Abstract
In order to investigate the influence of hydrogen on the fatigue strength of AISI type 304 meta-stable austenitic stainless steel, specimens were cathodically charged with hydrogen. A small hole with a diameter d =100 μm and a depth h =100 μm, was introduced into the specimen surface after hydrogen-charging. Fatigue crack growth rates of the hydrogen-charged specimens were compared with those of uncharged specimens in tension-compression fatigue tests under a constant stress amplitude, σ_a = 320MPa, at a stress ratio, R = -1, and a test frequency, f = 1.0 Hz. Hydrogen-charging led to a marked increase in fatigue crack growth rate. The crack growth path in the hydrogen-charged specimens was relatively linear, whereas the crack growth path in the uncharged specimens was more zigzag. In the uncharged specimens, a large number of slip bands were observed in the vicinity of the fatigue crack, whereas in the charged specimens relatively few slip bands were observed. To elucidate the behavior of hydrogen during fatigue process, the surfaces of both uncharged and charged specimens were examined by the hydrogen microprint technique (HMT). In the uncharged specimens, no hydrogen emission was observed. On the other hand, in the hydrogen-charged specimens, a hydrogen emission was observed, especially in the vicinity of fatigue crack. A comparison between HMT image and etched microstructure revealed that hydrogen was mainly emitted from slip bands, which were supposed to be a pathway of hydrogen. An analysis of the electron backscatter diffraction pattern (EBSD) showed that both α' and ε martensites were induced along the slip bands.
