I In the industrial world, steel is a component that has extensive functions and benefits because its role is very vital, so it is very important for us to understand the characteristics of the steel to be used. To obtain the mechanical properties and quality of steel as expected, it is necessary to carry out a heat treatment process. In this SLEIPNER steel, the heat treatment process carried out is hardening and aging. This heat treatment process is preceded by initial heating at a temperature of 850°C with a holding time of 30 minutes then the temperature is increased to the austenitization point with a temperature variation of 1035°C, 1055°C, 1070°C held for 30 minutes, then cooled in oil media for 5 minutes. Continued with a double aging process with a temperature of 580°C and a holding time of 120 minutes. The tests carried out include hardness testing using the Rockwell C method and microstructure observation by photographing using an optical microscope. It is hoped that this research will be able to determine the characteristics of steel and improve its mechanical properties according to the required hardness value, in this case the target requested as a material for making coin molds (coining dies) is 52 HRc.