It is a recent tendency that high temperature power plants with more power and higher efficiency, are more in demand, especially for the steam turbine installations. But such increase in power and efficiency is largely dependent on metallurgical advances. Designers and engineers need to know of limitations in regard to the senility of the metals they are considering of using. However, they are obliged to deal with the short time creep rupture data, for the long time creep rupture test requires

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... uch time and money. Such short time data needs extensive care in extrapolation for a longer period of time. Using many data collected by Nihon Gakujitsu Shinko-Kai, the long time creep rupture strength for 18-8 Ti steel and 18-8Cb steel was estimated statistically by various methods, i.e., the extrapolation method by one straight line, Larson-Miller parameter method, Dorn's parameter method, Manson-Haferd parameter method and the extrapolation method by broken line. Although the parameter methods are more convenient because the time and temperature can be expressed with one parameter, the reliability is rather low. The most reliable one in these methods was the extrapolation method by broken line. It is considered that metallurgical changes result in breaks on the break point of 18-8Ti steel and the second break point of 18-8Cb steel correspond to precipitating of each straight line obtained experimentally. Calculation was conducted with the use of this extrapolation method of 105hr rupture strength at various temperatures for 18-8Ti steel and 18-8Cb steel.