Estimation of Long-Term Creep Rupture Strength of ASME Grade 91 Steel Based on Z Phase Precipitation

The reasonable procedures for estimating long-term creep rupture strength are investigated for ASME Grade 91 steel using creep rupture data in the NIMS Creep Data Sheet. The target was set to the estimation of 500,000 hrs creep rupture strength at 550°C, taking a strong demand for fast reactor applications of this steel into account, and the estimation of 100,000 hrs creep rupture strength at 600°C for USC applications. Based on the analysis on the degradation in creep rupture strength of this steel, the regression analysis is carried out using rupture data located inside the time-temperature-precipitation (TTP) curve for Z phase precipitation, where the precipitation of Z phase and hence the degradation take place, and below the nose temperature of TTP curve, where the precipitation of Z phase is accelerated with increasing temperature. The second-order regression analysis by the Larson-Miller method gives us the estimated values of 100.1 and 73.4 MPa at 550°C and 500,000 hrs and at 600°C and 100,000 hrs, respectively, which are compared with those by the analysis using rupture data above 500 hrs, those by the Region splitting analysis method and those by the Muti-region analysis method.