Non-Destructive Detection of Spinodal Decomposition in Cast Austenitic Stainless Steels

Abstract

Embrittlement of cast austenitic stainless steels (CASS) in light water reactor (LWR) primary loop piping and components constitutes a significant limitation to extended operation of nuclear power plants. Existing methods to evaluate embrittlement in CASS LWR structural components are generally destructive, limiting their ability to continuously monitor material integrity for extended plant operation beyond the period of initial licensing. We have employed transient grating spectroscopy (TGS) as a non-destructive evaluation (NDE), non-contact, picosecond ultrasonic technique to assess spinodal decomposition-induced embrittlement in CASS samples by inference of multiple, correlated material properties. This is done by linking changes in surface acoustic wave (SAW) frequencies and thermal diffusivity values, simultaneously measured by TGS, to directly confirm incidences of spinodal decomposition as determined by detailed microstructural analysis. CASS alloys thermally aged for 1,500-30,000 hours at temperatures within the operation range of LWRs (280°C-340°C), as well as accelerated aging at temperatures up to 400°C, were examined thermo-acoustically via TGS and were microstructurally characterized using electron microscopy. Charpy impact energies were measured destructively to verify the extent of embrittlement after thermal aging. We confirmed that spinodal decomposition of delta ferrite was always present as an underlying mechanism of the SAW peak frequency splitting phenomenon observed in aged CASS CF8 TGS measurements, excluding carbide evolution by comparison with data obtained for low-carbon CASS (CF3). CASS microstructures exhibiting SAW peak frequency splitting have higher thermal diffusivities with a p-value <0.001. The results of this study indicate that TGS can therefore be used as a NDE technique for evaluating in-service LWR CASS structural materials by monitoring the progression of spinodal decomposition-induced embrittlement as an inference method to augment Charpy impact testing. Moreover, TGS has the potential to be adapted to characterize spinodal decomposition on CASS components in service, thereby circumventing the limitations of conventional destructive characterization approaches.