The aim of this work is to describe the thermochemical processes of steel processing in order to achieve the desired properties and their application. When talking about thermochemical processing, the change in the composition, structure and properties of the surface layers of the object is conditioned by simultaneous thermal and chemical action. Heat treatment is a procedure in which the object is deliberately subjected to temperature-time cycles with the aim of achieving the desired microstructure, and thus the desired properties. Cementation is a thermochemical treatment of steel whose goal is to enrich the surface layers of the object with carbon. It consists of carburizing, tempering and low-temperature tempering. Nitriding is a thermochemical treatment of steel whose goal is to enrich the surface layers of the object with nitrogen. It consists of hardening, high-temperature softening and nitriding. Boring is a thermochemical treatment of steel, the aim of which is to enrich the surface layers of the object with boron (diffusion), whereby iron borides of extremely high hardness and wax resistance to abrasive wear are formed.