Effect of High‐Pressure Natural Gas/Hydrogen Mixed Environment on the Fatigue Life of X65 Pipeline Steel Welded Joints

This study investigates the fatigue performance and hydrogen‐assisted failure behavior of X65 pipeline steel welded joints in a 10 MPa N 2 /H 2 mixed environment with hydrogen contents from 0 to 20 vol%. Through fatigue life testing, fractography, and electron backscatter diffraction (EBSD), the fatigue lives of base metal (BM), weld metal (WM), and heat‐affected zone (HAZ) were examined under different stress amplitudes. Results show that in the high stress range, hydrogen significantly accelerates fatigue damage. In the low stress range, hydrogen has negligible influence on fatigue life. The HAZ is the most vulnerable region due to stress concentration, higher hardness, and lower fraction of high‐angle grain boundaries. Crack‐tip deformation in hydrogen‐blended environments is stress‐level dependent, showing lower geometrically necessary dislocation (GND) density and narrower kernel average misorientation (KAM) distribution than N 2 at lower stress amplitudes, but higher GND density and broader KAM distribution at higher stress amplitudes.