Study on the Heat Indication Test of Steam Turbine Rotor Forgings : 5th Report, Discussion on the Thermal Diffusivity Theory as the Cause of Type D Deflection

Abstract

The authors have frequently pointed out the nonuniform thermal emissivity as the cause of type D deflection in the four reports previously issued. In the 5th report of the studies about the heat indication test, the thermal diffusivity theory which is generally believed to be a cause of type D deflection is fully discussd. Thermal conductivity of low alloy steels used for turbine and generator rotor forgings is accurately mesaured by the relative mthod. 1% Cr-1% Mo-V Steels tempered at 625°C and 675°C showed the difference of 5 per cent in thermal conductivity, and in case of 3%Ni-Mo-V Steels tempered at 600°C and 680°C the difference was 3.5 per cent. Timo and his co-worker have reported 2.5 per cent nonuniformity of thermal conductivity and explained it as the cause of the type D deflection. However, a model rotor which has the difference of thermal conductivity at diametrically opposite sides more than 5 per cent (twice the difference of thermal conductivity Timo and his co-worker refered to) did not show the type D deflection, but it showed only type C deflection (the heating and cooling rates were both 120°C/hr at the test). It was described in the 2nd report that an actual rotor forging with 2 per cent nonuniformity of thermal coefficient and model rotors with all sorts of metallurgical asymmetry revealed only the type C deflection, and never showed the type D deflection. In the authors' plant the type D deflections were all disappeared by mechanical improvement of rotor forging surfaces. Considering the above results, it is concluded that Timo and his co-worker's thermal conductivity theory about the type D deflection is not founded on fact.