Hardening of metals by high frequency currents. Hfc equipment for steel hardening

The high-frequency current is generated in the installation due to the inductor and allows heating the product placed in the immediate vicinity of the inductor. The induction machine is ideal for hardening metal products. It is in the HDTV installation that you can clearly program: the required depth of heat penetration, hardening time, heating temperature and cooling process.

For the first time, induction equipment was used for hardening after a proposal from V.P. Volodin in 1923. After long trials and testing, HFC heating has been used for steel hardening since 1935. HFC installations for hardening today are the most productive way of heat treatment of metal products.

Why an induction machine is better suited for hardening

HFC hardening of metal parts is performed to increase the resistance of the upper layer of the product to mechanical damage, while the center of the workpiece has an increased viscosity. It is important to note that the core of the product remains completely unchanged during HFC hardening.
The induction installation has many very important advantages in comparison with alternative types of heating: if earlier HFC installations were more cumbersome and inconvenient, now this drawback has been corrected, and the equipment has become universal for heat treatment of metal products.

Induction equipment advantages

One of the disadvantages of an induction hardening unit is the impossibility of processing some products with a complex shape.

Varieties of metal hardening

There are several types of metal hardening. For some products, it is enough to heat the metal and immediately cool it, while for others it is necessary to hold it at a certain temperature.
There are the following types of hardening:

• Stationary hardening: usually used for parts with a small flat surface. The position of the part and the inductor remains unchanged when using this hardening method.
• Continuous sequential hardening: used for hardening cylindrical or flat products. With continuous sequential hardening, the part can move under the inductor, or keep its position unchanged.
• Tangential hardening of products: excellent for machining small cylindrical parts. Tangential continuous sequential hardening rotates the product once during the entire heat treatment process.
• The HFC unit for hardening is an equipment capable of producing high-quality hardening of a product and at the same time saving production resources.

By agreement, heat treatment and hardening of metal and steel parts with dimensions larger than 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 the necessary equipment operated by qualified specialists. We carry out all orders with high quality and on time. We also accept and carry out orders for the heat treatment of steels and high frequency current coming to us and from other regions of Russia.

The main types of heat treatment of steel

Type I annealing:

Diffusion annealing of the first kind (homogenization) - Rapid heating up to t 1423 K, long holding and subsequent slow cooling. Alignment of chemical inhomogeneity of the material in large shaped alloy steel castings

Recrystallization annealing of the first kind - Heating to a temperature of 873-973 K, long holding and subsequent slow cooling. There is a decrease in hardness and an increase in plasticity after cold deformation (processing is interoperative)

Annealing of the first kind, reducing stress - Heating to a temperature of 473-673 K and subsequent slow cooling. It removes residual stresses after casting, welding, plastic deformation or machining.

Type II annealing:

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

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

Type II isothermal annealing - Heating up to a temperature of 30-50 K above the Ac3 point (for hypereutectoid steel) or above the Ac1 point (for hypereutectoid steel), holding and subsequent stepwise cooling. Accelerated machining of small rolled products or forgings made of alloy and high carbon steels in order to reduce hardness, improve machinability, 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, an improvement in machinability, an elimination of internal stresses in the tool steel before hardening, an increase in the ductility of low-alloy and medium-carbon steels before cold deformation

Light type II annealing - Heating in a controlled environment to a temperature above the Ac3 point by 20-30 K, holding and subsequent cooling in a controlled environment. Occurs Protection of the steel surface from oxidation and decarburization

Annealing of the second kind Normalization (normalization annealing) - Heating to a temperature above the Ac3 point by 30-50 K, holding and subsequent cooling in calm air. There is a correction of the structure of heated steel, removal of internal stresses in structural steel parts and improvement of their machinability, an increase in the depth of hardenability of tools. steel before hardening

Hardening:

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

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

Intermittent hardening - Heating up to t above the Ac3 point by 30-50 K (for hypoeutectoid and eutectoid steels) or between the Ac1 and Ac3 points (for hypereutectoid steel), holding and subsequent cooling in water, and then in oil. Reduces residual stresses and strains in high carbon tool steel parts

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

Step hardening - The same (differs from isothermal hardening in a shorter residence time of the part in the cooling medium). Reduces stresses, strains and prevents cracking in small carbon tool steel tools as well as larger alloy tool steel and HSS tools

Surface hardening - Heating by electric current or gas flame of the surface layer of the product to quenching t, 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

Self-tempering quenching - Heating to a temperature above the Ac3 point by 30-50 K, holding and subsequent incomplete cooling. Heat retained inside the part provides tempering of the hardened outer layer

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

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

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. Occurs Protection against oxidation and decarburization of complex parts of molds, dies and fixtures that are not subject to grinding

Vacation low - Heating in the temperature range 423-523 K and subsequent accelerated cooling. There is a release of internal stresses and a decrease in the brittleness of the cutting and measuring tools after surface hardening; for case-hardened parts after hardening

Average vacation - Heating in the interval 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

Vacation high - Heating in the temperature range 773-953 K and subsequent slow or fast cooling. Occurs Providing high ductility of structural steel parts, as a rule, during thermal improvement

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

Thermomechanical treatment - Heating, rapid cooling to 673-773 K, multiple plastic deformation, quenching and tempering. Provision for rolled products and parts of simple shape that are not welded, increased strength in comparison with the strength obtained by conventional heat treatment

Aging - Heating and long exposure at elevated temperatures. There is a stabilization of the dimensions of parts and tools

Carburizing - Saturation of the surface layer of mild steel with carbon (carburization). It is followed by a subsequent hardening with low tempering. The depth of the cemented layer is 0.5-2 mm. It imparts a high surface hardness to the product while maintaining a viscous core. Carbon or alloy steels with carbon content are subjected to cementation: for small and medium-sized products 0.08-0.15%, for larger ones 0.15-0.5%. Gear wheels, 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 undergo cyanidation, as a result of which, along with a solid surface, the product has a viscous core. Such products are characterized by high wear resistance and shock resistance.

Nitriding (nitriding) - Nitrogen saturation of the surface layer of steel products to a depth of 0.2-0.3 mm. Gives a high surface hardness, increased resistance to abrasion and corrosion. Gauges, gears, shaft journals, etc. are subjected to nitriding.

Cold Treatment - Chilled after quenching to sub-zero temperatures. There is a change in the internal structure of hardened steels. It is used for tool steels, case-hardened products, some high-alloy steels.

METALS HEAT TREATMENT (HEAT TREATMENT), a certain 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 that have a significant effect 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 that occur in the solid material. Heat treatment cycles are various combinations of heating, holding at a certain temperature and fast or slow cooling, corresponding to the structural and chemical changes that need to be caused.

Granular structure of metals. Any metal usually consists of many crystals in contact with each other (called grains), usually microscopic in size, but sometimes visible to the naked eye. Atoms inside each grain are arranged in such a way that they form a regular three-dimensional geometric lattice. The type of lattice, called crystal structure, is a characteristic of the material and can be determined by X-ray diffraction analysis methods. The correct arrangement of atoms is preserved throughout the entire grain, except for small violations, such as individual lattice sites that accidentally turn out to be vacant. All grains have the same crystal 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 the grain boundaries are easier to etch with chemical reagents. A polished flat metal surface treated with a suitable etchant usually exhibits a clear grain boundary pattern. The physical properties of a material are determined by the properties of individual grains, their effect on each other, and the properties of grain boundaries. The properties of a metallic material are critically dependent 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 rises, it becomes 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 diffusion rate, the process of transition of the nonequilibrium structure of a substance to an equilibrium one is accelerated. The temperature at which the diffusion rate noticeably increases is not the same for different metals. It is usually higher for metals with a high melting point. In tungsten, with its melting point equal to 3387 C, recrystallization does not occur even with red heat, while heat treatment of aluminum alloys melting at low temperatures, in some cases, it is possible to carry out at room temperature.

In many cases, heat treatment involves a very rapid cooling, called quenching, in order to preserve the structure formed at the elevated temperature. 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 this "metastable" structure.

The changes caused by heat treatment can be of two main types. First, both in pure metals and in alloys, changes affecting only the physical structure are possible. These can be changes in the stress 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 the smoothing of inhomogeneities in the composition and the formation of precipitates of another phase, in interaction with the surrounding atmosphere, created to purify the metal or impart specified surface properties to it. Changes of both types can occur simultaneously.

Relief of stress. Cold deformation increases the hardness and brittleness of most metals. Sometimes this "strain hardening" is desirable. Non-ferrous metals and their alloys are usually given some degree of hardness by cold rolling. Mild steels are also often cold worked hardened. High carbon steels that have been cold rolled or cold drawn to the increased strength required, for example, for the manufacture of springs, are usually subjected to stress relief annealing, heated to a relatively low temperature, at which the material remains almost as hard as before, but disappears in it. inhomogeneity of the distribution of internal stresses. This reduces the tendency to 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 by pressure, it is often required to greatly change the shape of the workpiece. If shaping is to be carried out in a cold state (which is often dictated by practical considerations), then the process has to be broken down into a number of stages, with recrystallization in between. After the first stage of deformation, when the material is hardened to such an extent that further deformation can lead to fracture, the workpiece is heated to a temperature higher than the stress relief annealing temperature and held for recrystallization. Due to the 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 the places where the old structure is most disturbed, namely at the old grain boundaries. Upon 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 previous shape, but it is now made of a soft, stress-free 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, room temperature meets this definition. If the deformation is carried out at a sufficiently high temperature so that recrystallization has time to follow the deformation of the material, then this treatment is called hot. As long as the temperature remains high enough, it can be deformed as much as you like. 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 readily recrystallizes, that is, its "hot" processing can be carried out 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 averaged over all grains. In the case of random grain orientation, the general physical properties are the same in all directions. If some crystal planes or atomic rows of most of the grains are parallel, then the properties of the sample become "anisotropic", ie, depending on the direction. In this case, the cup, obtained by deep extrusion from a circular plate, will have "tongues" or "scallops" on the upper edge, due to the fact that in some directions the material deforms more easily than in others. In mechanical shaping, anisotropy of physical properties is generally 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, in all grains coincides with the given direction of the magnetic flux. Thus, the "preferred orientation" (texture) may be desirable or undesirable depending on the purpose of the material. Generally speaking, as 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. A fine-grained structure almost always corresponds to the best mechanical properties. Reducing grain size is often one of the goals of heat treatment (as well as melting and casting). As the temperature rises, diffusion accelerates, and therefore the average grain size increases. The grain boundaries shift so that the larger grains grow at the expense of the smaller ones, which eventually disappear. Therefore, the final hot working processes are usually carried out at the lowest possible temperature so that the grain sizes are kept to a minimum. Low temperature hot working is often deliberately provided, mainly to reduce grain size, although the same result can be achieved by cold working followed by recrystallization.

Homogenization. The processes mentioned above take place both in pure metals and in alloys. But there are a number of other processes that are possible only in metallic materials containing two or more components. So, for example, in the casting of the 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 that forms at any given moment is not the same as in the liquid phase, which is in equilibrium with it. Consequently, the composition of the solid that appears at the initial moment of solidification will be different than at the end of solidification, and this leads to spatial inhomogeneity of the composition on a microscopic scale. This inhomogeneity is eliminated by simple heating, especially in combination with mechanical deformation.

Cleaning. Although the purity of the metal is primarily determined by the melting and casting conditions, the purification of the metal 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 portion of the oxides to pure metal. The depth of such cleaning depends on the ability of impurities to diffuse from the volume to the surface, and therefore is determined by the duration and temperature of heat treatment.

Isolation of secondary phases. One important effect underlies most modes of heat treatment of alloys. It is connected with the fact that the solubility in the solid state of the alloy components depends on the temperature. Unlike pure metal, in which all atoms are the same, in a two-component, for example solid, solution there are atoms of two different types, randomly distributed over the sites of the crystal lattice. If you increase the number of atoms of the second kind, then you can reach a state where they cannot simply replace the atoms of the first kind. 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, which differ in composition and structure from the initial grains and are usually scattered between them in the form of individual particles. Such second phase particles can have a profound effect on the physical properties of a 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 metal and alloy products by means of 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 of native gold and copper, primitive man faced the phenomenon of work hardening, which made it difficult to manufacture products with thin blades and sharp tips, and in order to restore plasticity, the blacksmith had to heat cold forged copper in the hearth. The earliest evidence of the use of softening annealing of hardened metal dates back to the end of the 5th millennium BC. NS. Such annealing was, in terms of the time of its appearance, the first operation of the heat treatment of metals. In the manufacture of weapons and tools from iron obtained using the raw-blown 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 took place, one of the varieties of chemical-thermal treatment. 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. Water quenching of carburized iron has been used since the end of the 2nd early 1st millennium BC. NS. Homer's Odyssey (8th-7th centuries BC) contains the following lines: "How a blacksmith immerses a red-hot ax or an ax into cold water, and the iron hiss with a gurgle is stronger than iron happens, being tempered in fire and water." In the 5th century. BC NS. Etruscans quenched high-tin bronze mirrors in water (most likely to improve brilliance during polishing). Cementation of iron in charcoal or organic matter, hardening and tempering of steel was 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 ascribed the obtaining 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 requirements for the development of technology, and, first of all, for the development of steel cannon production, led to the transformation of heat treatment of metals from art into science. In the middle of the 19th century, when the army was striving to replace bronze and cast-iron cannons with more powerful steel ones, the problem of making 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 Works in St. Petersburg, studying etched thin sections prepared from the muzzles of guns under a microscope, and observing the structure of fractures at the rupture site under a magnifying glass, concluded that steel is stronger, the finer its structure. In 1868 Chernov discovered internal structural transformations in cooling steel that occur at certain temperatures. which he called the critical points a and b. If the steel is heated to temperatures below point a, then it cannot be hardened, and to obtain a fine-grained structure, the steel must be heated to temperatures above point b. The discovery by Chernov of the critical points of structural transformations in steel made it possible to scientifically select the Heat Treatment mode to obtain the required properties of steel products.

In 1906 A. Wilm (Germany) discovered aging after hardening on the duralumin invented by him (see Aging of metals) the most important way strengthening of alloys on different bases (aluminum, copper, nickel, iron, etc.). In the 30s. 20th century thermomechanical treatment of aging copper alloys appeared, and in the 50s thermomechanical treatment of steels, which made it possible to significantly increase the strength of products. The 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 action was a harmonious theory of heat treatment of metals.

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

Nitriding - saturation of the surface of metal parts with nitrogen in order to increase hardness, wear resistance, fatigue limit and corrosion resistance. Steel, titanium, some alloys, most often alloyed steels, especially chromium-aluminum, as well as steel containing vanadium and molybdenum, are subjected to nitriding.
Steel nitriding occurs at t 500 650 C in ammonia. Above 400 C, the dissociation of ammonia begins according to the reaction NH3 '3H + N. The formed 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 Fe2N nitride, ³ Fe4N nitride, ± nitrogenous ferrite containing about 0.01% nitrogen at room temperature. and the ³-phase, which, as a result of slow cooling, decomposes at 591 C into a eutectoid ± + ³ 1. The hardness of the nitrided layer increases to HV = 1200 (corresponding to 12 H / m2) and remains with repeated heating up to 500 600 C, which ensures high wear resistance of parts at elevated temperatures. Nitrided steels are significantly superior in wear resistance to case-hardened and hardened steels. Nitriding is a long process, it takes 20-50 hours to obtain a layer with a thickness of 0.2 0.4 mm. subject to nitriding, tinning (for structural steels) and nickel plating (for stainless and heat-resistant steels) are used. The hardness of the nitriding layer 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 because of the increased brittleness of the metal).

During nitriding, an upper thin nitride layer and a solid solution of nitrogen in ± titanium are formed. The layer depth in 30 h is 0.08 mm with a surface hardness of HV = 800 850 (corresponds to 8 8.5 H / m2). The introduction of some alloying elements into the alloy (up to 3% Al, 3 5% Zr, etc.) increases the rate of nitrogen diffusion, 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 t up to 500 600 C (cylinder liners, crankshafts, gears, valve pairs, parts fuel equipment and etc.).
Lit .: Minkevich A.N., Chemical heat treatment of metals and alloys, 2nd ed., M., 1965: Gulyaev A.P. Metallovedenie, 4th ed., M., 1966.

Induction heating occurs by placing the workpiece close to an alternating electrical current conductor called an inductor. When a high-frequency current (HFC) passes through the inductor, an electromagnetic field is created and, if a metal product is located in this field, an electromotive force is excited in it, which causes an alternating current of the same frequency as the inductor current to pass through the product.

Thus, a thermal effect is induced, which causes the product to heat up. The heat power P, released in the heated part, will be equal to:

where K is a coefficient depending on the configuration of the product and the size of the gap formed between the surfaces of the product and the inductor; Iin - current strength; f - current frequency (Hz); r - electrical resistivity (Ohm · cm); m - magnetic permeability (H / E) of steel.

The process of induction heating is significantly influenced by a physical phenomenon called the surface (skin) effect: the current is induced mainly in the surface layers, and at high frequencies the current density in the core of the part is low. The depth of the heated layer is estimated by the formula:

Increasing the frequency of the current allows you to concentrate significant power in a small volume of the heated part. Due to this, high-speed (up to 500 C / sec) heating is realized.

Induction heating parameters

Induction heating is characterized by three parameters: specific power, heating duration and current frequency. Specific power is the power converted into heat per 1 cm2 of the surface of the heated metal (kW / cm2). The rate of heating of the product depends on the value of the specific power: the higher it is, the faster the heating is carried out.

The heating time determines the total amount of heat energy transferred, and therefore the temperature reached. It is also important to take into account the frequency of the current, since the depth of the hardened layer depends on it. The frequency of the current and the depth of the heated layer are in the opposite relationship (second formula). The higher the frequency, the smaller the heated metal volume. Choosing the value of the specific power, the heating duration and the current frequency, it is possible to vary the final parameters of induction heating within a wide range - the hardness and depth of the hardened layer during quenching or the heated volume when heated for stamping.

In practice, the controlled heating parameters are the electrical parameters of the current generator (power, current, voltage) and the heating duration. With the help of pyrometers, the heating temperature of the metal can also be recorded. But more often there is no need for constant temperature control, since the optimal heating mode is selected, which ensures a constant quality of hardening or heating of the HFC. The optimal hardening mode is selected by changing the electrical parameters. In this way, several parts are hardened. Further, the parts are subjected to laboratory analysis with fixing the hardness, microstructure, distribution of the hardened layer in depth and plane. When subcooled, residual ferrite is observed in the structure of hypoeutectoid steels; coarse acicular martensite arises when overheated. The signs of defects when the HDTV is heated are the same as when classical technologies heat treatment.

In the case of surface hardening with HFC, heating is carried out to a higher temperature than in the case of conventional bulk hardening. This is due to two reasons. First, at a very high heating rate, the temperatures of the critical points at which the transition of pearlite to austenite occurs increase, and secondly, this transformation must have time to complete in a very short heating and holding time.

Despite the fact that heating during high-frequency quenching is carried out to a higher temperature than during normal quenching, the metal does not overheat. This is due to the fact that the grain in steel simply does not have time to grow in a very short period of time. It should also be noted that, in comparison with volume quenching, the hardness after hardening with HFC is higher by about 2–3 HRC units. This provides a higher wear resistance and surface hardness of the part.

Advantages of high frequency quenching

• high process productivity
• ease of adjusting the thickness of the hardened layer
• minimal warpage
• almost complete absence of scale
• the ability to fully automate the entire process
• the possibility of placing a hardening unit in the flow of machining.

Most often, parts made of carbon steel with a content of 0.4-0.5% C are subjected to surface high-frequency hardening. These steels, after quenching, have a surface hardness of HRC 55-60. At higher carbon contents, there is a risk of cracking due to sudden cooling. Along with carbon steel, low-alloyed chromium, chromium-nickel, chromium-silicon and other steels are also used.

Equipment for performing induction hardening (HFC)

Induction hardening requires special technological equipment, which includes three main units: a power source - a generator of high-frequency currents, an inductor and a device for moving parts in the machine.

A high-frequency current generator is electrical machines that differ in the physical principles of the formation of an electric current in them.

1. Electronic devices operating on the principle of electronic tubes that convert direct current into alternating current of increased frequency - tube generators.
2. Electromachine devices operating on the principle of directing an electric current in a conductor, moving in a magnetic field, converting a three-phase current of industrial frequency into alternating current of increased frequency - machine generators.
3. Semiconductor devices operating on the principle of thyristor devices that convert direct current into alternating current of increased frequency - thyristor converters (static generators).

Generators of all types differ in frequency and power of the generated current

Generator types Power, kW Frequency, kHz Efficiency

Tube 10 - 160 70 - 400 0.5 - 0.7

Machine 50 - 2500 2.5 - 10 0.7 - 0.8

Thyristor 160 - 800 1 - 4 0.90 - 0.95

Surface hardening of small parts (needles, contacts, spring tips) is carried out using micro-induction generators. The frequency generated by them reaches 50 MHz, the heating time for hardening is 0.01-0.001 s.

HFC hardening methods

According to the performance of heating, induction continuous-sequential hardening and simultaneous hardening are distinguished.

Continuous sequential hardening used for long parts of constant cross-section (shafts, axles, flat surfaces of long products). The heated part moves in the inductor. The part of the part, which is at a certain moment in the zone of the inductor's influence, is heated to the hardening temperature. At the exit from the inductor, the section enters the spray cooling zone. The disadvantage of this heating method is the low productivity of the process. To increase the thickness of the hardened layer, it is necessary to increase the heating duration by reducing the speed of movement of the part in the inductor. Simultaneous hardening assumes a one-time heating of the entire surface to be hardened.

Self-tempering effect after quenching

After completion of heating, the surface is cooled by a shower or a stream of water directly in the inductor or in a separate cooling device. This cooling allows quenching of any configuration. By metering the cooling and changing its duration, it is possible to realize the effect of self-tempering in steel. This effect consists in the removal of heat accumulated during heating in the core of the part to the surface. In other words, when the surface layer has cooled and underwent martensitic transformation, a certain amount of thermal energy is still stored in the subsurface layer, the temperature of which can reach the low tempering temperature. After stopping cooling, this energy will be removed to the surface due to the temperature difference. Thus, there is no need for additional steel tempering operations.

Design and manufacture of inductors for HFC hardening

The inductor is made of copper tubes through which water is passed during the heating process. This prevents overheating and burnout of the inductors during operation. Inductors are also made, combined with a hardening device - a sprayer: on the inner surface of such inductors there are holes through which coolant flows to the heated part.

For uniform heating, it is necessary to manufacture the inductor in such a way that the distance from the inductor to all points on the surface of the product is the same. Usually this distance is 1.5-3 mm. When quenching a product of a simple shape, this condition is easily met. For uniform hardening, the part must be moved and (or) rotated in the inductor. This is achieved by using special devices - centers or hardening tables.

The development of the design of the inductor presupposes, first of all, the determination of its shape. In this case, they are repelled from the shape and dimensions of the hardened product and the hardening method. In addition, in the manufacture of inductors, the nature of the movement of the part relative to the inductor is taken into account. The economy and heating performance are also taken into account.

Parts cooling can be used in three ways: water spraying, water flow, part immersion in a quenching medium. Shower cooling can be carried out both in inductors-sprayers and in special quenching chambers. Cooling by a flow allows creating an overpressure of the order of 1 atm, which contributes to a more uniform cooling of the part. To ensure intensive and uniform cooling, it is necessary that the water moves along the cooled surface at a speed of 5-30 m / s.

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

Description of the method.

Induction heating is the heating of materials by electric currents that 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 an 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 more turns of wire (most often copper). In the inductor, with the help of a special generator, powerful currents of various frequencies (from ten Hz to several MHz) are induced, as a result of which an electromagnetic field arises 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 workpiece inductor system is a coreless transformer in which the inductor is the primary winding. The workpiece is a short-circuited secondary winding. The magnetic flux between the windings is closed in the air.

At a high frequency, eddy currents are displaced by the magnetic field formed by them into the 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 metal layers 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 a factor of e relative to the current density on the surface of the workpiece, while 86.4% of 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 depth of the skin layer for copper is about 0.25 mm, for iron ≈ 0.001 mm.

The inductor gets very hot during operation, as it absorbs its own radiation. In addition, it absorbs heat 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 burn-through or other depressurization of the inductor.

Application:
Ultrapure non-contact metal melting, brazing and welding.
Obtaining prototypes of alloys.
Bending and heat treatment of machine parts.
Jewelry making.
Processing small parts that can be damaged by flame or arc heating.
Surface hardening.
Quenching and heat treatment of complex-shaped parts.
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-conductive liquid, 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 the operation of the installation. Only electrically conductive material is heated - metal (including molten), carbon, conductive ceramics, electrolytes, liquid metals, etc.

Due to the arising MHD forces, the liquid metal is intensively mixed, up to keeping it suspended in air or shielding gas - this is how ultrapure alloys are obtained in small quantities (levitation melting, melting in an electromagnetic crucible).

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

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

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

Local and selective heating is easy.

Since the heating is most intense 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 control and retention, supply and removal of workpieces.

Induction heating installations:

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

Also, for heating small parts, installations of increased frequency on MOSFET transistors are being built for operating frequencies up to 1.7 MHz. Controlling transistors and protecting them at higher frequencies presents certain difficulties, therefore, higher frequency settings are still quite expensive.

An inductor for heating small parts has a small size and low inductance, which leads to a decrease in the quality factor of the operating oscillatory circuit at low frequencies and a decrease in efficiency, and also poses a danger to the master oscillator (the quality factor of the oscillating circuit is proportional to L / C, an oscillating 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:
- an increase in the operating frequency, which leads to the complication and rise in the cost of the installation;
- the use of ferromagnetic inserts in the inductor; gluing 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 powerful generator lamps. Before World War I, induction heating was of limited use. At that time, machine generators of increased frequency (the work of V.P. Vologdin) or spark discharge installations were used as generators.

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

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

Choose a scheme according to following criteria: reliability; stability of fluctuations; stability of the power released in the workpiece; ease of manufacture; ease of customization; the minimum number of parts to reduce cost; the use of parts that together give a reduction in weight and dimensions, etc.

For many decades, an inductive three-point was used as a generator of high-frequency oscillations (Hartley generator, generator with autotransformer feedback, circuit on an inductive loop voltage divider). This is a self-excited circuit of parallel power supply of the anode and a frequency-selective circuit made on an oscillatory 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 rollers of the T-34 tank was carried out on such installations.

Disadvantages of the 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 (≈700С) (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 TV-sets should operate in a narrow range of frequencies allowed by Rossvyazokhrankultura, 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, a smaller one for 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 from single-turn inductors to multi-turn inductors, 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%), as well as better maintain the operating frequency. Their principle of operation is as follows. Due to the use of coupled circuits and weakening the connection between them, a change in the inductance of the working circuit does not entail a strong change in the frequency of the frequency setting circuit. Radio transmitters are designed according to the same principle.

Modern TVF generators are inverters based on IGBT assemblies or powerful MOSFET transistors, usually made in a bridge or half-bridge scheme. Operate at frequencies up to 500 kHz. The gates of the transistors 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 starts to heat up worse. Also, the magnetic properties of the material disappear and the process of magnetization reversal stops - the workpiece begins to heat up worse, the load resistance abruptly decreases - this can lead to the "separation" of the generator and its failure. The control system monitors the transition through the Curie point and automatically increases the frequency when the load is suddenly reduced (or decreases the power).

Remarks.

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

Increasing the frequency dramatically decreases the power factor (proportional to the cube of the frequency).

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

When calculating the inductor, it is necessary to take into account the inductance of the buses supplying 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 a 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 loop resistance between the branch points becomes maximum, and the current (I total) through the load resistance Rн will be minimal (the current inside the loop I-1L and I-2c is greater than the generator current).

Ideally, the loop impedance is infinity - the circuit does not draw any current from the source. When the frequency of the generator changes in either direction from the resonant frequency, the total resistance of the circuit decreases and the line current (I total) increases.

Serial oscillatory circuit - voltage resonance.

The main feature of a series resonant circuit is that its impedance is minimal at resonance. (ZL + ZC - minimum). When the frequency is tuned to a value greater than or below the resonant frequency, the impedance increases.
Output:
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 is taken from the site http://dic.academic.ru/ and reworked into a text that is more understandable for the reader by the company "Prominductor".

Quenching installation for heating t. V. h. consists of a generator so-called. h.,

a step-down transformer, capacitor banks, an inductor, a machine tool (sometimes the machine is replaced with a device for driving a part or an inductor) and equipment carrying an auxiliary service (time relay, quenching liquid supply control relay, signaling, blocking and regulating devices).

In the considered installations, such generators t.v.ch. at medium frequencies (500-10000 Hz), machine generators, and recently static thyristor-type converters; at high frequencies (60,000 Hz and above) tube generators. A promising type of generators are ion converters, the so-called excitron generators. They allow you to keep energy losses to a minimum.

In fig. 5 shows a diagram of an installation with a machine generator. Except for the machine generator 2 and engine 3 with exciter 1, the installation contains a step-down transformer 4, capacitor banks 6 and inductor 5. The transformer lowers the voltage to a safe (30-50 V) and at the same time increases the current strength 25-30 times, bringing it to 5000-8000 A.

Picture 5 Picture 6

Table 1 Types and designs of inductors

In Fig. 6 shows an example of hardening with a multi-turn inductor. Quenching is carried out as follows:

The part is placed inside a stationary inductor. With the launch of the HDTV apparatus, the part begins to rotate around its axis and at the same time heats up, then, with the help of automated control, liquid (water) is supplied and cools down. The whole process lasts from 30-45 seconds.

HFC hardening is a type of metal heat treatment, as a result of which the hardness increases significantly and the material loses its ductility. The difference between HFC hardening and other hardening methods is that heating is performed using special HDTV installations which act on the part to be hardened with high-frequency currents. HFC quenching has many advantages, the main one being full control over heating. The use of these hardening complexes can significantly improve the quality of the products, since the hardening process is carried out in a fully automatic mode, the operator's work consists only in securing the shaft and starting the machine operation cycle.

5.1. Advantages of induction hardening complexes (induction heating installations):

HFC hardening can be performed with an accuracy of 0.1 mm

Providing uniform heating, induction hardening allows you to achieve an ideal distribution of hardness along the entire length of the shaft

High hardness of HFC quenching is achieved through the use of special inductors with water conduits, which cool the shaft immediately after warming up.

HFC quenching equipment (quenching furnaces) is selected or manufactured in strict accordance with the technical specifications.

6.Descaling in shot blasting machines

In shot blasting machines, parts are cleaned from scale with a jet of cast iron or steel shot. The jet is created by compressed air with a pressure of 0.3-0.5 MPa (pneumatic shot blasting) or fast-rotating impeller wheels (mechanical cleaning with shot blades).

At pneumatic shot blasting in installations, both shot and quartz sand can be used. However, in the latter case, a large amount of dust is formed, reaching 5-10% of the mass of the parts to be cleaned. Getting into the lungs of maintenance personnel, quartz dust causes an occupational disease - silicosis. Therefore, this method is used in exceptional cases. When blasting, the compressed air pressure should be 0.5-0.6 MPa. Cast iron shot is made by casting liquid iron into water by spraying a stream of cast iron with compressed air, followed by sorting on sieves. The shot must have the structure of white cast iron with a hardness of 500 HB, its dimensions are in the range of 0.5-2 mm. Cast iron shot consumption is only 0.05-0.1% of the mass of parts. When cleaning with shot, a cleaner surface of the part is obtained, a higher productivity of the apparatus is achieved and better working conditions are provided than when cleaning with sand. To protect the ambient atmosphere from dust, shot blasting machines are equipped with closed hoods with enhanced exhaust ventilation. According to sanitary standards, the maximum permissible dust concentration should not exceed 2 mg / m3. Shot transportation in modern installations is fully mechanized.

The main part of the pneumatic installation is a shot blasting machine, which can be injection and gravity. The simplest single-chamber injection shot blasting machine (Fig. 7) is a cylinder 4, with a funnel for shot at the top, hermetically sealed with a lid 5. At the bottom, the cylinder ends with a funnel, the hole from which leads to the mixing chamber 2. The shot is fed by a rotary flap 3. Compressed air is supplied to the mixing chamber through the valve 1, which captures the shot and transports it through a flexible hose 7 and a nozzle 6 for details. The shot is under the pressure of compressed air until it expires from the nozzle, which increases the efficiency of the abrasive jet. In the apparatus of the described single-chamber design, the compressed air must be temporarily turned off when it is replenished with shot.