Influence of Hardness and Microstructure on the Fracture Behavior of Vintage X46 Pipeline Steel in Hydrogen Gas

Abstract Global efforts to decarbonize the energy sector and achieve net zero emissions by 2050 have prompted significant interest in variable renewable energy resources (such as wind and solar) and the use of low (or no) carbon fuels, such as hydrogen, as a direct alternative to fossil fuels. One of the main challenges with transitioning to hydrogen is safely, reliably, and economically delivering hydrogen at the scale required to meet residential, commercial, and industrial needs. Repurposing the existing 483,000 km (300,000 miles) of natural gas transmission pipelines in the United States offers the potential to minimize cost associated with the built out of new infrastructure and leverage right of way access. However, as the network was largely installed prior to 1970, there is considerable variability in the quality, condition, and range of microstructures, particularly in the weld heat affected zones (HAZs) of pipelines already in the ground. Thus, significant research is on-going to quantify the influence of hydrogen on the fracture properties of vintage pipeline steels and, for base pipe (and weld metal), trends are being established. Due to weld geometry, microstructural complexities in HAZs, and the manner in which the fracture samples are extracted, data generated for weld HAZ fracture properties are likely to be non-conservative. Assessing the suitability of vintage pipeline steels in gaseous hydrogen requires that worse case scenarios be considered. In this study, specialist heat treatments were performed on bulk sections of a vintage X46 pipeline steel to produce a range of microstructures and hardness values that were identified within an ex-service ERW seam weld. Detailed microstructural and mechanical properties characterization was performed prior to elastic-plastic fracture toughness and fatigue crack growth testing to evaluate the influences that grain size, microstructural constituents, and hardness have on fracture properties in gaseous hydrogen. This paper will present the results of the microstructural characterization and assessment of mechanical properties; results of fracture testing will be reported later.