In order to clarify the effect of hydrogen-gas pressure on threshold of hydrogen-induced crack growth (K_{I,H}), elasto-plastic fracture toughness tests were performed on JIS-SM490B carbon steel, JIS-SCM435 low-alloy steel and JIS-SUS304 stainless steel. For JIS-SM490B and JIS-SUS304, the values of K_{I,H} at displacement rates of 2×10^{–5} and 2×10^{–4} mm/s were almost the same in hydrogen at pressures of p_{H2}= 20, 45 and 115 MPa (K_{I,H} ≒ 80 MPa·m^{1/2} for JIS-SM490B and 150 MPa·m^{1/2} for JIS-SUS304). In contrast, for JIS-SCM435, the values of K_{I,H} at displacement rates of 2×10^{–5} and 2×10^{–4} mm/s were the same in hydrogen gas at pressures of p_{H2} = 20, 45 and 80 MPa (K_{I,H} ≒ 100 MPa·m^{1/2}); however, those of K_{I,H} were degraded in 115-MPa hydrogen gas (K_{I,H} ≒ 50 MPa·m^{1/2}). Based on the HISCG (Hydrogen Induced Successive Crack Growth) model, an effect of p_{H2} on K_{I,H} and an existence of K_{I,H} were also discussed for JIS-SM490B, JIS-SCM435 and JIS-SUS304.
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