Unified interpretation of hydrogen-diffusion and various fracture properties in high-pressure hydrogen gas based on trap-site occupancy of hydrogen by dislocation—austenitic stainless steels and low-alloy steels

A binding energy, EB, and the number of trap sites, NX, of hydrogen trapped in cold-rolled austenitic stainless and quenched-tempered low alloy steels exposed in high-pressure hydrogen gas were determined from a linear relationship between the concentration of the trapped hydrogen, NHX, and the trap-site occupancy, θX, which is a function of EB and NX, being calculated from the Oriani’s local equilibrium theory. The determinations identified that EB = 28 kJ/mol and NX = 2.12×1025 /m3 for 30% cold-rolled SUS316L (heat of B); EB = 28 kJ/mol and NX = 3.83×1025 /m3 for 60% cold-rolled SUS316L (heat of B); EB = 24 kJ/mol and NX = 2.29×1025 /m3 for 30% cold-rolled SUS304 (heat of B); EB = 43 kJ/mol and NX = 2.68×1024 /m3 for SCM435 (heat JL); EB = 42 kJ/mol and NX = 2.80×1024 /m3 for SCM435 (heat KL); EB = 42 kJ/mol and NX = 2.20×1024 /m3 for SNCM439 (heat BL); EB = 42 kJ/mol and NX = 2.42×1024 /m3 for SNCM439 (heat GL). From binding energies reported in existing literatures and the size of dislocation cores, the trapped hydrogen in cold-rolled austenitic stainless and quenched-tempered low alloy steels was mainly trapped by dislocation cores. For the low alloy steels, furthermore, hydrogen-induced degradations (HIDs) of various fracture characteristics were linearly proportional to θX and the following values of EB were obtained: EB = 44 kJ/mol for fatigue crack growth characteristics in hydrogen gas at pressures, p, of 0.1 ~ 95 MPa and temperatures, T, of 25 ~ 95℃ for SCM435 (heat TS); EB = 44 kJ/mol for fracture toughness characteristics in hydrogen gas at p = 0.7 ~ 115 MPa and T = 25℃ for SCM435 (heat of KL); EB = 43 ~ 46 kJ/mol for slow strain rate tensile characteristics in hydrogen gas at p = 115 MPa and T = −45 ~ 120℃ for SCM435 (heats JL and KL) and SNCM439 (heats BL and GL). The values of EB obtained from the strength characteristics were nearly equal to those from the hydrogen-diffusion. A series of analyses implied that the hydrogen-diffusion characteristics and the HIDs of various facture characteristics were dominated by the interaction of hydrogen and dislocation cores; then, the characteristics under various combinations of p and T could be predicted by the unified parameter, θX.