The structure stability of carbide-free bainite wheel steel

Metallographic structures of carbide-free bainite steel wheel rim are mainly composed of supersaturated lath ferrite and retained austenite film among bainitic ferrite laths. It is suspected that supersaturated ferrite and retained austenite are likely to decompose under the influence of temperature change and mechanical stress. Stability of wheel rim structure is studied by means of x-ray diffraction, dye microscopy, and micro-hardness test. When the samples are tempered in the range of 150-350 degrees C, the retained austenite films are at the state of relative stability. Fifty percent of retained austenite is decomposed when the sample is tempered at 400 degrees C. Microhardness increases when the sample is tempered at 150 degrees C. The decrease in hardness is mild when the samples are tempered from 200 to 500 degrees C. The mechanical stability of retained austenite film is studied with tensile sample under the effect of tensile stress. The retained austenite appears to be stable in low and middle degree of deformation, and decomposition occurs at great amount of deformation. Diffraction peak of carbide is not found in all above experiments. The steel enriched silicon prevents the carbide precipitation during the transformation. It indicates the carbide-free bainite wheel steels have an excellent thermal and mechanical stability.