Probing the impact of structure on hydrogen embrittlement in ultra-high strength Cr–Mn–N austenitic stainless steels: study of the cold-deformed structure versus fine-grained structure

This study investigated the impact of micro-structure on hydrogen embrittlement (HE) in ultra-high strength Cr–Mn–N austenitic stainless steels fabricated via two distinct approaches: cold rolling (CR, deformation strengthening) and phase reversion annealing (fine-grained (FG), grain refinement). The CR specimen achieved a yield strength of 1.38 GPa but exhibited severe HE sensitivity due to strain-induced α′-martensite and dislocations acting as rapid hydrogen diffusion channels. In contrast, the FG specimen retained comparable strength (1.37 GPa) while demonstrating superior HE resistance, attributed to low-angle and Σ3 grain boundaries effectively suppressing H diffusion and inhibiting the propagation of cracks. Mechanistically, the CR specimen's high dislocation density (10.1 × 10¹⁴ m⁻²) and α′-martensite content (26 vol.-%) facilitated hydrogen ingress (30.13 ppm total H), resulting in 46.4% elongation loss. Conversely, the FG specimen's refined micro-structure (1.2 μm grain size) with abundant Σ3 boundaries (0.29 μm μm⁻² linear density) limited hydrogen penetration (16.98 ppm total H).