In this paper the austenitizing phase of differential quenching treatment for large steel forgings is presented. Time and temperature are the variables that could be industrially-controlled in each heat treatment; so it is working on them that makes possible to optimize the process and then to maximize the final product quality. The analyzed products are the forged back-up rolls: they are characterized by an accurate thermal cycle after plastic deformation. Actually, after a preliminary thermal cycle realized to facilitate the rough machining operations, they are differentially quenched in order to increase the working layer hardness, which thickness is function of customer’s specifications. Numerical analysis and, more precisely, FEM simulation, is a useful forecast tool to improve a scientific approach in industrial criticality resolution, especially when product and laboratory samples dimensions ratio is very high like in this case. In this field the obtainable results from a FEM analysis are thermal field, phase distribution and thermal induced stress distribution after heat treatment. In this case of study the FEM software used is Forge 2011.