Long-term embrittlement of cast duplex stainless steels in LWR systems

This progress report summarizes work performed by Argonne National Laboratory on long-term embrittlement of cast duplex stainless steels in LWR systems during the six months from April to September 1987. Microstructural studies were conducted to investigate the kinetics of spinodal decomposition and G-phase and γ2 precipitation of CF-8 and CF-8M grades of cast stainless steel. The results indicate that the presence of Mo in CF-8M steel accelerates spinodal decomposition as well as G-phase and γ2 precipitation. Examination of the long-term-aged CF-8M steels also revealed a ''spinodal-like'' decomposition of the austenite caused by segregation of Fe and Ni in the matrix. Preliminary results indicate that local regions of austenite are significantly hardened by the decomposition. Charpy-impact, tensile, and J-R curve data are presented for several heats of cast stainless steels aged at temperatures between 320 and 450 degree C for times up to 10,000 h. The results indicate that concentrations of carbon and nitrogen in the steel and the ferrite content and spacing are important parameters in controlling low-temperature embrittlement. The existing correlations for estimating the extent and kinetics of embrittlement do not accurately represent the properties of different grades and compositions of cast stainless steel after thermal aging. 36 refs., 24 figs., 5 tabs