Abstract
In view of inverse sigmoidal form of stress vs. time to rupture curve, long-term creep strength of ferritic heat resistant steels has been investigated using a large number of long-term creep data in the NRIM Creep Data Sheets. It was observed that the inverse sigmoidal form of stress vs. time to rupture curve was caused not by the change in mechanisms of strengthening or deformation, but by the decrease in creep strength and by the approach to steady internal stress due to microstructural change. At lower stresses, the complex form of creep rate vs. time curve which had two minima in creep rate was found and attributed to the inverse sigmoidal form of stress vs. time to rupture curve. Comparing the creep rupture strength for the different kind of steels, the large difference in creep rupture life of about four orders of magnitudes was found at lower temperatures and higher stresses. At higher temperatures and lower stresses, however, the creep rupture strength of all ferritic steels investigated was almost the same. It was concluded that the creep strength of the ferritic heat resistant steels was reduced by the microstructural change and approached to the inherent values which were nearly the same for all ferritic steels investigated.
