2014 Volume 100 Issue 3 Pages 414-420
Conventional time-temperature-parameter (TTP) methods often overestimate long-term creep rupture life of high Cr ferritic steels. The cause of the overestimation is examined paying attention to temperature and stress dependence of creep rupture life of Gr.91, 92 and 122 steels. In stress-rupture data of all the three steels there are four regions with different values of stress exponent n for rupture life. Activation energies Q for rupture life in the regions take at least three different values. The values of n and Q decrease in a longer-term region. The decrease in Q value is the cause of the overestimation of long-term rupture life predicted by the conventional TTP methods unable to deal with the change in Q value. Therefore, before applying a TTP analysis to stress-rupture data, the data should be divided into several data sets so that Q value is unique in each divided data set. When this multi-region analysis is adopted, all the data points of the steels can be described with higher accuracy, and their long-term rupture life can be evaluated more correctly.