Special Issue; Products for Industrial Machinery - NTN

NTN TECHNICAL REVIEW No.742006
3. Test Results and Discussion
3.1 Ultrasonic fatigue test results
3.1.1 SN characteristics
The results of the ultrasonic fatigue test are plotted
graphically in Fig. 6. Compared to the results obtained
without hydrogen pre-charging, the fatigue strength
with hydrogen pre-charging was much lower. In
addition, fatigue strength decreased with greater
current densities. As shown in Fig. 7, the mode of
fracture was a fish-eye fracture initiated at a non-metal
inclusion or other impurities within each test piece
regardless of whether or not they were subjected to
hydrogen pre-charging. Thus, no macro-scale
difference in fracture mode was found.
3.1.2 Relationship between fatigue strength and
diffusible hydrogen content
The relationship between the temperature and the
hydrogen release rate were obtained from the thermal
desorption analysis of hydrogen. The results are
shown graphically in Fig. 8. The amount of hydrogen
released was greater with greater hydrogen precharging
current densities, peaking at around 80˚C.
The hydrogen released at temperatures up to 200˚C
Stress amplitude, MPa
1400
1200
1000
800
600
400
1.E+04
0.4mA/cm 2
0.5mA/cm 2
1.E+05
No hydrogen pre-charging
0.1mA/cm 2 0.2mA/cm 2
1.E+06
0.3mA/cm 2
1.E+07
Number of load repetitions
Fig. 6 Results of ultrasonic fatigue testing
1.E+08
was defined as diffusible hydrogen and the amount of
such hydrogen is plotted in Fig. 8.
Based on the results shown in Figs. 6 and 8, the
effects of diffusible hydrogen on fatigue strength of
JIS-SUJ2 steel were evaluated considering the
following two points.
First, because scattering was present in fatigue test
data, a statistical approach for handling the data was
needed. Second, because premature flaking would
accidentally occur on JIS-SUJ2 steel accidentally in
real machines, it appeared reasonable to make the
above-mentioned evaluation based on the
characteristics of this material at around the fatigue
limit rather than based on the high loading
characteristics of this material, the latter being the
evaluation practice in conventional rolling fatigued
tests. Furthermore, the fatigue characteristics that can
be reflected in machine designs appear to be fatigue
strength-related characteristics rather than life-related
characteristics. In this context, based on the JSMS-
SD-6-02 SN curve regression method 6) of the Society
of Materials Science, Japan, the fatigue test data was
applied to a continuously-decreasing one-sided
logarithmic curve model. In this way, the 50% fatigue
strength curve and the 10% fatigue strength curve
were determined. The curve in Fig. 9 is the 50%
fatigue strength curve. The evaluation criterion was
10% fatigue strength after 10 7 load application cycles.
10 A NB D ABDconstants5
The relationship between the amount of diffusible
hydrogen and the 10% fatigue strength after 10 7 load
application cycles is illustrated in Fig. 10. Since there
is a good linear relation between these two factors, it
could be understood that the loss in fatigue strength is
governed by the diffusible hydrogen content.
Hydrogen release rate, ppm/min
0.25
0.20
0.15
0.10
0.05
0.5mA/cm 2 6.5wt-ppm
0.4mA/cm 2 5.3wt-ppm
0.3mA/cm 2 4.5wt-ppm
0.2mA/cm 2 3.5wt-ppm
0.1mA/cm 2 2.2wt-ppm
0.00
0 100 200 300 400
Temperature ˚C
Fig. 7 Photograph of fracture center
Fig. 8 Results of thermal desorption analysis of hydrogen
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