Toughness reduction in cast austenitic stainless steels (CASS) due to thermal embrittlement is a growing concern for flaw tolerance evaluation in nuclear power plants. As the fracture toughness diminishes due to thermal-aging, some aged CASS materials have the potential to reach much lower toughness values than at the beginning of life. CASS also shows a high variability in toughness even for a given chemical composition, operating temperature, ferrite number or chrome equivalent. This is mainly due to the combined ferritic and austenitic microstructure and potentially large grain sizes. This variability in toughness needs to be used when developing flaw evaluation criteria in Section XI of the ASME Boiler and Pressure Vessel Code.
In this work, deterministic critical flaw size evaluations were conducted for one of the low-toughness CASS materials with a ferrite number (FN) lower than 25. Six different methods were used to evaluate the critical flaw sizes for circumferential surface flaws. In order to account for the large variability in CASS material properties, a probabilistic fracture mechanics (PFM) procedure was developed. Based on all deterministic analyses, the Dimensionless Plastic-Zone Parameter analysis was used to perform the probabilistic critical flaw evaluation. As an example calculation, an actual toughness distribution from a single US PWR plant chemical composition (using a thermal-aging model) was used to perform the probabilistic analysis. The probabilistic analysis provides the critical flaw sizes at various stress ratios for Service Level A (10−6) and Service Level B (10−5) probability of failure for this particular case.