Fatigue crack growth behavior of X80 pipeline girth welded joints in natural gas/mixed hydrogen environment: Experimental and numerical investigations

Fatigue life evaluation of welded joints is essential for structural integrity management in hydrogen-mixed natural gas pipelines. In this study, the fatigue crack growth rate (FCGR) of X80 welded joints was examined in simulated natural gas environments under 12 MPa with hydrogen volume fractions of 0, 10, and 30 %. The experimental results demonstrate that the hydrogen-induced FCGR curve in double logarithmic coordinates exhibits non-linear behavior, comprised of transition and acceleration regimes. The FCGR in different regions of the welded joint ranks from high to low as follows: heat affected zone (HAZ), base metal (BM), and weld metal (WM). The WM exhibits superior hydrogen embrittlement (HE) resistance owing to high density of fine acicular ferrite (AF) microstructure, whereas the HAZ displays the highest HE susceptibility associated with bainitic structures and hard, brittle secondary phases. Based on experimental results, a finite element method was proposed to simulate cross-region fatigue crack growth in welded joints under hydrogen environments, explicitly incorporating region-specific variations in FCGRs across distinct zones of the weldment. The simulation results indicate that the semi-elliptical crack propagates from the WM through the HAZ to the BM, with its morphology transitioning from semi-ellipse to valley shape and then to semi-circle as fatigue cycles increase, resulting in stress concentration at the crack front of the WM zone.