In this paper, the processes of microstructure formation in 40С2 steel depending on various cooling rates were studied. The study began with constructing a polythermal section of the equilibrium phase diagram for the given chemical composition of steel using Thermo-Calc software. This made it possible to determine the temperatures at the start and end of phase transformations. Next, the microstructure of steel was examined at various cooling rates: 1, 10, 20, 50 and 100 °C/s. Each of these cooling rates led to the formation of different structural elements in steel, such as ferrite, pearlite and martensite. These structural changes directly affect the mechanical properties of the material, such as strength and hardness. As a result, a thermokinetic diagram of austenite decomposition for 40С2 steel was constructed. This diagram reflects the relationship between temperature and time required to transform austenite into more stable phases at various cooling rates. The results of the study are of significant importance for understanding and optimizing heat treatment technologies in accordance with the requirements for the specified mechanical properties of the final product.