2009 Volume 50 Issue 3 Pages 551-555
The successive ferrite (α) + pearlite (P) transformations from austenite (γ) were modeled to predict the presence of bainite in as-forged medium-carbon manganese steels. The kinetics of diffusional transformations were calculated based on classical nucleation and growth theory coupled with CALPHAD multi-component thermodynamics. The description of the growth rate of proeutectoid-α includes a time dependence due to the carbon enrichment in the remaining γ. The γ/α interface was assumed to be in negligible-partitioned local equilibrium (NPLE). The kinetics calculation of P nucleating on the α surface was integrated into the α model. Given the transformation temperature range in continuous cooling, the growth rate of P was also expressed in the NPLE constraint for γ/cementite. The concentration of untransformed γ (γU) can be monitored and should be dependent on the extent of the preceding transformations. Thus, the energies available for the nucleation and diffusionless growth of bainitic-α were evaluated from the thermodynamics of the γU single-phase system, which is proposed as a method to predict the inclusion of bainite in the final α+P microstructure.