Role of Inherent Heterostructure in Hydrogen-Induced Cracking of API X70 Pipeline Steel Under Pressurized CH4-H2 Mixed-Gas Environments

In this study, the hydrogen embrittlement (HE) susceptibility and hydrogen-induced cracking behavior of X70 pipeline steel were investigated in CH4–H2 mixed-gas environments containing 5–20% H2 at a total pressure of 7 MPa. Slow strain rate tensile tests, thermal desorption spectroscopy, fractography, and electron backscatter diffraction (EBSD) were employed to correlate hydrogen uptake, mechanical degradation, fracture morphology, and crack evolution. The results showed that increasing the hydrogen fraction progressively deteriorated the mechanical performance of X70 pipeline steel, as evidenced by reductions in tensile strength and elongation. This degradation was accompanied by a fracture-mode transition from micro-void coalescence to quasi-cleavage fracture. More importantly, EBSD analysis revealed a hydrogen-content-dependent change in the role of the heterogeneous microstructure. At relatively low hydrogen fractions, fine-grained regions and microstructural heterogeneity acted as effective barriers to hydrogen-induced crack propagation. However, at higher hydrogen fractions, this intrinsic crack-arresting capability was substantially weakened. Newly formed cracks were observed to initiate ahead of pre-existing cracks and propagate preferentially along grain boundaries, with limited local plastic deformation. These findings demonstrate that the increased HE susceptibility of X70 pipeline steel under high-H2 mixed-gas conditions is associated with the progressive loss of the crack-arresting resistance provided by its heterogeneous microstructure.