Thermal Aging Degradation Mechanisms of Cast Austenitic Stainless Steels

Cast austenitic stainless steels (CASSs) have been extensively used for the large components of light water reactor (LWR) power plants such as primary coolant piping and pump casing. Since a large number of massive CASS components are installed in every modern nuclear power plant, any significant degradation of CASS components will raise a serious concern on the integrity of the entire power plant. Thorough scientific understanding and systematic monitoring of the thermal aging degradation of CASS components, therefore, should be essential requirements for assessing the plant safety to provide clean energy production from nuclear LWRs.
In this research, we propose to use resources at EMSL to examine the links between thermal aging at various temperatures with the elemental and microstructural evolution of CASS materials and the resultant changes in mechanical behavior and performance through the usage of scanning transmission electron microscopy (S/TEM) and atom probe tomography (APT). Several different CASS steels have been aged at temperatures from 290 oC to 400 oC for 10,000 hours to simulate thermal aging in nuclear LWR components and accelerated thermal aging produced in previous research. Previous research of accelerated thermal aging at high temperatures (about 400 oC) has suggested that spinodal decomposition of the ferrite phase into iron-rich alpha-phase and chromium-rich alpha prime-phase is the primary embrittlement mechanism in the thermal aging of cast stainless steels. However, these studies have not been able to properly make the connection of thermal aging degradation found in experiments with any degradation in current LWR components that have aged for 40 years. Therefore, we will investigate the thermal aging degradation mechanisms at temperatures and times relevant to nuclear LWR CASS components to serve as one aspect in the lifetime extension of LWR to beyond 60-80 years of service life. We will investigate microstructural and elemental evolution of CASS steels as a function of aging temperature, and therefore we need the high spatial and chemical resolution that the capabilities of the S/TEMs, APT, and FIB/SEM at EMSL can provide. We will also study the effects of the high stress and strain found at fracture surfaces and crack fronts, particularly at high temperature, on the local microstructure and elemental distribution. The insights provided in this research will be instrumental both for a basic scientific understanding of aging mechanisms in steels and metal alloys as a function of temperature and also for the practical relevance to current and next generation nuclear power plant materials.