Cast stainless steel aging: Mechanisms and predictions

A program is being conducted to investigate the significance of low-temperature embrittlement of cast duplex stainless steels under LWR operating conditions and to evaluate possible remedies to the embrittlement problem for existing and future plants. The scope of the investigation includes the following goals: (1) establish the mechanism of embrittlement and validate the simulation of in-reactor degradation by accelerated aging, (2) evaluate the effects of key compositional and metallurgical variables on embrittlement, and (3) obtain fracture toughness data to predict the degree of toughness loss suffered by cast stainless steel components during normal and extended service life of reactors. Charpy-impact, tensile, and J-R curve tests were conducted on several experimental and commercial heats of cast stainless steel that were thermally aged up to 30,000 h at temperatures between 290 and 450 degree C (∼555 and 840 degree F). The results indicate that aging at these temperatures leads to an increase in tensile strength, a decrease in impact energy, JIC, and tearing modulus of the material, and the ductile-to-brittle transition curve shifts to higher temperatures. In general, the low-carbon CF-3 grades of cast stainless steels are the most resistant and molybdenum-containing high-carbon CF-8M grades are the most susceptible to low-temperature embrittlement. The effects of material variables on the embrittlement of cast stainless steels are evaluated. The ferrite morphology has a strong effect on the degree or extent of embrittlement, while material composition influences the kinetics of embrittlement. The kinetics of embrittlement can vary significantly with small changes in the constituent elements of the cast material