Effects of Maximum Temperature during Thermal Cycle and Mo Addition on Thermal Fatigue Behavior and Changes in Microstructure of Nb-Added Ferritic Stainless Steel

In this study, thermal fatigue tests in the temperature ranges 473-873 K, 473-973 K, and 473-1073 K, with a restriction ratio of 100%, were conducted using 13%Cr-Nb-Si added steel, which is a representative heat-resistant ferritic stainless steel for automotive exhaust systems. The effects on the thermal fatigue. i.e., the thermal fatigue life and response of stress and strain. and changes in the microstructure were investigated. Comparison of the thermal fatigue life in the different temperature ranges revealed that the fatigue life improved with a decrease in the maximum temperature. The relationship of the inelastic, elastic, and total strain with the thermal fatigue life was estimated using the Coffin -Manson and Basquin laws. Using these, the curves for predicting the life on the maximum temperature could be obtained. In addition, a detailed observation of the interrupted specimens at the maximum temperatures of 973 K and 1073 K using electron back scatter diffraction confirmed the occurrence of dynamic recovery and recrystallization. along with uniaxial and line grain formation. However, there was no dynamic recrystallization at the maximum temperature of 873 K. Changes in the microstructure during thermal fatigue were quantified by the frequency of the low-angle boundary, and the expression for dynamic recovery and recrystallization was formulated using the Zener-Hollo-mon parameter and cumulative inelastic strain range. Furthermore, together with 18%Cr Nb Mo added steel, from the consideration of the stress relaxation behavior during holding at the maximum temperature in the thermal fatigue and the prediction curves of fatigue life, it was shown that the thermal fatigue strength was improved by approximately 100 K in terms of temperature by the addition of 2% Mo.