Microstructural stability and its effect on ratchetting effect of advanced 9-12%Cr ferrite steel at high temperature

Abstract

Progressive accumulation of strain will occur in materials and structures under the asymmetrical cyclic stressing. This phenomenon is called as ratchetting or cyclic creep. In the case of static creep, based on the short-term rupture data of high chromium ferrite steels, life assessments with the conventional time–temperature-parameter (TTP) methods may lead to the risk of overestimating the long-term rupture life, which is the so-called –premature failure. It was found that a similar premature failure would occur during the ratchetting process for 9-12% chromium steels. In order to understand the physical origins of this premature failure, a study of microstructure evolutions in a high Cr ferrite steel under the creep-fatigue cyclic loadings by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations is carried out in this paper. Results show that the occurrence of additional fatigue damage in the long term tests, which seems to come from the unstable dislocation lath structure, is the main cause of premature breakdown. In addition, the contribution of substructure-level inhomogeneous deformation to the risk of overestimation was highlighted herein.