The effects of a hydrogen gas environment on the fatigue characteristics of a type 304 austenitic stainless steel were investigated and the following results were obtained. The hydrogen effect is not clearly seen by judging fatigue life diagram. However, crack initiation retards and crack propagation accelerates in hydrogen gas environment. The retardation seems to be caused by the absence of oxygen and water vapour. The acceleration seems to be caused by the intrinsic hydrogen effect.

In order to clarify the effects of a hydrogen gas environment on the fatigue crack growth characteristics of stable austenitic stainless steels, bending fatigue tests were carried out in a hydrogen gas, in a nitrogen gas at 1.0 MPa and in air on a SUS316L using the Japanese Industrial Standards (type 316L). Also, in order to discuss the difference in the hydrogen sensitivity between austenitic stainless steels, the fatigue tests were also carried out on a SUS304 using the Japanese Industrial Standards (type 304) metastable austenitic stainless steel as a material for comparison. The main results obtained are as follows. Hydrogen gas accelerates the fatigue crack growth rate of type 316L. The degree of the fatigue crack growth acceleration is low compared to that in type 304. The fracture surfaces of both the materials practically consist of two parts; the faceted area seemed to be brittle and the remaining area occupying a greater part of the fracture surface and seemed to be ductile. The faceted area does not significantly contribute to the fatigue crack growth rate in both austenitic stainless steels. The slip-off mechanism seems to be valid not only in air and in nitrogen, but also in hydrogen. Also, the main cause of the fatigue crack growth acceleration of both materials occurs by variation of the slip behaviour. The difference in the degree of the acceleration, which in type 316L is lower than in type 304, seems to be caused by the difference in the stability of the γ phase.

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In order to establish an appropriate method for measuring the local hydrogen content distribution around a fatigue crack tip in austenitic stainless steels, secondary ion mass spectrometry (SIMS) and the hydrogen micro-print technique (HMPT) were applied to a fatigue crack in a type 304 stainless steel fatigued in a hydrogen gas environment. The main results in this study are as follows. In the SIMS method, it is visualized that a high content of hydrogen exists in the plastic zone at a fatigue crack tip propagated in hydrogen gas, compared to that on a smooth area fatigued in hydrogen, though there is a measurement error based on false detection due to the edge effect regarding hydrogen in water vapor on the fatigue crack surface. On the other hand, hydrogen in the plastic zone is difficult to detect by HMPT. This is attributed to the difficulty for hydrogen atoms to be emitted from the sample in this case. To detect hydrogen, it is necessary to sputter the atoms forcibly. In addition, it is considered that to analyze the local hydrogen distribution around a fatigue crack tip with SIMS not only qualitatively but also quantitatively, reduction of the false detection due to the edge effect is necessary.

In order to investigate the hydrogen effect on fatigue crack growth (FCG) behavior in a few kinds of practical alloys; austenitic stainless steels (solution-treated metastable type 304 and stable type 316L), an aluminum alloy (age-hardened 6061) and a low carbon steel (annealed 0.13%C-Fe), FCG tests were carried out in hydrogen gas and in nitrogen gas. The FCG rates of these materials are enhanced by hydrogen, though the acceleration degrees are different. A crack grows across grains by slip-off in 316L stainless steel and in age-hardened 6061 aluminum alloys even in hydrogen. Faceted area increases in 304 stainless steel and in low carbon steel in hydrogen. In 304 stainless steel, the ratio of facets to the entire fracture surface was not so large. Thus, the FCG rate is not significantly affected through the facets in 304 stainless steel. In low carbon steel, facets were increased considerably, though a crack grows step by step or after a large number of loading cycles even along grain boundaries. Anyhow hydrogen enhances the FCG rate of these materials through the influence on slip behavior. Based on above-mentioned results, the effect of loading frequency on FCG rate in hydrogen of the age-hardened 6061 aluminum alloy was also investigated. The FCG rate increases as the testing frequency decreases, though the FCG rate in hydrogen shows the tendency to saturate.