Since many years, induction hardening has been successfully applied for the heat treatment of components, mainly in the aeronautical and automotive sectors, because of its peculiar advantages like high quality and repeatability of process and its easy automation. A multi-scale multiphysical finite element (FE) analysis is presented in this paper for the prediction of microstructural evolution during induction hardening processes. An ad hoc code has been developed in order to calculate the metallurgical phase changes that occur during heating and cooling steps. This routine has been coupled with commercial FEM codes able to solve the coupled electromagnetic and thermal problem that typically describes the induction heating processes. During the heating, the magnetic field generated by the coil induces currents in the workpiece and as consequence the heating of conductive material by Joule effect. In induction hardening of steels, an external layer of the piece is heated up to the austenitization temperature, then it is cooled down to obtain a layer of martensite. In thermo-metallurgical model, material properties depend on the temperature distribution but also on the microstructure since the material is a mixture of different phases. From the solution of the coupled steady-state, at a given frequency, electromagnetic and transient thermal problem, temperature distribution as well as heating and cooling rates are used for the evaluation of the existing metallurgical phases at every time step. The effect of latent heat of solid-solid phase transformations has been also considered.The model developed has been applied on a real complex case,e.g. an aeronautical spur gear, in order to predict the phase transformation during the whole process.The numerical results will be verified through experimental validation.