Evaluation and extension of allowable stress values of Grade 91 steel have been investigated with the updated creep database. Creep database has been updated and 2,046 of time to rupture data, 523 of time to initiation of tertiary creep data and 388 of time to 1% total strain data have been compiled. Accuracy of evaluation and extrapolation was examined on several type data analysis procedures. Larson-Miller and Orr-Sherby-Dorn parameters were used. Lot-centering method, data-censoring by 1,000 h, and region-splitting method by half-yield were investigated. Precise and conservative evaluation was obtained by region-splitting analysis with 3rd and 2nd order polynomials for high- and low-stress regimes, respectively. According to the data analysis, Minimum Stress-to-Rupture, Sr, and Allowable Stress Intensity Values of St and Smt have been evaluated and extended to 500,000 hours.

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