The effect of hydrogen on fatigue crack growth behavior and ductility loss of austenitic stainless steels

The effect of hydrogen on fatigue crack growth behavior and ductility loss after fatigue cycles of austenitic stainless steels, SUS 304 and SUS 316, was investigated. In the hydrogen-charged specimens, the crack growth rates were approximately twice higher than those of the uncharged specimens for crack growing from 100 μm to 1000 μm. The aspect ratio for the crack with 2a ≥ 800 μm was smaller in the hydrogen-charged specimens than in the uncharged specimens due to higher hydrogen content in the surface layer than in the subsurface. The slip band density around the crack in the hydrogen-charged specimens was less than in the uncharged specimens. The ductility loss was measured by tensile test carried out by interrupting fatigue test. There was a critical crack length for a decrease in ductility. There were no definite differences in ductility loss between the hydrogen-charged specimens and the uncharged specimens within the hydrogen content and the test strain rate of this study. The ductility loss in hydrogen-charged specimens was dependent only on the length of the main crack produced during fatigue cycles. Although many small cracks were observed on the surface of hydrogen-charged specimens, they did not give more influence on the ductility loss than fatigue cracks. These small cracks were nucleated by the synergetic effect of the hydrogen contained in specimen, the applied tensile stress and the martensitic transformation which was detected after tensile test.