Effect of hydrogen on deformation behavior in Ti-Mo and V-Mo precipitate-strengthened steels

Recent studies show that precipitate-strengthened microalloyed steels offer strong resistance to hydrogen embrittlement due to hydrogen trapping by nanoscale precipitates. This study investigates how these precipitates interact with dislocations under strain, simulating service conditions. Two model steels were used: Ti-Mo steel with coherent (Ti,Mo)C precipitates and V-Mo steel with semi-coherent (V,Mo)C precipitates. Dislocation density was measured during in-situ neutron diffraction tensile tests, with and without hydrogen charging. Hydrogen increased dislocation density before straining but suppressed dislocation multiplication during loading, reducing overall dislocation strengthening, especially in Ti-Mo steel. The Ti-Mo steel showed greater sensitivity to hydrogen, attributed to reversible hydrogen trapping at coherent precipitate interfaces, and a greater increase in dislocation density. In contrast, V-Mo steel exhibited more irreversible trapping and a smaller increase in dislocation density. During tensile testing, with reversible hydrogen released to diffuse, subgrain formation in Ti-Mo steel was restricted, as the higher concentration of diffusible hydrogen suppressed screw dislocation mobility. Consequently, fewer subgrain microstructures were generated, diminishing their effectiveness as barriers to crack propagation in Ti-Mo steel, compared to V-Mo steel. These results highlight the importance of selecting precipitate types that enhance irreversible hydrogen trapping, thereby improving the hydrogen resistance of steels.