Optimized chemical composition, working and heat treatment condition for resistance to irradiation assisted stress corrosion cracking of cold worked 316 and high-chromium austenitic stainless steel

The authors have reported that the primary water stress corrosion cracking (PWSCC) in baffle former bolts made of austenitic stainless steels for PWR after long-term operation is caused by irradiation-induced grain boundary s egregation. The resistance to PWSCC of simulated austenitic stainless steels whose chem ical compositions are simulated to the grain boundary chemical composition of 316 stainless steel after irradiation increased with decrease of the sil icon content, increases of the chromium content, and precipitation of M23C6 carbides at the grain boundaries. In order to develop resistance to irradiation assisted stress corrosion cra cking in austenitic stainless steels, optimized chemical compositions and h eat treatment conditions for 316CW and high-chromium austenitic stainless s teels for PWR baffle former bolts were investigated. For 316CW stainless steel, ultra-low-impurities and high-chromium content a re beneficial. About 20% cold working before aging and after solution treat ment has also been recommended to recover sensitization and make M23C6 carb ides coherent with the matrix at the grain boundaries. Heating at 700 to 72 5 degrees C for 20 to 50 h was selected as a suitable aging procedure. Cold working of 5 to 10% after aging produced the required mechanical propertie s. The optimized composition of the high-chromium austenitic stainless steel c ontents 30% chromium, 30% nickel, and ultralow impurity levels. This compos ition also reduces the difference between its thermal expansion coefficient and that of 304 stainless steel for baffle plates. Aging at 700 to 725 deg rees C for longer than 40 h and cold working of 10 to 15% after aging were selected to meet mechanical property specifications.