Prevention of the overestimation of long-term creep rupture life by multiregion analysis in strength enhanced high Cr ferritic steels

Long-term creep rupture life is often evaluated from short-term data by a time–temperature parameter (TTP) method. However the conventional TTP methods sometimes fail in understanding creep rupture behavior of strength enhanced high Cr ferritic steels and overestimate creep rupture life in long-term creep. In the present paper, creep rupture data of seven kinds of heat resistant steels with different W and Cr concentrations have been analyzed. The conventional TTP method like Orr–Sherby–Dorn analysis evaluates long-term creep rupture life assuming a unique value of activation energy for all the creep rupture data. This analysis is called single region analysis in this paper. The single region analysis can represent well the creep rupture data of steels containing less than 8% Cr. The creep rupture analyses of steels containing more than 8% Cr exhibit that apparent activation energy changes from a high value in short-term creep region to a low value in long-term creep region. In each case a creep data was divided into several data sets, and then the conventional single region analysis was applied to each divided data set. This analysis is referred to as multiregion analysis. The multiregion analysis describes very well all the data points, whereas regression curves of the single region analysis deviate from the data points, resulting in overestimation of long-term rupture life. The difference between the two activation energies of short-term and long-term creep increases with increasing Cr concentration. Therefore, the overestimation due to singles region analysis is expected to be more serious at higher Cr concentration.