Study on the transition of hydrogen embrittlement mechanisms in 316L stainless steel under the coupled effects of pre‑strain and TIG welding

Abstract

Abstract

Abstract Hydrogen embrittlement (HE) of TIG-welded 316L austenitic stainless steel is strongly affected by welding-induced microstructural heterogeneity and service-induced plastic pre-strain, but the associated mechanism transition across different regions of the welded joint remains insufficiently understood. In this study, in-situ slow strain rate tensile tests in a 5 MPa hydrogen environment, together with microscopic structure characterizations, were conducted on 316L base metal, weld metal and welded joint specimens with different pre-strain levels. The results show that the weld metal exhibits higher HE susceptibility than the base metal, and HE susceptibility increases with pre-strain in both regions. The welded joint exhibits the most severe degradation, with fracture preferentially occurring near the fusion line and heat-affected zone. In the base metal, pre-strain induces high-density dislocation pile-ups, deformation twins and local α'-martensite formation, while hydrogen promotes localized dislocation activity, indicating that hydrogen-enhanced localized plasticity is the dominant failure mechanism. In the weld metal, the δ-ferrite/austenite phase boundaries, heterogeneous dislocation distribution and strain incompatibility promote local hydrogen accumulation. The observation of slip localization, twin damage and quasi-cleavage features suggests that hydrogen enhanced decohesion may also contribute to failure, together with HELP. These findings clarify the microstructure-dependent transition of HE mechanisms in pre-strained TIG-welded 316L stainless steel and provide guidance for the integrity assessment of welded components in hydrogen service.