Steel is one of the most important and most used materials today. Properties such as high strength, toughness and hardness provide a wide range of applications. In the basic variant, steel is defined as iron and carbon alloy, but with the addition of certain elements with change in its properties is achieved. The basic division of the steel is based on the carbon content: low-carbon, medium-high and high-carbon. The greatest advantage lies in changing the crystal and chemical structure that is achieved by various methods. Heat treatment is one of the processes which changing the temperature of the material changes the microstructure and properties. The subject of the thesis research is the theoretical description of heat treatment and application on the machine part KS.17-DT.83. The machine part is a special ball bearings inner ring. It is necessary to increase the hardness of the surface in order to prevent wear of the ring due to its operation. Thermal treatment of carburization is applied. The heat treatment process is determined by the chemical composition of the steel, and by the application of a particular element. Carburising is part of the thermal-chemical treatment of steel by changing the chemical composition in the surface layers, with the aim of achieving hardness on the surface and core toughness. It consists of carburising, tempering and low temperature tempering. The first part of the thesis describes the basic mechanisms of phase conversion in steel, types of thermal processes. The central part further describes the thermal process of carburising, and further follows the practical work of machining the machine part KS.17-DT.83 as well as tests to improve its hardness to determine the performance of the heat treatment.