Volume 18, Issue 3 (May 1990)
Nondestructive Evaluation of In-Service Toughness Degradation for Cr-Mo-V Steam Turbine Rotor Steel
Embrittlement was investigated for five retired and three laboratory-aged steam turbine rotors by sampling Charpy V-notch test coupons and testing them. A nondestructive evaluation method of toughness degradation was discussed.
The investigation has revealed that Cr-Mo-V rotor steel becomes embrittled by phosphorus segregation at the grain boundaries over a long period of use at high temperatures. The phosphorus-enhanced toughness degradation can be nondestructively detected by the electrochemical polarization (ECP) test.
An approximate formula to evaluate the degraded fracture appearance transition temperature (FATT) of a specific Cr-Mo-V rotor was established that included the nondestructive information of the ECP minimum current density, grain size, and data given from the fabrication record. The nondestructive estimation procedure for the ductile fibrosity percentage versus the temperature curve for the specific rotor is also presented.