Hardening of HDTV equipment. High-frequency hardening – advantages of induction furnaces. Descaling in shot blasting plants

Tool soldering

Soldering aluminum

Heat treatment

CJSC "Modern Machine-Building Company" official representative CIEA (Italy) brings to your attention induction heating generators (HDTV units) for heat treatment of metal products.

High-frequency furnaces for hardening

Since its creation, in the late 60s, CEIA has been developing and manufacturing industrial equipment based on the use of the electromagnetic field effect. In the late 1980s, CEIA introduced the first solid-state induction heater to the specialty soldering equipment market. In 1995, CEIA introduced another innovation - the “Power Cube Family” range of induction heating devices, which includes:

generators (power from 2.8 kW to 100 kW and operating frequencies from 25 kHz to 1800 kHz) and heating heads;
control devices (controller, master controller, special programmer) ensuring operation in automatic or semi-automatic mode;
optical pyrometers with a measurement range from 80 to 2000 ºС;
stands for heating heads, pyrometers and solder feeders.

CIEA fully carries out all stages of production: from the development of devices and electronic boards to the assembly of generators. The production employs highly qualified personnel. Each device undergoes mandatory electromagnetic testing.

High-frequency furnaces for hardening from JSC “SMK”

Modular design HDTV installations induction heating allows you to configure workstations with different characteristics, corresponding to the technical and economic needs of the customer. This also makes it possible to change the original configuration (when changing the generator or controller model).

The company CJSC "Modern Machine-Building Company" has experience in automating heat treatment processes according to the conditions terms of reference Customer.

Principle of operation:

Induction heating is carried out using the energy of an electromagnetic field. An inductor loop of the required size is brought to the workpiece. Medium- and high-frequency alternating current (HFC) passing through the loop creates eddy currents on the surface of the workpiece, the magnitude of which can be controlled and programmed. Induction heating occurs without direct contact, and only metal parts are subjected to heat treatment. Induction heating is characterized by high efficiency of energy transfer without heat loss. The depth of penetration of induced currents directly depends on the operating frequency of the generator (high frequency induction heating installation) - the higher the frequency, the greater the current density on the surface of the workpiece. By lowering the operating frequency, you can increase the depth of HDTV penetration, i.e. heating depth.

Advantages:

Generators (High Frequency Induction Heating Installations) CEIA have the following advantages:

high efficiency;
small dimensions and the ability to be built into automated lines;
localization of the heating area (thanks to a precisely selected inductor);
microprocessor ensuring repeatability of the working cycle;
a self-diagnosis system that gives a signal and turns off the unit in case of a problem;
the ability to move only the heating head with an inductor into the work area (connecting cable up to 4 m long);
the equipment meets electrical safety requirements and has ISO certificate 9001.

Application:

Generators (High Frequency Induction Heating Installations) CIEA is used for various types heat treatment of all conductive products (metal alloys, non-ferrous metals, carbon and silicon compounds):

heating;
hardening;
annealing;
soldering of tools, including diamond or carbide;
soldering of microcircuits, connectors, cables;
aluminum soldering.

The strength of elements in particularly critical steel structures largely depends on the condition of the nodes. The surface of the parts plays an important role. To give it the necessary hardness, durability or viscosity, heat treatment operations are carried out. The surface of parts is hardened using various methods. One of them is hardening with high frequency currents, that is, HDTV. It is one of the most common and very productive methods during large-scale production of various structural elements.

Such heat treatment is applied both to entire parts and to individual sections. In this case, the goal is to achieve certain levels of strength, thereby increasing service life and performance.

Technology is used to strengthen nodes technological equipment and transport, as well as during hardening of various tools.

The essence of technology

High-frequency hardening is an improvement in the strength characteristics of a part due to the ability of an electric current (with variable amplitude) to penetrate the surface of the part, subjecting it to heating. The penetration depth due to the magnetic field can be different. Simultaneously with surface heating and hardening, the core of the assembly may not be heated at all or may only slightly increase its temperature. The surface layer of the workpiece forms the required thickness, sufficient for the passage of electric current. This layer represents the depth of penetration of the electric current.

Experiments have proven that increasing the frequency of the current helps to reduce the depth of penetration. This fact opens up opportunities for regulation and production of parts with a minimal hardened layer.

Heat treatment of HDTV is carried out in special installations - generators, multipliers, frequency converters, which allow adjustment in the required range. In addition to the frequency characteristics, the final hardening is influenced by the dimensions and shape of the part, the material of manufacture and the inductor used.

The following pattern has also been revealed - the smaller the product and the simpler its shape, the better the hardening process. This also reduces the overall energy consumption of the installation.

The inductor is copper. There are often additional holes on the inner surface designed to supply water during cooling. In this case, the process is accompanied by primary heating and subsequent cooling without current supply. The inductor configurations are different. The selected device directly depends on the workpiece being processed. Some devices do not have holes. In such a situation, the part is cooled in a special quenching tank.

The main requirement for the high-frequency hardening process is to maintain a constant gap between the inductor and the product. When maintaining a given interval, the quality of hardening becomes the highest.

Hardening can be done in one of the following ways::

Continuous-sequential: the part is stationary, and the inductor moves along its axis.
Simultaneous: the product moves, and the inductor moves vice versa.
Sequential: different parts are processed one after the other.

Features of the induction installation

The installation for high-frequency hardening is a high-frequency generator together with an inductor. The workpiece is located both in the inductor itself and next to it. It consists of a coil on which a copper tube is wound.

An alternating electric current passing through an inductor creates an electromagnetic field that penetrates the workpiece. It provokes the development of eddy currents (Foucault currents), which pass into the structure of the part and increase its temperature.

The main feature of the technology– penetration of eddy current into the surface structure of the metal.

Increasing the frequency opens up the possibility of concentrating heat on a small area of the part. This increases the rate of temperature rise and can reach up to 100 – 200 degrees/sec. The degree of hardness increases to 4 units, which is excluded during volumetric hardening.

Induction heating - characteristics

The degree of induction heating depends on three parameters - specific power, heating time, frequency of electric current. Power determines the time spent heating the part. Accordingly, with a larger value, less time is spent.

The heating time is characterized by the total volume of heat expended and the temperature developed. Frequency, as mentioned above, determines the depth of penetration of currents and the hardened layer formed. These characteristics have an inverse relationship. As the frequency increases, the volumetric mass of the heated metal decreases.

It is these 3 parameters that allow you to adjust the degree of hardness and layer depth, as well as the heating volume, over a wide range.

Practice shows that the characteristics of the generator set (voltage, power and current values), as well as the heating time, are monitored. The degree of heating of the part can be controlled using a pyrometer. However, in general, continuous temperature monitoring is not required because There are optimal HDTV heating modes that ensure stable quality. The appropriate mode is selected taking into account the changed electrical characteristics.

After hardening, the product is sent to the laboratory for testing. The hardness, structure, depth and plane of the distributed hardening layer are studied.

Surface hardening HDTV accompanied by high heat compared to the conventional process. This is explained as follows. First of all, high speed An increase in temperature contributes to an increase in critical points. Secondly, it is necessary to short term ensure completion of the transformation of pearlite into austenite.

High-frequency hardening, in comparison with the conventional process, is accompanied by higher heating. However, the metal does not overheat. This is explained by the fact that granular elements in the steel structure do not have time to grow in a minimum time. In addition, volumetric hardening has a lower strength of up to 2-3 units. After high-frequency hardening, the part has greater wear resistance and hardness.

How is the temperature selected?

Compliance with technology must be accompanied the right choice temperature range. Basically, everything will depend on the metal being processed.

Steel is classified into several types:

Hypoeutectoid – carbon content up to 0.8%;
Hypereutectoid – more than 0.8%.

Hypoeutectoid steel is heated to just above that required to convert pearlite and ferrite to austenite. Range from 800 to 850 degrees. After this, the part is cooled at high speed. After rapid cooling, austenite is transformed into martensite, which has high hardness and strength. With a short holding time, austenite with a fine-grained structure, as well as fine-needle martensite, is obtained. Steel gains high hardness and low brittleness.

Hypereutectoid steel heats up less. Range from 750 to 800 degrees. In this case, incomplete hardening is performed. This is explained by the fact that such a temperature makes it possible to retain in the structure a certain volume of cementite, which has a higher hardness compared to martensite. Upon rapid cooling, austenite is transformed into martensite. Cementite is preserved by small inclusions. The zone also retains carbon that has not fully dissolved and has turned into solid carbide.

Advantages of technology

Controlling modes;
Replacing alloy steel with carbon steel;
Uniform heating process of the product;
The ability not to heat the entire part completely. Reduced energy consumption;
High resulting strength of the processed workpiece;
There is no oxidation process, no carbon is burned;
No microcracks;
There are no warped points;
Heating and hardening of certain areas of products;
Reducing the time spent on the procedure;
Introduction of high-frequency installations into production lines during the manufacture of parts.

Flaws

The main disadvantage of the technology under consideration is the significant installation price. It is for this reason that the feasibility of use is justified only in large-scale production and excludes the possibility of doing the work yourself at home.

Study the operation and principle of operation of the installation in more detail in the presented videos.

Steel is hardened to make the metal more durable. Not all products are subject to hardening, but only those that are often abraded and damaged from the outside. After hardening, the top layer of the product becomes very durable and protected from corrosion and mechanical damage. Hardening with high frequency currents makes it possible to achieve exactly the result that the manufacturer needs.

Why HDTV hardening?

When given a choice, the question “why?” very often arises. Why should you choose HDTV hardening if there are other methods of hardening metal, for example, using hot oil?
High-frequency hardening has many advantages, which is why it has become actively used recently.

Under the influence of high-frequency currents, heating is uniform over the entire surface of the product.
The induction machine's software can fully control the hardening process for a more accurate result.
High-frequency hardening makes it possible to heat the product to the required depth.
The induction installation allows you to reduce the number of defects in production. If, when using hot oils, scale often forms on the product, then heating the HDTV completely eliminates this. High-frequency hardening reduces the number of defective products.
Induction hardening reliably protects the product and makes it possible to increase productivity in the enterprise.

Induction heating has many advantages. There is also one disadvantage - in induction equipment it is very difficult to harden a product that has a complex shape (polyhedrons).

HDTV hardening equipment

Modern induction equipment is used for hardening HDTV. The induction unit is compact and allows you to process a significant number of products in a short period of time. If the enterprise constantly needs to harden products, then it is best to purchase a hardening complex.
The hardening complex includes: a hardening machine, an induction installation, a manipulator, a cooling module, and if necessary, a set of inductors can be added for hardening products of different shapes and sizes.
HDTV hardening equipment is an excellent solution for high-quality hardening of metal products and obtaining accurate results in the process of metal transformation.

By agreement, heat treatment and hardening of metal and steel parts with dimensions larger than those in this table is possible.

Heat treatment (heat treatment of steel) of metals and alloys in Moscow is a service that our plant provides to its customers. We have all necessary equipment, which is staffed by qualified specialists. We complete all orders with high quality and on time. We also accept and carry out orders for heat treatment of steels and high-frequency materials coming to us from other regions of Russia.

Main types of heat treatment of steel

Annealing of the first kind:

First type diffusion annealing (homogenization) - Rapid heating to t 1423 K, long exposure and subsequent slow cooling. The chemical heterogeneity of the material in large shaped castings made of alloy steel is leveled out

First type recrystallization annealing - Heating to a temperature of 873-973 K, long exposure and subsequent slow cooling. There is a decrease in hardness and an increase in ductility after cold deformation (processing is interoperational)

Stress-reducing annealing of the first kind - Heating to a temperature of 473-673 K and subsequent slow cooling. Removal of residual stresses occurs after casting, welding, plastic deformation or machining.

Annealing of the second kind:

Complete annealing of the second type - Heating to a temperature above the Ac3 point by 20-30 K, holding and subsequent cooling. There is a decrease in hardness, improvement in machinability, removal internal stresses in hypoeutectoid and eutectoid steels before hardening (see note to the table)

Annealing of the second type is incomplete - heating to a temperature between points Ac1 and Ac3, holding and subsequent cooling. There is a decrease in hardness, improvement in machinability, removal of internal stresses in hypereutectoid steel before hardening

Type II isothermal annealing - Heating to a temperature 30-50 K above the Ac3 point (for hypoeutectoid steel) or above the Ac1 point (for hypereutectoid steel), holding and subsequent stepwise cooling. Accelerated processing of small rolled products or forgings from alloy and high-carbon steels occurs in order to reduce hardness, improve machinability, and relieve internal stresses

Type II spheroidizing annealing - Heating to a temperature above the Ac1 point by 10-25 K, holding and subsequent stepwise cooling. There is a decrease in hardness, improvement in machinability, removal of internal stresses in tool steel before hardening, increase in ductility of low-alloy and medium-carbon steels before cold deformation

Annealing of the second type, light - Heating in a controlled environment to a temperature above the Ac3 point by 20-30 K, holding and subsequent cooling in a controlled environment. Protects the steel surface from oxidation and decarburization

Annealing of the second type Normalization (normalization annealing) - Heating to a temperature above the Ac3 point by 30-50 K, holding and subsequent cooling in still air. The structure of heated steel is corrected, internal stresses are relieved in parts made of structural steel and their machinability is improved, and the depth of hardenability of the tools increases. steel before hardening

Hardening:

Continuous complete hardening - Heating to a temperature above the Ac3 point by 30-50 K, holding and subsequent sharp cooling. Obtaining (in combination with tempering) high hardness and wear resistance of parts from hypoeutectoid and eutectoid steels

Incomplete hardening - Heating to a temperature between points Ac1 and Ac3, holding and subsequent sharp cooling. Obtaining (in combination with tempering) high hardness and wear resistance of parts made of hypereutectoid steel

Intermittent hardening - Heating to a temperature above point Ac3 by 30-50 K (for hypoeutectoid and eutectoid steels) or between points Ac1 and Ac3 (for hypereutectoid steel), holding and subsequent cooling in water and then in oil. There is a reduction in residual stresses and deformations in parts made of high-carbon tool steel

Isothermal hardening - Heating to a temperature above the Ac3 point by 30-50 K, holding and subsequent cooling in molten salts, and then in air. Obtaining minimal deformation (warping), increasing ductility, endurance limit and bending resistance of parts made of alloy tool steel

Step hardening - The same (differs from isothermal hardening by the shorter residence time of the part in the cooling medium). There is a reduction in stress, deformation and prevention of crack formation in small tools made of carbon tool steel, as well as in larger tools made of alloy tool and high-speed steel

Surface hardening - Heating by electric current or a gas flame of the surface layer of a product to hardening temperature, followed by rapid cooling of the heated layer. There is an increase in surface hardness to a certain depth, wear resistance and increased endurance of machine parts and tools

Hardening with self-tempering - Heating to a temperature above the Ac3 point by 30-50 K, holding and subsequent incomplete cooling. The heat retained inside the part provides tempering of the hardened outer layer. Local hardening of an impact tool of a simple configuration made of carbon tool steel, as well as during induction heating

Hardening with cold treatment - Deep cooling after hardening to a temperature of 253-193 K. There is an increase in hardness and stable dimensions of parts made of high-alloy steel

Quenching with cooling - Before immersion in a cooling medium, heated parts are cooled for some time in air or kept in a thermostat with a reduced temperature. There is a reduction in the heat treatment cycle of steel (usually used after carburization).

Light hardening - Heating in a controlled environment to a temperature above the Ac3 point by 20-30 K, holding and subsequent cooling in a controlled environment. Protection against oxidation and decarburization of complex parts of molds, dies and fixtures that are not subject to grinding

Low tempering - Heating in the temperature range 423-523 K and subsequent accelerated cooling. Internal stresses are relieved and the fragility of cutting and measuring tools is reduced after surface hardening; for case-hardened parts after hardening

Medium tempering - Heating in the range t = 623-773 K and subsequent slow or accelerated cooling. There is an increase in the elastic limit of springs, springs and other elastic elements

High tempering - Heating in the temperature range 773-953 K and subsequent slow or rapid cooling. Occurs: Ensuring high ductility of structural steel parts, usually with thermal improvement

Thermal improvement - Quenching and subsequent high tempering. Complete removal of residual stress occurs. Ensuring a combination of high strength and ductility during the final heat treatment of structural steel parts operating under shock and vibration loads

Thermo-mechanical processing - Heating, rapid cooling to 673-773 K, repeated plastic deformation, hardening and tempering. Providing for rolled products and parts of simple shapes that are not subject to welding, increased strength compared to the strength obtained by conventional heat treatment

Aging - Heating and prolonged exposure at elevated temperatures. The dimensions of parts and tools are stabilized

Cementation - Saturation of the surface layer of mild steel with carbon (carburization). Accompanied by subsequent hardening with low tempering. The depth of the cemented layer is 0.5-2 mm. What happens is that the product is given high surface hardness while maintaining a viscous core. Carbon or alloy steels with a carbon content are subject to carburization: for small and medium-sized products 0.08-0.15%, for larger ones 0.15-0.5%. Gears, piston pins, etc. are subjected to cementation.

Cyanidation - Thermochemical treatment of steel products in a solution of cyanide salts at a temperature of 820. The surface layer of steel is saturated with carbon and nitrogen (layer 0.15-0.3 mm). Low-carbon steels are subjected to cyanidation, as a result of which, along with a hard surface, the products have a viscous core. Such products are characterized by high wear resistance and resistance to shock loads.

Nitriding (nitriding) - Saturation of the surface layer of steel products with nitrogen to a depth of 0.2-0.3 mm. There is an imparting of high surface hardness, increased resistance to abrasion and corrosion. Calibers, gears, shaft journals, etc. are subjected to nitriding.

Cold treatment - Cooling after hardening to a temperature below zero. Change Happens internal structure hardenable steels. Used for tool steels, case-hardened products, and some high-alloy steels.

METAL HEAT TREATMENT (HEAT TREATMENT), a specific time cycle of heating and cooling to which metals are subjected to change their physical properties. Heat treatment in the usual sense of the term is carried out at temperatures below the melting point. Melting and casting processes, which have a significant impact on the properties of the metal, are not included in this concept. Changes in physical properties caused by heat treatment are due to changes in the internal structure and chemical relationships occurring in the solid material. Heat treatment cycles are various combinations of heating, holding at a certain temperature, and rapid or slow cooling to suit the structural and chemical changes that are desired to be induced.

Grain structure of metals. Any metal usually consists of many crystals in contact with each other (called grains), usually having microscopic dimensions, but sometimes visible to the naked eye. Inside each grain, the atoms are arranged so that they form a regular three-dimensional geometric lattice. The lattice type, called crystal structure, is a characteristic of the material and can be determined by X-ray diffraction techniques. The correct arrangement of atoms is maintained throughout the entire grain, except for small disturbances, such as individual lattice sites that accidentally become vacant. All grains have the same crystalline structure, but, as a rule, are oriented differently in space. Therefore, at the boundary of two grains, atoms are always less ordered than inside them. This explains, in particular, that grain boundaries are more easily etched chemical reagents. A polished flat metal surface treated with a suitable etchant will usually reveal a clear grain boundary pattern. The physical properties of a material are determined by the properties of individual grains, their influence on each other, and the properties of grain boundaries. The properties of a metallic material depend significantly on the size, shape and orientation of the grains, and the purpose of heat treatment is to control these factors.

Atomic processes during heat treatment. As the temperature of a solid crystalline material increases, it becomes increasingly easier for its atoms to move from one site of the crystal lattice to another. It is on this diffusion of atoms that heat treatment is based. The most effective mechanism for the movement of atoms in a crystal lattice can be imagined as the movement of vacant lattice sites, which are always present in any crystal. At elevated temperatures, due to an increase in the rate of diffusion, the process of transition from a non-equilibrium structure of a substance to an equilibrium one accelerates. The temperature at which the rate of diffusion noticeably increases is different for different metals. It is usually higher for metals with a high melting point. In tungsten, with its melting point of 3387 C, recrystallization does not occur even at red heat, while the heat treatment of aluminum alloys, melting at low temperatures, in some cases can be carried out at room temperature.

In many cases, heat treatment involves very rapid cooling, called quenching, the purpose of which is to preserve the structure formed at elevated temperatures. Although, strictly speaking, such a structure cannot be considered thermodynamically stable at room temperature, in practice it is quite stable due to the low diffusion rate. Many useful alloys have a similar “metastable” structure.

The changes caused by heat treatment can be of two main types. Firstly, in both pure metals and alloys, changes are possible that affect only the physical structure. These may be changes in the stressed state of the material, changes in the size, shape, crystal structure and orientation of its crystal grains. Secondly, the chemical structure of the metal can also change. This can be expressed in smoothing out compositional inhomogeneities and the formation of precipitates of another phase, in interaction with the surrounding atmosphere created to clean the metal or give it specified surface properties. Changes of both types can occur simultaneously.

Relieving stress. Cold deformation increases the hardness and brittleness of most metals. Sometimes such "strain hardening" is desirable. Non-ferrous metals and their alloys are usually given one or another degree of hardness by cold rolling. Low-carbon steels are also often hardened by cold deformation. High-carbon steels, brought by cold rolling or cold drawing to the increased strength required, for example, for the manufacture of springs, are usually subjected to stress-relieving annealing and heated to a relatively low temperature, at which the material remains almost as hard as before, but disappears in it. heterogeneity of internal stress distribution. This reduces the tendency for cracking, especially in corrosive environments. Such stress relief occurs, as a rule, due to local plastic flow in the material, which does not lead to changes in the overall structure.

Recrystallization. With different methods of metal forming, it is often necessary to greatly change the shape of the workpiece. If forming must be carried out in a cold state (which is often dictated by practical considerations), then the process must be divided into a number of steps, with recrystallization carried out in between. After the first stage of deformation, when the material is so strengthened that further deformation can lead to destruction, the workpiece is heated to a temperature above the annealing temperature to relieve stress and held for recrystallization. Due to rapid diffusion at this temperature, a completely new structure arises due to atomic rearrangement. New grains begin to grow inside the grain structure of the deformed material, which over time completely replace it. First, small new grains are formed in places where the old structure is most disrupted, namely at the old grain boundaries. With further annealing, the atoms of the deformed structure are rearranged so that they also become part of new grains, which grow and eventually absorb the entire old structure. The workpiece retains its original shape, but it is now made of a soft, non-stressed material that can be subjected to a new deformation cycle. This process can be repeated several times if required by a given degree of deformation.

Cold working is deformation at a temperature too low for recrystallization. For most metals this definition corresponds to room temperature. If deformation is carried out at sufficiently high temperature, so that recrystallization manages to follow the deformation of the material, then such processing is called hot. As long as the temperature remains high enough, it can be deformed as much as desired. The hot state of a metal is determined primarily by how close its temperature is to its melting point. The high malleability of lead means that it recrystallizes easily, meaning that it can be “hot” worked at room temperature.

Texture control. The physical properties of a grain, generally speaking, are not the same in different directions, since each grain is a single crystal with its own crystal structure. The properties of a metal sample are the result of averaging over all grains. In the case of random grain orientation, the general physical properties are the same in all directions. If some crystalline planes or atomic rows of most grains are parallel, then the properties of the sample become “anisotropic,” i.e., direction-dependent. In this case, the cup, obtained by deep extrusion from a round plate, will have “tongues” or “scallops” on the top edge, due to the fact that the material deforms more easily in some directions than in others. In mechanical shaping, anisotropy of physical properties is, as a rule, undesirable. But in sheets of magnetic materials for transformers and other devices, it is very desirable that the direction of easy magnetization, which in single crystals is determined by the crystal structure, coincides in all grains with the given direction of the magnetic flux. Thus, the "preferred orientation" (texture) may or may not be desirable depending on the purpose of the material. Generally speaking, when a material recrystallizes, its preferred orientation changes. The nature of this orientation depends on the composition and purity of the material, on the type and degree of cold deformation, as well as on the duration and temperature of annealing.

Grain size control. The physical properties of a metal sample are largely determined by the average grain size. The best mechanical properties almost always correspond to a fine-grained structure. Reducing grain size is often one of the goals of heat treatment (and melting and casting). As the temperature increases, diffusion accelerates, and therefore the average grain size increases. The grain boundaries shift so that larger grains grow at the expense of smaller grains, which eventually disappear. Therefore, finishing hot working processes are usually carried out at as low a temperature as possible to keep grain sizes to a minimum. Low temperature hot working is often specifically used, mainly to reduce grain size, although the same result can be achieved by cold working followed by recrystallization.

Homogenization. The processes mentioned above occur in both pure metals and alloys. But there are a number of other processes that are possible only in metallic materials containing two or more components. For example, in a cast alloy there will almost certainly be inhomogeneities in the chemical composition, which is determined by the uneven solidification process. In a solidifying alloy, the composition of the solid phase formed at each this moment, is not the same as in a liquid that is in equilibrium with it. Consequently, the composition of the solid substance that appears at the initial moment of solidification will be different than at the end of solidification, and this leads to spatial heterogeneity of the composition on a microscopic scale. Such heterogeneity is eliminated by simple heating, especially in combination with mechanical deformation.

Cleaning. Although metal purity is determined primarily by melting and casting conditions, metal purity is often achieved by solid state heat treatment. The impurities contained in the metal react on its surface with the atmosphere in which it is heated; Thus, an atmosphere of hydrogen or other reducing agent can convert a significant part of the oxides into pure metal. The depth of such cleaning depends on the ability of impurities to diffuse from the volume to the surface, and is therefore determined by the duration and temperature of heat treatment.

Isolation of secondary phases. Most heat treatment regimes for alloys are based on one important effect. It is due to the fact that the solubility in the solid state of alloy components depends on temperature. Unlike pure metal, in which all the atoms are identical, in a two-component, for example, solid solution, there are atoms of two different types, randomly distributed among the nodes of the crystal lattice. If you increase the number of second-class atoms, you can reach a state where they cannot simply replace first-class atoms. If the amount of the second component exceeds this solubility limit in the solid state, inclusions of the second phase appear in the equilibrium structure of the alloy, differing in composition and structure from the original grains and usually scattered between them in the form of separate particles. Such second-phase particles can have strong influence on the physical properties of the material, which depends on their size, shape and distribution. These factors can be changed by heat treatment (heat treatment).

Heat treatment is the process of processing products made of metals and alloys by thermal action in order to change their structure and properties in a given direction. This effect can also be combined with chemical, deformation, magnetic, etc.

Historical background on heat treatment.
Man has been using heat treatment of metals since ancient times. Even in the Chalcolithic era, using cold forging native gold and copper, primitive man was faced with the phenomenon of work hardening, which made it difficult to manufacture products with thin blades and sharp tips, and to restore ductility, the blacksmith had to heat cold-forged copper in a hearth. The earliest evidence of the use of softening annealing of cold-worked metal dates back to the end of the 5th millennium BC. e. Such annealing in terms of appearance time was the first operation of heat treatment of metals. When making weapons and tools from iron produced using the cheese-blowing process, the blacksmith heated the iron blank for hot forging in a charcoal forge. At the same time, the iron was carburized, that is, cementation occurred, one of the types of chemical-thermal treatment. By cooling a forged product made of carburized iron in water, the blacksmith discovered a sharp increase in its hardness and an improvement in other properties. Quenching carburized iron in water was used from the end of the 2nd beginning of the 1st millennium BC. e. In Homer's "Odyssey" (8th-7th centuries BC) there are the following lines: "As a blacksmith plunges a red-hot ax or ax into cold water, and the iron hisses with a bubbling sound; iron is stronger than iron, being tempered in fire and water." In the 5th century BC e. The Etruscans hardened mirrors made of high-tin bronze in water (most likely to improve shine during polishing). Cementation of iron in charcoal or organic matter, hardening and tempering of steel were widely used in the Middle Ages in the production of knives, swords, files and other tools. Not knowing the essence of internal transformations in metal, medieval craftsmen often attributed the achievement of high properties during the heat treatment of metals to the manifestation of supernatural forces. Until the middle of the 19th century. Human knowledge about the heat treatment of metals was a set of recipes developed on the basis of centuries of experience. The needs of technological development, and primarily the development of steel cannon production, determined the transformation of heat treatment of metals from an art into a science. In the mid-19th century, when the army sought to replace bronze and cast iron cannons with more powerful steel ones, the problem of manufacturing gun barrels of high and guaranteed strength was extremely acute. Despite the fact that metallurgists knew the recipes for smelting and casting steel, gun barrels very often burst for no apparent reason. D.K. Chernov at the Obukhov Steel Plant in St. Petersburg, studying etched sections prepared from gun barrels under a microscope and observing under a magnifying glass the structure of fractures at the point of rupture, concluded that the stronger the steel, the finer its structure. In 1868, Chernov discovered internal structural transformations in cooling steel that occur at certain temperatures. which he called critical points a and b. If steel is heated to temperatures below point a, then it cannot be hardened, and to obtain a fine-grained structure, steel must be heated to temperatures above point b. Chernov's discovery of the critical points of structural transformations in steel made it possible to scientifically select the heat treatment mode to obtain the necessary properties of steel products.

In 1906, A. Wilm (Germany), using the duralumin he invented, discovered aging after hardening (see Aging of metals), the most important method of strengthening alloys on different bases (aluminum, copper, nickel, iron, etc.). In the 30s 20th century thermomechanical treatment of aging copper alloys, and in the 50s, thermomechanical processing of steels, which made it possible to significantly increase the strength of products. Combined types of heat treatment include thermomagnetic treatment, which allows, as a result of cooling products in a magnetic field, to improve some of their magnetic properties.

The result of numerous studies of changes in the structure and properties of metals and alloys under thermal influence was a coherent theory of the heat treatment of metals.

The classification of types of heat treatment is based on what type of structural changes in the metal occur when exposed to heat. Thermal treatment of metals is divided into thermal treatment itself, which consists only of thermal effects on the metal, chemical-thermal, combining thermal and chemical effects, and thermomechanical, combining thermal effects and plastic deformation. The actual heat treatment includes the following types: annealing of the 1st kind, annealing of the 2nd kind, hardening without polymorphic transformation and with polymorphic transformation, aging and tempering.

Nitriding is the saturation of the surface of metal parts with nitrogen in order to increase hardness, wear resistance, fatigue limit and corrosion resistance. Nitriding is applied to steel, titanium, some alloys, most often alloyed steels, especially chromium-aluminum ones, as well as steel containing vanadium and molybdenum.
Nitriding of steel occurs at a temperature of 500–650 C in an ammonia environment. Above 400 C, ammonia begins to dissociate according to the reaction NH3 3H + N. The resulting atomic nitrogen diffuses into the metal, forming nitrogenous phases. At a nitriding temperature below 591 C, the nitrided layer consists of three phases (Fig.): µ nitride Fe2N, ³" nitride Fe4N, ± nitrogenous ferrite containing about 0.01% nitrogen at room temperature. At a nitriding temperature of 600-650 C, more formation is possible and ³-phase, which, as a result of slow cooling, decomposes at 591 C into eutectoid ± + ³ 1. The hardness of the nitrided layer increases to HV = 1200 (corresponding to 12 H/m2) and is maintained during repeated heating to 500-600 C, which ensures high wear resistance of parts at elevated temperatures. Nitrided steels are significantly superior in wear resistance to cemented and hardened steels. Nitriding is a long process, it takes 20-50 hours to obtain a layer 0.2-0.4 mm thick. Increasing the temperature speeds up the process, but reduces the hardness of the layer. To protect places, not subject to nitriding, tinning (for structural steels) and nickel plating (for stainless and heat-resistant steels) are used.To reduce the fragility of the layer, nitriding of heat-resistant steels is sometimes carried out in a mixture of ammonia and nitrogen.
Nitriding of titanium alloys is carried out at 850-950 C in high-purity nitrogen (nitriding in ammonia is not used due to increased brittleness of the metal).

During nitriding, an upper thin nitride layer is formed and solid solution nitrogen in ±-titanium. Layer depth in 30 hours is 0.08 mm with surface hardness HV = 800 850 (corresponds to 8 8.5 H/m2). The introduction of some alloying elements into the alloy (Al up to 3%, Zr 3 5%, etc.) increases the rate of diffusion of nitrogen, increasing the depth of the nitrided layer, and chromium reduces the rate of diffusion. Nitriding of titanium alloys in rarefied nitrogen makes it possible to obtain a deeper layer without a brittle nitride zone.
Nitriding is widely used in industry, including for parts operating at temperatures up to 500-600 C (cylinder liners, crankshafts, gears, spool pairs, parts of fuel equipment, etc.).
Lit.: Minkevich A.N., Chemical-thermal processing of metals and alloys, 2nd ed., M., 1965: Gulyaev A.P..Metal science, 4th ed., M., 1966.

Induction heating is a method of non-contact heating with high frequency currents (RFH - radio-frequency heating, heating by radio frequency waves) of electrically conductive materials.

Description of the method.

Induction heating is the heating of materials electric currents, which are induced by an alternating magnetic field. Consequently, this is the heating of products made of conductive materials (conductors) by the magnetic field of inductors (sources of alternating magnetic field). Induction heating is carried out as follows. An electrically conductive (metal, graphite) workpiece is placed in a so-called inductor, which is one or several turns of wire (most often copper). Powerful currents of various frequencies (from tens of Hz to several MHz) are induced in the inductor using a special generator, as a result of which an electromagnetic field appears around the inductor. The electromagnetic field induces eddy currents in the workpiece. Eddy currents heat the workpiece under the influence of Joule heat (see Joule-Lenz law).

The inductor-blank system is a coreless transformer in which the inductor is the primary winding. The workpiece is the secondary winding, short-circuited. The magnetic flux between the windings is closed through the air.

At high frequencies, eddy currents are displaced by the magnetic field they themselves generate into thin surface layers of the workpiece Δ (Surface effect), as a result of which their density increases sharply, and the workpiece heats up. The underlying layers of metal are heated due to thermal conductivity. It is not the current that is important, but the high current density. In the skin layer Δ, the current density decreases by e times relative to the current density on the surface of the workpiece, while 86.4% of the heat is released in the skin layer (of the total heat release. The depth of the skin layer depends on the radiation frequency: the higher the frequency, the thinner skin layer It also depends on the relative magnetic permeability μ of the workpiece material.

For iron, cobalt, nickel and magnetic alloys at temperatures below the Curie point, μ has a value from several hundred to tens of thousands. For other materials (melts, non-ferrous metals, liquid low-melting eutectics, graphite, electrolytes, electrically conductive ceramics, etc.) μ is approximately equal to unity.

For example, at a frequency of 2 MHz, the skin depth for copper is about 0.25 mm, for iron ≈ 0.001 mm.

The inductor becomes very hot during operation, as it absorbs its own radiation. Moreover, it absorbs thermal radiation from a hot workpiece. Inductors are made from copper tubes cooled by water. Water is supplied by suction - this ensures safety in case of burnout or other depressurization of the inductor.

Application:
Ultra-clean non-contact melting, soldering and welding of metal.
Obtaining prototypes of alloys.
Bending and heat treatment of machine parts.
Jewelry making.
Processing of small parts that can be damaged by gas flame or arc heating.
Surface hardening.
Hardening and heat treatment of parts with complex shapes.
Disinfection of medical instruments.

Advantages.

High-speed heating or melting of any electrically conductive material.

Heating is possible in a protective gas atmosphere, in an oxidizing (or reducing) environment, in a non-conducting liquid, or in a vacuum.

Heating through the walls of a protective chamber made of glass, cement, plastics, wood - these materials absorb electromagnetic radiation very weakly and remain cold during operation of the installation. Only electrically conductive material is heated - metal (including molten), carbon, conductive ceramics, electrolytes, liquid metals, etc.

Due to the MHD forces that arise, intensive mixing of the liquid metal occurs, up to keeping it suspended in air or a protective gas - this is how ultra-pure alloys are obtained in small quantities (levitation melting, melting in an electromagnetic crucible).

Since heating is carried out through electromagnetic radiation, there is no contamination of the workpiece with torch combustion products in the case of gas-flame heating, or with the electrode material in the case of arc heating. Placing samples in an inert gas atmosphere and high heating rates will eliminate scaling.

Ease of use due to the small size of the inductor.

The inductor can be made of a special shape - this will allow it to be evenly heated over the entire surface of parts of a complex configuration, without leading to their warping or local non-heating.

It is easy to carry out local and selective heating.

Since the most intense heating occurs in the thin upper layers of the workpiece, and the underlying layers are heated more gently due to thermal conductivity, the method is ideal for surface hardening of parts (the core remains viscous).

Easy automation of equipment - heating and cooling cycles, temperature adjustment and maintenance, feeding and removal of workpieces.

Induction heating units:

For installations with an operating frequency of up to 300 kHz, inverters based on IGBT assemblies or MOSFET transistors are used. Such installations are designed for heating large parts. To heat small parts, high frequencies are used (up to 5 MHz, medium and short waves), high-frequency installations are built on vacuum tubes.

Also, to heat small parts, high-frequency installations are being built using MOSFET transistors for operating frequencies up to 1.7 MHz. Controlling transistors and protecting them at higher frequencies presents certain difficulties, so higher frequency settings are still quite expensive.

The inductor for heating small parts is small in size and has low inductance, which leads to a decrease in the quality factor of the working oscillatory circuit at low frequencies and a decrease in efficiency, and also poses a danger to the master oscillator (the quality factor of the oscillatory circuit is proportional to L/C, an oscillatory circuit with a low quality factor is too good “pumped” with energy, forms a short circuit in the inductor and disables the master oscillator). To increase the quality factor of the oscillatory circuit, two ways are used:
- increasing the operating frequency, which leads to more complex and expensive installations;
- use of ferromagnetic inserts in the inductor; pasting the inductor with panels made of ferromagnetic material.

Since the inductor works most efficiently at high frequencies, induction heating received industrial application after the development and start of production of high-power generator lamps. Before World War I, induction heating had limited use. High-frequency machine generators (works by V.P. Vologdin) or spark-discharge installations were then used as generators.

The generator circuit can, in principle, be anything (multivibrator, RC generator, generator with independent excitation, various relaxation generators), operating on a load in the form of an inductor coil and having sufficient power. It is also necessary that the oscillation frequency be high enough.

For example, to “cut” a steel wire with a diameter of 4 mm in a few seconds, an oscillatory power of at least 2 kW is required at a frequency of at least 300 kHz.

The scheme is selected according to the following criteria: reliability; vibration stability; stability of the power released in the workpiece; ease of manufacture; ease of setup; minimum number of parts to reduce cost; the use of parts that together result in a reduction in weight and dimensions, etc.

For many decades, an inductive three-point generator (Hartley generator, autotransformer generator) was used as a generator of high-frequency oscillations. feedback, circuit based on an inductive loop voltage divider). This is a self-exciting parallel power supply circuit for the anode and a frequency-selective circuit made on an oscillating circuit. It has been successfully used and continues to be used in laboratories, jewelry workshops, industrial enterprises, as well as in amateur practice. For example, during the Second World War, surface hardening of the T-34 tank rollers was carried out on such installations.

Disadvantages of three points:

Low efficiency (less than 40% when using a lamp).

A strong frequency deviation at the time of heating of workpieces made of magnetic materials above the Curie point (≈700C) (μ changes), which changes the depth of the skin layer and unpredictably changes the heat treatment mode. When heat treating critical parts, this may be unacceptable. Also, powerful HDTV installations must operate in a narrow range of frequencies permitted by Rossvyazohrankultura, since with poor shielding they are actually radio transmitters and can interfere with television and radio broadcasting, coastal and rescue services.

When changing workpieces (for example, from a smaller one to a larger one), the inductance of the inductor-workpiece system changes, which also leads to a change in the frequency and depth of the skin layer.

When changing single-turn inductors to multi-turn ones, to larger or smaller ones, the frequency also changes.

Under the leadership of Babat, Lozinsky and other scientists, two- and three-circuit generator circuits were developed that have a higher efficiency (up to 70%) and also better maintain the operating frequency. The principle of their operation is as follows. Due to the use of coupled circuits and weakening of the connection between them, a change in the inductance of the operating circuit does not entail a strong change in the frequency of the frequency-setting circuit. Radio transmitters are designed using the same principle.

Modern HDTV generators are inverters based on IGBT assemblies or high-power MOSFET transistors, usually made according to a bridge or half-bridge circuit. Operate at frequencies up to 500 kHz. The transistor gates are opened using a microcontroller control system. The control system, depending on the task at hand, allows you to automatically hold

A) constant frequency
b) constant power released in the workpiece
c) the highest possible efficiency.

For example, when a magnetic material is heated above the Curie point, the thickness of the skin layer increases sharply, the current density drops, and the workpiece begins to heat up worse. The magnetic properties of the material also disappear and the process of magnetization reversal stops - the workpiece begins to heat up worse, the load resistance decreases abruptly - this can lead to “spreading” of the generator and its failure. The control system monitors the transition through the Curie point and automatically increases the frequency when the load abruptly decreases (or reduces power).

Notes.

If possible, the inductor should be located as close to the workpiece as possible. This not only increases the electromagnetic field density near the workpiece (proportional to the square of the distance), but also increases the power factor Cos(φ).

Increasing the frequency sharply reduces the power factor (proportional to the cube of the frequency).

When heating magnetic materials, additional heat is also released due to magnetization reversal; heating them to the Curie point is much more efficient.

When calculating an inductor, it is necessary to take into account the inductance of the buses leading to the inductor, which can be much greater than the inductance of the inductor itself (if the inductor is made in the form of one turn of small diameter or even part of a turn - an arc).

There are two cases of resonance in oscillatory circuits: voltage resonance and current resonance.
Parallel oscillatory circuit – current resonance.
In this case, the voltage on the coil and on the capacitor is the same as that of the generator. At resonance, the circuit resistance between the branching points becomes maximum, and the current (I total) through the load resistance Rн will be minimal (the current inside the circuit I-1l and I-2s is greater than the generator current).

Ideally, the loop impedance is infinity—the circuit draws no current from the source. When the generator frequency changes in any direction from the resonant frequency, the circuit impedance decreases and the line current (I total) increases.

Series oscillatory circuit – voltage resonance.

The main feature of a series resonant circuit is that its impedance is minimal at resonance. (ZL + ZC – minimum). When tuning the frequency above or below the resonant frequency, the impedance increases.
Conclusion:
In a parallel circuit at resonance, the current through the circuit terminals is 0 and the voltage is maximum.
In a series circuit, on the contrary, the voltage tends to zero and the current is maximum.

The article was taken from the website http://dic.academic.ru/ and revised into a text that is more understandable for the reader by Prominductor LLC.