 1435–1440.
[66] S. Harish, A. Bensely, D. Mohan Lal, A. Rajadurai, Gyöngyvér B. Lenkey, J. Mater.
Process. Tech. 209 (2009) 3351–3357.
[67] S.C. Jung, D.J. Medlin, G. Krauss, SAE Technical Paper Series (No. 960313), Febru-
ary (1996) 147–158.
[68] D. Das, A.K. Dutta, K.K. Ray, Wear 266 (2009) 297–309.
[69] D. Das, A.K. Dutta, K.K. Ray, Cryogenics 49 (2009) 176–184.
[70] ASTM E975-00: Standard Practice for X-Ray Determination of Retained Austen-
ite in Steel with Near Random Crystallographic Orientation, ASTM Book of
Standards, vol. 03.01, West Conshohocken, PA, United States, 2004.
[71] B.D. Cullity, Elements of X-ray Diffraction, 2nd ed., Addison Wesley, Reading,
NY, 1978.
[72] T. Nykiel, T. Hryniewicz, Metall. Mater. Trans. A31 (2000) 2661–2665.
[73] P. Muro, S. Gimenez, I. Iturriza, Scr. Mater. 46 (2002) 369–373.
[74] A. Iost, R. Bigot, J. Mater. Sci. 31 (1996) 3573–3577.
[75] D.J. Kamody, US Patent 6105374, 2000.
[76] O.N. Mohanty, Mater. Sci. Eng. B32 (1995) 267–278.
[77] I. Alexandru, G. Alincai, C. Baciu, Mem. Etudes Sci. Rev. Metall. 87 (1990)
383–389.
[78] A. Stojko, PhD Thesis, Technical University of Denmark, Denmark, 2006.
[79] M. Pellizzari, la metallurgia italiana (September 2008) 17–22.
[80] A. Akhbarizadeh, A. Shafyei, M.A. Golozar, Mater. Des. 30 (2009) 3259–3264.
... Higher austenitization temperatures are related to dissolution of carbides whose alloys are able to improve hardness of matrix. The second limitation occurs as the decrease of Ms and Mf temperatures is due to matrix saturation increase [6]. Among the main purposes of the current experiment belong revealing how the alloying elements were partitioned between phase constituents present in material. ...
... In order to ensure statistical consistency, the input data were averaged from 5 measurements for each specimen. Among the main parameters: the mean number of microparticles per volume unit -Nk (1/mm 3 ), mean interparticle spacing lA (μm), calculated according formulas in [7] and quantity of carbides (%) evaluated by point method [6,7]. ...
... The average chemical compositions of these particles (in mass percent) are shown in Table 1. With respect to computed equilibrium, Figure 1a one can assume the presence of major amount of M7C3 and minor amount of M23C6 phases [6,8,9]. Besides that, one would also expect also the presence of M3C2 (see Figure 1a). ...
Conference Paper
Full-text available
The main goal of this article is to present the results of state the microstructure in chromium ledeburitic tool steel commercially named Sverker 3. This material was treated by austenitizing at various temperatures and then quenched in oil. Paper is focused on distribution and change in carbide particles depending on austenitizing temperature. Analysis was focused on population of these particles and classification particles by EDS analysis results and measuring of hardness. According to chemistry of particles were particle sorted by types. Beside that all results are compared to phase equilibrium calculated in Thermo-Calc. Keywords: Ledeburitic tool steel, quenching, microstructure, carbide particles
... In particular, sub-zero treatment (SZT) has proven to be more beneficial to convert retained austenite (γR) into martensite over CHT for Cr, and Cr-V ledeburitic steels (Jurči, 2011;Jurči et al., 2015a). For instance, many authors reported complete elimination of γR amount, when Cr and Cr-V ledeburitic steels are subjected to cryogenic treatments (Das et al., 2010a;Das & Ray, 2012;Das et al., 2009a;Das et al., 2009b). On the other hand, SZTs not only eliminates γR, but also refines the martensite (Tyshchenko et al., 2010;Jurči et al., 2015a;Villa et al., 2014), forms "extra" small globular carbides (SGCs) (Jurči et al., 2015a;Jurči et al., 2015b) and modifies the precipitation of nano-sized precipitates (Das et al., 2009b;Jurči et al., 2015b). ...
Conference Paper
Full-text available
Medium enterprise is major part of entrepreneurship in Ethiopia by means of generating opportunities for employment, innovation idea, and supporting the country’s economy. Developing countries such as Ethiopia must create a fertile ground for the medium enterprise to improve their productivity and competitiveness in the market specially during pandemic where uncertainty and susceptibility for the enterprise increase. Its highly important for Ethiopia to know the influence medium enterprise face during pandemics so that they can allocate the bounded resource they have to increase their performance. In the light of the current coronavirus crisis, medium enterprise faces a variety of challenges in a complex and fast changing environment. To understand the challenge faced by medium enterprise in Addis Ababa during the COVID-19 the study used quantitative research. Convenience sampling was used to draw 100 medium enterprise for the study which were engaged in manufacturing sector. The study analysed 100 medium enterprise in Addis Ababa between 2019 and 2021 using descriptive statistics. The evidence suggests that the influence of the coronavirus crisis on medium manufacturing enterprise in Addis Ababa during the first year was very strong in terms of loss of monthly sales and net income and employment both permanent and temporary. Surprisingly, the government response to the COVID-19 was swift and efficient based on the tight resource available, instead of locking down the country as most countries did during the pandemic the Ethiopian government used partial lock down and used its limited resource on prevention.
... It is known that cryogenic treatment (CT) is a necessary heat treatment procedure to improve the mechanical properties of SH-UHSS [5]. It can not only promote the uniform precipitation of high density of carbides [6], but also the refinement of martensite matrix [7]. Unfortunately, due to the discrepancy of cooling rate, there is often inhomogeneous microstructure and mechanical properties from the surface to the center for the engineering components with large cross-section [8]. ...
Article
Full-text available
Cryogenic treatment (CT) is an essential heat treatment process in improving the mechanical properties of secondary hardening ultra-high strength steel (SH-UHSS). The present study investigated the influence of cooling rate during CT on the hierarchical microstructure and strength-toughness of M54 SH-UHSS. Increasing cooling rate can synchronously refine martensite matrix and M2C-type (M = Mo, Cr, W, V) carbides, while there are the most refined martensite blocks and highest-density precipitation of the carbides at cooling rate of 3 °C·min⁻¹. Further increased cooling rate weakens the effect of CT on the refinement and precipitation of martensite matrix. The refinement is related to the high-level segregation of carbon atoms and favorable equilibrium concentration of vacancies during CT and pinning effect of carbides on mobile dislocation during tempering. Besides, uniform carbon clusters and high-nucleation rate by the refined martensite matrix and high-density of dislocations mostly contribute to the above beneficial precipitation of carbides. Precipitation and martensitic matrix strengthening mostly contribute to the ultra-high strength (yield strength of 1730 MPa, ultimate tensile strength of 2018 MPa) at the cooling rate of 3 °C·min⁻¹; meanwhile, the refined blocks as "effective" controlling unit is identified to be the major toughening mechanism contributing to a desirable impact toughness (V-notched Charpy impact energy of 30 J). This study would be instructive for processing the engineering components with large cross-section.
... Various coating systems are commonly used, such as physical vapor-deposited (PVD) TiAlN [19], AlCrN [20] and chemical vapor-deposited (CVD) coatings [21,22]. Recent work by Bensely et al. [23] and Das et al. [24,25] show that wear can also be significantly reduced by a deep cryogenic (DCT) heat treatment at temperatures < −150 • C of tools. This DCT procedure reduces residual stresses and retained austenite by a substantial amount and allows for an easier precipitation of nanocarbides in the following temper steps, in general [26]. ...
Article
Full-text available
Punching of ultra-high-strength spring steel causes critical stresses in the tools. Pronounced wear and even spontaneous failure may occur. Wear of the punches influences the quality of the cutting surfaces of the blanked parts, which is predominantly determined by the cutting edge radius. The radius differs with an increasing number of strokes depending on the punch material. However, there are no studies characterizing the influence of the cutting edge radius on the cutting surface quality on an industrial scale, i.e., considering a very high number of strokes. In the presented study, punches made of high-speed steel, powder metallurgical steel and carbide were used to punch the ultra-high-strength steel 1.4310 (Rm = 1824 MPa) up to 1,000,000 strokes. The experiments were stopped at defined number of strokes, the punches were removed, nondestructively characterized regarding cutting edge radius and wear and reinstalled. It turned out that the radius differed significantly over the number of strokes and, further, varied depending on the punch material. Remarkably, the most low-cost material, precisely the high-speed steel, showed the smallest cutting edge radius of 16 µm and brought the parts with the best cutting surface quality (more than 30% burnish zone) after the maximum number of strokes. The results indicate clearly that the cutting edge radius develops differently for each regarded material and at different number of strokes. Therefore, it is of utmost importance to perform wear tests on different numbers of strokes under industrial conditions. With the knowledge gained, it will be possible to design optimized punches with lower costs and increased lifetime.
... The highest hardness value in Q samples appears at 980°C (58.89 ± 0.22 HRC), while it appears at 1020°C (60.85 ± 0.48 HRC) after cryogenic treatment. It is generally believed the hardness value of die steel is related to the content of alloying elements, phase structure, and heat treatment parameters [20,21]. The investigated steel contains medium carbon content and strong carbideforming elements including Cr, Mo, and V, which promotes forming of hard carbides that significantly improve Microstructure evolution and mechanical properties of PESR 55Cr17Mo1VN plastic die steel during… hardness [22,23]. ...
Article
55Cr17Mo1VN high nitrogen martensitic stainless steel is usually applied to the high-quality mold, which is largely produced by the pressurized electro slag remelting process. The microstructure evolution of quenching and tempering heat treatment were investigated and an optimal heat treatment process to achieve excellent mechanical properties was found out. The main precipitates in the steel included carbon-rich type M23C6 and nitrogen-rich type M2N. With increasing austenitizing temperature, the equivalent diameter of the precipitates got fined, and retained austenite content increased significantly when the austenitizing temperature exceeded 1020 °C. The fracture mode gradually changed from brittle fracture to ductile fracture with increasing tempering temperature from 200 to 550 °C. The experimental steel tempered at 350 °C achieved a good combination of hardness (60.6 HRC) and strength (2299.2 MPa) to meet service requirements. Flake M23C6 precipitated along martensite lath boundaries and the secondary hardening phenomenon occurred when the tempering temperature was 450 °C. Due to the high nitrogen content, M2N precipitated from the inside of laths and matrix when tempered at 550 °C.
Article
Die and mould makers frequently employ AISI-D6 steel. First, 32 trials of electrical discharge machining (EDM) are carried out on the aforementioned material. A variety of EDM procedure performance measures, including tool wear ratio (TWR), material removal rate (MRR), and surface roughness (Ra), are taken into account, as well as the pulse time (Ton), pulse current (I), and pulse voltage (V). Operators are guided by process. Increasing MRR and Ra concentrations and decreasing TWR concentrations were found to increase pulse on-time values. As a result of increasing the pulse current, physical removal rate, tool wear ratio and surface unevenness increased. As a result of the increased voltage, the MRR, TWR, and Ra values were all reduced. The outcomes are predicted using ensemble machine learning models. The experimental data used to create estimation models for MRR, TWR, and Ra. In addition, an unknown set of experiments has validated the model-based predictions.
Technical Report
Full-text available
Deep cryogenic treatment (DCT) is a type of cryogenic treatment, where a material is subjected to temperatures below-150°C. When a metallic material is modified with DCT, changes in microstructure are induced due to change in grain size, formation of new grains, movement of dislocations, change of solubility of atoms, alteration of crystal structure, and new phase formation. Changes in the material can have positive or negative effects on the final properties of metallic materials. The metallic material's performance and later performance of manufactured components and tools from this specific material strongly depend on selection of proper material, proper design, accuracy with which the tool is made from untreated material and application of proper heat treatment, including DCT. Ferrous metals (different grades of steel) and non-ferrous alloys (aluminum, magnesium, titanium, nickel etc.) can be DCT handled. DCT treatment has shown to reduce density of defects in crystal structure, increase wear resistance of material, increase hardness, improve toughness, and reduce tensile strength and corrosion resistance. However, some researchers reported on results that show no change in properties (toughness, hardness, corrosion resistance, etc.) or even deterioration when subjected to DCT treatment. This leads to a lack of consistency and reliability of the treatment process, which is needed for integration in industry. Nevertheless, to prove with certainty the resulting outcome on the material properties and understanding the reason for the variation of this effect on metallic materials, a more systematic approach and testing with different variables should be conducted in the future.
Article
The objective of this review paper is to introduce Cryogenic temperature effect on different properties of materials. Results showed that cryogenic operation improves properties like hardness, fatigue strength, tensile strength, toughness and resistance to wear in comparison to conventional operations. The improvement is a result of microstructural changes at cryogenic temperature which helps in conversion of austenite to martensite and carbide nucleation. The results also showed that doing tempering before or after cryogenic operation has significant influence on material properties which helps to achieve better carbide distribution. Different heat treatment sequences which involve tempering before or after Deep Cryogenic Treatment (DCT), provides varied results. Soaking time at cryogenic temperature also has an important role in refinement of microstructure and affect material properties.
Article
Treatment of metals at temperatures below 0 °C is a developing technology. This paper describes recent studies of the influence of intricate variants of heat treatment (so called A B C G H M N kinds of treatment) through low temperatures, upon microstructures, hardness, toughness and life of high speed steels. Optimum heat treatments are those characterized by variants G and M: the first one including a single tempering leads to a better productivity.
Article
As opposed to conventional sub-zero treatment to transform retained austenite, deep cryogenic treatment at liquid-nitrogen temperatures has been claimed to enhance the wear resistance of tool steels by additional hitherto ill-defined phenomena. In summarising the current state of knowledge, this review clarifies the underlying mechanisms.
Article
Many experimental investigations reveal that it is very difficult to have a completely martensitic structure by any hardening process. Some amount of austenite is generally present in the hardened steel. This austenite existing along with martensite is normally referred as the retained austenite. The presence of retained austenite greatly reduces the mechanical properties and such steels do not develop maximum hardness even after cooling at rates higher than the critical cooling rates. Strength can be improved in hardened steels containing retained austenite by a process known as cryogenic quenching. Untransformed austenite is converted into martensite by this treatment. This conversion of retained austenite into martensite results in increased hardness, wear resistance and dimensional stability of steel. Wear can be defined as the progressive loss of materials from the operating surface of a body occurring as a result of relative motion at the surface. Hardness, load, speed, surface roughness, temperature are the major factors which influences wear. Many studies on wear indicate that increasing hardness decreases the wear of a material. With this in mind, to study the surface wear on a surface modified (Cryogenic treated) steel material an attempt has been made in this paper. In this study as a Part -I Hardening was carried out on carbon tool steel (AISI 1095) of different L/D ratio with conventional quenchants like purified water, aqueous solution and Hot mineral oil. As a Part -II hardening was followed by quenching was carried out as said in Part - I and the hardened specimen were quenched in liquid Nitrogen which is at sub zero condition. The specimens were tested for its microstructure, hardness and wear loss. The results were compared and analyzed. The alloying elements increases the content of retained austenite hence the material used was AISI1095 (Carbon 0.9%, Si 0.2%, Mn0.4% and the rest Iron).
Article
As an alternative to multi-tempering, an approach which is becoming widely adopted in the metallurgical/engineering industry is the use of liquid nitrogen temperature treatments at -196°C. The process currently marketed by The BOC Group using liquid nitrogen is referred to as Cryotough. Because liquid nitrogen is adopted as the cooling media, the temperature is sufficiently low to benefit all materials susceptible to austenite retention, providing a microstructure of homogeneous martensite. Practical results have been reported showing up to 600% improvements in tool life for some components.