Low Cycle Fatigue behaviour and anisotropy of two steels for turbogenerator coil retaining rings and rotors

Turbogenerator rotors and coil retaining rings (CRR) are typically subjected to low cycle fatigue (LCF). The rotor contains uniformly spaced longitudinal slots, where copper conductors are packed: at the nominal speed of 3000 rpm a great centrifugal force acts on the copper conductors that must be restrained by CRRs, shrunk fitted onto the body over the coils. Thus, at each on-off transitory (10,000 – 15,000 in the whole machine life) both the rotor and the CRR are subjected to a cyclic load. Several contributions report the mechanical properties of the typically used steels for rotor and CRR manufacturing, but none deals with their characterization under LCF in strain controlled conditions, while investigations on the anisotropy were performed just on CRR steels in load controlled conditions. This research aims at determining the main parameters describing the LCF performance of two widely applied steels for rotors and CRRs, investigating also the anisotropy in the dynamic behaviour: for this purpose an extensive experimental campaign was carried out on specimens machined (in the tangential and radial directions) from prolongations of trial rotors and CRRs. An original approach was used for misalignment compensation, strain control and data recording, sensitivity analyses were finally performed on results.