Effects of hydrogen and specimen thickness on fracture toughness of ferritic steel welded joint

In this study, fracture behaviors of base metal and weld metal of ferritic steel welded joint in the thick wall pipeline for hydrogen transportation were investigated and the effects of hydrogen and specimen thickness (B) on fracture toughness were comprehensively considered in detail. A series of single edge notched tensile (SENT) tests for base metal and weld metal with B/W (width) ratio of 0.5, 1 and 2 were conducted with and without pre-electrochemical hydrogen charging, and crack tip opening displacement (CTOD) of δ_m value was obtained by the double clip gauges method. It was found that fracture toughness (δ_m) of base metal and weld metal decreased with increasing specimen thickness or hydrogenating, and δ_m of hydrogenating specimen with B/W ratio of 2 was smallest. It was believed that large specimen thickness decreased fracture toughness by constraining plastic deformation at crack tip and restricting dislocations movement, while hydrogen promoted embrittlement by reducing cohesive energy of fracture. In the hydrogenating specimen with larger thickness, there was lower dislocation density near crack tip, leading to a decrease in trapped hydrogen and intensifying the cohesive energy reduction effect, which resulted in reduced δ_m and the worse fracture toughness, and it means that hydrogen and specimen thickness synergistically affected fracture toughness. In addition, effect of thickness and hydrogen on decreasing fracture toughness was more pronounced for weld metal than base metal, which could be attributed to poorer plastic deformation ability of weld metal and lower dislocation density at crack tip. It is concluded that the hydrogen and thickness effects should be taken into account for the structural integrity evaluation of welded joint in hydrogen transportation pipeline.