The asynchronous dual-frequency (ADF) induction hardening process was simulated based on the electromagnetic-thermal-metallic-mechanical coupled numerical model. Calculations of microstructure fraction and internal stress were introduced through the calculations of electromagnetic and temperature fields during heating, as well as through the calculation of temperature field during subsequent quenching. The isoconversional method and K-M equation were used to calculate the fraction of austenite and martensite, respectively. A thermal elastoplastic constitutive model associated with the transformation stress and transformation induced plasticity was developed to describe residual stress. The ADF induction hardening process for a 42CrMo spur gear of was studied to determine the microstructure and residual stress distributions. The cause of the gear tooth root hardening under medium frequency heating was revealed from the simulated temperature and microstructure results. The stress evolution showed that quenching cracks were most likely to occur at the beginning of martensite transformation. The effect of the residual stress distribution on the initiation and prolongation of the bending crack at gear root was analyzed in combination with the simulation of the gear mesh.