To improve the heat rate in the thermal power station, the development of an ultra super critical steam turbine has projected. A 12%Cr steel rotor forging which has higher creep rupture strength than conventional rotor forging has been required as the turbine rotor for a steam temperature of 593°C. Creep rupture strength at 593°C for 105 h has been required to be over 10 kgf/mm2.
To achieve this requirement, the effect of carbon and nitrogen contents on creep rupture properties were studied using small laboratory heats based on low Si-10Cr-1.5Mo-0.05Nb-0.17V series steels. The excellent creep rupture properties were observed at the range of C: 0.130.15%, N : 0.040.05%.
Based on the results of laboratory heats, the rotor forging for the demonstration tests was manufactured from 39 t ESR ingot. Creep rupture strength at the center core of the rotor forging was 12.4 kgf/mm2 at 593°C for 105 h. No drop of creep rupture ductility and no notch embrittlement were observed even for long term tests.
