In-Service Degradation of Toughness of Steam Turbine Rotors

The toughness of steam turbine rotors degrades with time of service exposure in the range of 340°–540°C. There is a need to estimate the toughness degradation that has occurred in order to assess the reliability, remaining life, and further operational parameters for the rotors. Since it is not possible to take large specimens from operating rotors to evaluate the rotor toughness, nondestructive or relatively nondestructive techniques involving removal of small samples are needed. To serve this need, several techniques such as composition based correlations, grain boundary analysis by Auger electron spectroscopy, analytical electron microscopy, and secondary ion mass spectroscopy, chemical and electrochemical etching, eddy current measurements and mechanical testing of miniature samples have been explored by the electric power industry. This paper presents an overview of the principal results emerging from the various on-going projects in this area.