Strain rate sensitivity of hydrogen-assisted ε-martensitic transformation and associated hydrogen embrittlement in high-Mn steel

Abstract

This study presents the degree of promotion of deformation-induced γ−ε martensitic transformation by hydrogen charging and the associated fracture behavior at various strain rates ranging from 10−5 to 10−2 s−1. A decrease in the strain rate from 10−2 to 10−5 s−1 promotes the deformation-induced γ−ε martensitic transformation regardless of hydrogen charging (65%→82% in area fraction for uncharged specimens, 68%→84% for hydrogen-charged specimens). Hydrogen charging, which provides 11.7 mass ppm hydrogen concentration, further promotes the γ−ε martensitic transformation. However, the degree of promotion of the transformation by hydrogen is insensitive to the strain rate. Corresponding to the promotion of the γ−ε martensitic transformation, the hydrogen embrittlement susceptibility increases with a decrease in the strain rate. For instance, the elongation of the hydrogen-charged specimens decreases from 36 to 32% by decreasing strain rate from 10−2 to 10−5 s−1. Hydrogen uptake deteriorates the resistance to crack initiation and propagation. Furthermore, the primary effect of the decrease in strain rate on hydrogen embrittlement is the acceleration of the crack propagation. In addition to the promotion of γ−ε martensitic transformation, a decrease in the strain rate in the presence of hydrogen may cause hydrogen localization at the crack tip, which assists brittle-like martensite cracking.