Thermomechanical processing. Chemical-thermal and thermomechanical processing Thermomechanical processing of metals and alloys

Thermomechanical processing includes plastic deformation, which affects the formation of the structure during thermal treatment of the metal. Plastic deformation changes the nature of the distribution and increases the density of defects in the crystal lattice, which, in turn, strongly affects the nature of structure formation during phase transformations. Thus, after TMT, a structure with an increased density of defects in the crystal structure is formed in the alloy, which leads to the formation of new mechanical properties.

For steel, two types of thermomechanical treatment are mainly used - low-temperature and high-temperature.

During LTMT, supercooled austenite is deformed in the region of its increased stability, but necessarily below the temperature of the onset of recrystallization. After that, it turns into martensite (Fig. 53). As a final heat treatment, low tempering is carried out.

The reason for the hardening of steel during LTMT is the inheritance of the dislocation structure of deformed austenite by martensite. Dislocations do not disappear during the formation of martensite, but are transferred from the initial phase to the new one, i.e. martensite inherits the deformed austenite substructure. The high density of dislocations fixed by carbon atoms and carbide inclusions results in high strength with an acceptable level of plasticity.

Rice. 53 Scheme of low temperature (LTMO)

thermomechanical processing of steel

LTMT is applicable only for alloy steels with a sufficient level of stability of supercooled austenite. In addition, LTMT requires powerful deforming equipment.

During HTMT, austenite is deformed in the region of its high-temperature stability, and then quenched for martensite (Fig. 54). Hardening is followed by low tempering.

Rice. 54 Scheme of high temperature (HTMT)

thermomechanical processing of steel.

The HTMT mode is chosen so that by the beginning of the martensitic transformation, the austenite has a developed polygonized structure. The degree of deformation should not be too large, so as not to cause recrystallization that reduces the hardening. After the end of deformation, immediate quenching is necessary to prevent static recrystallization and maintain the deformed structure by the beginning of the martensitic transformation. Martensite crystals do not go beyond the austenite subgrains, which causes their significant grinding and obtaining a high set of properties.

The most important advantage of HTMT is the ability to simultaneously increase both strength and fracture toughness. In addition, HTMT does not require powerful specialized equipment.

6. Chemical-thermal treatment of steel

6.1. general characteristics chemical-thermal treatment of steel

Chemical-thermal treatment (CHT) is the surface saturation of steel with some chemical elements, namely non-metals and metals (for example, carbon, nitrogen, aluminum, chromium, etc.) by their diffusion in the atomic state from external environment at high temperature. In the course of these processes, the chemical composition, microstructure and properties of the surface layers of products necessarily change. In CTO, the workpieces are heated in any chemically active environment. The main processing parameters are the heating temperature and the holding time. CTO is usually carried out for long time. The process temperature is chosen specifically for each type of processing.

The primary processes of any type of CTO are dissociation, absorption and diffusion.

Dissociation - decomposition of a chemical compound to obtain chemical elements in a more active, atomic state. Absorption - absorption by the surface of the part of the atoms of the specified non-metals. Diffusion - movement of the absorbed element deep into the product. The speeds of all three processes must necessarily be consistent with each other. For absorption and diffusion, it is necessary that the saturating element interact with the base metal to form either a solid solution or a chemical compound, since in the absence of this, chemical-thermal treatment is impossible.

The main types of chemical-thermal treatment of steel are carburizing, nitriding, nitrocarburizing, cyanidation and diffusion metallization.

The rate of diffusion of atoms into the iron lattice varies and depends on the composition and structure of the resulting phases. When saturated with carbon or nitrogen, which form interstitial solid solutions with iron, diffusion proceeds faster than when saturated with metals that form substitutional solid solutions. Therefore, in this case, higher temperatures and longer processing times are used, but despite this, a thinner layer thickness is obtained than with nitriding and especially carburizing.

When determining the thickness of the diffusion layer obtained by saturating steel with one or another element, usually not its full value with a changed composition is indicated, but only the depth to a certain hardness or structure (effective thickness).

As a rule, one of the last stages in the manufacture of a steel product is heat treatment. Heating to the required temperature with further cooling leads to significant changes in the internal structure of the metal. As a result, it acquires new properties that directly depend on the selected thermal regimes. Heat treatment of steel allows you to change its hardness, brittleness and toughness, as well as make it resistant to deformation, wear and tear. The main types of heat treatment include quenching, tempering and annealing. In addition, there are combined methods: chemical-thermal and thermomechanical treatments that combine heating and cooling with other types of influence on the metal structure. With all the variety of basic types and their varieties, the essence of all these technologies is the same - changing the internal phase and structural states of the metal in order to give it the required properties.

The main task of heat treatment of a steel product is to give it the required performance quality or a combination of such qualities. During heat treatment of cutting tools made of tool and alloy steels, a hardness of 63 HRC and increased wear resistance are achieved. And the impact tool after it should have a hard surface layer and a plastic impact-resistant core. Steels for the manufacture of springs and spring plates after heat treatment become bending strong and elastic, and metal for rails becomes resistant to deformation and wear. In addition, thermal methods are used to harden the surface layers of steel products by saturating them at high temperatures with carbon, nitrogen or other compounds, and also by hardening hardening after hot pressure working. Another purpose of heat treatment is the restoration of the original properties of the metal, which is achieved by annealing them.

Benefits of metal heat treatment

Heat treatment radically changes the operational properties of metals, using only the internal rearrangement of their crystal lattices. By alternating heating and cooling cycles, it is possible to significantly increase the hardness, wear resistance, ductility and impact strength of the product. In addition, heat treatment makes it possible to make structural changes only in the surface layer to a given depth or to affect only part of the workpiece. The combination of heat treatment with hot pressure treatment leads to a significant increase in the hardness of the metal, exceeding the results obtained separately during work hardening or hardening. With chemo- heat treatment the surface layer of the metal is saturated by diffusion with chemical elements, which significantly increase its wear resistance and hardness. At the same time, the main part of the product retains viscosity and plasticity. From a production point of view, heat treatment equipment is much simpler and cheaper than machine tools and installations in machining and foundry industries.

The principle of heat treatment

Heat treatment of metals is based on phase changes internal structure occurring when they are heated or cooled. AT general view The heat treatment process consists of the following steps:

heating, which changes the structure of the crystal lattice of the metal;
cooling, fixing the changes achieved during heating;
vacation, renting mechanical stresses and ordering the resulting structure.

A feature of steel heat treatment technology is that when heated to 727 ºC, it passes into the state of a solid melt - austenite, in which carbon atoms penetrate into the elementary cells of iron, creating a uniform structure. With slow cooling, the steel returns to its original state, and with fast cooling, it is fixed in the form of austenite or other structures. The properties of hardened steel depend on the method of cooling and further tempering. The principle is observed here: the faster the cooling and the lower the temperature, the higher its fragility and hardness. Heat treatment is one of the key technological processes for all iron-carbon alloys. For example, it can only be obtained by heat treatment of white cast iron.

Types of heat treatment of steel

Each type of heat treatment operations belongs to a certain group in accordance with its belonging to technological stage. The preliminary ones include normalization and annealing, the main ones are various methods of hardening and processing with heating, and the final ones are tempering in various media. Such a division of thermal operations is to some extent conditional, since sometimes tempering is carried out at the beginning of heat treatment, and normalization and annealing at the end. Hot metal working technology includes heating, holding operating temperature during the required period and cooling at a given rate. In addition, to improve the wear resistance of alloy steel products, cold heat treatment is used with the workpiece immersed in a cryogenic medium with cooling below -150 ºC.

Annealing

The main feature of annealing is the heating of products to a high temperature and very slow gradual cooling. Such thermal regimes contribute to the formation of a uniform crystal structure and the complete removal of residual stresses. Depending on the type of metal and the desired result, annealing is divided into the following types:

Diffusion. The part is heated to a temperature of about 1200 ºC, and then gradually cooled for tens of hours (for massive products - up to several days). Typically, such heat treatment eliminates dendritic inhomogeneities in the steel structure.
Full. The billet is heated beyond the critical point of austenite formation (727 ºC) followed by slow cooling. This type of annealing is the most commonly used and is applied mainly to structural steel. Its result is a reduction in the graininess of the crystal structure, an improvement in its plastic properties and a decrease in hardness, as well as the removal of internal stresses. Full annealing is sometimes used before hardening to reduce the grain size of the metal.
Incomplete. In this case, heating occurs to a temperature above 727 ºC, but not more than 50 ºC. The result with such annealing is practically the same as with complete annealing, although it does not provide a complete change in the crystal structure. But it is less energy-intensive, performed in a shorter period, and less scale is formed on the part. Such heat treatment is used for tool and similar steels.
Isothermal. Heating is carried out to a temperature slightly exceeding 727 ºC, after which the product is immediately transferred to a bath with a melt at 600÷700 ºC, where it is kept for a certain time until the formation of the required structure is completed.
Recrystallization. This kind of heat treatment is only used to eliminate work hardening after broaching, stamping, drawing, etc. In this case, the steel part is subjected to thermal heating below 727 ºC, kept in this state for a certain time, and then slowly cooled.
Spheroidizing. special kind annealing applied to high-carbon steels (more than 0.8%), during which the pearlite structure is transformed from lamellar to granular (spherical).

Another fairly common application of annealing, both in industry and in home workshops, is the restoration of the original properties of steel after unsuccessful hardening or trial heat treatment.

hardening

Hardening is the central link in most heat treatment processes, since it is it that ensures the required performance of the hardened metal. Hardening includes three main stages: heating the product above 727 ºC, maintaining the set temperature until the formation of the required crystal structure is completed, and rapid cooling to fix the result. The main technological parameters during hardening are the heating and cooling temperatures, as well as the speed of these thermal processes. The heating temperature of low-carbon (up to 0.8%) steel directly depends on the percentage of carbon (see the graph below): the lower it is, the more the product needs to be heated. For tool steels, heating 30÷50 ºC above 727 ºC is sufficient. The parameters of heat treatment of alloyed steels are highly dependent on their composition, so the choice of temperature regimes for them must be made according to technological reference books.

The heating rate during heat treatment depends entirely on the steel grade, the mass and shape of the part, the type of heat source and the desired result. Therefore, it can be selected either from reference tables or only empirically. The same applies to the cooling rate, which also depends on the listed characteristics. When choosing a cooling medium, they are primarily guided by the cooling rate, but at the same time, its other features are also taken into account. First of all, these include the stability and harmlessness of its composition, as well as the ease of removal from the surface of the product. In addition, during the operation of pumping and mixing equipment used in heat treatment, such characteristics as viscosity and fluidity are important.

Vacation

Vacation is, as a rule, the finishing operation of the heat treatment of the product. It is produced after hardening to relieve residual stresses in steel and reduce its brittleness, as well as increase toughness and resistance to shock loads. During tempering, the part is heated to a temperature below 727 ºC and then slowly cooled in air. Depending on the temperature ranges used, the following types of holidays are usually distinguished:

Short. Heating is carried out up to 200 ºC. Such tempering is applied to cutting tools and hardened steels to maintain high hardness and wear resistance.
Average. Products are heated to a temperature of 300÷450 ºC. This type of tempering is used to increase the elasticity and fatigue resistance of spring and spring steels.
High. The heating range is 460÷710 ºC. Heat treatment, which includes hardening with high tempering, is called improvement by thermists, because in this case the best ratio of ductility, wear resistance and toughness is achieved.

During low-temperature thermal heating, the metal is covered with colored oxide films, which change their color depending on the temperature from pale yellow to grayish-gray. This is a fairly reliable indicator of how hot a part is, and many people temper based on the color of the tint.

Chemical-thermal treatment

One of the varieties of combined heat treatment is high-temperature saturation of the upper layer of metal with chemicals that increase its hardness and wear resistance. Depending on the composition of the compounds used for such saturation, chemical-thermal treatment of steel is divided into the following types:

Cementation. Saturation of the top layer of steel with carbon at a temperature in the range from 900 to 950 ºC.
Nitrocarburizing. In this case, thermal saturation is carried out simultaneously with nitrogen and carbon from a gaseous medium when heated from 850 to 900 ºC.
Cyanidation. The surface layer is saturated with the same elements as during nitrocarburizing, but from the melt of cyanide salts.
Nitriding. It is carried out at a temperature not exceeding 600 ºC.
Saturation with solid compounds of metals and non-metals (boron, chromium, titanium, aluminum and silicon).

In the first four types, saturation occurs from gaseous media, and in the latter, from powders, melts, pastes and suspensions.

Thermomechanical processing

During mechanical pressure treatment, as a result of work hardening, the metal surface is compacted and hardened. This property of steel is used in thermomechanical processing, which combines hot rolling, drawing or stamping with rapid quenching. If the hot one is immediately immersed in a cooling medium, its densified structure does not have time to change, while its hardness is additionally increased due to hardening. Usually, two types of thermomechanical processing are distinguished: high and low temperature, which differ in heating (above and below the temperature of the onset of austenite formation). After both types, it is necessary to carry out additional heat treatment: tempering in the temperature range of 200÷300 ºC. Compared to conventional hardening, the combination of mechanical and heat treatment makes it possible to increase the strength of the metal by 30–40% with a simultaneous increase in its ductility.

Cryogenic processing

Cryogenic treatment consists in cooling steel to critically low temperatures, as a result of which the same processes occur in its crystal lattice as during thermal hardening to martensite. To do this, the part is immersed in liquid nitrogen, which has a temperature of -195 ºC and is kept in it for the estimated time, depending on the steel grade and the mass of the product. After that, it is naturally heated to room temperature, and then, as in conventional thermal hardening, it is tempered, the parameters of which depend on the desired result. In a steel product treated in this way, not only hardness increases, but also strength. In addition, after exposure to ultra-low temperatures, aging processes in it stop and over time it does not change its linear dimensions.

Applied equipment

The equipment used for heat treatment includes five main categories that are present in any heat shop:

heating installations;
hardening tanks;
devices for preparation and supply of liquid and gaseous media;
lifting and transport equipment;
measuring and laboratory equipment.

The first type includes chamber furnaces for heat treatment of metals and alloys. In addition, heating can be carried out by high-frequency inductors, gas-plasma installations and baths with liquid melts. separate view heating equipment are installations for chemical-thermal and thermomechanical processing. Loading and unloading of products is carried out with the help of overhead cranes, overhead cranes and other lifting mechanisms, and movement between the heat treatment operating units is carried out with special trolleys with fastening equipment. Devices that provide the process of heat treatment with liquid and gaseous media are usually located near the corresponding equipment or are connected to it by pipelines. The main measuring equipment of the thermal shop are various pyrometers as well as standard measuring instruments.

Features of heat treatment of non-ferrous alloys

The main differences in the heat treatment of non-ferrous metals and alloys are associated with the peculiarity of the structure of their crystal lattices, increased or decreased thermal conductivity, as well as chemical activity with respect to oxygen and hydrogen. For example, there are practically no problems with hardenability during heat treatment of aluminum and copper alloys, and for titanium this is one of the main engineering problems, since its thermal conductivity is fifteen times lower than that of aluminum. Copper alloys at high temperatures actively interact with oxygen, so their heat treatment must be carried out in protective environments. Aluminum alloys are practically inert to atmospheric gases, while titanium, on the contrary, has a tendency to hydrogenation, therefore, to reduce the proportion of hydrogen, it must be annealed in a vacuum environment.

During the heat treatment of products made of deformable aluminum alloys (profiles, pipes, angles), very precise observance of the heating temperature is required, while it is not very high: only 450÷500 ºC. And how can you solve this problem at home with minimal means? If anyone knows the answer to this question, please share the information in the comments.

Increasing the strength and other mechanical properties of metals is achieved in many ways, one of the most common is thermomechanical treatment. This method combines heat treatment and plastic deformation.

Thermomechanical processing of metals(TMO) has been used by man for a long time, blacksmiths in ancient times made blades using this technology, they heated the workpiece in a forge, then processed it with a hammer and cooled it sharply in cold water, the process was repeated several times.

In this way, it was possible to create strong, sharp and sufficiently resistant products. Nowadays, a similar effect is also used on metal and alloys; Let's consider what types of TMT exist, and what characteristics of workpieces they increase.

There are such types of thermomechanical processing:

High temperature;

Low temperature.

For each type of metal and alloy, a processing scheme is individually selected, since all materials differ in their physical and chemical properties. Let's get acquainted in more detail with the technology of these processes.

High-temperature thermomechanical processing of metals

The deformation of the metal in this type of processing occurs after its preliminary heating. The temperature of the material must be above the recrystallization temperature, in other words, it must be in the austenitic state.

Plastic deformation leads to the fact that hardening is formed on the austenite, after which the metal is subjected to quenching and tempering.

Thermomechanical processing of metal at high temperature gives the following results:

Lowering the temperature threshold of cold brittleness;

Increased resistance to brittle fracture;

The development of temper brittleness is eliminated;

Increasing impact strength;

Reduced sensitivity to cracking during heat treatment.

Such processing lends itself to alloyed, structural, spring, carbon and tool steels.

Low-temperature thermomechanical processing of metals

In this type of processing, the workpiece is also heated to the state of austenite, it is kept in this state, then cooling occurs. It is important that the temperature after cooling be lower than the recrystallization temperature and higher than the martensitic transformation temperature. In this state, plastic deformation of parts is carried out.

Also practiced is the deformation of austenite, which is in a supercooled state when its temperature is equal to the temperature of bainitic transformation.

Low-temperature thermomechanical processing of metal does not give material stability during tempering, in addition, plastic deformation is carried out using powerful equipment. These factors limit the scope this method in industry.

Where is thermomechanical processing of metals used?

There are quite a lot of areas in which thermomechanical metal processing is used, since it helps to significantly improve the quality of manufactured parts.

The main advantage of this technology is that it allows you to simultaneously increase the plasticity and strength of the material, which is a unique phenomenon.

In mechanical engineering, defense and transport industry such qualities are highly valued, because the technology is used quite often.

Since the metal is hardened and the defects of its crystal lattice are eliminated, the resistance to erosion and corrosion of the finished products increases, there is no residual stress in them, and the service life is significantly increased.

What equipment is used for thermomechanical processing of metals

Thermomechanical processing of metal involves the use of special devices for heating, cooling and pressure on the workpiece.

First of all, special furnaces are used to heat the parts, the temperature regime in them can be different, it all depends on the type of material to be processed.

Plastic deformation is carried out on special machines - it can be broaching, forging or stamping.

Powerful aggregates can be included in automatic lines, which greatly simplifies the processing process and makes it more productive.

Equipment for TMO at the exhibition

You can find out how TMT and other metal processing processes take place at, which will take place at the Moscow Expocentre.

The event will be interesting for owners of industrial plants and small workshops to visit, as representatives of more than 1000 companies will demonstrate the latest machines, tools and other equipment.

Also exhibitors from different countries present their guests innovative technologies, which help to optimize the business and increase its profitability.

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.

Essence of heat treatment

During the production of 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 and 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 all industries National economy must meet high wear resistance requirements. Metal, as a raw material, needs to enhance the desired performance properties, which can be achieved by exposing it to high temperatures. Thermal high temperatures change the original structure of a substance, redistribute its constituent components, transform the size and shape of crystals. All this leads to 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 to three simple processes: heating the raw material (semi-finished product) to the desired temperature, keeping it under the specified conditions for the required time and rapid cooling. AT modern production several types of heat treatment are used, differing from each other in some technological features, but the algorithm of the process in general remains the same everywhere.

According to the method of performing heat treatment, there are the following types:

Thermal (hardening, tempering, annealing, aging, cryogenic treatment).
Thermo-mechanical includes processing by high temperatures in combination with mechanical action on the alloy.
Chemical-thermal involves the heat treatment of metal, followed by enrichment of the surface of the product with chemical elements (carbon, nitrogen, chromium, etc.).

Annealing

Annealing - 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 slowly in a natural way. As a result of annealing, it is possible to eliminate inhomogeneities chemical composition substances, 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 the first and second kind.

Annealing of the first kind implies heat treatment, as a result of which changes in the phase state of the alloy are insignificant or absent at all. It also has its own varieties: homogenized - the annealing temperature is 1100-1200, under such conditions the alloys are kept for 8-15 hours, recrystallization (at t 100-200) annealing is used for riveted steel, that is, already deformed 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 above the critical temperature mark characteristic of a given substance and cooling at a specified rate (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 that, rapid cooling is carried out to a temperature slightly below the critical one and left to cool in air.
Normalized - complete annealing with subsequent cooling of the metal in air, and not in a furnace.

hardening

Hardening is a manipulation with an alloy, the purpose of which is to achieve a martensitic transformation of the metal, which reduces the ductility of the product and increases its strength. Quenching, as well as annealing, involves heating the metal in a furnace above the critical temperature to the quenching temperature, the difference lies in the higher cooling rate that occurs in the liquid bath. Depending on the metal and even its shape, different types hardening:

Hardening in one medium, that is, in one bath with a liquid (water for large parts, oil for small parts).
Discontinuous hardening - cooling takes place in two successive stages: first in a liquid (a sharper coolant) to a temperature of approximately 300, then in air or in another oil bath.
Stepped - after the product reaches the hardening temperature, it is cooled for some time in molten salts, followed by cooling in air.
Isothermal - the technology is very similar to step hardening, it differs only in the holding 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 area in the middle of the part. As a result of this 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

Tempering is the final stage in the heat treatment of alloys, which determines the final structure of the metal. The main purpose of tempering is to reduce the brittleness of a metal product. The principle is to heat the part to a temperature below the critical temperature and cool it down. Since the modes of heat treatment and the cooling rate of metal products for various purposes may differ, then there are three types of vacation:

High - the heating temperature is from 350-600 to a value below the critical one. This procedure is most often used for metal structures.
Medium - heat treatment at t 350-500, common for spring products and springs.
Low - the heating temperature of the product is not higher than 250, which allows achieving high strength and wear resistance of parts.

Aging

Aging is a heat treatment of alloys, which causes the processes of decomposition of a supersaturated metal after quenching. The result of aging is an increase in the limits of hardness, yield and strength of the finished product. Not only cast iron is subjected to aging, but also easily deformable aluminum alloys. If a metal product subjected to hardening is kept at normal temperature, processes occur in it that lead to a spontaneous increase in strength and a decrease in ductility. This is called natural. If the same manipulation is done at elevated temperatures, it will be called artificial aging.

Cryogenic processing

Changes in the structure of alloys, and hence their properties, can be achieved not only by high, but also by extremely low temperatures. Thermal treatment of alloys at t below zero is called cryogenic. This technology is widely used in various sectors of the national economy as a supplement to heat treatments with high temperatures, since it can significantly reduce the cost of thermal hardening of products.

Cryogenic treatment 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 anti-corrosion properties, as well as eliminate the need for re-treatments.

Thermomechanical processing

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 treatment (TMT) according to the method of completion can be of three types:

Low-temperature TMT consists of two stages: plastic deformation followed by quenching and tempering of the part. The main difference from other types of TMT is the heating temperature to the austenitic state of the alloy.
High-temperature TMT involves heating the alloy to the martensitic state in combination with plastic deformation.
Preliminary - deformation is carried out at t 20, followed by hardening and tempering of the metal.

Chemical-thermal treatment

It is also 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, hardness, and wear resistance of the product, is also to impart acid resistance 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 additional strength. The essence of the procedure is to saturate the metal with carbon. Carburizing can be done in two ways: solid and gas carburizing. 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 holding it in this environment and cooling. In the case of gas carburizing, the product is heated in a furnace up to 900 under a continuous stream of carbonaceous gas.
Nitriding is a chemical-thermal treatment of metal products by saturating their surface in nitrogen environments. The result of this procedure is an increase in the tensile strength of the part and an increase in its corrosion resistance.
Cyaniding is the saturation of a metal with both nitrogen and carbon at the same time. The medium can be liquid (molten carbon- and nitrogen-containing salts) and gaseous.
Diffusion metallization is modern method 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).

Features of heat treatment of cast iron

Cast iron alloys are subjected to heat treatment 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 irons), aluminizing (pearlitic cast irons), chromium plating. All these procedures as a result significantly improve the properties of the final products of cast iron: increase the service life, eliminate the likelihood of cracks during use of the product, increase the strength and heat resistance of cast iron.

Heat treatment of non-ferrous alloys

Non-ferrous metals and alloys have different properties from each other, therefore they are processed by different methods. Thus, copper alloys are subjected to recrystallization annealing to equalize the chemical composition. For brass, low-temperature annealing technology (200-300) is provided, since this alloy is prone to spontaneous cracking in a humid environment. Bronze is subjected to homogenization and annealing at t up to 550 . Magnesium is annealed, quenched and subjected to artificial aging (natural aging does not occur for quenched magnesium). Aluminum, as well as magnesium, undergoes three heat treatment methods: annealing, hardening and aging, after which the deformable ones significantly increase their strength. Processing of titanium alloys includes: hardening, aging, nitriding and carburizing.

Summary

Heat treatment of metals and alloys is the main technological process both in ferrous and non-ferrous metallurgy. Modern technologies have a variety of heat treatment methods 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 desired results, but also to a large extent rationalize production processes.

October 31, 2011

The figure shows the main schemes of TMT of aging alloys. Jagged lines indicate plastic deformation.

Low temperature thermomechanical processing (LTMT)

LTMO of aging alloys- this is the first time of appearance (30s) and the most widely used in the industry thermomechanical processing.

The main purpose of NTMO— increase in strength properties.

In LTMT, the alloy is first subjected to conventional quenching, and then cold deformation before aging.

Compared to aging without prior deformation, LTMT results in higher tensile and yield strengths, but also lower ductility values.

The figure shows the effect of the degree of cold deformation on the hardness of a quenched nickel alloy (curve 1) and the same alloy aged after deformation (curve 2).

The influence of the degree of reduction during drawing after hardening from 1000 °C on the hardness of cold-drawn and aged wire with a diameter of 4 mm from the Nimonic-90 alloy (according to W. Betteridge):

1 - cold-drawn;
2 - deformation + aging at 460 °C, 16 h.

Strengthening during LTMT is caused by two reasons. First, cold deformation creates work hardening, and subsequent precipitation hardening starts from a higher initial level of alloy hardness. Secondly, and most importantly, cold deformation increases the effect of precipitation hardening. Thus, in the absence of cold work hardening, the hardening of the Nimonic-90 alloy as a result of aging at 450°C is very small—only 15 kgf/mm2. With an increase in the degree of cold deformation, the hardening during aging continuously increases (curves 1 and 2 in the figure diverge).

With a reduction of 90%, the increase in hardness as a result of aging was 175 kgf/mm 2 . Consequently, in the case under consideration, cold hardening increased the hardening during aging by an order of magnitude (!). Such a strong effect of hardening from LTMT in comparison with hardening during heat treatment according to the usual scheme (quenching + aging) is a relatively rare phenomenon.

It is due to the fact that the aging temperature of 450 °C is too low for nimonic, and in the absence of cold work hardening, the decomposition of a supersaturated solution at this temperature develops very slowly. If, after hardening, aging is carried out at a temperature that is optimal for maximum hardening (about 700 °C), then the effect of the introduction of cold work hardening will be much less.

In the very first approximation, it can be argued that cold hardening, by increasing the density of imperfections in the crystals of a supersaturated solution, makes it thermodynamically less stable and accelerates aging. However, experimental facts and more detailed analysis show that the effect of hardening on aging can be quite complex. The nature of this influence depends on the modes of hardening, deformation, and aging, on the nature of the alloy, and, for one alloy, on the type of precipitates during aging.

"Theory of heat treatment of metals",
I.I. Novikov

During HTMT, austenite is deformed in the area of its thermodynamic stability and then quenched for martensite (see Figure Scheme of Alloy Steel Processing). After quenching, a low tempering is carried out. The main goal of conventional heat treatment with deformation (rolling forging) heating is to eliminate special heating for hardening and thereby obtain an economic effect. The main goal of HTMT is to improve mechanical properties...

Of great interest is the phenomenon of inheritance ("reversibility") of hardening from HTMT discovered by ML Bernstein during repeated heat treatment. It turned out that HTMT hardening is retained if the steel is re-hardened with a short exposure at the heating temperature for quenching or if the HTMT-hardened steel is first subjected to high tempering and then re-hardened. For example, the tensile strength of steel 37XH3A after HTMT according to the regime ...

The processes of TMT of steels have been intensively studied since the mid-1950s in connection with the search for new ways to increase the structural strength. Low-temperature thermomechanical treatment (LTMT) During LTMT, supercooled austenite is deformed in the region of its increased stability, but necessarily below the temperature of the onset of recrystallization and then (turns into martensite. After that, low tempering is carried out (not shown in the figure). Processing scheme ...