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In order to investigate the suitable test methods for determining threshold of hydrogen-induced crack growth, K I,H , J Ic , none-unloading J Ic , constant-load and constant-displacement tests were conducted in 115 MPa hydrogen gas at room temperature, using SCM435 low alloy steels whose tensile strengths were between 824 and 1127 MPa. Constant-load and constant-displacement tests have been standardized by ASME KD-10 for determining K I,H in hydrogen gas. J Ic test has been standardized by ASTM E1820 for determining fracture toughness J Ic or K Ic in air. Recently, J Ic test was done in hydrogen gas to obtain K I,H . In this study, there were many cases when J Ic tests were not conducted in hydrogen gas, because CT specimens broke before the first unloading. Then, none-unloading J Ic test, where unloading processes were eliminated in J Ic test, was employed in hydrogen gas. All four tests were done under small scale yielding in 115 MPa hydrogen gas. K I,H obtained by four tests was almost the same. KD-10 demands that constant-load and constant-displacement tests of ferritic steels such as SCM435 are done for 1000 hours. Contrary to this, none-unloading test at displacement rate of 2×10 -3 mm/s was finished within one hour. It was concluded from these results that none-unloading J Ic test was the best test for determining K I,H in 115 MPa hydrogen gas. On the other hand, J Ic test was the only test for determining K Ic in air, because K Ic was obtained under large scale yielding.

In order to study the hydrogen embrittlement behavior of austenitic stainless steel weld metals, slow strain rate tensile (SSRT) tests were performed in 106 MPa hydrogen gas at −45 ºC. Tensile specimens, in which whole of the gauge section consists of weld metals, were machined out from a TIG welded round bar. The base metal of multi-pass welded bars was SUS316 (hi-Ni), and the filler metals were 317L, 316 and 316L. The nickel equivalent values of SUS316 (hi-Ni) base metal, and 317L, 316 and 316L weld metals were 29.69, 29.99, 28.87 and 28.27 mass %, respectively. Two series of weld metals were tested; one was as-welded metal, and the other was post-welded solution-treated weld metal. The relative reduction of area, RRA, was 0.55 for 317L as-welded metal, and approximately 0.9 for SUS316 (hi-Ni) base metal and 317L, 316 and 316L post-welded solution-treated weld metals. The result indicated that the resistance against hydrogen embrittlement was recovered by the post-welded solution-treatment. The fracture surface of SUS316 (hi-Ni) base metal and 317L post-welded solution-treated weld metal was entirely covered with dimples, whereas the fracture surface of 316 and 316L post-welded solution-treated weld metals was covered with dimples and quasi-cleavages. Accordingly, it was concluded that 317L post-welded solution-treated weld metal with the highest nickel equivalent had excellent resistance against hydrogen embrittlement, which was recommended for use in high-pressure hydrogen components.

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It has been shown that rolling contact fatigue (RCF) test using specimen having a small drilled hole is a useful method for evaluating the effect of small defect on flaking strength of steels. In this study, RCF tests of rolling bearings having a small drilled hole were carried out. The flaking failure was considered as a problem of shear-mode fatigue crack emanating from the small defect. As a first step to quantify the crack-growth threshold on the basis of fracture mechanics, mode II stress intensity factor range, KII, of a ring-shaped crack emanated around the edge of a drilled hole under the passing of a rolling element was analyzed by using finite element method. And then, the obtained values were correlated with the KII values of penny-shaped cracks in an infinite body under uniform shear through the intermediary of a correlation factor, fdrill. The stress intensity factor of the ring-shaped crack was uniformly correlated with that of the penny-shaped crack by the single factor fdrill irrespective of hole diameter, d, depth of hole edge, h', and maximum contact pressure, qmax, within the ranges: d = 0.05 ~ 0.2 mm, h' = 0.05 ~ 0.345 mm and qmax = 2.0 ~ 3.0 GPa. The obtained results will be applied for the quantification of RCF test results shown in the subsequent paper.