Hydrogen gas can be transported via natural gas pipelines, either pure or blended with natural gas, to deliver clean energy. However, adsorbed hydrogen on the level of parts per million in the steel microstructure can cause abrupt failure of pipelines by fracture. Existing literature suggests that this problem is more severe in the case of heat-affected zones (HAZ) compared to the weld and base metal of pipelines. Therefore, it is essential to investigate the hydrogen-affected fracture toughness of pipeline steel HAZ and identify the microstructural reasons behind the fracture resistance. In the present work, three girth weld HAZ samples from X65-X70 steel pipelines, whose fracture toughness had been previously measured in high-pressure H2 gas, were studied under a scanning electron microscope to observe the fracture surface features. Subsequently, the samples were sectioned and again studied under SEM to reveal the microconstituents beneath the fracture surface features. It was observed that the girth weld HAZ microstructure is inhomogeneous and changes sharply over a few mm distance. The crack propagation path during the fracture tests through different steel microconstituents was identified. Moreover, numerical simulations and Gleeble® heat-treatments were used to produce HAZ microstructures to evaluate their influence on fracture toughness in hydrogen.
