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

Abstract

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 microstructures 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% H2 using the smooth tensile specimen. The notched reduction in the area (RA) decreased markedly under 10 MPa gas mixtures with 1% H2. 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 incompatibility and localized deformations.