Microstructure evolution and degradation of mechanical properties due to short-term thermal aging of cast austenitic stainless steels

Abstract

The thermal aging behavior of cast austenitic stainless steels (CASSs) is one of the material degradation phenomena to be evaluated for long-term operation of nuclear power plants. In this study, CF8 and CF8M with 10 to 25% of δ-ferrite were subjected to accelerated thermal aging at 400 °C for 100 h, 300 h and 1,000 h. The microstructural changes due to thermal aging were studied using scanning transmission electron microscopy / energy-dispersive X-ray spectroscopy (STEM/EDX) and high-resolution transmission electron microscopy (HRTEM). The mechanical behavior was evaluated by conducting Vickers hardness test, tensile test and fracture toughness test. In CF8M aged at 400 ºC for 1,000 h, both the spinodal decomposition of δ-ferrite and G-phase formation were observed. Whereas in other samples only spinodal decomposition was observed. The correlation between the microstructure evolution in δ-ferrite and loss of fracture resistance was discussed.