The effect of microstructure on the fatigue and fracture properties of vintage power rotor steels

Abstract

MIT's Alcator C-Mod fusion experiment makes use of a large spinning mass coupled to a generator as its power source. A significant fraction of this mass consists of a 225 MVA generator rotor that was retired from power plant service. As part of normal maintenance, a periodic inspection by non-destructive evaluation (NDE) is performed to determine if flaws have developed, or progressed from pre-existing conditions, that exceed safe limits for operation. In 2008, after over fifty years of service at a power plant and at Alcator, the organization that performed the most recent inspection recommended that the rotor should not be returned to service. The basis for the rejection of the rotor was not clear, since the inspection did not reveal any change in the distribution of indications by NDE. Moreover, an independent fitness-for-service evaluation concluded that the rotor was safe to operate.

However, the rejection was considered a serious enough event to warrant a more complete analysis of the condition of the rotor prior to continued operation. The cost of replacing or repairing the rotor was prohibitively expensive in terms of capital cost and experimental time lost. This prompted an exhaustive investigation into the current state of the main rotor in order to more accurately determine whether or not it was still fit for service. After an extensive test program which included ultrasonic (UT) scans, tensile tests, fracture toughness (FT) tests, crack growth (CG) tests and microstructural analysis, it was determined that the physical and mechanical properties of the rotor were more than adequate, and that neither its properties nor its microstructure had changed appreciably since it was first forged. The rotor was therefore shown to still be fit for operation.

While the UT, tensile, FT and CG tests documented the mechanical properties of the rotor, analysis of the microstructure and the crack path during fatigue and fracture testing further documented and verified the stability of the material with respect to cracking resistance. Detailed microstructural characterization determined the relationship between the indications as identified by NDE (assumed to be cracks by the fitness-for-service analysis) and their actual morphologies. These were ultimately shown to be benign microstructural features that did not degrade the operating capability or cracking resistance of the rotor. This thesis details the results of this microstructural analysis by optical and electron microscopy, as well as the conclusions based on said analysis. In addition, an explanation as to how features observed in the data arose gives more insight into the viability of similar rotors, the serviceability of which has come into question due to age alone.

The results of the analysis show that the rotor properties are more than adequate for continued operation. The results of the microstructural and fractographic analysis are consistent with the measured mechanical properties and demonstrate that further operation is justified.