TECHNICAL PAPER

Deformation-Induced Martensitic Transformation Behavior of
Retained Austenite during Rolling Contact
K. KANETANI T. MORONAGA T. HARA

K. USHIODA

The deformation-induced martensitic transformation is a phenomenon that significantly improves the mechanicalproperties of steels, and is well known to be beneficial for the rolling contact fatigue (RCF) of bearings. In thepresent study, the characteristics of the deformation-induced martensitic transformation in the RCF of carburized,
quenched and tempered SAE4320 steel were investigated in detail using scanning electron microscopy with electronbackscattering diffraction and transmission electron microscopy with automated crystal orientation mapping.
These analyses clarified that different variants of the extremely fine deformation-induced martensites as small as
several tens of nm were formed within an austenite grain with RCF, and the martensites were speculated to havethe Kurdjumov-Sachs or the Nishiyama-Wasserman relationship with the retained austenite. Furthermore, thedeformation-induced martensites were preferentially formed within the retained austenite grains rather than at
the interface between the tempered martensite and retained austenite. This suggests that the deformation-induced
martensites were formed from some localized regions that were plastically introduced within the retained austenite
grains.
Key Words: Rolling contact fatigue (RCF), Steel, Martensite, Austenite, Deformation-induced martensitic
transformation, SEM, EBSD, ACOM, TEM

1. Introduction
The deformation-induced transformation from austenite(c) into martensite (a’) while deforming steels containing
metastable austenite, is well known. The deformationinduced martensitic transformation demonstrates a highlevel of plasticity and occurs when deforming austenite at
a temperature of M S or higher. In addition to the chemicalcomposition of austenite, such as the amount of carbon
in solid solution, the M S temperature, stacking fault
energy1-7) and microstructural factors such as grain size7-11),morphology12-14) and crystal orientation15, 16) of austenite,
the transformation behavior is also heavily dependent on
the stress state17) and deformation temperature18-20). The
transformation mechanism varies but is mainly dependent
on the deformation temperature. As the temperature
exceeds M S, stress-induced martensitic transformation
occurs during the elastic deformation of austenite in asimilar manner as found in thermally induced regularmartensite. Additionally, since the mechanical driving
force required for martensitic transformation increases
with temperature, austenite initially displays plastic
deformation at temperatures above M r
S (temperature*1 This paper is reprinted from ISIJ International, Vol. 61 (2021),
No. 10, pp. 2629-2635 a part thereof is modified.

72

where the stress at which martensitic transformation
starts and the stress where austenite yields are equal).
The resulting local heterogeneous deformation areas(e.g. slip bands, shear bands, and deformation twins)
and their intersecting areas result in strain-induced
martensitic transformation 21) . Debate about themechanisms of the martensitic transformation in theplastic deformation regime is ongoing22); however, several
theories propose that martensitic transformation occurs
due to the concentration of stresses in the intersection
areas of deformation twins 23) and slip bands 24, 25)
introduced by plastic deformation. Regardless of theirformation mechanism, deformation-induced martensites
significantly improve the mechanical properties of steel.
TRIP (transformation-induced plasticity) steel, which
has better balances between strength and ductility, makes
use of this phenomenon. Since the proposal of high alloy
TRIP steels by Zackay et al. 26) in the 1960s, the field has
seen a wide range of developments, including that of
low alloy TRIP steels for practical applications. Previous
studies regarding the microstructural characteristics ofdeformation-induced martensite focus on single-phaseaustenitic high alloy TRIP steels. For instance, studies
using Fe-Ni alloys19, 20) showed that deformation-induced
martensite subjected to tensile deformation above the M Stemperature is typical lenticular martensite with a mid-

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Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact

rib. However, in contrast to the deformation mechanismof athermal martensite, deformation-induced martensite
subjected to tensile deformation at higher temperatures
has no mid-rib or deformation twins but instead forms
notably fine butterfly martensite. Additionally, deforming
Fe-Cr-Ni alloys with low stacking fault energy or
austenitic stainless steels results first in hcp e martensite,
followed by the generation of bcc a’ martensite fromthe e phase19, 27, 28) or a’ martensite from slip bands24, 25).
A study focusing on an Fe-Ni-Co-Ti alloy 29) reportsthat deformation-induced martensite will grow fromthin plate martensite produced in advance by sub-zeroprocessing. The martensite grows and simultaneously
maintains its thin plate when deforming just above M Stemperature. However, when the deforming temperature
is higher, it instead grows into a lenticular shape with adifferent morphology. Furthermore, a well-known generalfeature of martensitic transformation is the special crystal
orientation relationship with the parent austenite phase.
For ferrous metals, the martensite crystal orientation
relationship with the parent austenite phase is reported
to be the Kurdjumov-Sachs (K-S) relationship ((111)c//(011) a, [101] c//[111] a) or the Nishiyama-Wasserman(N-W) relationship ((111)c//(011)a, [112]c//[011]a) 30).
Although few reports exist for deformation-induced
martensite, a K-S-like relationship has been confirmed
regardless of its formation mechanism15, 19).
It has been reported that austenite not only improves
strength-related properties through the TRIP effect, butalso improves the fatigue-related properties of various
modes31-36). Quenched and tempered steel is used in the
bearing rings and rolling elements of rolling bearings.
Since the microstructure contains retained austenite
along with tempered martensite and cementite, numerous
studies have been conducted into the impact of thisretained austenite on the rolling contact fatigue (RCF)
life 37-50). These studies report that retained austenite
effectively improves RCF life, and may be related tothe deformation-induced martensitic transformation ofretained austenite as the underlying principle. Other than
the suppression of crack propagation like that found in
TRIP steel37), the possibilities of suppressing martensite
decomposition37, 38) and introducing compressive residual
stress39, 40) owing to the generation of new martensite havealso been reported. However, this previous work infers
the contribution of deformation-induced transformationof retained austenite from measurements of the hardness
and residual stress after RCF, and direct observation of
deformation-induced martensite is limited. Since studies
were mainly conducted using quenched and tempered
high carbon chromium bearing steel, the following
facts are believed to be the causes of this limitation.
For instance, the microstructure was extremely fine, theretained austenite content was low at approximately 10%,JTEKT ENGINEERING JOURNAL English Edition No. 1019E (2023)

and the stresses during rolling contact were complex.
Other studies conducted observations of deformationinduced martensites using carburized, quenched, andtempered low carbon steels containing relatively highlevels of retained austenite39, 48). However, the detailed
microstructural character of martensite and its crystal
orientation relationship with the parent austenite have notbeen clarified. The phenomenon of deformation-induced
martensitic transformation, which is essential for the
improvement of RCF life, has not been directly observed.
Compared to the study of high alloy TRIP steels, muchremains unclear.
This study intends to clarify both the microstructuralcharacter of deformation-induced martensite formed
during rolling contact and its formation mechanism.
We conducted crystal orientation analysis by electronbackscattering diffraction (EBSD) using field emissionscanning electron microscopy (FE-SEM), and automatedcrystal orientation mapping through nanobeam precession
electron diffraction using transmission electron
microscopy (ACOM-TEM) to reveal the morphology of
deformation-induced martensite and its crystal orientation
relationship with the parent austenite.

2. Experimental Methods
2. 1 Specimens
SAE4320 steel was selected as the sample owing to
its high Ni content, which is an austenite stabilizer. The
chemical composition is shown in Table 1. Retained
austenite can be easily obtained from SAE4320 steel
by carburizing and quenching. A manufactured hotrolled steel bar of 26mm diameter was cut and subjected
to heat treatment, then finished into a cylinder with adiameter of 20mm and width of 36mm by grinding. The
heat treatment involved carburizing, quenching, andtempering. For carburization, the specimen was kept for
26 h in a carburizing atmosphere of 960℃ and modified
such that the carbon concentration in the surface layer
was approximately 1.1%. After soaking for 70 min at
820℃, the specimen was quenched in oil at 80℃, thentempered at 180℃ for 2 h.
Table 1 Chemical composition of steel used (mass%)
C

Si

Mn

P

S

Cu

Ni

Cr

Mo

O

0.20 0.19 0.55

0.018

0.006

0.10 1.70 0.53 0.21 0.0009

2. 2 Rolling Contact Fatigue Test
The machine used for the RCF test is schematicallyshown in Fig. 1. This test machine was used to rotate one
specimen against two JIS-SUJ2 steel balls (31.75mm indiameter) supported by three guide rolls. The lubricating73Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact

oil was applied to the specimen surface through adriving roll. The test conditions are listed in Table 2.
The maximum orthogonal shear stress through rolling
contact is at a depth of 0.24mm (z 0) from the rolling
contact surface. Microstructural change due to RCF is
the most prominent at depth z 0 because it is affected byorthogonal shear stress48). The test was terminated after
3.7 × 106 cycles, at which point the sample was removed
and submitted for microstructural analysis and crystal
orientation analysis.
Driving rollSpecimenBall

3. Experimental Results

Guide rollFig. 1 Schematic of the radial type rolling contact fatigue test
machineTable 2 Rolling contact fatigue test conditions
Contact
condition

Herzian maximum pressure
Maximum orthogonal shear
stress (depth z 0)
Loading speed
LubricantOperating temperature

5.8 GPa
1.4 GPa(0.24mm)
285 HzMineral oil(ISO-VG100)
60 ± 5℃

2. 3 Microstructural Analysis and Crystal
Orientation Analysis
SEM-EBSD measurements of the untested (preRCF) and tested (post-RCF) sample were conducted.
The specimen for SEM-EBSD was buffed with a 1 Pm
diamond paste at a section parallel to the rolling contact
surface (perpendicular to the ND outlined later), at a
depth z 0 from the rolling contact surface, and polished
to a mirror finish using colloidal silica. Measurements
were taken using an EBSD data collection system (OIMby TSL) attached to an FE-SEM (JSM-7100F by JEOL)
at an acceleration voltage 15 kV, a working distance of
15mm and a step size of 40 nm or 10 nm. Grains smaller
than the step size cause the Kikuchi pattern to become
ill-defined and thereby reduce measuring accuracy. We
therefore excluded any low-accuracy data from the dataprocessing sets.
To obtain a more detailed analysis of the post-74

RCF microstructure, we conducted ACOM-TEM
measurements. When the thin film specimen used forthis was manufactured using the focused ion beam (FIB)
method, we confirmed the transformation of retained
austenite during the sample preparation process inadvance. Accordingly, we used a precision cutting device
to cut a thin film sample with the same cross-section asthat of the sample used for SEM-EBSD. To suppress thetransformation of retained austenite in the process of
producing the thin film specimen, we employed a twin-jet
electropolisher using an ethanol/perchloric acid solution(mixed at a volume ratio of 9:1) at a temperature of 10℃.
Subsequently, we performed Ar ion milling with a low
acceleration voltage to obtain a smooth surface suitablefor ACOM-TEM. The equipment used was ASTAR(NanoMEGAS) incorporated in a TEM (JEM-2800 by
JEOL). Measurements were taken at an acceleration
voltage of 200 kV, a precession angle of 0.5° and a step
size of 2 nm.

3. 1 Microstructural Analysis and Crystal
Orientation Analysis through SEM-EBSD
To determine the microstructures before and after
RCF testing, we analyzed the phase distribution using
SEM-EBSD. The measuring range was 16 Pm × 32 Pm,
and step size was 40 nm. Figure 2 shows the results forthe phase map overlaid on the image quality (IQ) map.
The rolling, transverse, normal directions (RD, TD,
and ND respectively) of the specimen are indicated inFig. 2. Movement of the load (steel ball) during rolling
contact is in the opposite direction to the RD. The aphase corresponding to tempered martensite, the cphase corresponding to retained austenite and cementite
can be confirmed in the pre-RCF sample (Fig. 2(a)).
Subsequently, refinement of the tempered martensite anda reduction in retained austenite in the post-RCF sample
can be seen in Fig. 2(b), although there is no change
in the cementite. This result is the same as previously
reported 49) ; however, the fine, elongated structure
corresponding to white bands (WB) as confirmed in
the previous report were not observed because different
sections of the material were observed. Moreover, thereduction in retained austenite due to RCF was similar
to the that observed by X-ray diffraction in previous
research40, 48, 50, 51). The X-ray diffraction and SEM-EBSD
results suggest the possibility that the deformationinduced transformation of retained austenite into
martensite occurs due to rolling contact. However, nocorresponding increase in the a phase was observed andthus we cannot conclude that martensite was generated
due to the deformation-induced transformation of retained
austenite.

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Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact

(a)

(b)

a phase(a)(b)

c phasecementiteRDTD

500 nma, ca phaseND

500 nmcementitec phase5 Pm

cementite

5 Pm

Fig. 2 Phase maps obtained using SEM-EBSD at z0 depth(a) Before RCF (b) 3.7 × 106 cycles

The morphology of the martensite generated throughthe deformation-induced transformation of austenite
during rolling contact has been unclear in preexisting
literature. It is therefore theorized that martensite exists
in an extremely fine form, with no EBSD patterns having
been obtained. To observe fine-grained deformationinduced martensites, we therefore conducted EBSD
measurements with a step size of 10 nm centering on
single retained austenite grains. The pre-RCF IQ +phase map and inverse pole figure (IPF) map are shownin Fig. 3. Tempered martensite is present around theretained austenite. Moreover, the grain size of the retained
austenite and the tempered martensite is of the order ofa few Pm. As it consists of high carbon martensite, it
is possible that microstructures such as transformation
twins are present in the tempered martensite, but this
could not be confirmed by the EBSD measurements we
conducted. The post-RCF microstructures are shownin Fig. 4. Microstructural change due to RCF was
confirmed by comparing the results to those in Fig. 3.
The grain size of the tempered martensite around retained
austenite has been refined to submicron proportions due
to rolling contact and a variety of crystal orientations
were observed. The same results have been obtained in
microstructure observations using TEM, and it has beenreported that dislocation cells are formed within thetempered martensite through the introduction of plasticstrain associated with rolling contact and that fine grains
with random orientations are generated52). Furthermore,
the grain size of the retained austenite remains in theorder of a few Pm and is not subjected to grain refinement
like the tempered martensite. However, the a phase,
which was not present prior to RCF and which is evenfiner than the surrounding tempered martensite, can beobserved inside the retained austenite grain. We surmisethat this fine a phase is deformation-induced martensite,which was generated from the retained austenite due torolling contact.

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Fig. 3 SEM-EBSD results centered on retained austenite at z0
depth before RCF(a) Phase map overlaid on IQ map (b) IPF map
(a)

(b)

500 nma phase

500 nma, ccementitec phasecementite

Fig. 4 SEM-EBSD results centered on retained austenite at z0
depth after RCF(a) Phase map overlaid on IQ map (b) IPF map

3. 2 Microstructural Analysis and Crystal
Orientation Analysis by ACOM-TEM
To further clarify the morphology of deformationinduced martensite, we conducted an ACOM-TEM
analysis. To obtain crystal data from the electron
diffraction patterns when irradiating a specimen with
an electron probe of a few nm, the spatial resolution of
ACOM-TEM is one order of magnitude higher than thatof SEM-EBSD. In the bright field image in Fig. 5(a), a
microstructure flatter than the surrounding microstructures
was confirmed, which is believed to be a mix of retained
austenite and deformation-induced martensite 48). The
phase distribution analysis (Fig. 5(b)) shows that theparent phase in the flat microstructure is retained austenitewith fine deformation-induced martensites existinginside. Furthermore, the IPF map in Fig. 5(c) shows thatthe retained austenite consists of a single grain with aconstant crystal orientation, and that the deformationinduced martensite present inside are individual grains
with different crystal orientations. Although it is difficult
to quantitatively assess the size of the deformationinduced martensites that were measured with ACOMTEM, grains smaller than 5 pixels are present and we can75Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact

surmise that the smallest measures a few nm. The ACOMTEM results clearly revealed the presence of extremely
fine deformation-induced martensites. In the Discussionsection, we will outline the crystal orientation relationshipbetween the retained austenite and deformation-induced
martensite, obtained from the SEM-EBSD and ACOMTEM analysis results.
(a)a phasec phasecementitea, ccementite

300 nm(b)(c)

200 nm

200 nmFig. 5 ACOM-TEM results centered on retained austenite atz 0 depth after RCF.
(a) TEM bright field image (b) Phase map(c) IPF map(b)(c) were measured in the white dotted line area of (a)4. Discussion

relationship, it is believed that several of these variants
have occurred. In the ACOM-TEM results (Fig. 6(f)),
which had many measurement points, variants close to
K-S or N-W relationships are presumed to occur as well,although the variation in crystal orientation is large. The
accuracy of crystal orientation analysis performed using
electron diffraction patterns in ACOM-TEM is lower
than that in SEM-EBSD, which uses Kikuchi patterns.
Furthermore, since martensite is very fine, diffraction
patterns for retained austenite are superimposed, which
reduces the accuracy of the analysis. Therefore, it should
be considered that the ACOM-TEM results may have alower crystal orientation accuracy than the SEM-EBSD
results. Although SEM-EBSD has few measuring points
because very fine deformation-induced martensite was
not detected, martensite with K-S or N-W relationships
with the parent retained austenite was confirmed,
demonstrating that deformation-induced martensite
was newly formed during rolling contact. Furthermore,
although the accuracy of crystal orientation measurements
is not high, similar crystal orientation relationships wereconfirmed from the ACOM-TEM results, supporting
the hypothesis that the nm-sized fine a grains are
deformation-induced martensite. The orientational
difference between the K-S and N-W relationships was
only 5.3°. Therefore, distinguishing between them isdifficult under experimental conditions. Since we werealso unable to do so in this study, this requires further
study and theoretical evidence.
(a)

(b)

(c)

(e)

(f)

4. 1 Crystal Orientation Relationship between
Parent Austenite and Deformation-induced
Martensite
Martensite is generated with a K-S or N-W crystal
orientation relationship with the parent austenite30). A
K-S relationship has been confirmed for deformationinduced martensitic transformation15, 19), but no reports
exist regarding deformation-induced martensitic
transformation during rolling contact, and the details of
this phenomenon remain unclear. We therefore studied
the crystal orientation relationship between retained
austenite and deformation-induced martensite generated
during rolling contact. To this end, we extracted the range
believed to be a single retained austenite from Fig. 4(SEM-EBSD) and Fig. 5 (ACOM-TEM). Crystals werethen rotated so that the c <001> direction formed thebase axis on the stereoprojection and {001}a pole figures
were obtained. Results are shown in Fig. 6. Although
the number of SEM-EBSD measuring points are low,
it is clear from Fig. 6(c) that multiple variants of thedeformation-induced martensites are present. Since there
are 24 types in a K-S relationship and 12 types in an N-W

76

300 nm(d)

200 nma, ccementite

Fig. 6 Crystal orientation relationships between retained
austenite and deformation-induced martensites afterRCF(a)-(c) SEM-EBSD analyses (d)-(f) ACOM-TEM
analyses(a)(d) IPF maps (b)(e) {001}c pole figures(c)(f) {001}a pole figures

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Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact

4. 2 Mechanism of Deformation-induced
Martensitic Transformation
Olson and Cohen21) demonstrated that if deformationinduced martensite is transformed during the elastic
deformation of retained austenite, pre-existing interfaces
between tempered martensite and retained austenite(hereinafter referred to as tempered a’/retained cinterface) form the nucleus for stress-induced martensitic
transformation. However, if the retained austenite has
been subject to plastic deformation, then martensitic
transformation occurs from the heterogeneous
deformation areas formed inside the retained austenite21).
The newly formed nuclei of deformation-induced
martensite for the latter are surmised to have formed
from intersections with shear bands, and it is suggested
that this is a strain-induced martensitic transformation.
According to Onodera et al . 23), Suzuki et al . 24), and Ueda
and Fujita25), however, these types of areas are local areasof stress concentration where there is intersection with
deformation twins and slip bands. In this case, a type ofstress-induced martensitic transformation is expected.
The mechanisms introduced in these previous studies
indicate that the deformation-induced martensitic
transformation is greatly affected by elastoplastic
deformation behavior of the retained austenite. We studied
the mechanism of the deformation-induced martensitic
formation during rolling contact from the perspective ofstress conditions. During rolling contact, principal shear
stress is the highest at a particular depth from the rolling
contact surface49). Using the Herzian contact theory, theprincipal shear stress at depth z 0, at which we conducted
the microstructural analyses, is determined to beapproximately 1 807 MPa. Additionally, since no known
mean critical resolved shear stress (sCRSS) is available
for austenite in SAE4320 steel or equivalent steels, weused yield stress (rY) of austenite for Fe-Si-Mn-C alloy
as reported by Jacques et al . 53), who showed that the rYof austenite increases as carbon content increases. rY for
austenite with approximately 1.1% carbon content, whichcorresponds to the retained austenite used in this study,
is estimated to be approximately 650 MPa. If sCRSS (212MPa) is used as a reference, which was calculated by
Eq. ⑴ using Taylor factor M = 3.07 for fcc polycrystal
metals, then the yield criteria for austenite are adequately
met, even when considering the stress distribution
between the tempered martensite, cementite and theirinterfaces.
sCRSS ＝ rY/M⑴

To facilitate the observation of microstructural change
during rolling contact, we conducted RCF testing at
higher contact stress settings than those found underregular usage conditions. We also obtained results for the

JTEKT ENGINEERING JOURNAL English Edition No. 1019E (2023)

rolling bearing used under standard contact conditions.
Using a 6206 deep groove ball bearing (bore diameter
30mm, outside diameter 62mm, width 16mm) as the
typical rolling bearing type, we performed the calculations
for loading with half the radial load (9.75 kN) of the basic
dynamic radial load rating (19.5 kN54)). Results showedthat principal shear stress at depth z 0 was 840 MPa (z 0 =0.10mm) for the inner ring and 1 041 MPa (z 0 = 0.17mm)
for the outer ring, which were both far higher than theaustenite sCRSS. In other words, plastic deformation of theretained austenite is believed to possibly take place forrolling bearings that are used under standard conditions.
Based on the above, we surmise that during rolling
contact, plastic deformation of the retained austenite
initially occurs followed by the deformation-induced
martensitic transformation inside retained austenite
grains. In other words, the deformation-induced
martensites presumably nucleate in locally formed
heterogeneous areas or in the intersecting areas of slip
bands, shear bands, and deformation twins. Notably,most deformation-induced martensites appear to haveformed inside retained austenite grains (Fig. 6(a)and (d)). To obtain a 3D estimation of the generated
deformation-induced martensite, we conducted EBSD
measurements at the section perpendicular to the TD.
Although this retained austenite grain is different fromthat in Fig. 4, analysis was conducted in the same way as
shown in Fig. 6. Results are shown in Fig. 7. Figure 6(a)and Fig. 7(b), confirm that, regardless of the section
measured, deformation-induced martensites are formed
inside the retained austenite grains. Based on theseobservations, we surmise that very fine deformationinduced martensites measuring a few nm in size that areformed inside retained austenite grains due to plastic
deformation contribute to improved RCF life.
Unevenness in the shape of the post-RCF tempered a’/retained c interface is more pronounced than in the preRCF interface (Fig. 3), as was confirmed by the highspatial resolution ACOM-TEM results (Fig. 5(b)). These
results suggest the possibility that deformation-induced
martensites are formed from the tempered a’/retained cinterface. Olson and Cohen21) reported that these kindsof grain boundaries potentially formed the nucleus oftransformation when austenite had undergone elastic
deformation. The mechanism by which deformationinduced martensite present near the tempered a’/retainedc interface is formed will be the focus of our future work.
In the current study we focused on ascertaining
the microstructural character of deformation-induced
martensite. The systematic analysis of the relationshipbetween the formation of deformation-induced martensite,
the deformation behavior of austenite and its deformed
structures, along with the study of the 3D shape of
deformation-induced martensite inside identical retained77Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact

(b)

(a)

200 nm
500 nma phasea, ccementitec phasecementite(c)(d)

Fig. 7 SEM-EBSD results centered on retained austenite on asection perpendicular to TD at z0 depth before RCF(a) Phase map overlaid on IQ map(b) IPF map of selected single retained austenite grain(c) {001}c pole figure (d) {001}a pole figure

austenite grains will be the focus of future work. A more
complete understanding of the formation and morphology
of deformation-induced martensite may be obtained byascertaining the morphology of deformation-induced
martensite through 3D EBSD measurements using serialsectioning with FIB-SEM. However, as outlined above,
surface grinding of microstructures containing retained
austenite that is thermally and mechanically unstable due
to FIB has many issues for the purpose of microstructuralobservation, and remains to be investigated in future.

austenite. Therefore, it is highly likely that plastic
deformation of the retained austenite occurs during
rolling contact. Accordingly, we surmised that
deformation-induced martensites are formed fromlocally heterogeneous deformation areas or theseintersecting areas introduced inside retained austenite
grains through plastic deformation.
(3) Future work will aim to define the mechanism ofmartensitic transformation associated with plastic
deformation. Clarification of the transformation
mechanism will require the deformed structure
which forms the nucleus of the transformation andthe 3D microstructures of the deformation-induced
martensites to be ascertained. We intend to study thisin the future.

References
1)
2)

3)

4)

5)

5. Conclusion
After measuring the morphology of deformationinduced martensite generated through rolling contact
using SEM-EBSD and ACOM-TEM, we arrived at thefollowing conclusions:(1) Detailed SEM-EBSD and ACOM-TEM measurements
revealed that deformation-induced martensites consistof very fine grains with varying crystal orientations,the smallest of which are only a few nm long. They
are formed in various locations but mainly insideretained austenite grains. The crystal orientation
relationship between the parent retained austenite andthe deformation-induced martensite is similar to K-S
or N-W relationships, which is a typical feature ofmartensitic transformation.
(2) The principal shear stress estimated from the RCF
test conducted for this study and the regular usage
conditions for rolling bearings is substantially higher
than the critical resolved shear stress of retained

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and ratings life.

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Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact

K. KANETANI* T. MORONAGA**T. HARA***K. USHIODA*****

Material R&D Dept., Research & Development
Headquarters, Doctor of Engineering** Electron Microscopy Analysis Station, National Institutefor Materials Science*** Electron Microscopy Analysis Station, National Institutefor Materials Science, Doctor of Engineering**** Division of Mechanical Science and Engineering,
Graduate School of Natural Science and Technology,
Kanazawa University, Doctor of Engineering

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