Effects of thermal aging on the fracture toughness of cast stainless steel CF8

Cast stainless steels are widely used in the primary coolant systems of nuclear power plants because of their high strength, toughness, and corrosion resistance. Mechanical property degradation due to thermal embrittlement of the ferrite phase is of major concern in long-term operations. To investigate the aging-induced loss of fracture resistance in cast stainless steels, static fracture resistance (J-R)testing was performed for three different CF8 stainless steels with significantly different δ-ferrite contents. The materials were thermally aged at 290–400 °C for up to 10,000 h and tested in static fracture at 25–400 °C. It was found that the fracture toughness in high-ferrite materials generally decreased with thermal aging, while the relationship between aging and fracture toughness in low-ferrite materials was complicated, showing non-monotonic variation with the degree of aging. While significant degradation was observed for long-term aged high-ferrite materials, no material tested showed a K_0.2mm value less than 100 MPa√m.