7.3.1 Thermal stability
During the manufacture of turbine shafts, great efforts are made to ensure that the physical properties of the forging do not change in service. There are three types of instability:
- Permanent, due to asymmetrical coefficients of expansion across the diameter. This is avoided by close metallurgical control of the forging process.
- Temporary, due to residual stresses in the rotor. This is eliminated by stress relieving the rotor, both before and after machining, in a special furnace in which the shaft is rotated.
- Transient, due to differences in conductivity and emissivity. The use of ferritic materials throughout has obviated the difficulties that occur.
An unstable shaft is likely to develop a 'bow' in service, producing out-of-balance forces and consequential vibration.