マルテンサイト系ステンレス鋼の疲労強度に及ぼす水素チャージの影響

抄録

The effect of hydrogen on high cycle fatigue properties of a martensitic stainless steel was investigated. In the Fuel Cell (FC) system, many components are exposed to high pressure hydrogen environment under cyclic loading. In this study, hydrogen was artificially charged into specimen of a martensitic stainless steel, 0.44C-13Cr-0.13N steel, and the fatigue properties were compared with those of the specimens without hydrogen charge. Fatigue strength and fatigue life decreased with increasing hydrogen content. It implies that the fatigue threshold of the microstructure which contains high hydrogen content is much lower than that of the as-heat-treated microstructure. Specimens having a longer fatigue life had a particular morphology named ODA (Optically Dark Area) besides the inclusion at fracture origin. The fracture origin of hydrogen charged specimens showed smaller ODAs than as-heat-treated specimens. The upper bound of the critical hydrogen content level resulting ODA lies between 1.0ppm and 2.0ppm. The condition for the critical size of ODA for the start of conventional fatigue crack growth was analyzed by the fracture mechanics. The critical stress intensity factor range ΔK_ODA for the critical size of ODA of as-heat-treated specimens can be correlated with the threshold stress intensity factor range ΔK_th expressed by the √area parameter model for small cracks.