In order to develope a new steel for a large steam turbine rotor which has higher creep rupture strength than 1Cr-l Mo-0.25V steel, an investigation was carried out on the effects of chemical com- positions, such as tantalum, nitrogen and carbon, and heat treatments on creep rupture strength of12Cr-l Mo-0.2V steel.
1) The addition of tantalum or nitrogen is effective to increase creep rupture strength. The addition of both elements simultaneously is more effective owing to a fine precipitate of Ta (CxN1-x). The creep rupture strength is related to the addition of these elements, according to the following equation:σRup.=A+αTa%+βN%(Ta: up to 0.2%, N: up to 0.1% in the range of C+N: 0.16-0.2%, A: constant, α and β: coefficient).
2) The higher austenitizing temperature has also some beneficial effect in obtainning higher short time creep rupture strength, but it's effectiveness is small on long time creep rupture strength. The higher austenitizing temperature results in the decrease in rupture ductility, which is desired for large steam turbine rotor. The optimum austenitizing temperature is found to be 1050° to obtain good creep rupture strength with ductility.
