Heat treatment represents a set of operations of heating, cooling and aging metal hard alloys to obtain the necessary properties due to changes in structure and internal structure. Heat treatment is used as an intermediate operation in order to improve machinability by cutting, pressure, or as the final process operation that provides the required level of part properties. During heat treatment, an operation such as billet heating, which is very responsible. The final quality of the product depends on proper heating. When conducting heating, it is important to take into account that the metal changes its structure, properties and characteristics of the surface layer of the metal, as well as the interaction of the metal with atmospheric air forms scale. The thickness of the scale depends on the chemical composition of the metal itself, temperature and duration of heating. All steels are oxidizable in different ways.. Alloy steels, for example, form a dense thin layer of scale that protects the metal from further oxidation. Carbon steel they lose carbon from the surface layer 2-4 mm thick, which threatens the metal with a decrease in the hardness and strength of steel. High alloyed and high carbon steels are heated slowly to prevent cracking.. Lamely steel is called heat-resistant, because it is practically not subject to oxidation. When heating the metal, it is necessary to take into account the temperatures for the beginning and end of the forging, because severe overheating of the metal leads to burnout, due to which there is a violation of the bonds between the grains and the complete destruction of the metal. Among the main types of heat treatment it should be noted: 1. Annealing – metal processing, in which the metal is first heated, then slowly cooled in a furnace. Annealing can be of different types and depends on the heating temperature, the cooling rate of the metal. 2. Quenching. It is a type of heat treatment of various materials (metals, glass, alloys), consisting in heating them above the critical temperature with rapid subsequent cooling. It is performed to obtain nonequilibrium structures with an increased cooling rate. Quenching can be both with polymorphic transformation, and also without polymorphic transformation. 3. Vacation – This is a technological process, the essence of which is the heat treatment of martensite hardened metal or alloy, the main processes in which are the decay of martensite, recrystallization and polygonization. Vacation is carried out after hardening, in order to relieve internal stresses, to give the material the necessary operational and mechanical properties. 4. Normalization. In this case, the product is heated to an austenitic state and then cooled in calm air. As a result of normalization, internal stresses are reduced, and steel is recrystallized. Compared to annealing, normalization is a shorter and more productive process. There are also such types of heat treatment as chemical-heat treatment, cementation, nitriding, nitrocarburizing (cyanidation), steel boronation. Heat treatment of metals is carried out in furnaces.
