Special Issue; Products for Industrial Machinery - NTN

NTN TECHNICAL REVIEW No.742006
Direction of load transfer
Direction of load transfer
aDriving roller side, current
density of 1.0 mA/cm 2
bDriving roller side, current
density of 0.3 mA/cm 2
Fig. 12 Photographs of surface cracks and their circumferential cross sections
characteristics of penetration of diffusible hydrogen on
the possibility of premature flaking. This effect was
examined by repeatedly applying circumferential
tensile stress to the rolling surface in a double-roller
test using an ultrasonic fatigue test.
The dynamics of contact between the rollers were
calculated under the loading conditions of the doubleroller
test. The calculation results are illustrated in
Figs. 13(a) and 13(b). Contact pressure distribution is
provided in Fig. 13(a), while Fig. 13(b) shows
circumferential tensile stress distribution on the rolling
surface along the X-X line where the tensile stress
maximizes on the edge of the contact ellipsoid. When
the rolling friction coefficient is 0, the maximum tensile
stress of both contact edges is 214 MPa. When the
rolling friction coefficient is 0.1, the maximum tensile
stress of one contact edge becomes 531 MPa.
From the results of the ultrasonic fatigue test, as
shown in Fig. 14, the 10% fatigue strength after 10 7
load application cycles is 981 MPa when there is no
diffusible hydrogen penetration. This strength is
sufficiently greater than the results shown in Fig.
13(b). In contrast, the 10% fatigue strength dropped to
460 MPa when diffusible hydrogen penetrated the
material, as in the result of the ultrasonic fatigue test
with a hydrogen pre-charging current density of 0.3
mA/cm 2 (4.5 wt-ppm). Consequently, if hydrogen
penetrates the material, cracks can occur at a tensile
stress less than those shown in Fig. 13(b).
As summarized above, if for any reason the rolling
friction coefficient becomes greater and a large
amount of diffusible hydrogen penetrates the material
at the same time, there is a higher possibility of crack
occurrence on the rolling surface, which can lead to
premature flaking. However, to verify this assumption,
it will be necessary to prove that diffusible hydrogen
penetrates the material in an amount sufficient enough
to lower the fatigue strength of the material in the
rolling contact mode. Detailed research efforts for
quantitative verification of the effects of hydrogen on
rolling fatigue were made from the 1950’s to the
1970’s. Grunberg et al. proposed a theory that the
loss in rolling life of a steel material due to ingress of
water is caused by hydrogen derived from the water. 9)-
11)
This theory was supported by Scharzberg et al. 12),13)
Axial coordinate
1.0
0.5
0.0
-0.5
-1.0
-0.5 0.0 0.5GPa
(a) Contact pressure distribution between two rollers
Stress, MPa
1400
1200
1000
800
600
400
200
X
531MPa
214MPa
0.1
0
0
-0.5 0.0 0.5
Circumferential coordinate mm
(b) Distribution of circumferential tensile stress working on
the contact surface along the X-X line
Fig. 13 Distribution of circumferential tensile stress
at contact surface along X-X line
X
2.5
2.0
1.5
1.0
0.5
0.0
-60-