Hydrogen environment assisted cracking in X70 welding heat-affected zone under a high-pressure hydrogen gas

Abstract

In this study, the susceptibility of API X70 weld joints to hydrogen gas was investigated using slow strain rate tensile (SSRT) tests. The microstructure of the subregion of the actual heat-affected zone (HAZ) weld joint was reproduced by weld thermal-cycle simulation techniques. The SSRT results showed that the weld and cross-weld specimens had the highest hydrogen embrittlement (HE) susceptibility, followed by the subregions of the fine-grained heat-affected zone (FGHAZ) and coarse-grained heat-affected zone (CGHAZ) specimens and the base metal. The HE index ranking of the samples was as follows (in increasing sequence): weld, cross-weld, simulated FGHAZ, simulated CGHAZ, and base metal specimens. The rupture location of the cross-weld specimen in ambient air is different to that tested in 10 MPa H2. The higher susceptibility of the weld and cross-weld specimens was primarily attributed to synergic effects between the microstructural characteristics, stress localization, and microhardness distribution – particularly, microhardness plays an important role in the nucleation of microcracks and the hydrogen diffusion rate induced by stress/strain localization.