Effect of Vanadium-alloying on Hydrogen Embrittlement of Austenitic High-nitrogen Steels

The effect of hydrogen on tensile behavior and fracture mechanisms of V-alloying and V-free high-nitrogen austenitic steels was evaluated. Two steels with the chemical compositions of Fe-23Cr–17Mn–0.1C–0.6N (0V-HNS) and Fe-19Cr–22Mn–1.5V–0.3C–0.9N (1.5V-HNS) were electrochemically hydrogen-charged in NaCl water-solution for 100 hours. According to X-ray diffraction analysis and TEM researches, V-alloying promotes particle strengthening of the 1.5V-HNS. Despite differences in chemical compositions, namely, carbon and nitrogen concentrations, a solid solution hardening is similar for both steels because of precipitate-assisted depletion of austenite by interstitial atoms (carbon and nitrogen) in 1.5V-HNS. For hydrogen-free state, the values of the yield stress and the tensile strength are higher for particle-strengthened 1.5V-HNS as compared to 0V-HNS. Hydrogen-charging increases both the yield stress and the tensile strength of the steels, but hydrogen-assisted fracture micromechanisms are different for 0V-HNS and 1.5V-HNS. Hydrogen-charging drastically reduces a total elongation in 0V-HNS but provides insufficient embrittlement in 1.5V-HNS. Hydrogen-assisted brittle layers form on lateral surfaces of the specimens, and the widths and fracture micromechanisms in them are different for two steels. For 0V-HNS, surface layers of 84 μm in width possess transgranular brittle fracture mechanism (quasi-cleavage mode). For 1.5V-HNS, the brittle surface layers (31 μm width) destroy in intergranular brittle fracture mode. The central parts of steel specimens show dimple fracture similar to hydrogen-free steels. The possible reasons for different hydrogen-induced effects in 0V-HNS and 1.5V-HNS are discussed.