Nowadays the aeronautical industry keeps strict quality standards in its dimensional specifications, mechanical properties and microstructural characteristics. Therefore, the involved manufacturing processes require keeping high standards. The nickel based superalloys are present in many components of the jet engines, being the Inconel 718MR superalloy the most common, making up to 50% of the jet engine. This is designed to resist high temperatures, corrosion and creep. The process of rotary forging is a manufacturing process that is currently under scientific and technological development in the aeronautical industry. An Avrami model coupled with a commercial FEM platform (DEFORMTM 3D) was developed to evaluate the average grain size, as a function of the working conditions at 980 °C and 1000 °C. The results provide a better understanding of the influence of temperature in the grain size evolution during the rotary forging process, compared with previous reports.