Martensitic microstructures in steels provide the strength and toughness required for the dynamic loads experienced by construction and mining machines. Such microstructures are produced with appropriate heat treatments. A physics based model has been developed to represent the microstructure evolution during the martensitic transformation. This modeling has been used to understand the role of as-quenched microstructure on subsequent processing. This paper describes modeling the martensitic transformation in steels under different cooling rates. The model described in this paper has been validated with a medium carbon, low alloy steel.