Analysis Of Creep Rupture Data Research Articles

Creep Behavior and Microstructural Evolution of a Fe-20Cr-25Ni (Mass Percent) Austenitic Stainless Steel (Alloy 709) at Elevated Temperatures

Understanding creep properties and microstructural evolution for candidate materials of the next-generation nuclear reactors is essential for design and safety considerations. In this work, creep tests were carried out at temperatures ranging from 973 to 1073 K and stresses 40 to 275 MPa followed by microstructural examinations of a Fe-20Cr-25Ni (mass pct) austenitic stainless steel (Alloy 709), a candidate structural material for the Sodium-cooled Fast Reactors. The apparent stress exponent and activation energy were found to be 6.8 ± 0.4 and 421 ± 38 kJ/mole, respectively. The higher activation energy relative to that of lattice self-diffusion together with the observation of dislocation-precipitate interactions in the crept specimens was rationalized based on the concept of threshold stress. The threshold stresses were estimated using a linear extrapolation method and found to decrease with increased temperature. By invoking the concept of threshold stresses, the true stress exponent and activation energy were found to be 4.9 ± 0.2 and 299 ± 15 kJ/mole, respectively. Together with the observation of subgrain boundary formation, the rate-controlling mechanism in the Alloy 709 was conclusively determined to be the high-temperature dislocation climb. Three types of precipitates were identified in the crept samples: Nb(C, N), Z-phases of sizes between 20 and 200 nm within the matrix and M23C6 with sizes between 200 and 700 nm within the matrix and on grain boundaries. Further, the analysis of creep rupture data at high stresses indicated that the Alloy 709 obeyed Monkman–Grant and modified Monkman–Grant relationships with creep damage tolerance factor of ~ 5. Using the Larson–Miller parameter, it was concluded that the Alloy 709 exhibited superior creep strengths relative to the other advanced austenitic steels.

Prediction of Long-Term Creep Rupture Life of Grade 122 Steel by Multiregion Analysis

Conventional time-temperature-parameter (TTP) methods often overestimate long-term rupture life of creep strength enhanced ferritic steels. Decrease in activation energy Q for rupture life in long-term creep is the cause of the overestimation, since the TTP methods cannot deal with the change in Q. Creep rupture data of a heat of Gr.122 steel (up to 26,200 h) were divided into several data sets so that Q was unique in each divided data set. Then a TTP method was applied to each divided data set for rupture life prediction. This is the procedure of multiregion analysis of creep rupture data. The predicted rupture lives have been reported in literature. Long-term rupture lives (up to 51,400 h) of the same heat of the steel have been published in 2013. The multiregion analysis of creep rupture life can predict properly the long-term lives reported. Stress and temperature dependences of rupture life show similar behavior among different heats. Therefore, database on results of the multiregion analyses of various heats of the steel is helpful for rupture life estimation of another heat.

Reliability Design for Allowable Stress of Creep Rupture Strength of 4Cr25Ni35 Steel

Creep rupture data plays vital role in life prediction and safety assessment of high temperature components. In order to describe the scattering of the data, a statistical analysis of creep rupture data for 4Cr25Ni35 steel was performed by Z-parameter method. With the application of Z-parameter, reliability design for allowable stress of creep rupture strength was carried out according to design life. It is found that Manson-Haferd method appears better correlation results with experimental data. Statistical analysis shows that the scattering of Z-parameter for 4Cr25Ni35 steel is supported by normal distribution. Compared with safety factor method, the method based on Z-parameter can perform reliability design for allowable stress of creep rupture strength by considering the dispersibility of the rupture data. Reliability design based on Z-parameter is more agree with experimental data.

Reliability Design for Creep Rupture Strength of 2.25Cr-1Mo Steel

A statistical analysis of creep rupture data for 2.25Cr-1Mo steel was performed. The scattering of creep rupture data was represented by Z-parameter method based on Manson-Haferd method. With the application of Z-parameter, reliability design for allowable stress of creep rupture strength was carried out according to design life. The higher the value of confidence level, the lower the allowable stress. In comparison with safety factor method and minimum rupture strength method, it can be seen that reliability design based on Z-parameter is more agree with experimental data than other methods. Reliability design provides more precise results by considering the real distribution of creep rupture property and provides more flexible choice for design due to the need of safety and economy.

Multiregion analysis of creep rupture data of 316 stainless steel

A creep rupture data set of 316 stainless steel containing 319 data points at nine heats was subjected to a conventional single-region analysis and a multiregion analysis. In the former, the conventional Larson–Miller analysis was applied to the whole data set. In the latter, a data set of a single heat is divided into several data sets, so that the Orr–Sherby–Dorn (OSD) constant Q takes a unique value in each data set, and the conventional OSD analysis was applied to each divided data set. A region with a low value of Q appears in long-term creep of eight heats. Predicted values of the 10 5 h creep rupture stress of three heats were lower than the 99% confidence limit evaluated by the single-region analysis, suggesting that the single-region analysis is error prone. The multiregion analysis is necessary for the correct evaluation of the long-term creep properties of 316 stainless steel.

Influence of Data Analysis Method and Allowable Stress Criterion on Allowable Stress of Gr.122 Heat Resistant Steel

Long-term creep rupture life is usually evaluated from short-term data by a time-temperature parameter (TTP) method. The allowable stress of Gr.122 steel listed in the ASME code has been evaluated by this method and is recognized to be overestimated. The objective of the present study is to understand the causes of the overestimation and propose appropriate methodology for avoiding the overestimation. The apparent activation energy Q for rupture life of the steel changes from a high value of short-term creep to a low value of long-term creep. However, the decrease in Q is ignored in the conventional TTP analyses, resulting in the overestimation of rupture life. A multiregion analysis of creep rupture data is employed to avoid the overestimation; in the analysis creep rupture data are divided into a couple of regions so that the Q value is unique in each divided region. The multiregion analysis provides a good fit to the data and the lowest value of 105h creep rupture strength among the three ways of data analysis examined. A half of 0.2% proof stress cannot provide an appropriate boundary for dividing data to be used in the multiregion analysis. In the 2001 edition of the ASME code an F average concept has been proposed as a substitution for the safety factor of 2∕3 for average rupture stress. The allowable stress of Gr.122 steel changes significantly depending on the allowable stress criteria as well as the methods of rupture data analysis: i.e., from 74MPato48MPa.

蒸気タービンのリスクを考慮した寿命予測と保守管理法

Reliability analyses were conducted on field inspection data of steam turbines and creep data of turbine rotor materials to show how risk-based maintenance was introduced into the life assessment and maintenance of steam turbine plants. The cumulative hazard function method was applied to the field events requiring recovery actions, in order to obtain unreliability functions of operation hours or start cycles. The rotor bowing phenomena were investigated as the typical examples of creep deformation. The event tree and unreliability functions were obtained for those phenomena. Moreover, a unified unreliability function was obtained by substituting operation hours to time fraction t⁄tr, where, tr was time to rupture. As the time to rupture was expressed by the function of stress, temperature and Vickers hardness, this unified unreliability function could be used for various machine and material heats. Unreliability functions were also obtained from the statistical analysis of creep rupture data, especially using the normalization of stress by hardness. The risk functions were derived from the sum of unreliability functions multiplied by recovery costs along the event tree. Maintenance intervals could be determined based on cost analyses among risk costs, preventive maintenance costs and operational benefits. This procedure was proved to be effective for economical and reliable operation & maintenance for steam turbine plants.

Some features and potentialities of the generalized analysis of creep-rupture data for metallic materials

The method of basic diagrams was used to analyze over 200 creep-rupture diagrams obtained in long-term experiments (100,000 h or more). The analysis of known experimental data and the prediction of creep rupture based on the departure of some portions of experimental diagrams from the corresponding protions of basic ones offers strong possibilities for increasing the accuracy and reliability of predictions.

Recent Developments in the Analysis of Creep Rupture Data

Recent Developments in the Analysis of Creep Rupture Data

Statistical Modeling of Creep Rupture Data

A flexible statistical modeling framework for the analysis of creep rupture data is proposed, which offers an improvement on traditional methods of deriving creep rupture strength values and confidence limits. The paper reviews a family of models that can be used to represent the trend relationship between failure times about the trend line, and examines the reliability of extrapolations. Areas of statistical research which would lead to model improvement are discussed, such as variance heterogeneity, left censoring and allowance for the cluster (cast) structure of the data.

Design parameters for high temperature creep and the minimum-commitment method

Materials used in energy conversion systems are often subjected to stresses at elevated temperature where resistance to creep and creep/fatigue damage is important. Consequently, more sophisticated and accurate techniques are needed to characterize elevated-temperature material behaviour. A new approach in the analysis of elevated-temperature creep data has recently been developed, called the Minimum-Commitment-Method (MCM), which allows the development of design equations from analysis of creep-rupture data using a station to station or data point to data point approach. The power and general applicability of the minimum-commitment, station-function approach to the analysis of creep-rupture data is demonstrated. It is shown that equivalence in the best value for the variable constant A in the generalized time-temperature-stress equation, logt +AP(T) logt +P(T) =G (log σ), is obtained, whether derived from convergence of isostress creep-rupture data or by the focal-point convergence of isothermal creep-rupture data. Data conforming exactly to restrictions imposed by several common time-temperature parameters were analysed, and the corresponding values for A were derived by computer analysis of the isostress and the isothermal data. Multiple heat analyses on real data for Type 316 stainless steel was undertaken as well. In all cases, the analyses yielded equivalence in the A-value whether it was obtained from isothermal or isostress creep data. Finally, it is demonstrated that application of the Minimum-Commitment-Method using the derived Avalues yields a minimum in error of log-time for interpolation and extrapolation of the data.

A Quarter-Century of Progress in the Development of Correlation and Extrapolation Methods for Creep Rupture Data

A brief review is presented of developments in analysis of creep-rupture data, including Time-Temperature Parameters, the Minimum Commitment Method, and Successive Regression. The role of Dr. Robert Goldhoff in this progress is emphasized.

Fracture as the limit of flow

Careful measurement of the shape of the creep curve for a Ni-Cr alloy shows that it closely follows a modifiedAndrade equation. Departures from this form in the near neighbourhood of rupture are shown to be due to the development of cracks so that the initiation of cracking is associated with no observable departure from the preceding regular curve of flow. Results of the analysis of creep rupture data support the indication that cracking is initiated when one or other of the terms of the modifiedAndrade equation reach limiting values.