The role of fine defect clusters in irradiation-assisted stress corrosion cracking of proton-irradiated 304 stainless steel

The influence of small defect clusters on irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steels has been examined. A CP-304 SS alloy was irradiated with 3.2 MeV protons to 1.0 dpa at 360 degrees C and then annealed for 45 min. to 1 h at temperatures ranging from 400-600 degrees C. Dislocation microstructure was analyzed using both bright field and dark field techniques and cracking susceptibility was measured in an oxygenated water environment. Intergramilar cracking was mitigated following annealing at 500 degrees C/45 min. while only the smallest dislocation loops had been removed. Analysis of the slip step bands on the surface of deformed samples indicates that deformation mode may be important in determining cracking susceptibility. Low-temperature irradiation (< 75 degrees C) was also used to create a high population of small defects without radiation-induced segregation. Samples were then annealed at 350 degrees C for 8 or 15 min. to grow the dislocation loop microstructure, resulting in an irradiated hardness similar to the 1.0 dpa at 360 degrees C case. No segregation was measured in the as-irradiated specimens. The change in hardness resulting from 0.3 dpa at 75 degrees C was 73.8 kg/mm(2), greater than that for 1.0 dpa at 360 degrees C. Following annealing at 350 degrees C for 8 min., the change in hardness was reduced to 60.8 kg/mm(2), similar to 1.0 dpa at 360 degrees C (65.3 kg/mm(2)). Samples were also strained to 29.6 % strain in an oxygenated water environment at 288 degrees C. No cracking was observed on any specimen. The small defect damage may have been annealed during the constant extension rate test.