Hardening is a heat treatment process of steel, the purpose of which is to achieve a martensitic structure in order to obtain the maximum possible hardness depending on the carbon content in the steel and to make it as uniform as possible. It consists of heating the workpiece to the austenitization temperature, holding it for a certain time at that temperature, and subsequent quenching in order to achieve a hard martensitic microstructure. Hardening increases the hardness of steel, thus allowing it to be widely used in industry, especially in cutting tools, gears, and other machine parts where wear resistance is important. Process parameters, such as austenitizing temperature, holding time, and quenching agent, are carefully controlled to achieve the desired hardness and mechanical properties for a particular application. In this paper, the hardening process is described and the Fe-Fe3C diagram, Upton diagram, TTT diagram and Burns diagram are presented and explained, which are essential for understanding and properly performing the hardening process.