Ageing Effect on Creep-fatigue Properties of Super-clean 9% CrMoV Steel for Steam

Focusing on the heating degradation of material properties, which is one of the main factors controlling the lives of steam turbine rotors of power plants, the ageing effect on the creep-fatigue properties of a newly developed super-clean 9% CrMoV steel was investigated in this work, comparing with that of the present 1% CrMoV steel for high-pressure steam turbine rotors. Creep-fatigue tests for the as-received steels and their aged materials were mainly conducted at 566 deg C in air. The creep-fatigue strengths of the two as-received steels show almost the same values. Although the decreasing of creep-fatigue strengths of the aged 1% CrMoV steel is extremely remarkable due to heating degradation, that of the aged super-clean 9% CrMoV steel is very small. By observing the fracture surfaces and the longitudinal sections of the creep-fatigue ruptured specimens, and basing on the microstructural changes, the effects of ageing treatments on the creep-fatigue strengths are discussed. From the viewpoint of creep-fatigue strength, the super-clean 9% CrMoV steel will be a very promising rotor material for the ageing corresponded to the long-term service.