Heat treatment of alloys is an integral part of the production process of ferrous and non-ferrous metallurgy. As a result of this procedure, metals are able to change their characteristics to the required values. In this article, we will consider the main types of heat treatment used in modern industry.
The essence of heat treatment
In the manufacturing process, semi-finished products, metal parts are subjected to heat treatment to give them the desired properties (strength, resistance to corrosion and wear, etc.). Heat treatment of alloys is a set of artificially created processes during which structural and physical-mechanical changes occur in alloys under the influence of high temperatures, but the chemical composition of the substance is preserved.
Purpose of heat treatment
Metal products that are used daily in any sector of the national economy must meet the high requirements for wear resistance. Metal, as a raw material, needs to be strengthened by the required operational properties, which can be achieved by exposure to high temperatures. Heat treatment of alloys with high temperatures changes the initial structure of a substance, redistributes its constituent components, and transforms the size and shape of crystals. All this leads to minimizing the internal stress of the metal and thus increases its physical and mechanical properties.
Types of heat treatment
The heat treatment of metal alloys boils down to three simple processes: heating the raw material (semi-finished product) to the desired temperature, keeping it under specified conditions for the required time, and rapid cooling. In modern production, several types of heat treatment are used, which differ in some technological features, but the process algorithm generally remains the same everywhere.
By the method of making heat treatment is of the following types:
- Thermal (hardening, tempering, annealing, aging, cryogenic treatment).
- Thermo-mechanical includes high temperature treatment in combination with mechanical action on the alloy.
- Chemical-thermal involves the heat treatment of metal with subsequent enrichment of the surface of the product with chemical elements (carbon, nitrogen, chromium, etc.).
Annealing
Annealing is a manufacturing process in which metals and alloys are heated to a predetermined temperature, and then, together with the furnace in which the procedure took place, cool very naturally in a natural way. As a result of annealing, it is possible to eliminate inhomogeneities in the chemical composition of the substance, relieve internal stress, achieve a granular structure and improve it as such, as well as reduce the hardness of the alloy to facilitate its further processing. There are two types of annealing: annealing of the first and second kind.
Annealing of the first kind involves heat treatment, as a result of which changes in the phase state of the alloy are insignificant or absent altogether. It also has its own varieties: homogenized - the annealing temperature is 1100-1200, under such conditions the alloys can withstand for 8-15 hours, recrystallization (at t 100-200) annealing is used for riveted steel, that is, deformed already being cold.
Annealing of the second kind leads to significant phase changes in the alloy. It also has several varieties:
- Full annealing - heating the alloy 30-50 higher than the critical temperature characteristic for a given substance and cooling at the indicated speed (200 / hour - carbon steels, 100 / hour and 50 / hour - low alloy and high alloy steels, respectively).
- Incomplete - heating to a critical point and slow cooling.
- Diffusion - annealing temperature 1100-1200.
- Isothermal - heating occurs in the same way as with complete annealing, however, after this, rapid cooling is carried out to a temperature slightly below critical and left to cool in air.
- Normalized - complete annealing followed by cooling of the metal in air, and not in a furnace.
Quenching
Quenching is the manipulation of an alloy, the purpose of which is to achieve martensitic transformation of metal, which ensures a decrease in the ductility of the product and an increase in its strength. Quenching, as well as annealing, involves heating the metal in the furnace above the critical temperature to the quenching temperature, the difference is the greater cooling rate that occurs in the bath with the liquid. Depending on the metal and even its shape, different types of hardening are used:
- Quenching in one medium, that is, in the same bath with liquid (water - for large parts, oil - for small parts).
- Intermittent quenching - cooling takes place in two successive stages: first in a liquid (sharper cooler) to a temperature of about 300, then in air or in another bath with oil.
- Stepwise - when the product reaches the quenching temperature, it is cooled for some time in molten salts, followed by cooling in air.
- Isothermal - according to the technology it is very similar to step hardening, it differs only in the exposure time of the product at the temperature of martensitic transformation.
- Hardening with self-tempering differs from other types in that the heated metal is not completely cooled, leaving a warm section in the middle of the part. As a result of such manipulation, the product acquires the properties of increased strength on the surface and high viscosity in the middle. This combination is essential for percussion instruments (hammers, chisels, etc.)
Vacation
Vacation is the final stage of the heat treatment of alloys, which determines the final structure of the metal. The main purpose of tempering is to reduce the fragility of a metal product. The principle is to heat the part to a temperature below critical and cool. Since the modes of heat treatment and the cooling rate of metal products for various purposes can differ, there are three types of tempering:
- High - heating temperature from 350-600 to a value below critical. This procedure is most often used for metal structures.
- Medium - heat treatment at t 350-500, common for spring products and springs.
- Low - the temperature of the product not higher than 250 allows to achieve high strength and wear resistance of parts.
Aging
Aging is the heat treatment of alloys that causes the decomposition of a supersaturated metal after quenching. The result of aging is an increase in the hardness, yield strength and strength of the finished product. Not only cast iron, but also non-ferrous metals, including easily deformable aluminum alloys, undergo aging. If the metal product subjected to hardening is maintained at normal temperature, processes occur in it that lead to a spontaneous increase in strength and a decrease in ductility. This is called natural aging of the metal. If the same manipulation is done under conditions of elevated temperature, it will be called artificial aging.
Cryogenic treatment
Changes in the structure of alloys, and hence their properties, can be achieved not only at high, but also at extremely low temperatures. The heat treatment of alloys at t below zero is called cryogenic. This technology is widely used in various sectors of the national economy as a complement to heat treatments with high temperatures, since it can significantly reduce the cost of heat hardening products.
Cryogenic processing of alloys is carried out at t -196 in a special cryogenic processor. This technology can significantly increase the service life of the machined part and the anti-corrosion properties, as well as eliminate the need for repeated treatments.
Thermomechanical treatment
A new method of processing alloys combines the processing of metals at high temperatures with the mechanical deformation of products in a plastic state. Thermomechanical processing (TMT) by the method of commissioning can be of three types:
- Low-temperature TMT consists of two stages: plastic deformation with subsequent hardening and tempering of the part. The main difference from other types of TMT is the temperature of heating to the austenitic state of the alloy.
- High-temperature TMT involves heating the alloy to a martensitic state in combination with plastic deformation.
- Preliminary - deformation is performed at t 20 with subsequent quenching and tempering of the metal.
Chemical heat treatment
It is possible to change the structure and properties of alloys with the help of chemical-thermal treatment, which combines thermal and chemical effects on metals. The ultimate goal of this procedure, in addition to imparting increased strength, toughness, and wear resistance to the product, is also to impart acid and fire resistance to the part. This group includes the following types of heat treatment:
- Cementation is carried out to give the surface of the product extra strength. The essence of the procedure is to saturate the metal with carbon. Cementation can be performed in two ways: solid and gas cementation. In the first case, the processed material, together with coal and its activator, is placed in a furnace and heated to a certain temperature, followed by its exposure to this medium and cooling. In the case of gas cementation, the product is heated in the furnace up to 900 under a continuous stream of carbon-containing gas.
- Nitriding is the chemical-thermal treatment of metal products by saturating their surface in nitrogen media. The result of this procedure is to increase the tensile strength of the part and increase its corrosion resistance.
- Cyanidation - saturation of a metal at the same time with nitrogen and carbon. The medium may be liquid (molten carbon and nitrogen salts) and gaseous.
- Diffusion metallization is a modern method of imparting heat resistance, acid resistance and wear resistance to metal products. The surface of such alloys is saturated with various metals (aluminum, chromium) and metalloids (silicon, boron).
Features of heat treatment of cast iron
Cast iron alloys are heat treated using a slightly different technology than non-ferrous metal alloys. Cast iron (gray, high-strength, alloyed) undergoes the following types of heat treatment: annealing (at t 500-650), normalization, hardening (continuous, isothermal, surface), tempering, nitriding (gray cast iron), aluminization (pearlitic cast iron), chromium plating. All these procedures as a result significantly improve the properties of the final cast iron products: increase the service life, eliminate the likelihood of cracks when using the product, and increase the strength and heat resistance of cast iron.
Heat treatment of non-ferrous alloys
Non-ferrous metals and alloys have excellent properties, therefore they are processed by different methods. So, copper alloys are subjected to recrystallization annealing to equalize the chemical composition. The technology of low temperature annealing (200-300) is provided for brass, since this alloy is prone to spontaneous cracking in a humid environment. Bronze undergoes homogenization and annealing at t up to 550. Magnesium is annealed, hardened and subjected to artificial aging (natural aging does not occur for hardened magnesium). Aluminum, like magnesium, undergoes three heat treatment methods: annealing, hardening and aging, after which wrought aluminum alloys significantly increase their strength. Processing of titanium alloys includes: recrystallization annealing, quenching, aging, nitriding and cementation.
Summary
The heat treatment of metals and alloys is the main technological process, both in ferrous and non-ferrous metallurgy. Modern technologies have many heat treatment methods that allow you to achieve the desired properties of each type of processed alloys. Each metal has its own critical temperature, which means that heat treatment should be carried out taking into account the structural and physico-chemical characteristics of the substance. Ultimately, this will not only achieve the desired results, but also significantly streamline production processes.