Mechanism of hydrogen-assisted surface crack growth of austenitic stainless steels in slow Strain rate Tensile test

Several criteria based on reduction in area (RA) or relative RA (RRA) are proposed for determining the hydrogen compatibility of austenitic stainless steels; however, the mechanism of hydrogen-induced degradation in RA and RRA is not necessarily clear. The degradation in the RA and RRA of the austenitic stainless steels is attributed to hydrogen-assisted surface crack growth (HASCG) accompanied by quasi-cleavages; therefore, a mechanism of the HASCG should be elucidated to establish novel criteria for authorizing various austenitic stainless steels for use in high-pressure gaseous hydrogen. To elucidate the HASCG mechanism, this study performed slow strain rate tensile (SSRT), elasto-plastic fracture toughness (J_IC), fatigue crack growth (FCG) and fatigue life tests on Types 304, 316 and 316L in high-pressure hydrogen gas. Experimental results of Type 304 were provided in this paper as a representative of Types 304, 316 and 316L. The results demonstrated that the SSRT surface crack grew via the same mechanism as for the J_IC and fatigue cracks, i.e., these crack growths could be uniformly explained on the basis of the hydrogen-induced successive crack growth (HISCG) model, which considers that cracks successively grow with a sharp shape under the loading process, due to local slip deformations near the crack tip by hydrogen. Accordingly, the HIS crack is ductile, not brittle.