Effect of Initial Grain Size on Thermal Fatigue Behavior and Changes in Microstructure of Ferritic Stainless Steel

摘要

To understand the effect of grain size on the thermal fatigue behavior and dynamic recrystallization during the thermal fatigue process in ferritic stainless steel, interrupted testing of thermal fatigue in the temperature range from 473K (minimum) to 1073K (maximum) with a restriction ratio of 50% was conducted for fine-grained (diameter: 38 mu m) and coarse-grained (diameter: 221 mu m) specimens in a Nb- and Si-added ferritic stainless steel tube. The fine-grained specimen showed a small inelastic strain range during the thermal fatigue process and a longer life than the coarse-grained specimen. Microstructure observation by electron back-scatter diffraction revealed that dynamic recovery and recrystallization occurred during the thermal fatigue process. However, the coarse-grained specimen showed lower nucleus density and retardation of the recrystallization in comparison with the fine-grained specimen. From a supposition of the accumulation of the inelastic strain range during thermal fatigue cycles, the relationship between the accumulation strain and Zener-Hollomon parameter for dynamic recovery and recrystallization almost accorded with previous knowledge on this tendency. In addition, we found that for the steady-state recrystallized grain size in thermal fatigue cycles, there was little initial grain size dependence.