Song-Nan Yin

Reliability Prediction of Long-term Creep Strength of Gr. 91 Steel for Next Generation Reactor Structure Materials

This paper focuses on reliability prediction of long-term creep strength for Modified 9Cr-1Mo steel (Gr. 91) which is considered as one of the structural materials of next generation reactor systems. A “Zparameter” method was introduced to describe the magnitude of standard deviation of creep rupture data to the master curve which can be plotted by log stress vs. The larson-Miller parameter (LMP). Statistical analysis showed that the scattering of the Z-parameter for the Gr. 91 steel well followed normal distribution. Using this normal distribution of the Z-parameter, the various reliability curves for creep strength design, such as stress-time temperature parameter reliability curves (σ-TTP-R curves), stress-rupture time-reliability curves (σ-tr-R curves), and allowable stress-temperaturereliability curves ([σ]-T-R curves) were reasonably drawn, and their results are discussed. (Received January 24, 2011)

Creep-Life Prediction and Its Error Analysis by the Time Temperature Parameters and the Minimum Commitment Method

To predict long-term creep life from short-term creep life data, various parametric methods such as Larson-Mille. (L-M), Orr-Sherby-Dorn (O-S-D), Manson-Haferd (M-H) parameters, and a Minimum Commitment Method (MCM) were suggested. A number of the creep data were collected through literature surveys and experimental data produced in KAERI. The polynomial equations for type 316LN SS were obtained by the time-temperature parameters (TTP) and the MCM. Standard error (SE) and standard error of mean (SEM) values were obtained and compared with the each method for various temperatures. The TTP methods showed good creep-life prediction, but the MCM was much superior to the TTP ones at and . It was found that the MCM were lower in the SE values when compared to the TTP methods.

Suggestion and Evaluation of a Multi-Regression Linear Model for Creep Life Prediction of Alloy 617

Creep life prediction has been commonly used by a time-temperature parameter (TTP) which is correlated to an applied stress and temperature, such as Larson-Miller (LM), Orr-Sherby-Dorn (OSD), Manson-Haferd (MH) and Manson-Succop (MS) parameters. A stress-temperature linear model (STLM) based on Arrhenius, Dorn and Monkman-Grant equations was newly proposed through a mathematical procedure. For this model, the logarithm time to rupture was linearly dependent on both an applied stress and temperature. The model parameters were properly determined by using a technique of maximum likelihood estimation of a statistical method, and this model was applied to the creep data of Alloy 617. From the results, it is found that the STLM results showed better agreement than the Eno’s model and the LM parameter ones. Especially, the STLM revealed a good estimation in predicting the long-term creep life of Alloy 617.

Reliability Assessment of Creep Rupture Life for Gr.91 Steel by an Interference Model

This paper focuses on reliability assessment of creep rupture life under the service conditions at high temperatures and stresses for Gr. 91 steel which is considered as one of the prime structural materials for next-generation nuclear reactors. An interference model based on Z parameter, which is considered for the fluctuations of both service temperature and stress besides the scattering of rupture data, was analyzed and used to assess the reliability on creep rupture life of Gr. 91 steel. The scattering distribution of the creep rupture data of the Gr. 91 steel was investigated by using the Z parameter. It appeared that the Z parameter of creep rupture data for Gr.91 steel exhibited normal distribution. Using the normal distribution, a Monte-Carlo simulation (MCS) was carried out to generate a number of random variables for Zs and Zcr , and the reliability of the creep rupture life under the fluctuations of service temperature and stress conditions was estimated by using the interference model. It showed that the value of reliability decreased with increasing service time. Higher temperature caused the trend of faster deterioration. The value of reliability decreased rapidly at higher temperature fluctuation amplitude, and the reliability decreased as the scattering of the creep rupture data became serious.Copyright

Taylor Series-Based Long-Term Creep-Life Prediction of Alloy 617

In this study, a Taylor series (T-S) model based on the Arrhenius, McVetty, and Monkman-Grant equations was developed using a mathematical analysis. In order to reduce fitting errors, the McVetty equation was transformed by considering the first three terms of the Taylor series equation. The model parameters were accurately determined by a statistical technique of maximum likelihood estimation, and this model was applied to the creep data of alloy 617. The T-S model results showed better agreement with the experimental data than other models such as the Eno, exponential, and L-M models. In particular, the T-S model was converted into an isothermal Taylor series (IT-S) model that can predict the creep strength at a given temperature. It was identified that the estimations obtained using the converted IT- S model was better than that obtained using the T-S model for predicting the long-term creep life of alloy 617.