Fracture Resistance of High Strength Steel Welds in Gaseous Hydrogen Determined Using Potential Difference and Unloading Compliance

Abstract High-strength ferritic steels are widely used in the current hydrogen infrastructure for the construction of seamless and welded high-pressure vessels. In mobility applications (such as refueling stations), hydrogen is stored in the gaseous state at high pressures (typically between, 500 bar and 1000 bar for refilling of fuel cell vehicles), and seamless vessels are currently the only viable solution. To lower emissions in emerging and hard-to-abate industries, hydrogen storage is expected at lower pressures (up to 350 bar) but in much larger volumes. In this scenario, large, welded vessels are required. The fracture toughness of these welds in hydrogen is a primary concern, as welding is known to produce microstructural and property gradients across the joint, potentially resulting in highly variable fracture resistance. In a previous work, the fracture resistance of welds fabricated from a high strength, EN 10216-3 grade P690 QL2 steel was investigated [1], showing greatly reduced fracture resistance due to the presence of local hard spots in the heat affected zone. This current work complements previous work with new fracture results for welds in gaseous hydrogen at pressure of 200 bar. Tests were conducted under rising displacement conditions as per ASTM E1820 standard [2], similar to the previous study, except that the crack extension was monitored with (single-specimen) potential difference measurements for comparison to the previous study using unloading compliance. While the J-integral resistance curve can be determined by both methods, the potential difference method, when properly implemented, can resolve the initiation of crack extension more precisely than the unloading-compliance technique, but the potential difference method is more difficult to implement and interpret. Additionally, displacement rate effects were investigated and compared between the two methods for crack-extension monitoring. The advantages and appropriateness of each test methodology are discussed in the context of structural integrity of welded pressure vessels.