Internal pressure fatigue properties of Cr-Mo steel (JIS G 4052, SCM435H) in high-pressure hydrogen or nitrogen atmosphere were investigated by the internal pressure fatigue test using a specially designed apparatus. Internal pressure was varied in the range from 0 to 40.5MPa, and test temperature was 20°C. The cycle length of internal pressure was varied up to 1000 seconds. We obtained the following results. 1. The number of fracture cycles in 45MPa hydrogen atmosphere was much less than those of in the 45MPa nitrogen atmosphere. 2. The number of fracture cycles was not depended on the cycle length. 3. In the fracture surface region where quasi-cleavage and intergranular surfaces are coexisting, the crack growth behavior is different from that of in the quasi-cleavage region.

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