Mechanism analysis of hydrogen induced crack formation in welded joint of X80 pipeline steel

Hydrogen-induced cracking is one of the most common welding defects in welded joints of high-strength steel. Because of its delayed cracking characteristics, major safety accidents are prone to occur during the use of the structure. Therefore, for analyzing the causes of hydrogen-induced cracking, it is of great significance to study the mechanism of hydrogen diffusion and enrichment in welded joints of pipeline steel. In this paper, firstly, the welded joint of pipeline steel is taken as the research object, the finite element simulation of welding process of X80 pipeline steel considering solid phase transformation, which coupled the heatmicrostructure-mechanical-hydrogen diffusion, is established. It makes the numerical simulation of hydrogen diffusion and enrichment in the whole welding process possible. Then, taking the butt joint of diameter 422mm steel pipe as an example, the groove form of butt joint is designed, and the multi pass welding process is determined. Through the established finite element calculation method, the distribution of residual stress and hydrogen concentration during the welding process are calculated. It is found that the residual stress concentration is obvious at the welded joint, and the tensile and compressive stress changes obviously. Finally, the effects of microstructure, stress distribution and temperature gradient on hydrogen diffusion and enrichment were analyzed. It was found that the stress gradient and temperature gradient played a leading role in hydrogen diffusion at different welding stages. The research results reveal some rules, which can provide technical support for the safety and reliability evaluation of offshore pipeline steel structures.