A Comparative study of hydrogen embrittlement susceptibility of L245MH and L360MH pipeline steels in high-pressure gaseous hydrogen

The hydrogen embrittlement (HE) susceptibility of the L245MH and L360MH pipeline steels was evaluated in a 6.3 MPa high-pressure hydrogen environment, through slow strain rate tensile (SSRT) tests and comprehensive microstructural characterization. SSRT results in both 6.3 MPa H 2 environment showed that L360MH generally shows higher HE susceptibility than L245MH steel under tested conditions studied. The microstructure of both steels was characterized by optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM), and these analytical tools were also used to study the crack path and the fracture morphology of samples fractured in hydrogen. The microscopic study revealed that some of the lamellar deformation layers in the microstructure along the rolling direction in L360MH were associated with hydrogen-induced serrated cracking seen on the fracture surface of SSRT samples failed in hydrogen environment, which negatively influences HE susceptibility index of the steels. Moreover, the crystallographic analysis of the fracture surfaces and secondary cracks in both steels exposed to gaseous hydrogen environments reveals a fundamental difference in their hydrogen-induced cracking mechanisms: The crack paths in L245MH are more consistent with slip-assisted hydrogen cracking, as the analyzed crack segments preferentially align with {110}-type slip-planes. In contrast, the cracking behavior of L360MH may involve the combined contribution of cleavage- or quasi-cleavage-related cracking and localized slip-assisted propagation. Some crack segments show preferential alignment with {100}-type plane traces, suggesting a quasi-cleavage-related cracking, whereas others are closer to {110}-type slip-planes, indicating possible crack path transition associated with localized plastic deformation.