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

[+] Author and Article Information
Kouichi Maruyama

Graduate School of Environmental Studies,  Tohoku University, 6-6-02 Aobayama, Aoba-ku, Sendai 980-8579, Japanmaruyama@material.tohoku.ac.jp

Kyosuke Yoshimi

Graduate School of Environmental Studies,  Tohoku University, 6-6-02 Aobayama, Aoba-ku, Sendai 980-8579, Japan

J. Pressure Vessel Technol 129(3), 449-453 (Jun 20, 2006) (5 pages) doi:10.1115/1.2748825 History: Received October 06, 2005; Revised June 20, 2006

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 23 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.

Copyright © 2007 by American Society of Mechanical Engineers
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Grahic Jump Location
Figure 1

Evaluation procedure of long-term creep rupture life

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Figure 2

(a) Stress versus rupture life plot; and (b) temperature dependence of rupture life of Gr.122 steel. Solid regression curves: multiregion analysis taking account of the change in activation energy Q between Regions H and L. Dashed regression curves: single region analysis neglecting the change in Q.

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Figure 3

Stress versus rupture life plot of Gr.122 steel. Solid curves are the regression curves for the solid data tested at stresses lower than σ0.2∕2.

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Figure 4

Comparison of regression curves obtained by a multiregion analysis and conventional single region analyses of the whole data and of the data of σ<σ0.2∕2.



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