Cyclic Deformation Behaviors under Isothermal and Thermomechanical Fatigue Conditions in Nb and Mo Added 15Cr Ferritic Stainless Steel

This paper deals with cyclic stress and strain responses during isothermal low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) loadings on Nb and Mo containing 15Cr stainless steel, which is used for exhaust manifolds in automobiles. The test temperatures (T-i) of the isothermal LCF were 600 and 800 degrees C. The minimum temperature of the TMF test was 100 degrees C and the maximum temperaures (T-p) were varied between 500 and 800 degrees C. In both loading conditions, weak cyclic softening is observed at T-i=T-p=800 degrees C, but the transition to strong cyclic hardening is completed with the temperature decrease below T-i=600 similar to 700 degrees C for LCF and T-p=500 similar to 600 degrees C for TMF. The stress-strain hysteresis loops in the TMF loading show a significant stress relaxation during compressive (heating) half cycle at T-p>500 degrees C, which develops tensile mean stress during cycling. Due to the stress relaxation, the TMF test sample reveals much lower dislocation density than the isothermally fatigued sample at the same temperature with T-p. A detailed correlation between fatigue microstructure and cycling deformation behavior is discussed.