Evaluation of low-temperature fracture toughness and minimum allowable temperature of SA-372 Gr. J Class 70 steel for hydrogen vessels

SA-372 Gr. J Class 70 steel has been widely deployed in more than one thousand 90 MPa-class hydrogen pressure vessels across South Korea, where its structural integrity has been validated under conventional service conditions ranging from –40 °C to + 85 °C. However, as hydrogen infrastructures expand toward liquefied and solid hydrogen systems, SA-372 is increasingly being considered as a candidate material for buffer storage tanks that may experience transitional sub-ambient temperatures (down to –40 °C). In this transitional role, existing exemption rules and accumulated service data are insufficient to ensure safe operation. To address this gap, this study experimentally evaluated the low-temperature fracture toughness of SA-372 Gr. J Class 70 using Charpy V-notch impact tests between + 23 °C and –80 °C. The absorbed energy data were analyzed through hyperbolic tangent fitting to construct a ductile-to-brittle transition curve, from which key parameters such as transition temperature and lower shelf energy were derived. These results were incorporated into a Level 3 Option C procedure of API 579–1 / ASME FFS-1, and combined with the Master Curve methodology to estimate temperature-dependent fracture toughness K mat (T). Fracture Assessment Diagram (FAD) analyses were then performed to determine the Minimum Allowable Temperature (MAT) under both inert and hydrogen-charged conditions, with hydrogen embrittlement incorporated via a conservative threshold toughness of K IH = 40 MPa√m. The results showed that while SA-372 maintains sufficient toughness below –60 °C in air, the MAT for 90 MPa service pressure was constrained to –24.1 °C when considering hydrogen exposure. This indicates that conventional exemption criteria (27 J at –40 °C) may be non-conservative for buffer tank applications bridging current gaseous systems and emerging cryogenic hydrogen infrastructures. By quantifying this critical temperature shift, the present work provides a mechanics-based safety framework for SA-372 vessels, supporting risk-informed inspection and design strategies in next-generation hydrogen energy systems.