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

This progress report summarizes work performed by Argonne National Laboratory on longterm thermal embrittlement of cast duplex stainless steels in LWR systems during the six months from October 1989 to March 1990. The results from Charpy-impact tests and microhardness measurements of the ferrite phase for several heats of cast stainless steel aged up to 30,000 h at 290--400 degree C are analyzed to establish the kinetics of thermal embrittlement. Correlations are presented for predicting the extent and kinetics of thermal embrittlement of cast stainless steels from material information that can be determined from the certified material test record. The extent of embrittlement is characterized by the room-temperature ''normalized'' Charpy-impact energy. Based on the information available, two methods are presented for estimating the extent of embrittlement at ''saturation,'' i.e., the minimum impact energy that would be achieved for the material after long-term aging. The first method utilizes only the chemical composition of the steel. The second method is used when metallographic information on the ferrite morphology, i.e., measured values of ferrite content and mean ferrite spacing of the steel, is also available. The change in Charpy-impact energy as a function of time and temperature of reactor service is then estimated from the extent of embrittlement at saturation and from the correlations describing the kinetics of embrittlement, which is expressed in terms of the chemical composition and aging behavior of the steel at 400 degree C. 37 refs., 16 figs., 4 tabs