Graphite Precipitation from Austenite in Hypo-Eutectoid Steel

Transformation behavior of structural steel S50C during isothermal treatment was compared with that of hypo-eutectoid alloy steels containing silicon, nickel and cobalt, focusing on a graphite precipitation, and a nucleation mechanism of graphite that precipitated from austenite was discussed according to the classical nucleation theory and the theory of reaction kinetics.It was suggested that the machanism of graphite nucleation in the steels containing silicon, nickel and cobalt consists of a series of processes such as clustering of solute carbon atoms, the diffusion of iron atoms and the formation of critical nuclei. The clustering of solute carbon atoms is stimulated by the addition of silicon, nickel and cobalt, because the activity of carbon is raised by the existence of these atoms.The rate of graphite nucleation mainly depends on the free energy of formation of critical nuclei just below the eutectoid temperature of the iron-graphite system, and on the diffusion rate of iron atoms at lower temperatures. The phenomenon that the curve for graphite precipitation in TTT relationship results in a “C” curve was successfully explained by this temperature dependence of graphite nucleation rate.