Abstract: In this study, we investigated the correlation between the pre-strain and hydrogen embrittlement (HE) mechanisms in medium-Mn steel. Intercritically annealed Fe-7Mn-0.2C-3Al (wt.%) steel, which showed a two-phase microstructure comprising α ferrite and γ_R retained austenite, was used as a model alloy. As the pre-strain level increased from 0 % to 45 %, the volume fraction of γ_R gradually decreased owing to the strain-induced α′ martensite transformation accompanied by an increase in dislocation density. The HE resistance decreased with increasing the pre-strain level because the sample with a higher pre-strain level revealed a higher amount of dissolved hydrogen, combined with a more extensive brittle fracture region owing to the enhanced diffusion and permeation of hydrogen from the reduced γ_R fraction. Additionally, the H-assisted crack in the sample without pre-strain was initiated and propagated from the γ_R grains when the strain-induced α′ phase was formed, because most of the dissolved hydrogen was concentrated in the γ_R grains, and these grains were predominantly deformed compared to the other phases. However, the pre-strained sample showed more pronounced multiple H-assisted cracking at the constituent phases, such as α and α′, because it exhibited relatively well-dispersed hydrogen atoms and reduced microstrain localization at the γ_R grains, due to the reduced γ_R fraction.

Key words: Medium-Mn steel, Transformation-induced plasticity, Hydrogen embrittlement, Advanced high-strength steels, H-assisted crack