Kinetic study of austenite formation during continuous heating of unalloyed ductile iron

The austenite formation kinetics in unalloyed cast ductile iron was studied on the basis of dilatometry measurements, and Avrami's equation was used to estimate the material's kinetic parameters. A continuous heating transformation diagram was constructed using heating rates in the range of 0.06 to 0.83℃·s^-1. As the heating rate was augmented, the critical temperatures, A_c1 and A_α, as well as the intercritical range, which was evaluated as the difference between the critical temperatures, ΔT=A_α - A_c1, increased. At a low heating rate, the kinetics of austenite formation was slow as a consequence of the iron's silicon content. The effect of heating rate on k and n, the kinetic parameters of Avrami's equation, was also determined. Parameter n, which is associated with nucleation sites and growth geometry, decreased with an increase in heating rate. In addition, parameter k increased with the increase of heating rate, suggesting that the nucleation and growth rates are carbon- and silicon-diffusion controlled during austenite formation under continuous heating.