Monotonic Tensile and Cyclic Deformation Behaviors of Carbide-Free Bainitic Steel Subjected to Austempering and Tempering after Air Cooling

The monotonic tensile and tension-compression cyclic deformation behaviors of carbide-free bainitic steel with two microstructures obtained by isothermal treatment (ISO) and air cooling followed by tempering (ACT) are investigated. Tensile results demonstrate that the ISO sample exhibits a lower yield strength but longer elongation due to more and bigger blocky retained austenite than that of the ACT sample, which can lead to a higher plastic deformation capacity for the former sample. Fatigue results indicate that the ACT sample shows a higher cyclic hardening capacity but lower cyclic softening capacity at lower strain amplitudes compared to the ISO sample, whereas an opposite phenomenon can be found at higher strain amplitudes. At given total strain amplitudes, the fatigue life of the ISO sample is shorter, which is mainly attributed to its lower yield strength and more brittle martensite. In comparison with the ACT sample, the ISO sample exhibits similar or slightly longer fatigue life at lower plastic strain amplitudes but shows shorter fatigue life at higher plastic strain amplitudes. Accordingly, a bilinear Coffin--Manson relationship is revealed for the ISO sample, which is closely associated with significantly more transformation at higher strain amplitudes compared to the lower strain amplitudes.