Fatigue and Fracture Performance of Vintage Pipeline Welds in Gaseous Hydrogen Environments

Abstract The existing natural gas infrastructure can be used to transport a mixture of gaseous hydrogen and natural gas as a lower-carbon alternative to natural gas alone. However, pipeline steels show increased rates of fatigue crack growth and diminished resistance to fracture when exposed to gaseous hydrogen. Whereas previous work suggests that hydrogen-assisted fatigue crack growth is relatively insensitive to the hardness/strength of carbon steels, hydrogen-assisted fracture is sensitive to the hardness of the material. Since welds and their heat affected zones (HAZ), especially vintage welds, often feature higher hardness than the base metal, hydrogen-assisted fracture can be further exacerbated near a weld. In this study, fatigue crack growth and fracture behavior of the base metal, weld metal, and HAZ of several vintage pipeline steels were investigated in gaseous hydrogen at a pressure of 210 bar using the compact tension geometry. The location of the crack with respect to the weld was verified in each specimen by etching the surface of the specimen. This study aims to correlate the microstructure and corresponding hardness to fatigue and fracture performance of pipeline welds. Furthermore, the fracture process and role of microstructure are examined using fractography.