Prediction of J-R curves and thermoelectric power evolution of cast austenitic stainless steels after very long-term aging (200,000 h) at temperatures below 350 °C

Abstract Cast austenitic stainless steels (CASS) are materials used to fabricate many important safety-related components in the primary circuits of light water reactors since the early 1970’s. The primary circuit, which transports heated water by nuclear reaction to steam generators, is subjected to in-service temperatures between 285 °C and 325 °C. Under these conditions, CASS undergo thermal aging which may significantly affect their mechanical properties and more especially their fracture toughness. From a metallurgical point of view, the changes in mechanical properties are attributable to several solid-state phase transformation processes including the spinodal decomposition of the ferritic phase and the precipitation of G phase. The kinetics of these phase transformations depends primarily on the time and temperature of aging, secondly on the chemical composition of the heat (chromium, molybdenum, silicon and nickel contents) and thirdly on the heat treatments performed during component manufacturing. The prediction of the long-term behavior of CASS is an important industrial issue for nuclear power plant operators. In the early 1980’s, Electricite De France (EDF) engaged an unparalleled laboratory aging program that is still in progress. Presently, some materials have been aged for more than 200,000 h at low temperatures. The results of this program allow the development of prediction models for Charpy-impact energies, J − R curves and thermoelectric power values much more precise than those proposed so far, based on aging at 400 °C and rarely exceeding 30,000 h. These prediction models are described in this paper.