Effects of sensitization on hydrogen embrittlement behavior in 304 stainless steel

• Sensitization forms Cr carbides and reduces chromium both near grain boundaries and within grains. • Hydrogen pre-charging significantly degraded strength, elongation, and relative reduction of area. • Fracture shifts from dimples to quasi-cleavage and intergranular. • Chromium depletion lowers SFE and raises strain induced martensite roughly twofold. • Martensite with high dislocation density promotes hydrogen diffusion and intergranular cracking. This study investigated the hydrogen embrittlement behavior of 304 stainless steel under the combined condition of sensitization and hydrogen pre-charging. Specifically, hydrogen trapping analysis and martensite transformation mapping were used to examine the respective roles of carbide precipitation and chromium depletion, and key factors were identified through fractographic observations. Sensitization was simulated at 650 °C for 50 h, followed by hydrogen pre-charging at 250 °C under 50 MPa for 3.5 days. Under hydrogen pre-charging, sensitized specimens showed a 9.3 % drop in ultimate tensile strength, a 17.3 % reduction in elongation, and a 16 % decrease in relative reduction of area, indicating higher hydrogen embrittlement susceptibility. Hydrogen desorption analysis revealed a redistribution of hydrogen from reversible to irreversible traps consistent with 139 nm coarsened Cr 23 C 6 carbides, while phase mapping revealed extensive formation of strain-induced martensite along grain boundaries and within grains. These martensitic regions accelerated hydrogen transport and promoted strain localization, leading to the disappearance of intragranular dimples and the development of intergranular cracking. The results demonstrate that strain-induced martensite formed in chromium-depleted regions is the dominant factor governing post-sensitization hydrogen embrittlement, emphasizing the necessity of controlling chromium depletion to maintain the stability of the austenitic matrix in hydrogen environments.