Heat and mechanical treatment. Metallic materials

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 humans for a long time; even in ancient times, blacksmiths made blades using this technology, they heated the workpiece in a forge, then processed it with a hammer and sharply cooled it in cold water, the process was repeated several times.

In this way, it was possible to create durable, sharp and sufficiently resistant products. Nowadays, a similar effect on metal and alloys is also applied; consider what types of TMT exist, and what characteristics of the processed 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

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

Plastic deformation leads to the formation of work hardening on the austenite, after which the metal is quenched and tempered.

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

• Lowering the temperature threshold of cold brittleness;


• Increased resistance to brittle fracture;


• Eliminates the development of temper brittleness;


• Increased impact strength;


• Decreases sensitivity to cracking during heat treatment.

Alloy steels, structural steels, spring steels, carbon steels and tool steels lend themselves to such processing.

Low-temperature thermomechanical processing of metals

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

The deformation of austenite is also practiced, which is in a supercooled state when its temperature is equal to the temperature of bainite transformation.

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

Where is thermomechanical processing of metals used?

There are quite a few 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 industries, such qualities are highly valued, therefore the technology is used quite often.

Since the metal is hardened and defects in its crystal lattice are eliminated, the finished products increase their resistance to erosion and corrosion, 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 workpieces.

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 - this can be broaching, forging or stamping.

Powerful units 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 the Moscow Expocentre.

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

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

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, etc. The main types of heat treatment include hardening, tempering and annealing. In addition, there are combined methods: chemical-thermal and thermomechanical treatment, combining heating and cooling with other types of effects on the metal structure. With all the variety of basic types and their varieties, the essence of all these technologies is the same - a change in the internal phase and structural states of a 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 operational quality or a combination of such qualities. Heat treatment of cutting tools made of tool and alloy steels achieves a hardness of 63 HRC and increased wear resistance. And the percussion tool after it must have a hard surface layer and a plastic impact-resistant core. After heat treatment, steels for the manufacture of springs and spring plates become strong in bending and elastic, and metal for rails becomes resistant to deformation and wear. In addition, the surface layers of steel products are hardened by thermal methods, saturating them at high temperatures with carbon, nitrogen or other compounds, as well as strengthening by hardening the autofrettage after hot working with pressure. Another purpose of heat treatment is to restore the original properties of the metal, which is achieved by annealing them.

Advantages of heat treatment of metals

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 increase the hardness, wear resistance, plasticity and toughness of the product by several times. In addition, heat treatment makes it possible to make structural changes only in the surface layer at a given depth or to affect only a part of the workpiece. The combination of heat treatment with hot working pressure leads to a significant increase in the hardness of the metal, exceeding the results obtained separately during autofrettage or quenching. During chemical-thermal treatment, the surface layer of the metal is saturated by the diffusion method with chemical elements, which significantly increase its wear resistance and hardness. At the same time, the main part of the product retains its viscosity and plasticity. From a production point of view, equipment for heat treatment is much simpler and cheaper than machine tools and installations of machining and foundries.

Heat treatment principle

Heat treatment of metals is based on phase changes in the internal structure that occur when they are heated or cooled. In general, the heat treatment process consists of the following stages:

• heating, which changes the structure of the crystal lattice of the metal;
• cooling, fixing the changes achieved during heating;
• tempering, relieving mechanical stress and arranging the resulting structure.

A feature of the technology of heat treatment of steel is that when heated to 727 ºC, it transforms into a state of 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 the 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 the 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 environments. This division of thermal operations is to some extent arbitrary, 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, maintaining the operating temperature for the required period and cooling at a given rate. In addition, to increase the wear resistance of alloy steel products, cold heat treatment is used with immersion of the workpiece in a cryogenic environment with cooling below -150 ºC.

Annealing

The main feature of annealing is the heating of products to a high temperature and a 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:

1. Diffusion The part is heated to a temperature of about 1200 ºC, and then gradually cooled down for tens of hours (for massive items - up to several days). Usually, such heat treatment removes dendritic inhomogeneities of the steel structure.
2. 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 mainly used for structural steel. Its result is a decrease in the grain size 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 quenching to reduce the grain size of the metal.
3. 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-consuming, it is performed in a shorter period, and less dross is formed on the part. This heat treatment is used for tool steels and similar steels.
4. Isothermal. Heating is carried out to a temperature slightly exceeding 727 ºC, after which the product is immediately transferred to a melt bath at 600 ÷ 700 ºC, where it is kept for a certain time until the end of the formation of the required structure.
5. Recrystallization. This type of heat treatment is used only to eliminate auto-fretting after broaching, stamping, drawing, etc. In this case, the steel part is thermally heated below 727 ºC, kept in this state for a certain time, and then cooled slowly.
6. Spheroidizing. A special type of annealing applied to high-carbon steels (more than 0.8%), in which the pearlite structure transforms 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 quenching or a trial heat treatment.

Hardening

Quenching is the central link in most heat treatment processes, since it is this that ensures the required performance of the hardened metal is obtained. 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 obtained result. The main technological parameters during quenching are the heating and cooling temperatures, as well as the rates 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 heat treatment parameters of alloy steels strongly depend on their composition; therefore, 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, mass and shape of the part, the type of heat source and the desired result. Therefore, it can be selected either according to reference tables or only empirically. The same applies to the cooling rate, which also depends on the listed characteristics. When choosing a cooling medium, first of all, they are guided by the cooling rate, but at the same time take into account its other features. 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, in the operation of pumping and mixing equipment used in heat treatment, characteristics such 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 remove residual stresses in steel and reduce its brittleness, as well as increase the toughness and resistance to shock loads. On 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:

1. Short. Heating is carried out up to 200 ºC. This tempering is applied to cutting tools and case-hardened steels to maintain high hardness and wear resistance.
2. 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.
3. High. The heating range is 460 ÷ 710 ºC. Heat treatment, including hardening with high tempering, is called improvement by thermists, since in this case the best ratio of ductility, wear resistance and toughness is achieved.

With 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 the heating of the part, and many produce tempering, focusing on the tarnish color.

Chemical heat treatment

One of the types of combined heat treatment is the high-temperature saturation of the top layer of the 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:

1. Cementation. Saturation of the top layer of steel with carbon at temperatures ranging from 900 to 950 ºC.
2. Nitrocarburizing. In this case, thermal saturation is performed simultaneously with nitrogen and carbon from a gaseous medium when heated from 850 to 900 ºC.
3. Cyanide treatment. The surface layer is saturated with the same elements as in nitrocarburizing, but from a melt of cyanide salts.
4. Nitriding. It is carried out at a temperature not exceeding 600 ºC.
5. 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 treatment

During mechanical processing by pressure, as a result of autofrettage, 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 a hot one is immediately immersed in a cooling medium, its compacted structure does not have time to change, while its hardness is additionally increased by quenching. Usually, two types of thermomechanical treatment 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 with conventional hardening, the combination of mechanical and heat treatment allows increasing the strength of the metal by 30 ÷ 40% with a simultaneous increase in its ductility.

Cryogenic treatment

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 quenching for martensite. To do this, the part is immersed in liquid nitrogen, which has a temperature of -195 ºC and is kept in it for a calculated time, depending on the steel grade and the weight of the product. After that, it is naturally heated to room temperature, and then, as with conventional thermal hardening, it is tempered, the parameters of which depend on the desired result. In a steel product processed in this way, not only the hardness increases, but also the strength. In addition, after exposure to ultra-low temperatures, aging processes stop in it 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 treatment plant:

• heating installations;
• quenching tanks;
• devices for the preparation and supply of liquid and gaseous media;
• lifting and transport equipment;
• measuring and laboratory technology.

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. A separate type of heating equipment is installations for chemical-thermal and thermomechanical treatment. Loading and unloading of products is carried out with the help of overhead cranes, girder cranes and other lifting mechanisms, and movement between the operating units of heat treatment 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 connected to it by pipelines. The main measuring technology of the thermal shop is various pyrometers as well as standard measuring instruments.

Features of heat treatment of non-ferrous alloys

The main differences in 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 in relation 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, therefore, 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.

Heat treatment of wrought aluminum alloy products (profiles, pipes, corners) requires very precise adherence to the heating temperature, 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.

One of the technological processes of hardening treatment is thermomechanical treatment (TMT).

Thermomechanical treatment refers to the combined methods of changing the structure and properties of materials.

Thermomechanical treatment combines plastic deformation and heat treatment (hardening of pre-deformed steel in the austenitic state).

The advantage of thermomechanical treatment is that with a significant increase in strength, the ductility characteristics decrease slightly, and the impact toughness is 1.5 ... 2 times higher than the impact toughness for the same steel after quenching with low tempering.

Depending on the temperature at which deformation is carried out, high-temperature thermomechanical treatment (HTMT) and low-temperature thermomechanical treatment (HTMT) are distinguished.

The essence of high-temperature thermomechanical treatment consists in heating steel to the temperature of the austenitic state (above BUT 3 ). At this temperature, deformation of the steel is carried out, which leads to work hardening of austenite. Steel with this state of austenite is hardened (Figure 16.1 a).

High-temperature thermomechanical treatment practically eliminates the development of temper brittleness in a dangerous temperature range, weakens irreversible temper brittleness and sharply increases the impact toughness at room temperature. The temperature threshold of cold brittleness is lowered. High-temperature thermomechanical treatment increases brittle fracture resistance, reduces sensitivity to cracking during heat treatment.

Rice. 16.1. Scheme of modes of thermomechanical treatment of steel: a - high-temperature thermomechanical treatment (HTMT); b - low-temperature thermomechanical treatment (NTMO).

High-temperature thermomechanical processing is effectively used for carbon, alloy, structural, spring and tool steels.

Subsequent tempering at a temperature of 100 ... 200 o C is carried out to maintain high strength values.

Low-temperature thermomechanical processing (ausforming).

The steel is heated to an austenitic state. Then it is kept at a high temperature, cooled to a temperature higher than the temperature of the onset of martensitic transformation (400 ... 600 o C), but below the recrystallization temperature, and at this temperature, pressure treatment and quenching are carried out (Fig. 16.1 b).

Low-temperature thermomechanical treatment, although it gives a higher hardening, does not reduce the tendency of steel to temper brittleness. In addition, it requires high degrees of deformation (75 ... 95%), so powerful equipment is required.

Low temperature thermomechanical treatment is applied to martensite hardened medium carbon alloy steels that have secondary austenite stability.

The increase in strength during thermomechanical treatment is explained by the fact that as a result of deformation of austenite, its grains (blocks) are fragmented. Block sizes are reduced by two to four times compared to conventional quenching. The dislocation density also increases. During the subsequent quenching of such austenite, smaller martensite plates are formed, and stresses are reduced.

The mechanical properties after different types of TMT for machine-building steels, on average, have the following characteristics (see Table 16.1):

Table 16.1. Mechanical properties of steels after TMT

The degree of influence of the liquid metal medium on the deformable material depends on its thermal and thermomechanical treatment. To a large extent, this effect is determined by the level of strength and grain size that materials acquire as a result of processing. Ho the effect of heat and thermomechanical treatment is also associated with some features of the structural state of the material.
VG Markov investigated the effect of the action of liquid tin on pearlitic chromium-molybdenum-vanadium steels subjected to tempering at different temperatures. Quenching in all cases was carried out from 990 ° C, and tempering - at 270, 370, 470, 570, 670 and 770 ° C; the duration of the tempering at each temperature was 1.5 hours. Specimens with a cylindrical working part with a diameter of 6 mm were made from steel blanks that passed the indicated heat treatment modes, which were then tested for tension at a speed of 1.25 mm / min. The samples were tested in a bath with liquid tin and in air at a temperature of 250/650 ° C.
It was found that steel is exposed to the greatest impact of liquid metal after low and medium tempering (at a temperature of 270/470 ° C). Specimens that have undergone such heat treatment fail brittle, without plastic deformation, their strength limit is 1.5-2 times lower than the yield strength in air. Specimens tempered at 570 ° C are destroyed in tin by some plastic deformation, their tensile diagram breaks off in the region of uniform deformation. Tempering at 670 ° C leads to a further weakening of the effect of tin on steel. In this case, the yield strength, tensile strength and uniform elongation of the samples tested in air and in tin are the same; the influence of the liquid metal is expressed only in a decrease in the concentrated elongation. The samples tempered at 770 ° C did not show any influence of the liquid metal environment.
Thus, an increase in the tempering temperature leads to a decrease in the effect of liquid metal on the mechanical properties of pearlitic steel. The main reason for the weakening of the effect is due in this case, apparently, to a decrease in the strength of the steel. Thus, the ultimate strength in air changes continuously from approximately 130 kg / mm2 after tempering at 270 ° C to 55 kg / mm2 after tempering at 670 ° C.
Similar regularities of the influence of heat treatment of steel 30KhGSA on the magnitude of the effect of the action of liquid tin and tin-lead solder were established in the works, their results were considered above (see Table 35). It is noted in the work that high-temperature tempering of pearlitic chromium-nickel and carbon steels reduces their sensitivity to the action of molten solders.

The authors of the work investigated the effect of mercury at room temperature on the mechanical properties of precipitation-hardening aluminum alloys, depending on the duration of aging. In fig. 88 shows the test results of an aluminum alloy alloyed with 4.5% Cu, 0.6% Mn and 1.5% Mg. It is seen that an increase in the aging time of the alloy, accompanied by hardening in air, leads to a sharp drop in its strength in a liquid mercury environment. It is interesting that even a slight hardening of the alloy at the beginning of the aging process causes a strong influence of the liquid metal. This indicates the dependence of the effect of a liquid metal medium on the structural state of the material.
A somewhat different character of the influence of liquid metal (mercury with 2% Na) was observed during aging of the Cu - 2% Be alloy. Fig. 89 it follows that testing an alloy in a liquid metal does not distort (qualitatively) the nature of the effect of aging on its yield strength. In this case, the usual stages of hardening are observed and then softening (with increasing exposure), associated with over-aging of the alloy. As for the influence of the liquid metal on the relative elongation of the material, it was similar to the effect on the strength established in the work, i.e., the effect of the influence of the environment, which manifested itself in a decrease in the relative elongation, increases with the hardening of the alloy and has the greatest value with the maximum hardening. Over aging of the alloy leads to a decrease in the embrittlement effect of the liquid metal coating.

In fig. 89 also gives the results of testing a copper-beryllium alloy, subjected to work hardening after quenching. Such processing contributes to an even greater hardening of the alloy during aging, while the decrease in the relative elongation is much less pronounced. For example, the largest reduction in elongation after quenching and work-hardening was about 60%, while after one quenching it was close to 100%.
The use of work hardening after heat treatment of the alloy, as shown in the works, usually does not cause a change in the degree of exposure to liquid metal. Thus, work hardening of a copper-beryllium alloy after quenching and aging at 370 ° C for 0.5 and 12 hours, i.e., up to and after the peak of hardening (see Fig. 89), does not lead to either strengthening or to weakening the influence of the liquid metal environment. The alloy that underwent maximum hardening during heat treatment (quenching and aging at 370 ° C for 1 h) showed an increase in the effect of the environment with an increase in the degree of work hardening.
Thermomechanical treatment of a material in some cases makes it possible to increase its strength in a liquid metal environment. The works investigated the effect of thermomechanical treatment on the mechanical properties of steel 40X in air and in contact with the Pb-Sn eutectic. Cylindrical specimens with a diameter of 10 mm with a circular notch were tested. The material was processed in the area of ​​the stress concentrator. The sample was installed on a special machine and heated by passing an electric current through it to the austenitizing temperature; then it was cooled to a temperature of 400/600 ° C, at which the concentrator was rolled with profile rollers. The initial depth of the notch made on the lathe was 1 mm, the radius at the top was 0.2 mm, and the angle was 0.8 rad. Rolling with rollers increased the notch depth to 1.5 mm, the radius remained unchanged. After running in, the sample was subjected to oil quenching followed by tempering. In addition to thermomechanical processing with rolling in rollers, processing with the deformation of the sample by torsion was also used. The effect of work hardening at room temperature on the effect of the action of liquid metal on steel after quenching and normalization was also evaluated.

From those shown in Fig. 90 tension diagrams show that at temperatures of 400 and 500 ° C the quenched specimens are destroyed under the action of liquid metal in the elastic region, experiencing a multiple decrease in strength. Some increase in strength is achieved by work hardening, rolling with rollers at room temperature, and thermomechanical processing by means of torsion. The greatest increase in strength results from thermomechanical treatment using rolling of samples with rollers. However, although when tested in air, this treatment gives a sharp increase in the ductility of the samples, when tested in the melt, the samples fail brittle. It should be avenged that the method of thermomechanical treatment, which turned out to be effective for steel 40X, did not give a positive result for steel 2X13 either when tested in air or in the melt of the Pb-Sn eutectic. The degree of influence of the liquid metal in this case was approximately the same as after quenching and tempering, imparting the same level of strength and ductility to the steel.
The above data show that an increase in the strength of a material as a result of thermal or thermomechanical treatment leads, as a rule, to an increase in the effect of liquid metal. The effect of hardening of 40Kh steel in the Pb-Bi eutectic after rolling the stress concentrator with rollers is obviously associated mainly with the appearance of compressive stresses in the surface layer of the sample, since thermomechanical treatment in the same mode, but with the deformation of the sample by torsion, does not lead to similar results. The structural factor seems to have an effect on the degree of exposure to a liquid metal medium in the case of testing dispersion-hardened alloys. An increase in the influence of the environment on these alloys should be expected, since they may contain significant stress concentrations in the region of finely dispersed precipitates, which are serious obstacles to the movement of dislocations.

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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 look at 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 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 every day in all sectors of the national economy must meet high requirements for resistance to wear. Metal, as a raw material, needs to be enhanced with the required performance properties, which can be achieved by exposing it to high temperatures. Thermal with high temperatures changes the initial structure of a substance, redistributes its constituent components, transforms 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.

Heat treatment types

Heat treatment of metal alloys comes down to three simple processes: heating the raw material (semi-finished product) to the required temperature, keeping it under the specified conditions for the required time and rapid cooling. In modern production, several types of heat treatment are used, differing from each other in some technological features, but the process algorithm 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 treatment implies heat treatment of the metal with subsequent enrichment of the product surface with chemical elements (carbon, nitrogen, chromium, etc.).

Annealing

Annealing is a production process in which metals and alloys are heated to a predetermined temperature, and then, together with the furnace in which the procedure took place, they cool naturally very slowly. As a result of annealing, it is possible to eliminate inhomogeneities in the chemical composition of a substance, to remove internal stress, to achieve a granular structure and improve it as such, and also to 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, in such conditions the alloys are kept 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 comes in several varieties:

• Full annealing - heating of the alloy by 30-50 above the critical temperature mark characteristic of a given substance and cooling at a specified rate (200 / h - carbon steels, 100 / h and 50 / h - 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 full 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 followed by cooling the metal in air, not in a furnace.

Hardening

Quenching is the manipulation of the 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 is in a higher cooling rate that occurs in a bath with a liquid. Depending on the metal and even its shape, different types of hardening are used:

• Quenching in one medium, that is, in one bath with liquid (water - for large parts, oil - for small parts).
• Intermittent quenching - cooling takes place in two successive stages: first in liquid (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 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.
• Self-tempering hardening 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 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

Tempering is the final stage of heat treatment 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 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, there are three types of tempering:

• High - heating temperature from 350-600 to values ​​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 heating temperature of the product is not higher than 250 allows to achieve 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. Aging results in an increase in the limits of hardness, flowability and strength of the finished product. Not only cast iron is subject to aging, but also easily deformable aluminum alloys. If a metal product subjected to quenching 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 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 by high, but also by extremely low temperatures. 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 supplement to heat treatment with high temperatures, since it can significantly reduce the cost of thermal hardening of 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 anti-corrosion properties, as well as eliminate the need for repeated treatments.

Thermomechanical treatment

The new method of processing alloys combines the processing of metals at high temperatures with mechanical deformation of products in a plastic state. Thermomechanical treatment (TMT) can be of three types according to the method:

• 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 implies heating the alloy to the martensitic state in combination with plastic deformation.
• Preliminary - deformation is carried out at t 20, followed by quenching and tempering of the metal.

Chemical heat 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, wear resistance to 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 keeping it in this environment and cooling. In the case of gas carburizing, the product is heated in a furnace to 900 under a continuous stream of carbon-containing 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.
• Cyanidation is the saturation of a metal with both nitrogen and carbon. The medium can be liquid (molten carbon- and nitrogen-containing 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, quenching (continuous, isothermal, surface), tempering, nitriding (gray cast irons), aluminizing (pearlitic cast irons), chromium plating. As a result, all these procedures significantly improve the properties of the finished cast iron products: they 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 different properties from each other, therefore they are processed by different methods. So, 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 humid environment. Bronze is subjected to homogenization and annealing at t up to 550. Magnesium is annealed, tempered and artificially aged (natural aging does not occur for hardened magnesium). Aluminum, like magnesium, undergoes three heat treatment methods: annealing, quenching and aging, after which the deformed 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 in both ferrous and non-ferrous metallurgy. Modern technologies have a variety of heat treatment methods to achieve the desired properties for 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 physicochemical characteristics of the substance. Ultimately, this will allow not only to achieve the desired results, but also to significantly rationalize production processes.