Insight into the influence of pre-charging hydrogen on mechanical behavior and fracture morphology of X52MS pipeline steel

The hydrogen supply pipelines can lead to failure behaviors such as hydrogen-induced cracking and reduced plasticity when it is exposed to hydrogen-containing media during service, which is related to the migration feature of hydrogen atoms within the matrix. In this contribution, the tensile property evolution of X52MS pipeline steel before and after hydrogen charging is investigated by combining electrochemical hydrogen charging test with slow stretching experiment. The results show that as the per-charging time increases, the number and size of hydrogen blisters rise, which is caused by hydrogen accumulation on the surface. For the test steel with the thickness of 7.1 mm, the hydrogen blister size reaches about 3 mm after 12 h of pre-charging, and the hydrogen embrittlement sensitivity index is the highest of 54.6%. In contrast, the test steel with the thickness of 6.3 mm has the lowest index of 14.7%, indicating the better resistance to hydrogen embrittlement. It can be concluded that the hydrogen blister size is positively correlated with the hydrogen embrittlement sensitivity index, and surface hydrogen accumulation is the key reason for hydrogen-induced crack initiation. Furthermore, the tensile strengths of the three test specimens change little after pre-charging, but the elongation gradually decreases. The shear lips on the side of the fracture are all featured by a zigzag shape, and the cracks appear in the necking area, with a relatively large fracture diameter, showing the obvious hydrogen damage features. The fracture center morphology gradually changes from dimple one to cleavage surface one after hydrogen charging. These insights provide theoretical basis and experimental guidance for the design and development of pipeline steel for hydrogen energy transportation.