LCF characterization with sensitivity analyses of materials for turbogenerator coil retaining rings and rotors

A turbogenerator rotor contains uniformly spaced slots, lodging copper conductors and insulation materials, here held in place by non magnetic metal wedges. The copper conductors emerge at the slot ends to form a coil, wound around the rotor. As the rotor spins (at the nominal speed of 3,000 rpm, up to 3,600 in set-up conditions), the copper conductors, together with the insulation material are subjected to great centrifugal forces. As a consequence, both conductors and insulators are pushed outside and must be restrained. Along most of the rotor length, this resistant is provided by the slot wedges. At the two ends coil retaining rings (CRRs) are shrunk fitted onto the rotor body over these conductor coils. Their function is to retain the coil assembly, reacting against the high centrifugal forces during rotation. Both the rotor and the CRRs are typical examples of components, being subjected to low cycle fatigue (LCF). The loads acting on such components are static in the nominal conditions, i.e. at the constant rotational speed of 3000 rpm, while a cycle is completed at any machine switch on and switch off. During the switch on transitory, the centrifugal forces are increased, meanwhile the friction components at the shrink-fit are reduced. An opposite effect occurs, as the machine is switched off. In the whole life of a turbogenerator (approximately 50 years) from 10,000 to 15,000 transitories are expected to cycle the most loaded regions of the CRR and of the rotor. Some papers deal with the main features of materials for CRR manufacturing. From the mechanical point of view, these materials, such as 18Mn18Cr, have high static and fatigue strength, high corrosion resistance and a great fracture toughness [1-2]. Other studies [3] regarded the mechanical properties of rotor steels, such as 26 NiCrMoV 14 5. However, despite the typical LCF load, none of these studies applies the LCF models, (for instance the Manson-Coffin model), for design purposes or structural analysis. Moreover, no data are often available, concerning the coefficients for the analytical description of LCF curves. Another question to be investigated involves the behavior of these materials along different forming directions, i.e. the anisotropy. Just few contributions [4-5] deal with experimentations focused on this item, but they regard just 18Mn18Cr and tests were performed by controlling load, instead of strain.