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

摘要

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 deg C. The minimum temperature of the TMF test was 100 deg C and the maximum temperaures (T_p) were varied between 500 and 800 deg C. In both loading conditions, weak cyclic softening is observed at T_i=T_p=800 deg C, but the transition to strong cyclic hardening is completed with the temperature decrease below T_i=600-700 deg C for LCF and T_p = 500-600 deg 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 deg 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.