Crystallographic study of hydrogen-induced twin boundary separation in type 304 stainless steel under cyclic loading

•We used small compact-tension specimens with twinned crystals of stainless steel.•This study focused on the martensitic transformation during fatigue crack growth.•The crack extended in martensite formed at crack tip in the uncharged specimen.•Hydrogen-induced twin boundary separation occurred at martensite/austenite interface.•Hydrogen can facilitate a slip-off crack growth mechanism in high stress intensity. This study focused on the martensitic transformation during fatigue crack growth in twinned crystals using small compact-tension specimens to elucidate the hydrogen-induced twin boundary separation in type 304 stainless steel. In the uncharged specimen, martensite variants were formed with their habit planes parallel to the most highly shear-stressed slip plane. The crack extended in martensite that was earlier formed ahead of the crack tip and deflected from the twin boundary. Hydrogen-induced twin boundary separation occurred predominantly at the interface between martensite and austenite in a medium stress intensity range. As the stress intensity range increased, martensite variants symmetrically arranged with respect to the twin plane dominated, suggesting that a slip-off crack growth mechanism was facilitated by hydrogen.