Microstructure-strength interplay governing hydrogen embrittlement in micro-alloyed high-Mn steels

Abstract

High-Mn steels (HMSs) exhibit excellent mechanical properties, yet their relatively low yield strength (YS) and susceptibility to hydrogen embrittlement (HE) remain critical limitations for structural applications. To address these challenges, grain refinement and precipitation hardening are promising approaches, as they enhance YS and introduce immobile hydrogen trapping sites. In this study, we elucidate the relationship between HE behavior and multiple strengthening mechanisms in micro-alloyed HMS by systematically varying the annealing temperature after cold rolling. Controlled microstructures with varying grain size and precipitate fraction successfully improved YS without loss of ductility under hydrogen-free conditions. However, after hydrogen charging, the susceptibility to HE increased significantly with lower annealing temperatures. High-resolution transmission electron microscope (HR-TEM) and geometric phase analysis (GPA) revealed that annealing conditions influenced hydrogen migration into strain-concentrated regions, leading to different crack initiation sites such as γ-austenite/ε-martensite and mechanical twin/matrix interfaces. These differences originated from variations in γ-austenite phase stability caused by carbide precipitation and hydrogen charging. Furthermore, the higher YS confined the plastic zone in front of the crack, accelerating hydrogen concentration and crack propagation in the plastic zone. Overall, the results show that HE susceptibility in HMSs is governed not only by matrix composition but also by matrix strength, providing important insights for designing high-strength, HE-resistant HMSs.