Evaluation equation for creep rupture life of Gr. 91 steel considering region classification and difference in creep strength between heats

Abstract

A creep rupture life evaluation equation for Gr. 91 steel was proposed, taking account of the region classification and the creep strength difference between heats. The creep rupture life evaluation equation consists of three parts. Each part is related to the region H, ML, or G. The factor dominating the difference in creep strength between heats is hardness in region H, Cr, and C concentrations in region ML, and grain size in region G. The minimum value among the three calculated creep rupture lives is used to draw the creep rupture curve of Gr. 91 steel. By substituting the hardness, Cr and C concentrations, and grain size of the material into the proposed equation, one can easily estimate the temperature and stress conditions under which each region (H, ML, or G) will appear, as well as the creep rupture life in each region. The accuracy of creep life description was compared among the creep rupture life evaluation equation proposed in this study, that of the Ministry of Economy, Trade and Industry (METI) and that of the ECCC. The accuracy of the proposed equation was the highest. The region classification by the proposed equation is helpful to reduce the discrepancy between the actual rupture life and the evaluation result in region G. In addition, the evaluation accuracy was improved by considering the heat dependence of creep strength.

Keywords

• high chromium steels
• grade 91 steel
• creep rupture life
• heat dependence
• larson-miller parameter
• stepwise regression
• data science