With 14 samples of various chemical compositions of 12% Cr steel and one 1% CrMoV steel, the effect of back stress on creep rate was studied in relation with activation energy and stress exponent. Back stress is depended on applied stress and temperature. The equation can be formulated. Effective stress exponent, ηο is not fixed value for all materials as reported but strong dependence of temperature and microstructure ηο value represents the stability of a material at the elevated temperature and no value is expected to be a criterion of stability of material in the other atmospheres. Elements of Nb, Zr, Ti, W, La, Pd and Ru are found to be effective to form very stable microstructure in 12% Cr steel and the elements are expected to improve creep rupture strength.
In order to improve the thermal efficiency of a steam turbine in the power generation system, there is a trend towards increasing the operating temperature and pressure. 1% Cr-Mo-V steel which has high creep rupture strength and good toughness up to 550" C of steam temperature has been successfully applied for the material of turbine rotor. However, to meet higher creep rupture strength above 600t, 12% Cr steel is considered a promising material to replace the conventional 1% Cr-Mo-V steel. An intensive study of 12% Cr steel for the application of turbine rotor was made by General Electric, Climax Molybdenum Company" in U. S. A in 1960s and Mitshubishi, Tokyo Electric PowerCo", and Tokyo University in Japan in 1980s. Hitachi studied" mould design for ingot making to reduce segreagation in ingot. The high creep rupture strength of 12% Cr steel was reported due to the secondary hardening by increasing coherent strains or increasing the volume fraction of precipitates such as M2X or MX carbides.
