Low Cycle Fatigue Life of Turbine Generator Retaining Rings Determined by Stress Cycling Tests

In a turbine generator rotor, the field winding extensions beyond the main body of the rotor forging are supported by high strength retaining rings, which are the most highly stressed components of the rotor. The availability of suitable high strength retaining rings determines the maximum diameter of the rotor which can be built. In modern generators of large rating it is desirable to use rings made of nonmagnetic material. Austenitic steel alloys strengthened by cold expansion or precipitation hardening are generally used. Extensive tests have been made to determine the necessary quality level and methods of evaluation which will assure adequacy of the rings during the entire life of the generator. Among these have been stress cycling tests of slices cut from rings simulating the expected life of a machine. Stress raisers including drilled holes, artificial cracks, non-metallic inclusions, and stress corrosion cracks were tested. As a result of these tests it has been determined that the widely used cold expanded austenitic steel rings at the stress levels used, can withstand large numbers of stress cycles with large stress raisers and thus provide the desired factor of safety.