Stress Corrosion Cracking of 18Mn-4Cr Generator Rotor End-Retaining Ring Steel

18Mn-4Cr austenitic steel has virtually been the standard material for the rotor end-retaining rings in power generation plants during the period of 1960–1980, before it was replaced by the 18Mn-18Cr variety. A good number of cases of failure of the 18Mn-4Cr steel ring by stress corrosion cracking (SCC) led to the switchover to the higher chromium variety. The cases of failure have been documented in the report of EPRI Workshop on Retaining Rings for Electrical Generators (October 1982). SCC has been attributed to the presence of moisture, and though the sources of humidity have been identified as imperfectly dried hydrogen, leakages from water-fed stator winding, oil in the hydrogen-tightness circuit and moisture condensation during shut downs, storage and transportation of the rings, no light was thrown on the mechanism of SCC or any possible effect of the stress relieving temperature on SCC. The present study was undertaken with these considerations in view.