Abstract
In this study, thermal fatigue tests at maximum temperature 1073 K were performed using 13%Cr-Nb-Si and 18%Cr-Nb-Mo steels as representative heat-resistant ferritic stainless steels for automotive exhaust systems. The changes in the microstructure, the crystal orientation and the hysteresis loop during thermal fatigue in the temperature range from 473 K to 1073 K were investigated. As a result of comparing thermal fatigue life under these conditions, 18%Cr-Nb-Mo steel with high temperature strength was found to have a longer thermal fatigue life than 13%Cr-Nb-Si steel. During the thermal fatigue process, the material was softened by reducing of the amount of solute Nb, and the coarsening of Nb precipitation. By this softening, the form of the hysteresis loops changed with the increase in cycles. By considering the softening of the material, the change in the hysteresis loops could be predicted to some extent. Furthermore, by EBSD analysis, it was recognized that the dynamic recovery and recrystallization accompanied by the uniaxial and fine grain formation occurred during the thermal fatigue process. From the viewpoint of change of the microstructure, the thermal fatigue damage was quantified by the ratio of the low-angle grain boundary, and the change of this index with the progress of the cycle in 18%Cr-Nb-Mo steel had a smaller than 13%Cr-Nb-Si steel. It was thought that this point was caused by the retardation of recrystallization by solute Mo.
