Abstract
Temper embrittlement is one of the typical material degradations of Cr-Mo-V steel high and intermediate pressure steam turbine rotors used at high temperature in thermal power plants, and so it is important to know the embrittlement characteristics of the turbine rotors serviced for a long term. For this purpose, FATTs (50% fracture appearance transition temperatures) at around the center hole of retired turbine rotors serviced for 150,000-240,000 h were investigated.
It was found that Cr-Mo-V turbine rotors were embrittled at the estimated service temperature of 350-460°C, and the maximum shift in FATT (ΔFATT) occurred at around 400°C. This embrittlement has been well correlated with an embrittling factor K2 (K2=(2Si+Mn+Cu+Ni) · X, where X=(10P+5Sb+4Sn+As)). It is thus concluded that the reduction of impurities is effective to reduce the susceptibility to the emberittlement during the service.
With this findings, the equation, which could estimate the amount of emberittlement ΔFATT of Cr-Mo-V steel steam turbine rotor, was obtained as a function of the embrittling factor K2, the service time and the service temperature. According to the equation, it was estimated that the embrittlement increased rapidly until about 100,000 h, but slowly after about 150,000 h because of approaching the saturation of temper embrittlement.
