Special Issue; Products for Industrial Machinery - NTN

NTN TECHNICAL REVIEW No.742006
Technical Paper
The Influence of Hydrogen on Tension-Compression and
Rolling Contact Fatigue Properties of Bearing Steel
Hiroshi HAMADA
Yukio MATSUBARA
In this report, the fatigue properties of JIS-SUJ2 bearing steel
were investigated under various hydrogen pre-charging conditions.
The results demonstrated that the fatigue strength substantially
decreased by the influence of hydrogen and that the decrease of
the fatigue strength was in direct correlation with the diffusible
hydrogen content. Additionally, double-roller rolling-sliding contact
fatigue tests were conducted to investigate the influence of
hydrogen on surface crack initiation life and microstructural change beneath contact surfaces. As a
result, surface cracking occurred much earlier as compared to the calculated life. The crack initiation
life decreased as the amount of the diffusible hydrogen increased. Unusual microstructural changes
were distinctly observed when a relatively high concentration of the diffusible hydrogen had
penetrated. In contrast, a relatively low amount of diffusible hydrogen did not always cause unusual
microstructural changes, although only marginal difference was seen in the surface crack initiation
life for varying levels of diffusible hydrogen content above 0 ppm.
1. Introduction
In certain bearing applications with automotive
auxiliary equipment, it has been known that premature
flaking accompanied by unusual microstructural
changes can occur under rolling surfaces, and that
one typical cause of this problem appears to be
hydrogen embrittlement. 1) In addition, though a very
rare occurrence, premature flaking sometimes occurs
in actual applications, and the cause is difficult to
determine. Currently, many researchers believe that
hydrogen-induced failures accompany microstructural
changes. However, assuming that microstructural
changes result only from the penetration of a large
amount of hydrogen, then there is the possibility that
hydrogen can induce premature flaking without
accompanying microstructural change.
The effects of hydrogen on the fatigue
characteristics of bearing steel were evaloated using
an ultrasonic fatigue test. High-speed load application
during this test inhibits dissipation of hydrogen. This
paper reports the results of this test.
In addition, this paper also reports the results of the
investigations into the effects of hydrogen on surface
crack initiation life and microstructural change
behavior using the double-roller rolling-sliding test.
2. Test Methods
2.1 Material
The chemical composition of JIS-SUJ2 bearing
steel, which was the material analyzed, is summarized
in Table 1. The material was subjected to a standard
quenching/tempering process under the heat
treatment conditions summarized in Table 2, and was
finished to the specified test piece dimensions through
a grinding process. The surface hardness after heat
treatment was HV747 and the residual content,
determined through an X-ray diffraction method, was
9.3%. These values mean that the test pieces feature
standard heat treatment quality.
2.2 Ultrasonic fatigue test method
The ultrasonic fatigue test executed was an axial
load fatigue test (stress ratio: R = –1) in which each
test piece was brought into a resonant state by
Elemental Technological R&D Center
New Product Development Dept. New Product Development R&D Center
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