Abstract
A theoretical model for the presence of bainite (B) in ferrite (α) + pearlite (P) microstructures was validated experimentally for commercial-grade medium-carbon manganese steels. The energies concerning the nucleation and growth of bainitic-α were used as the criteria for the B formation during the α+P transformations, which has been first applied to the changing composition of untransformed austenite (γU). Three steels were chosen to study the effect of Mn content on the B fraction at various cooling rates. To account for local variation of austenite grain size (d0), a log-normal distribution was employed. In this model, C enrichment in γU proceeds faster in smaller grains, and the shift to P or B transformation occurs in a specific temperature range. Calculations predicted the onset of B at around 603 K, with a fraction that increases with increasing cooling rate. In general, the agreement was good for all steels in terms of both calculated/observed kinetics and final α⁄P⁄B fractions. Thus, the model provides a practical prediction of the critical cooling rate in order to avoid B in as-forged product.
