Abstract: Microstructural characteristics such as morphology, dislocation density and crystal orientation were studied by using metallographic microscope, transmission electron microscope, electron backscatter diffraction technology and X-ray diffraction in a high strength low alloy steel with high vanadium content under both hot-rolled and quenched-tempered states, respectively. At the same time, the behavior of hydrogen diffusion and crack propagation were also studied by means of electrochemical hydrogen permeation technique and fatigue crack growth test. The aim was to establish the relationship among the microstructure, hydrogen diffusion and crack growth rate. The results show that compared with the hot-rolled state, retained austenite decomposes, the dislocation density decreases, and a large number of vanadium carbides precipitate, leading to an increase of irreversible hydrogen traps in the quenched-tempered state. Consequently, the hydrogen diffusion coefficient decreases, and the diffusion activation energy increases, making hydrogen diffusion more difficult and reducing the susceptibility to hydrogen embrittlement. In air and under low hydrogen charging current density conditions, the presence of retained austenite in the hot-rolled state provides superior crack growth resistance compared to the quenched-tempered state. However, at higher current densities, hydrogen embrittlement becomes dominant, resulting in an accelerated crack growth rate.

Key words: low alloy high strength steel, V microalloying, quenching and tempering treatment, hydrogen diffusion, crack growth