Hydrogen embrittlement susceptibility of X80 steel substrate and HAZ in simulated coal gas environment

Coal gas contains a certain amount of hydrogen gas, so hydrogen-induced failure becomes the potential safety problem of pipelines, especially in the heat-affected zone (HAZ) of welded joint. HAZ samples of X80 pipeline steel were experimentally prepared using a welding simulator. The hydrogen embrittlement susceptibility of X80 pipeline steel substrate and different sub-regions of HAZ were evaluated by tensile test under high pressure coal gas environment. The results show that the mechanical properties of X80 pipeline steel substrate and different sub-regions of HAZ slightly decreased under coal gas environment, and this means hydrogen gas has a negative effect on the steel performance. Coarse grained heat-affected zone (CGHAZ) had the highest hydrogen embrittlement susceptibility, since obvious grain growth occurred and high angle grain boundaries decreased in this region based on analysis with optical microscope (OM) and electron backscatter diffraction (EBSD). The hydrogen diffusion speed in CGHAZ and hydrogen concentration at crack tip would increase, comparing to steel substrate, and thus the crack arrest property decreased. Consequently, the fracture surface presented apparent brittle features.