Cyclic deformation behaviors of a high strength carbide-free bainitic steel

Abstract

The cyclic deformation behaviors of low-temperature bainite, lower bainite and upper bainite obtained on a high-strength carbide-free bainitic steel were examined through low-cycle fatigue testing. The relationship between the bainitic microstructure and fatigue behavior was systematically studied using scanning electron microscopy, transmission electron microscopy, atom probe technology, and electron back-scattered diffraction analyses. The results show that low-cycle fatigue undergoes three stages, namely, cyclic hardening, saturation or cyclic softening, and fracturing. The low-temperature bainite exhibits a long fatigue life under the total strain amplitudes, because of its high strength and larger high-angle misorientation distribution of the packets of bainitic ferrite plates. The low-temperature bainite also presents bilinearity in the Coffin–Manson plots. Coordinate deformation and high-uniform elongation lead to prolonged fatigue life at low plastic strain amplitude, whereas phase component primarily affects the fatigue life at high plastic strain amplitude. Highly stable film-like retained austenite is beneficial to arresting fatigue crack propagation. Blocky type retained austenite easily transforms to martensite, resulting in high compatible deformation capability. The hardening ability of the low-temperature bainite is higher than that of the upper bainite, which is attributed to its high pre-existent density of dislocations transformed from movable to immovable during initial hardening stage.