Long-term creep rupture data of 0.5Cr-0.5Mo (STBA20), 1Cr-0.3Mo (KA-STBA21) and 1Cr-0.5Mo (STBA22) steels were analyzed to estimate 100,000 h creep rupture strength. The creep rupture data were fitted to the regression equation of logarithmic stress using the time-temperature parameters of Larson-Miller (LM), Orr-Sherby-Dorn (OSD) and Manson-Haferd (MH) to estimate the 100,000 h creep rupture strength. The appropriate parameter and degree of regression equation was MH-2nd degree and LM-4th

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... e for 1Cr-0.3Mo and 1Cr-0.5Mo steels, respectively. For 0.5Cr-0.5Mo steel, the creep rupture data under stresses lower than 80% of tensile strength was analyzed because the scatter band of the creep rupture data was very large under high stress regimes. The appropriate parameter and degree of regression equation was LM-3rd degree for the 0.5Cr-0.5Mo steel. It was confirmed that the creep rupture data up to 550°C should be used for evaluation of 100,000 h creep rupture strength because oxidation scale decreased creep rupture strength at 600°C or above. The allowable stress of the steels studied was estimated based on average and minimum values of 100,000 h creep rupture strength. The estimated allowable stresses were equal to or greater than current allowable stress of ASME boiler and pressure vessel code and Technical Standard for Thermal Power Plant in Japan.