Recent progress on interpretation of tensile ductility loss for various austenitic stainless steels with external and internal hydrogen

Slow-strain rate tensile (SSRT) tests on various metals having γ-Fe phase; Type 304 and 316L stainless steels, HP160 high strength stainless steel, and A286 Fe-based super alloy were conducted in external hydrogen and with internal hydrogen. The external hydrogen indicates non-charged specimens tested in high-pressure hydrogen-gas environment, whereas the internal hydrogen indicates hydrogen-charged specimens, with uniform distribution of hydrogen, tested in inert gas. The hydrogen distribution was calculated based on the measured hydrogen diffusivity and solubility. The fracture morphologies were observed by scanning electron microscopy (SEM). For Types 304, 316L, and HP160 the relative reduction in area (RRA of the steels was successfully reproduced by the nickel equivalent, Ni_eq, showing the higher Ni_eq, the lager RRA. Furthermore, at a low Ni_eq, the RRA of the steel with external hydrogen was nearly equal to that with internal hydrogen. In contrast, at a high Ni_eq, the RRA of the steel with internal hydrogen was slightly degraded by hydrogen, RRA ∼ 0.8, whereas that in external hydrogen was not degraded, RRA ∼ 1. For A286, despite a high Ni_eq, the RRA of the alloy with internal hydrogen was significantly degraded by hydrogen, RRA ∼ 0.5. The fracture morphologies were categorized into four types: quasi-cleavage fracture associated with hydrogen-assisted surface cracks; ordinary void formation with no hydrogen effect; small-void formation associated with void sheet enhanced by hydrogen; facet formation induced by hydrogen. These categorized morphologies could be interpreted in terms of hydrogen distribution (internal or external hydrogen), austenitic stability (a low or high Ni_eq), and microstructure (solution or precipitation-hardened treatment).