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Influence of hydrogen on the tensile properties of X65 D pipeline steel

Abstract:

Natural-gas pipelines converted to carry hydrogen could have reduced mechanical properties due to hydrogen embrittlement. The tensile properties of the X65 D pipeline steel from the Dampier-Bunbury Natural Gas Pipeline were assessed in air and in hydrogen. The microstructure consisted of ferrite and pearlite elongated in the transverse and longitudinal directions by the hot and cold rolling during pipe production. Tensile tests were conducted using the linearly increasing stress test with cathodic hydrogen charging. Specimens were from the base metal, weld metal and heat-affected zone, from varying depths in the pipeline wall and for longitudinal and transverse orientations. There was no influence of hydrogen on steel strength, and no sub-critical crack growth. Hydrogen produced a similar reduction in the ductility for the base metal, weld metal, heat-affected zone, and at different depths in the pipe wall. The ductility reduction in the transverse direction was greater than in the longitudinal direction which correlated with more brittle quasi-cleavage facets, and which was attributed to the more pronounced pearlite banding in the transverse direction. The reduction of ductility by hydrogen was attributed to hydrogen causing a change in fracture mode from ductile micro-void coalescence in air to the altered micro-void coalescence fracture mechanism in hydrogen and to the introduction of quasi-cleavage in the presence of hydrogen which might have been by the HELP + HEDE mechanism.