Study on microstructures and properties of carbide-free and carbide-bearing bainitic steels

The microstructural characteristics, the mechanical and fatigue properties of carbide-free lower bainite and carbide-bearing lower bainite were studied. Moreover, the effects of the bainitic ferrite, retained austenite, and carbide phases on the properties of the tested steels were analyzed. Results show that the carbide-free lower bainite consists of fine bainitic ferrite plates and metastable retained austenite. Large amounts of C atoms are distributed in the metastable retained austenite. The carbide-bearing lower bainite consists of thick bainitic ferrite plates and stable carbides, which contain numerous C atoms. Carbide-free lower bainite exhibits higher strength, plasticity, and toughness than carbide-bearing lower bainite, and the former also exhibits longer fatigue life under total strain amplitude, the latter exhibits longer fatigue life under plastic strain amplitude. The metastable retained austenite of large size in the carbide-free lower bainite obviously undergoes strain-induced martensitic transformation during deformation. Martensitic transformation delays crack propagation by absorbing the energy required for crack propagation, thus increasing the fatigue life of carbide-free lower bainite under total strain amplitude. The stable carbide and the coarse bainite ferrite plates play positive roles for the fatigue life under plastic strain amplitude, while the fine bainite ferrite plates and metastable retained austenite are negative. The stable secondary carbide phase strengthens carbide-bearing lower bainite under uniaxial tensile deformation but may shorten fatigue life under cyclic fatigue by acting as locations of stress concentration.