Deformation behaviour of X70 pipeline steel under in-situ hydrogen charging: Effects of current density, pre-strain, and stress concentration

Ensuring the safe repurposing of X70 pipelines for hydrogen transport requires understanding how hydrogen interacts with deformation and local stress state. Two specimen geometries—a standard miniature tensile specimen and one with a hole at the gauge centre—are evaluated under in-situ electrochemical hydrogen charging. The effects of current density, pre-strain, and high stress triaxiality (η ≈ 0.48) are systematically examined. Increasing hydrogen flux reduces ductility sharply, whereas yield strength remains nearly unchanged. Pre-strained specimens exhibit higher embrittlement due to enhanced hydrogen trapping, and stress-concentrated specimens exhibit the most severe loss of plasticity and brittle fracture. Fractographic and cross-sectional observations reveal hydrogen-assisted cracks at surface for higher hydrogen fugacity and a transition from ductile to quasi-cleavage fracture. Under similar saturated hydrogen concentration, these results demonstrate that hydrogen fugacity, plastic deformation, and local stress state act synergistically to control embrittlement severity in X70 pipeline steels exposed to hydrogen environment.