Influence of hydrogen gas pressure on the fracture mechanisms of hollow X65 steel specimens

The hydrogen-induced fracture mechanisms in pipeline steels intended for hydrogen transport are strongly dependent on gas pressure at ambient temperature, necessitating a detailed understanding of its effects on crack initiation and propagation. This study systematically investigates the hydrogen embrittlement (HE) behavior of API X65 pipeline steel exposed to hydrogen pressures of 1, 5, and 10 MPa, employing slow strain rate tensile (SSRT) testing of hollow specimens. Subsequent fractographic analyses of fracture surfaces and longitudinal sections were performed to elucidate damage features. The results demonstrate significant degradation of ductility attributed to hydrogen exposure, as evidenced by reductions in elongation and area reduction percentages, with these effects intensified at elevated pressures. HE susceptibility was quantified by the elongation loss index (IEl) and reduction of area loss index (IRA), both exceeding 24% at 5 and 10 MPa and being discernible even at 1 MPa. Fractographic examination revealed the presence of brittle features, including quasi-cleavage and secondary cracks, which consistently initiated at the inner wall surface of the hollow specimens, indicating hydrogen-assisted cracking as the primary failure mechanism. Increasing hydrogen pressure resulted in more linear crack paths and sharper crack tips, indicative of enhanced embrittlement severity. A clear pressure-dependent transition in fracture morphology was observed, characterized by a diminishing ductile-to-brittle gradient and an increased prevalence of brittle regions. The transition from ductile to brittle fracture features was interpreted as being consistent with the conceptual framework of hydrogen-assisted cracking involving both plasticity-mediated and decohesion-related processes. At 1 MPa, fracture features suggested that hydrogen primarily influenced localized plastic deformation, whereas the increased quasi-cleavage at 5 and 10 MPa indicated a shift toward more brittle, low-ductility fracture behavior.