Analysis of Thermal Aging Behavior and Activation Energy of Austenitic Stainless Steel Weld for Reactor Vessel Internal

This study examines the thermal aging behavior and associated changes in mechanical properties of austenitic stainless steel welds (ASSW) used within reactor vessel internals (RVI). ASSW specimens were fabricated employing the Gas Tungsten Arc Welding (GTAW) technique, adhering to the welding procedure specifications for RVI, and were thermally aged at 340°C, 400°C, and 450°C for durations up to 15,000 hours. Assessments included strength, impact toughness, and delta-ferrite hardness, while thermal embrittlement activation energy was estimated using nano-indentation combined with the Cahn-Hilliard model. Microstructural analysis revealed a delta-ferrite content of approximately 7%, which appeared as fine dendritic structures (thickness <2 μm) within the austenite matrix. Spinodal decomposition in the delta-ferrite led to the formation of Cr-rich α′ phases at all thermal aging temperatures, with a noted increase in Cr concentration over time. Upon aging at 340°C for 14,000 hours, Ni-rich clusters were observed, indicating that extended aging could be necessary for G-phase precipitation at lower temperatures. Conversely, Ni-rich precipitates identified as a face-centered cubic (FCC) G-phase emerged in delta-ferrite aged at 400°C and 450°C. The absence of Cr carbide precipitation supports a consistent aging mechanism across the 340–450°C range. Aging resulted in a slight increase in tensile strength, a decrease in elongation, and a significant reduction in impact toughness. Nano-indentation confirmed an increase in delta-ferrite hardness, which was normalized using the austenite-to-delta-ferrite hardness ratio. Activation energy for thermal embrittlement was calculated to be 100 kJ/mol for the 340– 400°C range and 77 kJ/mol for the 340–450°C range.