Prediction of Bainite Intervention in Ferrite-Pearlite Forging Steel I. Modeling

The successive ferrite (α) + pearlite (P) transformations from austenite (γ) have been modeled to predict the intervention of bainite in as-forged medium-carbon manganese steels. The kinetics of the diffusional transformations were calculated based on the classical nucleation-growth theory coupled with the CALPHAD multi-component thermodynamics. The description of growth rate of proeutectoid-α includes a time dependency due to the carbon enrichment in remaining γ, and the γ/α interface has been assumed to be in the non-partitioned local equilibrium (NPLE). The kinetics calculation of pearlite (P) nucleated on the α surface has been integrated to the α model. Considering the transformation temperature range in continuous cooling, the growth rate of P has also been 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 these 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.