Thermal treatment of alloys. Types of heat treatment
Heat treatment of alloys is an integralPart 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 production process, semi-finished products,metal parts are heat treated to give them the right properties (strength, resistance to corrosion and wear, etc.). Thermal treatment of alloys is a combination of artificially created processes in which alloys under the action of high temperatures undergo structural and physico-mechanical changes, but the chemical composition of the substance remains.
Purpose of heat treatment
Metal products that are useddaily in any branch of the national economy, must meet the high demands of resistance to wear. Metal, like raw materials, needs to enhance the necessary operational properties, which can be achieved by exposure to high temperatures. Thermal treatment of alloys with high temperatures changes the original structure of the substance, redistributes its constituent components, converts the size and shape of the crystals. All this leads to a minimization of the internal stress of the metal and thus increases its physical and mechanical properties.
Types of heat treatment
Heat treatment of metal alloys is reduced tothree unpretentious processes: heating the raw material (semi-finished product) to the desired temperature, keeping it in the given conditions the necessary time and rapid cooling. In modern production, several types of heat treatment are used, differing in some technological features, but the process algorithm generally remains the same everywhere.
By the method of fulfillment, the thermal treatment is of the following types:
- Thermal (hardening, tempering, annealing, aging, cryogenic treatment).
- Thermo-mechanical includes treatment with high temperatures in combination with mechanical action on the alloy.
- Chemical-thermal implies the thermal treatment of metal with subsequent enrichment of the product surface with chemical elements (carbon, nitrogen, chromium, etc.).
Annealing is a production process in whichmetals and alloys are heated to a specified temperature, and then, together with the furnace in which the procedure took place, very slowly cool naturally. As a result of annealing, it is possible to eliminate the heterogeneity of the chemical composition of the substance, to remove the internal stress, to achieve the granular structure and improve it as such, and to 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 implies thermaltreatment, as a result of which changes in the phase state of the alloy are insignificant or absent altogether. It also has its own variants: homogenized - the annealing temperature is 1100-1200, under these conditions the alloys are held 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 alloy 30-50 abovethe critical temperature point characteristic for a given substance and cooling at the specified rate (200 / h - carbon steels, 100 / h and 50 / h - low-alloy and high-alloy steels, respectively).
- Incomplete - heating to the critical point and slow cooling.
- Diffusion - annealing temperature 1100-1200.
- Isothermal - heating occurs in the same way as during complete annealing, but after this, a rapid cooling is carried out to a temperature somewhat below the critical temperature and left to cool in the air.
- Normalized - complete annealing followed by cooling of the metal in air, and not in the furnace.
Hardening is manipulation with fusion, the purposewhich is the achievement of the martensitic transformation of the metal, which reduces the plasticity of the product and increases its strength. Hardening, as well as annealing, involves heating the metal in the furnace above the critical temperature to the quenching temperature, the difference is in the higher cooling rate that occurs in the bath with the liquid. Depending on the metal and even its shape, different types of hardening are used:
- Hardening in one environment, that is, in one bath with a liquid (water for large parts, oil for small parts).
- Intermittent quenching - cooling takes two consecutive steps: first in a liquid (a sharper cooler) to a temperature of about 300, then in air or in another oil bath.
- Stepwise - when the product reaches the quenching temperature, it is cooled for a time in molten salts, followed by cooling in air.
- Isothermal - by technology is very similar to stepped hardening, differs only in the holding time of the product at the temperature of the martensitic transformation.
- Hardening with self-release differs from other typesin 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 extremely necessary for percussion instruments (hammers, chisels, etc.)
Vacation - this is the final stage of thermalprocessing of alloys, which determines the final structure of the metal. The main purpose of tempering is to reduce the brittleness of the metal product. The principle is to heat the part to a temperature below critical and cooling. Since the modes of heat treatment and the cooling rate of metal products for different purposes may differ, three types of tempering are distinguished:
- High - the heating temperature is from 350-600 to 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 heating the product no higher than 250 allows to achieve high strength and wear resistance of parts.
Aging is the thermal treatment of alloys,which determines the decay of supersaturated metal after quenching. The result of aging is an increase in the limits of hardness, fluidity and strength of the finished product. Not only cast iron is exposed to aging, but also non-ferrous metals, including easily deformable aluminum alloys. If a metal product, subjected to hardening to withstand at normal temperature, processes occur in it, leading to a spontaneous increase in strength and a decrease in ductility. This is called natural aging of metal. If the same manipulation is performed in an elevated temperature, it will be called artificial aging.
Changes in the structure of alloys, and therefore theirproperties can be achieved not only by high, but also by extremely low temperatures. Thermal treatment of alloys at t below zero was called cryogenic. This technology is widely used in a variety of sectors of the national economy as a supplement to heat treatment with high temperatures, since it allows to significantly reduce the costs for the processes of thermal hardening of products.
Cryogenic treatment of alloys is carried out at t-196 in a special cryogenic processor. This technology allows to significantly increase the service life of the treated part and anticorrosive properties, as well as eliminate the need for repeated treatments.
A new method of processing alloys combinestreatment of metals at high temperatures with mechanical deformation of products in a plastic state. Thermomechanical treatment (TMS) by the method of committing can be of three types:
- Low-temperature TEM consists of two stages: Plastic deformation followed by hardening and tempering the part. The main difference from other types of TMO is the heating temperature to the austenitic state of the alloy.
- High-temperature TMO means heating the alloy to a martensitic state in combination with plastic deformation.
- Preliminary - deformation is performed at t 20 with subsequent hardening and tempering of the metal.
Change the structure and properties of alloys is possible andusing chemical-thermal treatment, which combines the thermal and chemical effects on metals. The ultimate goal of this procedure in addition to imparting increased strength, toughness, wear resistance of the product is the addition of a component of acid resistance and fire resistance. This group includes the following types of heat treatment:
- Cementation is carried out to impart a surfaceproducts of additional strength. The essence of the procedure is to saturate the metal with carbon. Cementation can be performed in two ways: solid and gas carburizing. In the first case, the material to be treated, together with the coal and its activator, is placed in an oven and heated to a certain temperature, followed by holding it in a given medium and cooling. In the case of gas carburizing, the product is heated in an oven to 900 under a continuous stream of carbon-containing gas.
- Nitriding is a chemical-thermal treatmentmetal products by saturation of their surface in nitrogen media. The result of this procedure is an increase in the tensile strength of the part and an increase in its corrosion resistance.
- Cyanidation is the saturation of the metal simultaneously with nitrogen and carbon. The medium may be liquid (molten carbon and nitrogen-containing salts) and gaseous.
- Diffusion metallization is aModern method of giving metal products heat resistance, acid resistance and wear resistance. The surface of such alloys is saturated with various metals (aluminum, chromium) and metalloids (silicon, boron).
Peculiarities of heat treatment of cast iron
Cast iron alloys are subject to thermalprocessing by a slightly different technology than alloys of non-ferrous metals. Cast iron (gray, high-strength, alloyed) passes the following types of heat treatment: annealing (at t 500-650), normalization, quenching (continuous, isothermal, surface), tempering, nitriding (gray cast iron), aluminizing (pearlitic cast iron), chrome plating. All these procedures as a result significantly improve the properties of the end products of cast iron: increase the service life, exclude the possibility of cracks when using the product, increase the strength and heat resistance of cast iron.
Heat treatment of non-ferrous alloys
Non-ferrous metals and alloys have an excellent friendfrom other properties, so they are processed by different methods. Thus, copper alloys are subjected to recrystallization annealing to equalize the chemical composition. For brass, a low-temperature annealing technology (200-300) is provided, since this alloy is prone to spontaneous cracking in a moist environment. Bronze is subjected to 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, as well as magnesium, is subjected to three methods of heat treatment: annealing, hardening and aging, after which the deformable aluminum alloys significantly increase their strength. Treatment of titanium alloys includes: recrystallization annealing, hardening, aging, nitriding and carburizing.
Thermal treatment of metals and alloys isthe main technological process, both in black and non-ferrous metallurgy. Modern technologies have a lot of methods of heat treatment, which allow to achieve the desired properties of each type of processed alloys. For each metal has its own critical temperature, which means that the heat treatment must take into account the structural and physicochemical features of the substance. Ultimately, this will not only achieve the desired results, but also greatly streamline production processes.