TECHNICAL PAPER

Development of Medium Carbon Bearing Steel Achieving Life
Improvement and Resource Saving
K. KANETANI T. MIKAMI T. SADA

In recent years, the growing interest in protecting the global environment has given rise to a demand for saving
resources in bearing manufacturing. In particular, a large amount of rare metals are added to the NiCrMo steel
used to manufacture rolling bearings for industrial machinery and the like. If less rare metals were used, resourcesaving could be achieved. Furthermore, if the replacement frequency could be decreased by extending the life of
the bearing, even greater resources-saving could be achieved. Increasing the amount of retained austenite on thesurface is known to be an effective method of extending bearing life and, in this study, we concluded that life canbe further improved by controlling the stability of retained austenite. We have developed a new rolling bearing steel
which enables both reduced rare metal usage and longer bearing life by controlling the quantity and stability ofretained austenite.
Key Words: rolling bearing, rolling contact fatigue, retained austenite, precipitate, resource-saving

1. Introduction
High-strength steel is used for races and rolling
elements that are major components of rolling bearings, asthese are required to be capable of cyclic contact at a highsurface pressure of several GPa. A steel typically used
for races and rolling elements is high carbon chromium
bearing steel subjected to a process of through hardening
in order to harden the overall part. Meanwhile, regarding
those applications which demand high impact resistance,case hardening steel is used to increase toughness, andthis steel is subjected to surface-hardening processes
such as carburized hardening or carbonitrized hardening,then used with the interior in a low-hardness state. Ifcase hardening steel is to be used for large bearings, steel(nickel chromium molybdenum steel) containing manyrare metals, such as nickel, chromium and molybdenum,
is used in order to secure hardenability. In recent years,
concern about protecting the global environment has
been growing around the world, and there is lively action
being taken by the steel industry to stop using so muchrare metal in order to enhance steel properties throughsubstitution with metals that can be easily refined and areavailable in an abundance on the planet. Even in regards
to bearing steel, resource-saving could be achieved if itwere possible to substitute nickel chromium molybdenum
steel with a material containing minimal rare metals.
Furthermore, if bearing life was extended, replacement
frequency would be reduced, thus also contributing toresource-saving.

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This report introduces the results of JTEKT’s
development of a bearing steel with reduced rare metal
content and excellent life performance compared toconventional products, developed with the aim of shifting
toward an environmentally-friendly society.

2. Approach to Improving Life
Failure of bearings caused by rolling contact fatigue(RCF) can be broadly divided into two types; sub-surface
initiated spalling caused by non-metallic inclusions
present in steel and surface initiated spalling caused bylubricant becoming contaminated with foreign matter orpoor lubrication. Bearings made from nickel chromium
molybdenum steel which has undergone surfacehardening are often used in industrial machinery such as
construction machinery and steelmaking equipment. As
such, improving a bearing’s durability against not only
sub-surface initiated spalling but also surface initiatedspalling (dent initiated spalling) which originates fromdents created when wear debris becomes trapped, would
be effective in extending bearing life. Dent initiatedspalling is caused by a mechanism of a dent forming in
the race surface, stress becoming concentrated aroundthe edges of this dent when the rolling element passes by
and the microstructure becoming fatigued. Therefore, inorder to improve bearing life by addressing dent initiatedspalling, there is a need to increase microstructural
fatigue strength, and one effective means of doing so
is to increase the volume fraction of retained austenite

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

−Development of Medium Carbon Bearing Steel Achieving Life Improvement and Resource Saving−

Ball movement direction
Dent

100 µmMicrostructural changes(a) Hardness 65HRC, retained c=14%, 20 hours of operation
Ball movement direction
Dent

Microstructural changes
100 µm(b) Hardness 64HRC, retained c=46%, 20 hours of operation

150
SUJ2
CrMoV steel
L10 life, h

100

50

0

0

10

20

30

40

50

Volume fraction of retained c, %Fig. 2 Relationship between volume fraction of retained cand dent-initiated spalling life1)
1 000

SUJ2
SNCM420L10 life ratio(c) of the surface1). Figure 1 shows the cross-section
microstructures of two ball bearing inner rings withequivalent hardness and different volume fraction ofretained c manufactured by adjusting materials and heat
treatment, then artificially given dents and rolled for acertain period of time under identical conditions. For the
inner ring with a high volume of retained c in the surface,in the portion where stress is concentrated around theedge of the dent, microstructural fatigue is suppressedby the retained c transforming into martensite through aprocess of strain-induced martensitic transformation. In
this way, by increasing the volume fraction of retainedc, it is possible to increase bearing life relating to dent
initiated spalling (Fig. 2). Furthermore, this technique
is being applied to improve bearing life relating to sub2)
surface initiated spalling . Figure 3 shows the evaluation
results for sub-surface initiated spalling life using a
specimen in which the volume fraction of retained c has
been intentionally changed by adjusting materials andheat treatment. In this way, by increasing the volume
fraction of retained c of the zone affected by shear stresslocated directly under the rolling contact surface, which
is where cracks initiate from, it is possible to promotestrain-induced martensitic transformation during rolling,
and improve bearing life. Bearing steel unavoidably
contains non-metallic inclusions which cause sub-surface
initiated spalling therefore in order to further improve thesub-surface initiated spalling life of high-cleanliness steel,utilizing retained c is believed to be an effective method.

100

10

1

0

10

20

30

40

50

Volume fraction of retained c, ％Fig. 3 Relationship between volume fraction of retained cand sub-surface initiated spalling life2)

As described above, it is possible to improve
bearing life by utilizing the strain-induced martensitic
transformation of retained c, and this is the perspective
which we applied to this steel development project. In
this development, in order to achieve a life performance
better than conventional products, we focused not only on
the volume of retained c, but also its stability, clarified
the alloy components which are more likely to undergo
the process of strain-induced martensitic transformation
during rolling contact, and adjusted the components ofthe developed steel. Furthermore, increasing the volume
of low hardness retained c leads to reduced bearing
hardness, therefore we incorporated a strengthening
mechanism through nanoscale carbonitrides to achieveboth a higher volume of retained c and high hardness.
Through these efforts, we succeeded in developing a steel
that uses less rare metals overall than the typical nickel
chromium molybdenum steel, which conforms to JISSNCM420.

Fig. 1 Microstructural changes in the vicinity of the dent
(Nital etching)1)

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)23

−Development of Medium Carbon Bearing Steel Achieving Life Improvement and Resource Saving−

The stability of retained c is closely related toalloying element content and an evaluation indicator ofthis is Ms temperature (martensite start temperature).
If the content of each alloying element is expressed asa mass percentage (%), for example the experimental
equation shown in Equation ⑴ below is proposed as therelationship between Ms temperature and the alloying
3)element .

Ms =550−361×（%C ）−39×（%Mn ）−35×（%V ）−20×（%Cr ）−17×（%Ni ）−10×（%Cu ）−5×（%Mo ＋%W ）＋15×（%Co ）＋30×（%Al ）⑴

Higher Ms temperature means lower stability of
the retained c, making it more likely to transform into
martensite. Most alloying elements in steel material
have the effect of lowering Ms temperature. Therefore,
in order to lower the stability of retained c and promote
the process of strain-induced martensitic transformation
during rolling contact, it is preferable to add the barest
minimal alloying element necessary to secure sufficientvolume fraction of retained c and prevent Ms temperaturefrom dropping as much as possible.

We performed a twin-disc test to evaluate the stabilityof retained c in relation to load. The test discs weremade by turning each evaluated steel, subjecting these toheat treatment, then grinding to ultimately create a discshape with an O.D. of 30mm and width of 8mm. The
heat treatment adopted was carbonitrized hardening andtempering which involves distribution of a high amountof nanoscale carbonitrides and each of the test discs were
given a volume fraction of retained c of around 30% in
order not to affect the stability of retained c. Table 2
shows the volume fraction of retained c for the surface ofeach test disc measured using X-Ray Diffraction (XRD).
As Fig. 4 shows, each test disc was put in contact with adrive cylinder made by through hardening and temperinghigh carbon chromium bearing steel (JIS-SUJ2) (surface
hardness: 750 HV) and rolled under the conditions shownin Table 3. The post-rolling volume fraction of retained cof the surface was measured, and the stability of retainedc evaluated by calculating the reduction rate of retained cusing Equation ⑵.
Retained c reduction rate ＝Post-rolling volume
fraction of retained c
1−×100，％
Pre-rolling volume
fraction of retained c

3. 2 Evaluated Steels

A

37%

B

32%

C

30%

D

27%

E

27%

8

u16

In order to clarify the effect of alloying elements on
the stability of retained c, we prepared the five types of
evaluated steels shown in Table 1. For the evaluation,
we selected the typical alloying elements of bearing
steel; namely Si, Cr, Mo, and V, which is formed fromnanoscale carbonitrides, and adjusted the content of thesein the evaluated steels. The types and amounts of alloying
elements not written in the table are the same for all of
the evaluated steels.

⑵

Table 2 Volume fraction of retained c in each steel typeu30

3. 1 Concept

3. 3 Evaluation Method

Test discu30

3. Controlling the Stability of Retained c
Through Alloying Element Adjustment

Drive disc
(SUJ2)

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Si

Cr

Mo

V

A

0.1

1.2

0

add.

B

0.1

1.2

add.

add.

C

0.1

1.2

0

much

D

0.1

1.5

0

add.

E

0.3

1.2

add.

add.

u16

Table 1 Nominal composition of evaluated steel (mass %)5

Fig. 4 Schematic of twin-disc test

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

−Development of Medium Carbon Bearing Steel Achieving Life Improvement and Resource Saving−

Table 3 Twin-disc test conditions
Contact stress

1.5 GPa

Rotational speed

100 min−1 (drive disc)

Slip ratio

9.1％

Lubricant

ISO-VG8

Lubrication method

Dripped (2 mL/min)

Test duration

8 hours

3. 4 Evaluation Results
Figure 5 shows the reduction rate of retained c in eachsteel type obtained through the experiment. For Steels
A and B, even though there is no distinct different in
reduction rate, there is a difference in the reduction rates
with the other steels, therefore it is clear that the type ofalloying element has an effect on the stability of retainedc. Here, if we sort out the effects of the alloying elements,
we observe what is shown in Table 4. “↑” represents
elements which increase the stability of retained c,“↓” represents elements which decrease the stabilityof retained c and “−” represents elements which haveno effect. By comparing Steels A and B, we see that Mo
has no effect on the stability of retained c, by comparing
Steels A, C and D, we see that V and Cr increase thestability of retained c, and by comparing Steels B and E,
we see that Si decreases the stability of retained c.

Retained c reduction rate, %50
40

The effects of Cr, Mo and V can be explained by
the Ms temperature equation shown in Equation ⑴. Inother words, Cr and V are the elements which majorly
reduce Ms temperature therefore, by adding these, it isbelieved that retained c is made more stable. Although
the coefficient of V is larger than that of Cr, the stabilityof Steel C, which has an increased amount of V, is lower
than that of Steel D, which has an increased amountof Cr, and this is believed to be due to carbonitrides
forming easily through the carbonitrization of V, therefore
reducing the content in retained c. Furthermore, from
Equation ⑴, it is believed that Mo slightly lowers Mstemperature therefore there was no obvious effect on thestability of retained c.
Si is not included in Equation ⑴, therefore its effecton Ms temperature is unknown. However, unlike Cr, Mo
and V, Si is known to be an element that increases carbon
4)
activity , therefore it is believed that when permeating thesteel with carbon through the process of carbonitrization,compared to the other evaluated steels, there is less carbon
content in the matrix. In other words, while the effect of
Si itself is unknown, by adding Si, it is possible to reduce
the post-carbonitrization carbon content compared to theother evaluated steels and this suppresses reduction of theMs temperature, which is therefore believed to reduce thestability of retained c.
From the above results, we understood that increasing
the amount of Si, which reduces the stability of retained c
is an effective approach to encouraging the strain-induced
martensitic transformation of retained c during rolling
contact and, ultimately, improving life.

30
20

4. Developed Steel for Long-life,
Resource-saving Bearings

10

4. 1 Features of the Developed Steel

0

A

B

C

D

E

Fig. 5 Reduction rate of retained c in each steel type

Table 4 Influence of alloy elements on retained c stability
Evaluation steel used Eﬀect on the stability
for comparisonof retained c

Si

Steels B and E↓

Cr

Steels A and D↑

Mo

Steels A and B−

V

Steels A and C↑

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

We determined the components of the developed steel
based on the results of the preceding section. Figure 6
shows the main alloying elements added to the developed
steel and their respective roles. If the developed steel
is used to make bearings, it undergoes a process ofcarbonitrized hardening and tempering in order toprecipitate nanoscale carbonitrides. Figure 7 shows theresults of observing precipitates by using Transmission
Electron Microscope (TEM). Primarily, the carbonitrides
created from V and Cr are distributed evenly, and the sizeof these particles are around several tens of nm. Throughthe distribution of these fine precipitates, precipitation
strengthening is achieved, and both a higher volume ofretained c and high hardness are achieved. Furthermore,
the existing SNCM420 contains not only chromium andmolybdenum, but also a high amount of nickel in order tosecure the hardenability required for large bearings. In thecase of the developed steel, however, rather than adding25

−Development of Medium Carbon Bearing Steel Achieving Life Improvement and Resource Saving−nickel, hardenability was compensated by using otherelements, thus reducing the overall rare metal content
by more than 20% and achieving resource-saving. By
increasing the base material carbon content more than
SNCM420 to the extent not to reduce impact resistance tocreate a medium carbon bearing steel, the time taken forthe carbonitrization is reduced and less energy is requiredfor heat treatment.

shown in Table 5. Figure 8 shows the results of thisevaluation. This bearing has a life over two times longer
than a bearing made from SNCM420 that has undergone
carburized hardening. This was accomplished by not only
balancing an increased amount of retained c and highhardness, but also by controlling the stability of retainedc.
Table 5 Life test conditions

C

Reduction of time taken for carbonitrization

Si

Promote the strain-induced martensitic
transformation of retained c

Mn

Increased retained c

30306D(O.D. 72mm, I.D. 30mm, assembled width 20.75mm)

Test bearing
Axial load

25.9 kN

Rotational speed−12 000min (inner ring rotation)Cr

Lubricant85W-90 gear oil

Mo

Lubrication
methodOil bath

Contaminants

High-speed steel atomized powder(730 HV, 100 to 150 μm)

Consentration ofcontaminants

0.02 mass% (0.1g/500 mL)

Formation of nanoscale carbonitrides

V

Fig. 6 Effects of alloy elements in the developed steel

Red : V
Green : Cr
Yellow : V+Cr(a) Element analysis results using Energy Dispersive
X-Ray Spectroscopy (EDS)

Cumulative probability of failure, %

2 µm

90
Developed steelcarbonitrization
SNCM420carburization

50

10
5
2
1

10

100

Life, hFig. 8 Dent-initiated spalling life

4. 3 Basic Performances of Bearings Made from
Developed Steel200 nm(b) Observation results using the extraction replica method

Fig. 7 Precipitate observation results

4.2 Life Performance of Bearings Made from
Developed SteelThe dent initiated spalling life of a bearing made fromthe developed steel was evaluated under the conditions

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In order to apply the developed steel to bearings, there
are certain performance requirements other than life
which must be satisfied. In particular, we evaluated theanti-brinelling and dimensional stability, as there was aconcern these could be reduced with an increased volumeof retained c in the surface.
Anti-brinelling performance was evaluated by pressing
a 3/8 inch steel ball into a flat plate at a predeterminedload and measuring the depth of the resulting dent. The
evaluation results are shown in Fig. 9. The developed
steel has a greater amount of retained c due to undergoinga process of carbonitrized hardening, however at thesame time it achieves precipitation strengthening through

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

−Development of Medium Carbon Bearing Steel Achieving Life Improvement and Resource Saving−nanoscale carbonitrides therefore it is apparent that thedeveloped steel is capable of anti-brinelling performanceequivalent to conventional steel that has undergone
carburized hardening.

Dent depth, µm

2Developed steelcarbonitrization
Conventional steelcarburization

1.5

1

0.5

5. Conclusion
Through precipitation strengthening with nanoscalecarbonitrides and incorporating the new method ofcontrolling the stability of retained c, we succeeded atdeveloping a bearing steel with superior life performancecompared to conventional products. This developed
steel, in terms of properties other than life performancealso, has performance equivalent to standard carburized
bearings, therefore can be applied to any application,
however can contribute to resource-saving particularly
when used in bearings for industrial machinery as analternative to SNCM420 and other nickel chromium
molybdenum steel.

0
0

0.5

1

1.5

2

2.5

Pressing load, kN

Fig. 9 Anti-brinelling test results

Dimensional change rate, %Dimensional stability was evaluated by press-fitting around bar with dimensions that make hoop stress equal
90 MPa into the bearing’s inner ring then holding for aset period of time at an ambient temperature of 120℃,
removing the round bar and measuring the dimensionalchange rate of the inner diameter compared to thestart. The evaluation results are shown in Fig. 10. The
surface of the developed steel subjected to carbonitrized
hardening has a high amount of thermally-unstableretained c, however by turning it into medium carbon
bearing steel, the internal volume fraction of retained c
is reduced and a dimensional stability equivalent to the
carburized hardening of conventional steel is obtained.

6. Special ThanksThis development was the result of joint researchw i t h N I P P O N S T E E L & S U M I TO M O M E TA LCORPORATION, to whom I wish to express my sincere
appreciation.

References
1)

2)

0.025

3)

0.02

4)

0.015

K. KIZAWA, T. MIKAMI: Effect of Retained Austenite
on Microstructural Change in Surface-Initiated Flaking
Process of Rolling Bearings, Proc. World Tribology
Congress, Kyoto (2009) 351.
K. KANETANI, T. MIKAMI, T SADA, K. KIZAWA:
Effect of Retained Austenite on Subsurface Initiated
Spalling in Rolling Contact Fatigue, Proceedings of
JAST Tribology Conference, Tokyo, (May 2016) 296 (inJapanese).
H . S U D O , I . TA M U R A , T . N I S H I Z AWA :Kinzokusoshikigaku, Maruzen (1972) 166 (in Japanese).
T. NISHIZAWA: Bull. Jpn. Inst. Met., 12 (1973) 401 (inJapanese).

0.01
Developed steel-carbonitrization

0.005
0

Conventional steel-carburization
0

250

500

750

1 000

1 250

Duration, hFig. 10 Dimensional stability test results

K. KANETANI**JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

T. MIKAMI*

T. SADA*

Material R&D Dept., Research & Development
Headquarters

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