During modern surface heat treatments of steels such as laser or induction hardening and tempering, heating rates up to 5000 °C/s or even more can occur in the material. These high heating rates affect the development of the microstructure which in turn affects the material properties. This study presents the results of the investigation of the tempering process of martensitically hardened AISI 4140 at different heating rates up to 1200 K/s. The kinetics of the occurring precipitations were modeled using a modified JMAK equation. The influence on the final microstructure after tempering with different heating rates is investigated using SEM images. The hardness of the different samples was measured in order to show the influence of the heating rate on the mechanical properties. Finally, a comparison to conventional tempering is given.