This is a numerical and experimental study of the hardness curves in the precipitation layer of gaseous nitrided 31CrMoV9 alloy steel. Hardness has been measured with an automatic micro-hardness meter according to the Vickers method. It is known that gas nitriding as a diffusion process can be described by the differential equation known as Fick's second law. This equation is solved with the Finite Element (FE) method. Modeling is based on the Orowan strengthening theory and the classic Johnson-Mehl-Avrami theory of precipitation. The model is constructed in the ANSYS application in the temperature field and experimental results are compared with theory (the model). The results prove that diffusion processes like nitriding can be modeled with FE methods.