Metallographic Analysis and Kinetic Equation of the Austenization of Graphitized High-Carbon Steel

The solubility of graphite inclusions in the austenite of graphitized steel during its austenization should positively affect its properties. To control the properties of graphitized steel better, the process of its austenization should be studied in details. The subject of research in this work is samples of hot-forged graphitized high-carbon steel (element content, %: 0.66 C, 1.5 Si, 0.43 Mn, 0.004 B, 0.007 N) produced by vacuum induction melting. Austenization was carried out at 850, 880, and 900°C. Metallographic and kinetic analyses demonstrated that, during austenization of steel samples, austenite after nucleation at ferrite grain boundaries and interfaces between graphite inclusions and ferrite grains grows mainly toward ferrite; after complete transformation of ferrite into austenite, the carbon of graphite inclusions continues to dissolve in austenite and the austenite components are homogenized. Microscopic pores after austenization are the result of dissolution of graphite inclusions: the morphology and size of the microspores are similar to those of the initial graphite inclusions. The austenization of the tested steel corresponds to the kinetic JMAK model (Johnson–Mehl–Avrami–Kolmogorov equation); n ≈ 1.8 in the equation shows that the interface is an effective austenite nucleation site. According to the Arrhenius equation, the activation energy of austenite transformation in this steel is 207.16 kJ/mol.