Nondestructive Evaluation of In-Service Toughness Degradation of CrMoV Steam Turbine Rotor Steel

Older thermal power plants are increasing in number today which are to be used beyond their design lives. In order to keep their high performance and to extend their useful lives, it is quite necessary to fully understand the degradation phenomena of the component materials during service operation and to make more accurate life assessment. The embrittlement, one of the typical material degradations, was investigated for the CrMoV steam turbine rotors retired after long time service at high temperatures in thermal power plants, as well as for the laboratory aged rotors. The V notch Charpy impact tests showed a remarkable increase in the fracture appearance transition temperature (FATT) in some rotors. The Auger electron spectroscopy analysis indicated that this increase was attributable to the phosphorus segregation to the grain boundary. The electrochemical polarization (ECP) test was applied to the rotor samples and the value of ECP minimum current density, which was corrected for the grain size obtained by the metallographical investigation of replica taken from the ECP test surface, was found to be a very good measure of the embrittlement. Based on these informations from ECP test, replica observation and rotor production record, a nondestructive method for evaluating in-service toughness degradation of CrMoV steam turbine rotors was presented. According to this method, the degraded FATT of a CrMoV rotor could be estimated within±20°C.