Generator rotor retaining rings: stress corrosion problems and solutions

The specific service conditions which have led to recent stress corrosion service problems with generator rotor retaining rings are pointed out. This includes early stress corrosion service failures with ferromagnetic high-strength steels, subsequent failures with carbon strengthened cold worked austenite, and the recent failures due to stress corrosion cracking of high nitrogen - high manganese cold worked austenite. The development of retaining ring steels is traced with emphasis on present and future retaining ring materials. Future retaining ring materials include molybdenum-containing austenitic stainless steels with high resistance to localized corrosion and thus with resistance to stress corrosion crack initiation. Also future prospects are chromium-base high nickel alloys with improved resistance to stress corrosion crack growth. This is based partially on the well known resistance of high nickel - high chromium alloys to stress corrosion cracking in chloride containing aqueous solutions. The advantages of the newly developed retaining ring materials are compared with the presently worldwide used materials. The difference in localized corrosion resistance and stress corrosion cracking resistance is illustrated quantitatively. (authors)