Simulation of the effect of internal pressure on the integrity of hydrogen pre-charged BCC and FCC steels in SSRT test conditions

Three effects of hydrogen have been reported on an austenitic stainless steel (Type-316L), a Cr-Mo steel (JIS-SCM435) and a carbon steel (JIS-SGP) during slow strain rate tensile (SSRT) tests performed in air on H-charged, smooth round-bar specimens. Two H-charging conditions were considered: exposure to 100-MPa hydrogen gas (270 °C, 200 h for Type-316L), and immersion in a NH₄SCN solution (40 °C, 48 h for other steels). Modifications of the micro-void coalescence (MVC) mechanism were observed for each steel: decrease of dimple size (Type-316L), increase of dimple size (JIS-SGP), and formation of quasi-cleavage (QC) surfaces (JIS-SCM435). To clarify the contribution of the hydrogen-induced cracking (HIC) mechanism to these failures, the pressure build-up in preexisting cavities and its impact on the material strength was simulated by finite difference method (FDM) and finite element method (FEM). The failure criterion was defined based upon the elastoplastic fracture mechanics parameter: J-integral. For Type-316L, no effect of internal pressure on the fracture was expected. For JIS-SCM435 and JIS-SGP, although an effect of internal pressure exists, its relatively low value cannot lead to failure. SSRT tests were performed on Type-316L under the following conditions: (I) non-charged in vacuum; (II) H-charged in vacuum; (III) H-charged in 115 MPa nitrogen gas; (IV) non-charged in 115 MPa nitrogen gas. The experimental results successfully supported the simulation-based conclusions.