Abstract
Demand for hydrogen gas is expected to rise in the following years as the transition towards carbon neutrality continues. Reliable infrastructure for transport and storage of gaseous hydrogen is therefore needed. To this end, the current natural gas grid could be repurposed for hydrogen service. However, hydrogen-assisted degradation of pipeline steels poses a potential problem, and the pipeline network should be assessed for its compatibility with hydrogen to ensure safe operation. As a complication, the current pipeline grid encompasses a wide variety of steel grades used for its construction, with different chemistry and microstructures as the pipeline technology has developed over the years. A more fundamental understanding on the ways in which hydrogen affects the mechanical behavior of these types of steels is therefore needed. In this work, the hydrogen effect on the fracture micromechanism of an X70 grade pipeline steel is assessed through fractographic analysis of single edge notched tenstion fracture toughness tests, supported by X-ray micro computed tomography (µCT) scans. The fracture micromechanism shifts from microvoid coalescence to (partially) quasi-cleavage after hydrogen charging. Additionally, hydrogen promotes splitting along the banded microstructure, which can significantly influence the measured fracture toughness.