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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