Irradiation-assisted stress corrosion cracking of austenitic stainless steels: recent progress and new approaches

Irradiation-assisted stress corrosion cracking (IASCC) of several types of BWR field components fabricated from solution-annealed austenitic stainless steels (SSs), including a core internal weld, were investigated by means of slow-strain-rate test (SSRT), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), and field-emission-gun advanced analytical electron microscopy (FEG-AAEM). Based on the results of the tests and analyses, separate effects of neutron fluence, tensile properties, alloying elements and major impurities identified in the American Society for Testing and Materials (ASTM) specifications, minor impurities, water chemistry, and fabrication-related variables were determined. The results indicate strongly that minor impurities not specified by the ASTM-specifications play important roles, probably through a complex synergism with grain-boundary Cr depletion. These impurities, typically associated with steelmaking and component fabrication processes, are very low or negligible in solubility in steels and are the same impurities that have been known to promote intergranular SCC significantly when they are present in water as ions or soluble compounds. It seems obvious that IASCC is a complex integral problem which involves many variables that are influenced strongly by not only irradiation conditions, water chemistry, and stress but also iron and steelmaking processes, fabrication of the component, and joining and welding. Therefore, for high-stress components in particular, it would be difficult to mitigate IASCC problems at high fluence based on the consideration of water chemistry alone, and other considerations based on material composition and fabrication procedure would be necessary as well. (orig.)