TECHNICAL PAPER

Improvement of Bearing Life through Bonding of Non-metallic*1
Inclusions with Matrix
T. SADA Y. NONAKA T. MIKAMI

K. KIZAWA

Non-metallic inclusions in the material of rolling bearings are well-known to induce subsurface initiatedspalling. Among these inclusions, oxides in particular are believed to harm bearing life due to the weak adhesion
between the oxide and the matrix. In this study, life tests were carried out for the inner rings of deep groove ballbearings made from low cleanliness bearing steel to which hot isostatic pressing (HIP) had been applied. From theexperimental results, we confirmed that life improvement of radial bearings can be achieved by bonding the oxides
with the matrix. Furthermore, void closing analyses were carried out using a model for ideal void shape. From thecorrelation between the void closing rates obtained by these analyses and the interface bonding rates measured forthe inner rings, we found that void closing analysis is effective in predicting the state of the interface between theoxide and the matrix.
Key Words: rolling bearing, rolling contact fatigue, non-metallic inclusion, hot isostatic pressing, finite element
analysis

1. Introduction
Non-metallic inclusions in the material of rolling
bearings are well-known to induce subsurface-initiatedspalling. Among these inclusions, oxides are believed
to be particularly detrimental to bearing life due to the1), 2)
weak bonding between the oxide and the matrix .
For this reason, Japanese steelmakers have establisheda technology to reduce the amount of oxygen, which
generates oxides, in steel to the maximum extent possible,
in order to achieve stable production of bearing steel
with high cleanliness (meaning that inclusions are small
3-5)
and few) . However, looking around the world, bearing
steel of such high cleanliness is not readily available. As
such, if a technology which enabled a bearing made with
low-cleanliness bearing steel to have a life equivalent toa bearing made with high-cleanliness bearing steel wasestablished, it would be possible to manufacture highquality bearings anywhere in the world.
Tsunekage and colleagues have published research
results focused on the bonding of oxides and matrix asa factor other than cleanliness which affects the rolling6), 7)
contact fatigue (RCF) life of bearing steel . Theirresearch confirms that rolling contact fatigue life isimproved using bearing steel in which oxides have been
bonded with the matrix using hot isostatic pressing (HIP).
This was verified in a thrust-type RCF test. In this type of
test, the direction of the contact load relative to the steel
bar rolling direction differs to radial bearings (e.g. deep
groove ball bearings, which are the most common type ofrolling bearing used). Moreover, although actual bearings
are normally used under a contact load which will not
result in the occurrence of plastic deformation, the thrusttype RCF test uses a large contact load at which plastic
deformation will occur.
In this study, in order to confirm if bonding oxides tothe matrix would be effective in increasing the life of an
actual bearing, life tests with radial load were carriedout for the inner rings of deep groove ball bearings madefrom low-cleanliness bearing steel to which hot isostaticpressing (HIP) had been applied. By comparing the lives
of inner rings with three different levels of HIP processedsteel, the pressurization condition necessary to obtainlife equivalent to mass-produced bearings was clarified.
Furthermore, the authors investigated a method ofpredicting the bonding state of oxides with the matrix byconducting an FEM analysis using a simple void model.

*1 This paper was prepared based on Journal of Japanese Societyof Tribologists No. 61 Vol. 4 (2016) 264-274.

22

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

−Improvement of Bearing Life through Bonding of Non-metallic Inclusions with Matrix−

2. Life Test on the Effect of Pressurizationshows an example for the temperature and pressure
measured during HIP processing.

2. 1 Test Bearings
Inner rings of deep groove ball bearings (I.D.:30mm,
O.D.: 62mm, width: 16mm - bearing no. 6206) madefrom low-cleanliness, high-carbon chrome bearing
steel (JIS-SUJ2) with the oxygen content intentionallymade high were produced. This steel was fabricated in a
vacuum induction melting (VIM) furnace with a 100 kg
capacity. The oxygen content of this steel was 27 ppm,
which is high compared to the SUJ2 steel mass produced
in Japan, which has an oxygen content of only a few
8)ppm . The inner rings were fabricated using the steel
bars made by applying the three levels of HIP processing
shown in Table 1 (HIP) and the steel bar to which HIP
had not been applied (non-HIP). These steel bars werenormalized, annealed and then turned to achieve the innerring shape. After this, the blanks were quenched andtempered, then grinded and given raceways with superfinishing to manufacture the final inner rings using thesame processes as mass production. The surface hardnessof these inner rings ranged between 62 and 63 HRC.
Finally, these inner rings were assembled with massproduced outer rings, balls and retainers to form test
bearings.
The HIP-2 shown in Table 1 used practically the same
conditions as the study by Tsunekage and colleagues inwhich rolling contact fatigue life was confirmed to have6), 7)(1 150℃, 147 MPa, 5 h) (the duration was
improved
shortened). HIP-3 had higher temperature conditions than
HIP-2 and a pressure close to the upper limit of most HIP
9)processing equipment (200 MPa) , while HIP-1 had thesame pressure as HIP-2 but lower temperature. Figure 1

2. 2 Bonding State between Inclusions and Matrix
Figure 2 shows examples of oxides observed in the
cross-sections of inner rings. All of the oxides observed
are Al2O3 and the maximum diameter (circular equivalent2
diameter) in 8 600 mm was predicted to be 14 μm in
10)accordance with the extreme value statistics method .
Most of the oxides contained in the non-HIP inner ring
had cavities between the matrix and the inclusion in theaxial direction (longitudinal direction of the steel bar).
Meanwhile, the majority of oxides in the HIP inner rings
were bonded with the matrix. The bonding state wasquantified in accordance with the following steps in order
to elucidate difference in bonding states depending on
the implementation or lack of HIP processing, and HIP
processing conditions.
First, the authors used a non-metallic inclusion
measurement device (METALSPECTOR II-C made by
Toshiba Solutions) on the samples to detect the oxides
in the cross-sections of the axial direction of the nonHIP and the three different HIP inner rings. Then, of
the oxides with an equivalent circle diameter of 4 µm or
greater detected in the four cross-sections of each inner2
ring (where each cross-section had an area of 34.32 mm ),the largest fifty were made the subjects of this evaluation.
Of the inclusions comprising of both oxides and sulfides,
the researchers only used those where the cross-sectional
area of the oxide was one-third or more of the overallcompound inclusion’s cross-sectional area. The existenceof cavities in the individual oxides included in theevaluation was determined with an optical microscope
and the interface bonding rate (%), as defined by thefollowing equation, was calculated.

Table 1 HIP conditions
Condition

Temperature

Pressure

Duration

HIP-1

1 050℃

150 MPa4h

HIP-2

1 150℃

150 MPa4h

HIP-3

1 200℃

193 MPa

4h

180
150

1 000
800

Pressure

600

120
90

400

60

200

30

0

0

2

4

6

8

10

12

0
14

Time, hFig. 1 Example of HIP processing log (HIP-2)

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

Longitudinal directionof steel bar

Temperature, ℃

1 200

⑴

210

Temperature

Pressure, MPa

1 400

No. of bonding inclusions
Interface×100≡
bonding rate
Total no. of examined
inclusions

CavityCavity(a) Non-HIP

Oxide

Oxide

5 µm

5 µm
(b) HIP-2

Fig. 2 Examples of observed oxides

23−Improvement of Bearing Life through Bonding of Non-metallic Inclusions with Matrix−

Table 2 Life test conditions

Interface bonding rate, %70
60

Radial load

50

Rotational speed

40

10
0

Non-HIP

HIP-1

HIP-2

−1

(inner ring)

LubricantMineral oil (ISO-VG68)

Lubrication

Oil circulation with on-line ﬁlter(Nominal ﬁltration rating : 2 μm)

Operating temperature

65±5℃

30
20

9 000 N
2 500 min

HIP-3

Fig. 3 Change in interface bonding rate of oxide according to
the HIP process

Figure 3 shows the derived interface bonding rate. For
the non-HIP inner ring, the interface bonding rate is 6%,
meaning that there are cavities between the matrix formost oxides. If HIP processing is applied, the numberof bonded inclusions increases and the bonding rateincreases with the temperature and pressure. However,there was no clear difference in the bonding rates of
HIP-2 and HIP-3.

shown in Table 2, and spalling occurrence was detectedusing the accelerometer mounted on its top. The test
conditions were set so that sufficient lubricant film would
form in order to ensure the occurrence of subsurfaceinitiated spalling. The bearing temperature was made
65℃, and, using the procedures shown in the appendix ofreference 11), the lubricant film parameter at the contact
between ball and inner race was 4.5. The life test was
conducted five times for each bearing steel condition(non-HIP, HIP-1, HIP-2 and HIP-3); as two bearings per
test are investigated, 10 bearing of each type were tested.

2. 4 Target Life
2. 3 Life Test Method
Figure 4 is a schematic of the life test rig used for thelife test. This test rig’s structure is one whereby a radialload can be applied equally to four bearings and in thecase of this test, the two inside bearings were establishedas the test bearings, while the two outside bearings
were made support bearings. Bearings with long-life
specifications were established as the support bearings
so that the test bearings would be prioritized in regard todamage. The test rig was operated using the conditions
Accelerometer

Compression
spring

The aim of this study was to raise the life of a bearing
made with low-cleanliness steel to that equivalent to a
mass-production bearing made with high-cleanlinesssteel. For that reason, the actual capability of a massproduction bearing is used as the target life. The life ofa mass-production bearing under the conditions shownin Table 2 is almost ten times that of basic rating life,
however this test only considers the inner ring, therefore
the basic rating life of the inner ring was calculated andten times this figure was set as the target life.
The basic dynamic load rating of inner ring 6206
12)
13)
calculated by ISO 281 and ISO/TR 1281-1 , is 20 070
N. Based on this, the basic rating life of the inner ringunder the conditions shown in Table 2 will be 73.9 hours.
This result multiplied by ten gives 739 hours, therefore
this was used as the target life for the inner ring madewith low-cleanliness steel.

2. 5 Life Test Results
Test bearing

Supportbearing

Test bearing

Supportbearing

Fig. 4 Schematic diagram of life test rig

24

Figure 5 is a Weibull distribution plot of the life test
results. As this test only focuses on the inner ring, thetest was suspended if spalling occurred on any otherpart of the bearing and the data taken up until that point
was made suspended data. From their appearance, all of
the spalling that occurred in the inner ring was judged
to have been initiated from subsurface of the raceway.
For the non-HIP inner ring, spalling occurred on thethree tests, therefore the median ranks of the spalling
data was determined in consideration of two suspended
14)
data and the population was estimated. Meanwhile,for the HIP inner ring tests, spalling only occurred once

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

−Improvement of Bearing Life through Bonding of Non-metallic Inclusions with Matrix−

for all three HIP levels when the test was conducted for
3 000 hours therefore it was determined difficult to obtain
anymore spalling data and the tests were suspended when
operating time exceeded fifty times the basic rating lifeof the bearing (3 390 hours). As such, it was not possible
to use linear regression of the spalling data to estimate
the population, therefore linear regression was applied to
five points of all data, including the suspended data, toestimate the population for the sake of convenience.
Table 3 shows the lives (L 10iE ) of the inner rings with90% reliability of each estimated population. Neither
the non-HIP nor the HIP-1 inner ring reached the target
life. Meanwhile, the HIP-2 inner ring, which had thesame conditions applied as that used by Tsunekage andcolleagues, and the HIP-3 inner ring, which was treatedat the highest pressure, achieved four times the target life,
clearly demonstrating that the life had been improved.
The above results verified that HIP processing could
improve bearing life on radial bearings in the same way as
it had been confirmed in thrust-type RCF tests. Moreover,by making the inclusions in the material bond with thematrix, the life of a bearing made with low-cleanlinesssteel can be made equivalent to a bearing manufactured
with high-cleanliness steel.

Cumulative probability of failure, %

90
80
70
60
50
40
30
20

Non-HIP
HIP-1
HIP-2
HIP-3

Failed Suspended
―
●
◆
◇
■
□
△
▲
HIP-3
HIP-2
HIP-1

5
4
3
2
1
100

An analysis was conducted using the finite element
method (FEM) aimed at a more detailed investigation of
the relationship between the HIP processing conditions
and the state of oxide interface bonding mentioned in theprevious section (Fig. 3). However, a model which only
had voids in the material was adopted in order to simplify
the phenomenon as much as possible. Because theactual internal conditions of the cavities in the material
are unknown (whether they are full of some type of gas
or entirely empty) for the analysis of this study, it was
assumed that pressure did not occur inside the void even
if the material was subjected to compressive stress.
As seen in Fig. 1, HIP processing involves increasing
both temperature and pressure simultaneously, meaningthat material strength will reduce and deformation will
occur. Moreover, temperature increase will lead tophase transformation. In order to reflect these complex
phenomena, a heat treatment simulation software(DEFORM-HT) was used for the analysis.

3. 1 Analysis Conditions
Figure 6 shows the void closing analysis model. The
shape of the void located in the middle of the area wasdefined as a cosine shape with Equation ⑵ so that it
would simulate a cavity when the material was stretchedin the rolling direction.

y ＝±

Non-HIP

10

3. Void Closing Analysis Using FEM

1 000

10 000Life, hFig. 5 Life test results of inner rings

Table 3 Estimated lives of inner rings
Specimen

L 10iE , h

Non-HIP

504.4

HIP-1

577.0

HIP-2

2 894.4

HIP-3

2 977.6

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

1
p
cos
x
4
2

⑵

The temperature and pressure combinations shown in
Table 4 were applied in the analysis of void deformationaccompanying HIP processing. The conditions given in
the table for HIP-1, HIP-2 and HIP-3 are equivalent to the
HIP processing applied to the inner rings material used
for the life test described in the previous section. HIP-3
pressure was 193 MPa according to the actual processing
log, however it is approximated as 200 MPa in order toposition at equal intervals for ease of analysis.
Because it is necessary to assume the changes intemperature and pressure inputted in the analysis against
conditions with no processing results, the researchers firstprepared a model of the actual processing log, as shownin the example provided in Fig. 7. For the temperatureand pressure increase phase and holding phase the interval
between critical points of the temperature and pressure in
the processing log respectively were approximated using astraight line. For the temperature and pressure drop phase,the interval between each critical point was approximatedwith a catenary, except for the final stage of the pressure
drop which was approximated with a parabola.

25

r ＝ ry ＋H ' e

⑶

20

2

0.5

p

x

20

Table 4 Parameters for void closing analysis
Temperature
1 050℃ 1 100℃ 1 150℃ 1 200℃
Pressure
150 MPa
200 MPa
＊

＊

＊

＊

HIP-1

＊

＊

＊

＊

＊

HIP-2

＊

＊

＊

＊

HIP-3

：Analyzed conditions.
HIP-1 to 3：Equivalent to the conditions applied to thetest bearings

26

0

2

4

Pressure, MPa

210
180
150
120
Pressure
90
60
30
0
6
8
10

Fig. 7 Example of modeled HIP processing log(1 150℃, 150 MPa)
600

800℃

500
400
300
200
100
0

900℃
1 000℃

00.05 0.1 0.15 0.2 0.25 0.3

1 100℃
1 200℃

Equivalent plastic strain e

Fig. 8 Relationship between equivalent plastic strain andflow stress for each temperature

Here, −
e is equivalent plastic strain. Figure 8 shows therelationship between equivalent plastic strain and flow
stress at temperatures of 800℃ or higher.

3. 2 Analysis Results

Void closing rate≡ 1−

Fig. 6 Plane strain model for void closing analysis

100 MPa

Temperature

Figure 9 shows examples of void deformation caused
by HIP processing as obtained through the analysis. It
can be seen that the voids after HIP has been applied
are smaller than pre-HIP. In order to compare the degreeof void deformation under each processing condition,
the opening width, b , of the void’s y direction (refer toFig. 10) was made the characteristic dimension of a voidand the void closing rate (%) was defined using Equation
⑷.

p

y

1 400
1 200
1 000
800
600
400
200
0

Time, hFlow stress r, MPa

For the purposes of this analysis, creep broughton by holding at high temperature is not taken into
consideration, therefore the duration is shortened.
Moreover, in the model shown in Fig. 6, the size ofthe voids relative to the area are bigger than the actual
cavities around the inclusions in the material therefore
pressure is corrected by multiplying a coefficient
determined in a pre-analysis. Based on the processing log
of the three HIP levels applied to the inner rings for thelife tests of Section 2, the conditions with no processing
results were determined through either interpolation or
extrapolation of the critical points and input data wascreated by connecting the interval of each critical point in
the abovementioned model.
In regards to the elastic-plastic property of the material
dependent on temperature, a database prepared by the
Research Group on Database of Cooling Power ofQuenchant affiliated with the Japan Society for Heat
Treatment focused on the cooling performance of15), 16)
was used. The linear hardening
quenching coolant
rule of Equation ⑶ was applied to the flow stress,
r , which expresses plastic behavior. However, in theabovementioned database, yield stress, ry , and the strain
hardening coefficient, H ', are not defined at temperatures
exceeding 900 ℃ ; therefore for the purposes of thisanalysis, these values were extrapolated based on data up
to the maximum temperature of 1 200℃.

Temperature, ℃−Improvement of Bearing Life through Bonding of Non-metallic Inclusions with Matrix−b Post-processingb Pre-processing

×100

⑷

Figure 11 shows the void closing rates for eachcombination of temperature and pressure shown in
Table 4, as obtained in the analysis. While it goes
without saying, the closing rate increases as pressure andtemperature increase.
Figure 12 shows the relationship between void closing
rate obtained in the analysis and the measured interface
bonding rate (Fig. 3) relative to the three HIP processing
levels applied to the inner rings for the life tests ofSection 2. The void closing rate of the non-HIP material
has also been plotted in the figure as zero. The figure
shows there is correlation between the void closing rate

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

−Improvement of Bearing Life through Bonding of Non-metallic Inclusions with Matrix−

1

y, mm

y, mm

1
0
−1
−1.5

0
x, mm

0
−1
−1.5

1.5

(a) Before HIP

0
x, mm

1.5

(b) After HIP

Fig. 9 Change in void shape due to HIP processing(1 050℃, 200 MPa)band interface bonding rate. The interface bonding rate,
which expresses the percentage of the 50 inclusions being
assessed that are bonded to the interface with the matrix,
and the void closing rate, which expresses deformation
relative to a schematically simple void, are related totwo different phenomena. Despite this, due to the factthat correlation between these two was recognized, it
can be considered that they are both indicators of howeasily a cavity or void “collapses” relative to the degreeof compressive stress and that their behavior underpressurization conditions is qualitatively consistent.
Consequently, it is believed possible to relatively predict
the bonding state of the inclusions which actually exist
in the material with the matrix through a void closing
analysis, as discussed in this section.

4. Conclusion

100
80
60
40
1 20
0
1 15
0
1 10
0
1 05
0

e,
℃

20

ra
t

pe

Te
m

sure
, MP
a

100

150

Pres

ur

0

200

Void closing rate, %Fig. 10 Characteristic dimension of cosine-shaped voidInterface bonding rate, %Fig. 11 Void closing rate for each HIP condition

70
60
50
40
30
20
10
0

0

20

40

60

80

In this study, life tests were performed on inner ringsof deep groove ball bearings made from steel with a highoxide content which had been subjected to three different
levels of HIP processing. The result of these tests
showed that service life could be improved even in radial
bearings, just as it had been confirmed in thrust-type RCF
tests, which differed in respect to load level and direction.
Moreover, this study also confirmed that the degree ofinclusion and matrix bonding differed depending on HIP
processing conditions (temperature and pressure) andthat this, in turn, affected service life. It can be deemedthat the necessary pressure conditions to raise the life of abearing made with low-cleanliness bearing steel to that ofa mass-production bearing are 1 150℃ and 150 MPa.
Using a cosine-shaped void model, a coupled analysis ofheat, phase transformation and elastic-plastic deformation
was conducted. A correlation was recognized between theinterface bonding rate measured on the inner rings using alife test (percentage of inclusions bonded with the matrix)
and the void closing rate obtained through the analysis(extent to which the void is closed). Therefore, it wasclarified that this analysis is effective in predicting thebonding state of inclusions and matrix.

100

Void closing rate, %Fig. 12 Relationship between void closing rate andinterface bonding rate

References
1)

2)

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

T. Uesugi: Recent Development of Bearing Steel inJapan, Tetsu-to-Hagané, Vol. 74, No. 10 (1988) 1889 (inJapanese).
T. Udagawa, K. Suga, T. Kozuka, T. Nishikawa: The
Influence of the Non-metallic Inclusions on the Rolling
Contact Fatigue Damage Behavior of Bearing Steel,
Aichi Steel technical review, Vol. 30, No. 1 (2013) 29 (inJapanese).

27

−Improvement of Bearing Life through Bonding of Non-metallic Inclusions with Matrix−

3)

J. Nakashima, T. Toh: Improvement of Continuously CastSlabs Quality by Decreasing of Nonmetallic Inclusions,
NIPPON STEEL TECHNICAL REPORT, No. 104(2013) 42.
4) T. Emi: Steelmaking Technology for the Last 100 Years:
Toward Highly Efficient Mass-Production Systems forHigh-Quality Steels, Tetsu-to-Hagané, Vol. 100, No. 1(2014) 31 (in Japanese).
5) K. Semura, H. Matsuura: Past Development and Future
Prospects of Secondary Refining Technology, Tetsu-toHagané, Vol. 100, No. 4 (2014) 456 (in Japanese).
6) N. Tsunekage, K. Hashimoto, T. Fujimatsu, K. Hiraoka:
Initiation Behavior of Crack Originated from NonMetallic Inclusion in Rolling Contact Fatigue, Sanyo
Technical Report, Vol. 18, No. 1 (2011) 23 (in Japanese).
7) K. Hashimoto, T. Fujimatsu, N. Tsunekage, K. Hiraoka,
K. Kida, E. C. Santos: Effect of inclusion/matrix
interface cavities on internal-fracture-type rolling contact
fatigue life, Materials and Design, Vol. 32, No. 10 (2011)
4980.
8) K. Kawakami: Generation Mechanisms of Non-Metallic
Inclusions in High-Cleanliness Steel, Sanyo Technical
Report, Vol. 14, No. 1 (2007) 22 (in Japanese).
9) T. Nakai: Recent Trend in HIP Technology, JOURNAL
OF THE JAPAN WELDING SOCIETY, Vol. 65, No. 8(1996) 622 (in Japanese).
10) K. Kizawa, M. Gotoh: Estimation of Rolling Contact
Fatigue Life through Evaluation of Cleanliness in
Bearing Steel, Koyo Engineering Journal English
Edition, No. 163E (2003) 37.
11) Technical Committee on Life of Rolling Bearings: Life
Test Manual for Rolling Bearings (Part 3), Journal ofJapanese Society of Tribologists, Vol. 53, No. 5 (2008)
326 (in Japanese).
12) ISO 281:2007, Rolling bearings - Dynamic load ratings
and rating life.
13) ISO/TR 1281-1:2008, Rolling bearings - Explanatory
notes on ISO 281 - Part 1: Basic dynamic load rating andbasic rating life.
14) L. G. Johnson: The Statistical Treatment of Fatigue
Experiments, Elsevier (1964).
15) M. Narazaki: Present State and Scope on Heat Treatment
Simulation, NETSUSHORI, Vol. 50, No. 4 (2010) 343(in Japanese).
16) Y. Watanabe: SOKEIZAI, Vol. 55, No. 2 (2014) 42 (inJapanese).

T. SADA*

T. MIKAMI*

K. KIZAWA*
*
**

28

Y. NONAKA**

Material R&D Dept., Research & Development
Headquarters
Heat treatment Production Engineering Dept.,
Production Engineering Headquarters

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)