Influence of Tempering Treatment on Precipitation Behavior, Microstructure, Dislocation Density and Hydrogen-Induced Ductility Loss in High-Vanadium Hot-Rolled X80 Pipeline Steel

In this study, precipitation behavior with different vanadium contents before and after tempering process and their effects on hydrogen diffusion and hydrogen-induced plastic loss were investigated in high-vanadium hot-rolled X80 pipeline steel. The results showed that the microstructure transformed from lath bainite, ferrite to granular bainite, massive ferrite and a small amount of M/A constituents after tempering treatment. Besides, the number of nanoscale precipitates obviously increased. These fine precipitates can fix a large amount of diffusible H atoms as traps and reduce the damage of hydrogen. The hydrogen diffusion coefficient in as-tempered sample decreased using hydrogen permeation test. Moreover, after tempering treatment, the dislocation density distinctly decreased, which significantly improved the resistance to hydrogen-induced ductility loss.