Metallographic analysis and kinetic equation of the graphitization process of graphitized steel

Graphitized steel can have good machinability and formability, or high strength through controlling microstructure. The graphitization process is formation of graphite particles in graphitized steel, which is key to control the microstructure and properties of the steel. In this paper, the quenched high carbon steel with 0.66% carbon (mass fraction) was graphitized at 650, 680, and 710℃, respectively. The microstructure formed during the graphitization process was analyzed by a field emission scanning electron microscope, electron probe microanalysis, X-ray diffraction, and a transmission electron microscope. According to the dynamic theory of phase transformation, the kinetic curve of the graphitization process was drawn, and the corresponding kinetic equation was established. The results show that in the graphitization process, the quenched martensite is first transformed to the stable state of precipitation carbide. When the carbide is cementite Fe₃C, the precipitation rate of graphite particles increases significantly. The acicular α-Fe in the matrix recrystallizes, and is gradually replaced by equiaxed ferrite. With prolonged graphitization time, the carbon content in ferrite decreases gradually; that is, it changes from a supersaturated state to a stable state. The carbon content increases to the peak value in graphite particles, whereas that of Fe decreases to the valley value. These changes show that the decomposed carbon of cementite, Fe₃C, diffuses into the graphite core, whereas Fe diffuses from the graphite core, and then graphite particles are formed. Additionally, when steel is graphitized, the curve of graphite particle area fraction with time is an S shape; that is, the dynamic process of the tested steel is in accordance with the JMAK (Johnson-Mehl-Avrami-Kolmogorov) equation, and the value of n in the equation is between 1.5 and 1.7.