Special Issue; Products for Industrial Machinery - NTN

The Influence of Hydrogen on Tension-Compression and Rolling Contact Fatigue Properties of Bearing Steel
Stress amplitude
50
10
10% fatigue strength after
10 7 load application cycles
10 7
Number of load applications
1.282s
Fig. 9 Estimation of 10% fatigue strength at 10 7 cycles
10% fatigue life after 10 7 load application cycles
MPa
1100
1000
900
800
700
600
500
400
300
200
No hydrogen pre-charging
0.1mA/cm 2 0.2mA/cm 2
0.3mA/cm 2
0.4mA/cm 2 0.5mA/cm 2
0 1 2 3 4 5 6 7
Diffusible hydrogen content wt-ppm
Fig. 10 Relationship between diffusible hydrogen
concentration and 10% fatigue strength at 10 7 cycles
3.2 Results of roller rolling-sliding test
3.2.1 Crack initiation life
The relationship between the hydrogen precharging
current densities and the number of rolling
test runs before occurrence of surface cracking in the
double-roller rolling-sliding test is illustrated in Fig. 11.
Because the calculated life under the conditions of this
test was 36,511 hours (approx. 4 years), 7),8) cracking
must not occur during the effective bearing life.
However, as shown in Fig. 11, in the result of each
test run that underwent hydrogen pre-charging,
surface cracking occurred before 80 hours had
elapsed, which is much shorter than the calculated
life. The accumulated test duration (the number of
load application cycles) before the occurrence of
cracking was shorter with greater current densities,
that is, with greater amounts of penetrated hydrogen.
3.2.2 Relationship between microstructural
changes and diffusible hydrogen contents
Photos of surface cracks and their circumferential
cross sections are shown in Fig. 12. Fig. 12(a) shows
the test result from a current density of 1.0 mA/cm 2
and Fig. 12(b) shows the test result from a current
density of 0.3 mA/cm 2 . In the case of Fig. 12(a), what
is distinctive about 12(a), that is not observed in 12(b)
were observed under the rolling surface. In contrast, in
the case of Fig. 12(b), no distinctive microstructural
changes were observed. Judging from these results, it
appears that distinctive microstructural changes occur
with greater amounts of penetrated hydrogen, but they
do not always occur with lower amounts of penetrated
hydrogen.
3.2.3 Correlation between the results of the
ultrasonic fatigue tests and the results of
the double-roller rolling-sliding test
As previously described in the introduction, with the
bearing products on the market, though a very rare
occurrence, surface-initiated flaking sometimes occurs
on the contact surface without apparent damage. The
cause of this problem could be tensile stress that is
repeatedly applied circumferentially to the rolling
contact surface. This tensile stress is greater with a
greater rolling friction coefficient. It could be easily
imagined that, under such conditions, premature
flaking would tend to occur if diffusible hydrogen
penetrates the material and the fatigue strength of the
material deteriorates. Therefore, an investigation was
conducted to determine the effect of the SN
Current density
mA/cm 2
0.3
Roller subjected
to hydrogen
pre-charging
Driving side
Following side
Number of test repetitions (accumulated test duration)
1 (20h) 2 (40h) 3 (60h) 4 (80h)
0.5
1.0
Driving side
Following side
Driving side
Following side
Fig. 11 Relationship between the current density for hydrogen pre-charging and the number of repetitions to failure
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