Effects of solid solution and grain-boundary segregation of Mo on hydrogen embrittlement in 32MnB5 hot-stamping steels

Hydrogen embrittlement (HE) has become an important issue in ultra-strong automotive steel applications. The addition of Mo to commercial 32MnB5 hot-stamping steel is preferred to enhance the strength levels with little ductility loss. However, the effects of solute Mo on HE have been rarely studied for developing 32MnB5 steel, and most studies on the alloying of Mo for interfacial cohesion have been conducted theoretically by calculating the cohesive energies in the Fe lattice. In this study, 0.15 wt.% Mo was added to the 32MnB5 steel and the resistance to HE was evaluated experimentally via electrochemical H-charging. The H-charged reference steel shows a large ductility loss (50–79%) after H-charging, while the addition of Mo significantly reduces the loss (17–26%) with sufficient post-elongation, indicating a higher resistance to HE. This is because the solute Mo decreases the H diffusivity, resulted from the high H affinity and repulsive strain field owing to the large atomic size of Mo. The direct observation of crack propagation reveals that the H-induced crack path changes from the prior austenite grain boundaries (PAGBs) to the grain interiors of H-enhanced slip planes. This is attributed to the reduced H- and strain-localization on the PAGBs by the solute Mo and the enhanced grain-boundary cohesion by Mo segregation. This work thus demonstrates the beneficial effects of the addition of Mo on the tensile properties and the intrinsic resistance to HE for the development of ultra-high-strength steels.