Creep Crack Growth of 316H Stainless Steel in a Simulated AGR Environment

This study investigates the role of environmental exposure on creep crack growth in AISI 316H stainless steel, a material widely used in Advanced Gas-cooled Reactors (AGRs). Notched specimens subjected to sustained loading under simulated AGR conditions (CO₂ with 1% CO, 500 ppm H₂O, 300 ppm CH₄, and 100 ppm H₂) revealed enhanced intergranular cracking at stress intensity factors above 13 MPa√m., accompanied by grain boundary cavitation and crack branching. To distinguish the individual contributions of creep, oxidation, and carburization, comparative tests were conducted in inert argon and hydrogenated steam environments. The inert atmosphere was used to isolate the mechanical effects of creep alone, while the hydrogenated steam environment allowed oxidation to be studied independently of carbon ingress. Both conditions resulted in significantly reduced crack growth compared to AGR exposure, indicating that carburization plays a critical role in accelerating damage. EPMA analysis suggests elevated carbon concentrations along cavitated grain boundaries, suggesting that carbon segregation contributes to embrittlement and facilitates crack propagation under creep conditions. These findings provide insights into the environmental degradation mechanisms affecting reactor-grade stainless steels. Importantly, the insights gained from this study are directly relevant to High-Temperature Gas-cooled Reactors (HTGRs), where similar alloy-environment interactions may influence long-term structural integrity under service conditions.