No.8 January 27, 2017

INVOLVEMENT OF HYDROGEN PERMEATION INTO STEEL IN SHORTENEDFATIGUE LIFEIn our previous issues, KYTB 6 and 7, the rolling four-ball tests demonstrated that the fatigue life in hydrogen was associated withwhite structure formation and the life can be effectively extended by use of rust inhibitors and anti-wear additives. In this issue, backto the basics yet again, similar tests were conducted in deuterium to study how surrounding deuterium permeates into steel. Deuterium was detected in areas around cracks at levels of up to 70 to 110 times higher than natural hydrogen, indicating that it permeates
into the cracked area of the steel surface to be concentrated.
Fig.1 show the rolling four-ball tester used in this study. In thecup filled with lubricating oil, three freely-rolling bearing balls
were rotated by another bearing ball placed on the top of themto make an evaluation of fatigue life in rolling contact under
static load. Unlike our previous tests in hydrogen H2 (KYTB 6),
the tests in this study were conducted in deuterium to draw aclear distinction from hydrogen present inherently in the steelball. Deuterium is a hydrogen isotope and accounts for about
0.015% of all the naturally occurring hydrogen.
Similar to the previous tests in H2, flaking occurred at the surface of the upper ball in a very short time and cracks wereaccompanied by white structure in their vicinity.
Cross section
Observing
断面観察方向Upperball direction
上部球（Φ15.88 SUJ2）

Fig.3 (a) shows the cross section of flaked surface and Figs. 3(b) and (c) show the chemical images of deuterium D- anddeuterated iron FeD+ for the surface, respectively, whichsuggest higher levels of D- and FeD+ in the cracked area.

シール
Seal
試験油
OilLower
下部球 balls

Exhaust
排気TOF-SIMS analysis was carried out on the cross section asshown in the upper right in Fig.1 to study the existence andcondition of hydrogen in tested steel ball. Fig.2 shows thedifference in the abundance ratio, RD/H between deuterium inthe steel ball and that in nature. Deuterium existed just beneath the ball surface at levels up to 70 to 100 times higher
than hydrogen in nature but gradually reduced with depth to inthe central part of the ball almost the same level as hydrogenin nature (1.6 times). Although deuterium in iron exhibited
diffusion coefficients of about 0.7 to 0.9 times that of hydrogen,repetitive rolling contacts seemed to have induced scatteringof hydrogen into the steel ball to produce a high-density subsurface layer.

（Φ15.00 SUJ2）

HH22or
D2 gas
又はD
供給supply
2

Thermocouple
熱電対荷重
LoadFig.1

Main part of rolling four-ball tester

(a)

120

contact surface
転走面
D- Rolling

100

FeD+R D/H

80

Crackedarea
き裂部60
40

RD/H =200Fig.2050

D- /H- (TOF-SIMS intensity ratio)
D/H (abundance ratio in nature)

100150Depth from rolling
contact surface,
転走面からの深さ,

200

250

30010μm(b)(c)
Fig.3 Chemical image of cracked area

10μmμmDeuterium abundance ratio for tested steel ball (RD/H)

Okada, Nanao, Mori, Nakazawa, Ikejima, Muto, Endo, Imai, Aida: Proceedings of JAST Tribology Conference (Tokyo 2012-5) 277.

Kyodo Yushi Co., Ltd., 2-2-30 Tsujido-kandai, Fujisawa, 251-8588 Japan

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