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

Abstract The reasonable procedures for estimating long-term creep rupture strength are investigated for ASME grade 91 steel using creep rupture data in the National Institute for Materials Science (NIMS) creep data sheet. The target was set to the estimation of 500,000 h 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 h creep rupture strength at 600 °C for ultrasupercritical (USC) applications. A regression analysis is carried out using rupture data located inside the time-temperature-precipitation (TTP) curve for Z phase precipitation and below the nose temperature of the TTP curve. Inside the TTP curve, the degradation takes place. Below the nose temperature of the TTP curve, the precipitation of the 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 MPa and 73.4 MPa at 550 °C and 500,000 h and at 600 °C and 100,000 h, respectively. These values are compared with those by the analysis using rupture data above 500 h, those by the Region splitting analysis method, and those by the Multiregion analysis method.