Hydrogen embrittlement susceptibility of X70 pipeline steel weld under a low partial hydrogen environment

The present study investigates the effects of a low partial pressure of hydrogen in a mixture with methane gas on the mechanical properties of welded API X70 pipeline steel using a series of smooth tensile and notched tensile, testing approaches. The micro-structures and mechanical property performances of the weld metal are compared to the results obtained for the base metal. The weld metal was found to be more vulnerable to hydrogen-induced crack initiation than the base metal. A significant loss of ductility occurred in the weld metal, even when tested under 10 MPa of a gas mixture with 1% H-2 using the smooth tensile specimen. The notched reduction in the area (RA) decreased markedly under 10 MPa gas mixtures with 1% H-2. The microstructural inhomogeneity in the metal matrix was associated with a more severe hydrogen-assisted fracture in the weld metal. The hydrogen-induced crack initiation primarily occurred at the interface between the dendritic pro-eutectoid ferrite and bainitic ferrite and propagated with the dendritic ferrite in which the diffusion of hydrogen was promoted by the stress/strain in-compatibility and localized deformations. (c) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.