Investigation of the through-thickness hydrogen embrittlement inhomogeneity in X65 seamless pipeline steel

Understanding the through-thickness inhomogeneity of hydrogen embrittlement (HE) is crucial for accurately assessing the HE susceptibility and hydrogen-induced failure behavior of pipeline steels . In this study, the outer and inner layers of X65 seamless pipeline steel were investigated. Compared to the inner layer, the outer layer exhibits finer grains, a higher density of grain boundaries and dislocations, and more precipitates. The electrochemical hydrogen permeation tests show that the outer layer has lower hydrogen diffusivity and more hydrogen traps. The thermal desorption spectroscopy and microstructure observation results indicate that the reversible traps can be related to low angle grain boundaries (LAGBs) and dislocations, and the irreversible traps can be attributed to high angle grain boundaries (HAGBs) and precipitates. Compared to the inner layer, the LAGBs and dislocations in the outer layer trap more hydrogen and hinder the hydrogen diffusion significantly. The outer layer shows less hydrogen concentration at HAGBs and precipitates with high hydrogen induced cracking risks. For these reasons, the HE inhomogeneity was shown in the slow strain rate test after hydrogen charging for 24 h. The HE index of the outer and inner layer showed a significant difference, which is 7.79 % an 19.59 %, respectively. The through-thickness HE inhomogeneity can be attributed to uneven deformation and cooling conditions in the manufacturing process of seamless pipeline steel. Since the inner layer is the surface exposed to hydrogen under service condition, the HE inhomogeneity with higher HE susceptibility in the inner layer can reduce the overall HE resistance and increase the risks of fracture failure of seamless pipeline steels. • The difference in outer and inner layer of seamless pipeline steel was studied. • The outer layer exhibits lower hydrogen diffusivity and more hydrogen traps. • The outer layer has a lower hydrogen embrittlement (HE) susceptibility. • Grain boundaries, dislocations and precipitates can act as hydrogen traps. • The effect of manufacturing process on HE of seamless pipes was investigated.