Study of hydrogen-induced delayed damage in medium manganese steel during in situ hydrogen charging: Role of heterogeneous structure in crack initiation and propagation

Medium manganese (Mn) steel stands out as an exemplary representative of the latest generation of advanced high-strength steels intended for for automotive applications, attracting considerable interest due to its well-optimized microstructure and superior mechanical properties. Nonetheless, its pronounced vulnerability to hydrogen embrittlement (HE) poses a significant challenge. This research meticulously explores the hydrogen-induced delayed cracking behavior of thermomechanically processed medium Mn steel by conducting in situ hydrogen-charging slow strain rate tensile (SSRT) experiments, supported by post-mortem characterization methods such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and first-principles calculations. Findings from the SSRT tests reveal that medium-Mn steel exhibiting a complex multi-component heterogeneous structure, with variations in its structure, phases, and composition, demonstrates reduced susceptibility to HE. The regions of brittle fracture in the hydrogen-exposed specimens predominantly display cleavage or quasi-cleavage characteristics. Additionally, first-principles calculations indicate that the Mn partitioning-induced compositional heterogeneity significantly influences hydrogen trapping behavior, even in the absence of tensile strain. Within body-centered cubic (BCC) structures, ’Mn-lean’ areas exhibit a lower hydrogen binding energy. Moreover, cracks caused by hydrogen in medium-Mn steel commence at the interface between martensite (not surrounded by austenite) and ferrite, propagating transgranularly along the pathways of transformed martensite.