Installing HDTV for hardening the principle of operation. Hardening of metals by high frequency currents

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 operated by qualified professionals. We carry out all orders with high quality and on time. We also accept and fulfill orders for heat treatment of steels and HDTV coming to us from other regions of Russia.

The main types of heat treatment of steel

Annealing of the first kind:

Annealing of the first kind diffusion (homogenization) - Rapid heating to t 1423 K, long exposure and subsequent slow cooling. Alignment of the chemical heterogeneity of the material in large shaped castings from alloy steel

Annealing of the first kind recrystallization - 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 inter-operational)

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

Annealing of the second kind:

Annealing of the second kind is complete - 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 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 points Ac1 and Ac3, exposure and subsequent cooling. There is a decrease in hardness, improvement of machinability, removal of internal stresses in hypereutectoid steel before hardening

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

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

Annealing of the second kind bright - Heating in a controlled environment to a temperature above the Ac3 point by 20-30 K, exposure 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, exposure and subsequent cooling in still air. There is a correction of the structure of heated steel, the removal of internal stresses in parts made of structural steel and an improvement in their machinability, an increase in the depth of tool hardenability. steel before hardening

Hardening:

Full continuous hardening - Heating to a temperature above the Ac3 point by 30-50 K, holding and subsequent rapid 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, exposure and subsequent rapid cooling. Obtaining (in combination with tempering) high hardness and wear resistance of parts from hypereutectoid steel

Intermittent hardening - Heating to t above the Ac3 point by 30-50 K (for hypoeutectoid and eutectoid steels) or between Ac1 and Ac3 points (for hypereutectoid steel), exposure and subsequent cooling in water, and then in oil. There is a decrease 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 plasticity, endurance limit and bending resistance of parts made of alloyed tool steel

Step hardening - The same (it differs from isothermal hardening by a shorter time spent in the cooling medium). Reduction of stresses, deformations and prevention of cracking in small tools made of carbon tool steel, as well as in larger tools made of alloyed tool and high speed steel

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

Quenching 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

Hardening with cold treatment - Deep cooling after hardening to a temperature of 253-193 K. An increase in hardness and obtaining stable dimensions of high-alloy steel parts occurs

Hardening with cooling - Heated parts are cooled in air for some time before being immersed in a cooling medium or kept in a thermostat with reduced t. There is a reduction in the heat treatment cycle of steel (usually used after carburizing).

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

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

Holiday medium - 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

Holiday high - Heating in the temperature range of 773-953 K and subsequent slow or fast cooling. Provision of high ductility of parts made of structural steel, as a rule, with thermal improvement

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

Thermomechanical processing - Heating, rapid cooling to 673-773 K, multiple plastic deformation, hardening and tempering. There is a provision for rolled products and parts of a simple shape that are not subjected to welding, increased strength compared to the strength obtained by conventional heat treatment

Aging - Heating and prolonged exposure to elevated temperatures. Parts and tools are dimensionally stabilized

Carburizing - Saturation of the surface layer of mild steel with carbon (carburization). Accompanied by subsequent quenching with low tempering. The depth of the cemented layer is 0.5-2 mm. There is a Giving to a product of high surface hardness with preservation of a viscous core. Carburizing is carried out on carbon or alloy steels with a carbon content: for small and medium-sized products 0.08-0.15%, for larger ones 0.15-0.5%. Gear wheels, piston pins, etc. are carburized.

Cyaniding - 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 (0.15-0.3 mm layer). Such products are characterized by high wear resistance and resistance to impact loads.

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

Cold treatment - Cooling after hardening to a temperature below zero. There is a change in the internal structure of hardened steels. It is used for tool steels, case-hardened products, some high-alloy steels.

HEAT TREATMENT OF METALS (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 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, corresponding to the structural and chemical changes that are required to cause.

Grain structure of metals. Any metal usually consists of many crystals (called grains) in contact with each other, usually microscopic in size, but sometimes visible to the naked eye. Inside each grain, the atoms 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 a material and can be determined by X-ray diffraction analysis. The correct arrangement of atoms is preserved within the entire grain, except for small disturbances, such as individual lattice sites that accidentally turn out to be vacant. All grains have the same crystal structure, but, as a rule, are differently oriented in space. Therefore, at the boundary of two grains, the atoms are always less ordered than inside them. This explains, in particular, the fact that grain boundaries are easier to etch with chemical reagents. On a polished flat metal surface treated with a suitable etchant, a clear pattern of grain boundaries is usually revealed. The physical properties of a material are determined by the properties of individual grains, their interaction with each other, and the properties of grain boundaries. The properties of the metallic material are highly dependent on the size, shape and orientation of the grains, and the aim of heat treatment is to control these factors.

Atomic processes during heat treatment. With an increase in the temperature of a solid crystalline material, 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 efficient 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 a non-equilibrium structure of a substance into 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 of 3387 C, recrystallization does not occur even at red heat, while heat treatment aluminum alloys, melting at low temperatures, in some cases it is possible to carry out at room temperature.

In many cases, heat treatment involves very rapid cooling, called quenching, in order to preserve the structure formed at 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. Very many useful alloys have a similar "metastable" structure.

Changes caused by heat treatment can be of two main types. First, both in pure metals and in alloys, changes are possible that affect only the physical structure. These can be changes in the stress state of the material, changes in 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 compositional inhomogeneities and the formation of precipitates of another phase, in interaction with the surrounding atmosphere, created to clean the metal or give it the desired surface properties. Changes of both types can occur simultaneously.

Relieve stress. Cold deformation increases the hardness and brittleness of most metals. Sometimes such "work hardening" is desirable. Non-ferrous metals and their alloys are usually given some degree of hardness by cold rolling. Mild steels are also often hardened by cold forming. High-carbon steels that have been cold-rolled or cold-drawn to the increased strength required, for example, for making springs, are usually subjected to stress-relieving 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 crack, 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 shaping must be carried out in a cold state (which is often dictated by practical considerations), then it is necessary to break the process into a number of steps, in between them carrying out recrystallization. After the first stage of deformation, when the material is strengthened to such an extent that further deformation may lead to fracture, the workpiece is heated to a temperature above the stress relief annealing temperature and allowed to recrystallize. Due to rapid diffusion at this temperature, a completely new structure is formed due to atomic rearrangement. Inside the grain structure of the deformed material, new grains begin to grow, which over time completely replace it. First, small new grains are formed in places where the old structure is most disturbed, namely, at the old grain boundaries. Upon further annealing, the atoms of the deformed structure rearrange themselves in such a way that they also become part of the new grains, which grow and eventually absorb the entire old structure. The workpiece retains its former shape, but it is now made of a soft, unstressed material that can be subjected to a new cycle of deformation. Such a 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 the deformation is carried out at a sufficiently high temperature so that recrystallization has time 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 arbitrarily. The hot state of a metal is determined primarily by how close its temperature is to the 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 crystalline structure. The properties of the 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, on the other hand, some crystal 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 upper edge, due to the fact that in some directions the material is deformed more easily than in others. In mechanical shaping, the anisotropy of physical properties is, as a rule, undesirable. But in sheets of magnetic materials for transformers and other devices, it is highly 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, "preferred orientation" (texture) may or may not be desirable, 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, and also 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 corresponds to a fine-grained structure. Grain size reduction 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 as small as possible. Low-temperature hot working is often deliberately provided, mainly to reduce the grain size, although the same result can be achieved by cold working followed by recrystallization.

Homogenization. The processes mentioned above occur 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 an alloy, there will almost certainly be inhomogeneities in the chemical composition, which is determined by an uneven solidification process. In a hardening alloy, the composition of the solid phase formed in each this moment, is not the same as in the liquid, which is in equilibrium with it. Therefore, the composition of the solid formed in initial moment solidification will be different than at the end of solidification, and this leads to spatial heterogeneity of the composition on a microscopic scale. Such inhomogeneity is eliminated by simple heating, especially in combination with mechanical deformation.

Cleaning. Although the purity of the metal is determined primarily by the conditions of melting and casting, metal purification 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 a 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 the heat treatment.

Separation of secondary phases. Most of the regimes of heat treatment of alloys are based on one important effect. It is related to the fact that the solubility in the solid state of the alloy components depends on temperature. Unlike a 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 nodes of the crystal lattice. If you increase the number of second-class atoms, you can reach a state where they cannot simply replace the first-class atoms. If the amount of the second component exceeds this limit of solubility 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 strong influence on the physical properties of the material, depending on their size, shape and distribution. These factors can be changed by heat treatment (heat treatment).

Heat treatment - the process of processing products made of metals and alloys by thermal exposure 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 Eneolithic era, using cold forging native gold and copper, primitive man encountered 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. e. Such annealing was the first operation of heat treatment of metals by the time of its appearance. In the manufacture of weapons and tools from iron obtained using the cheese-blowing process, the blacksmith heated the iron billet for hot forging in a charcoal furnace. At the same time, iron was carburized, that is, cementation occurred, 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 improvement in other properties. Hardening of carburized iron in water was used from the end of the 2nd to the beginning of the 1st millennium BC. e. In the "Odyssey" of Homer (8-7 centuries BC) there are such lines: "How a blacksmith plunges a red-hot ax or an ax into cold water, and the iron will hiss with a gurgle stronger iron sometimes, hardening in fire and water. "In the 5th century BC, the Etruscans hardened mirrors made of high-tin bronze in water (most likely to improve polishing gloss). Cementation of iron in charcoal or organic matter, hardening and tempering of steel were widely used in the Middle Ages in the manufacture of knives, swords, files, and other tools. Not knowing the essence of internal transformations in metal, medieval craftsmen often attributed the obtaining of high properties during the heat treatment of metals to the manifestation of supernatural forces. Until the middle of the 19th century. man's knowledge of the heat treatment of metals was a collection of recipes developed on the basis of centuries of experience. The needs of the development of technology, and primarily the development of steel cannon production, led to the transformation of heat treatment of metals from art to science. In the middle of the 19th century, when the army sought 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 plant in St. Petersburg, studying etched sections prepared from gun barrels under a microscope and observing the structure of fractures at the point of rupture under a magnifying glass, concluded that steel is the stronger, 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 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. Chernov's discovery of critical points of structural transformations in steel made it possible to scientifically justify the choice of 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 quenching (see Aging of metals), the most important method for hardening alloys based on various bases (aluminum, copper, nickel, iron, etc.). In the 30s. 20th century thermomechanical treatment of aging copper alloys appeared, and in the 1950s thermomechanical treatment of steels, which made it possible to significantly increase the strength of products. Combined types of heat treatment include thermomagnetic treatment, which makes it possible, as a result of cooling products in a magnetic field, to improve some of their magnetic properties.

Numerous studies of changes in the structure and properties of metals and alloys under thermal action have resulted in a coherent theory of heat treatment of metals.

The classification of types of heat treatment is based on what type of structural changes in the metal occur during thermal exposure. 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. Actually 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, as well as steel containing vanadium and molybdenum.
Nitriding of steel occurs at t 500 650 C in ammonia. Above 400 C, the dissociation of ammonia begins 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 Fe2N nitride, ³ "Fe4N nitride, ± nitrogenous ferrite containing about 0.01% nitrogen at room temperature. At a nitriding temperature of 600 650 C, more and 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 Gn/m2) and is retained upon repeated heating up to 500-600 C, which ensures high wear resistance of parts at elevated temperatures Nitriding steels are significantly superior in wear resistance to hardened and hardened steels Nitriding is a long process, it takes 20-50 hours to obtain a layer of 0.2-0.4 mm thickness Raising 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. The elasticity 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 due to the increase in the brittleness of the metal).

During nitriding, an upper thin nitride layer and a solid solution of nitrogen in ±-titanium are formed. Layer depth for 30 hours 0.08 mm with surface hardness HV = 800 850 (corresponds to 8 8.5 H/m2). The introduction of some alloying elements (Al up to 3%, Zr 3 5%, etc.) into the alloy increases the diffusion rate of nitrogen, increasing the depth of the nitrided layer, and chromium reduces the diffusion rate. 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 treatment of metals and alloys, 2nd ed., M., 1965: Gulyaev A.P. Metallurgy, 4th ed., M., 1966.

High frequency currents are able to ideally cope with a variety of metal heat treatment processes. The HDTV installation is perfect for hardening. To date, there is no equipment that could compete on equal terms with induction heating. Manufacturers began to pay more and more attention to induction equipment, acquiring it for processing products and melting metal.

What is a good HDTV installation for hardening

The HDTV installation is a unique equipment capable of processing metal with high quality in a short period of time. To perform each function, you should select a specific installation, for example, for hardening, it is best to purchase a ready-made HDTV hardening complex, in which everything is already designed for comfortable hardening.
The HDTV installation has a wide list of advantages, but we will not consider everything, but will focus on those that are specifically suitable for HDTV hardening.

The HDTV installation heats up in a short period of time, starting to quickly process the metal. When using induction heating, there is no need to spend additional time on intermediate heating, as the equipment immediately begins to process the metal.
Induction heating does not require additional technical means, such as the use of quenching oil. The product is of high quality, and the number of defects in production is significantly reduced.
The HDTV installation is completely safe for the employees of the enterprise, and is also easy to operate. There is no need to hire highly qualified personnel to run and program the equipment.
High-frequency currents make it possible to work deeper hardening, since heat under the influence of an electromagnetic field is able to penetrate to a given depth.

The HDTV installation has a huge list of advantages, which can be listed for a long time. Using HDTV heating for hardening, you will significantly reduce energy costs, and also get the opportunity to increase the level of productivity of the enterprise.

HDTV installation - the principle of operation for hardening

The HDTV installation works on the basis of the principle of induction heating. The Joule-Lenz and Faraday-Maxwell laws on the conversion of electrical energy were taken as the basis of this principle.
Generator feeds electrical energy, which passes through the inductor, transforming into a powerful electromagnetic field. The eddy currents of the formed field begin to act and, penetrating into the metal, are transformed into thermal energy starting to process the product.

Hardening of steels by high frequency currents (HF) is one of the most common methods of surface heat treatment, which makes it possible to increase the hardness of the surface of workpieces. It is used for parts made of carbon and structural steels or cast iron. HFC induction hardening is one of the most economical and technologically advanced methods of hardening. It makes it possible to harden the entire surface of the part or its individual elements or zones that experience the main load.

In this case, non-hardened viscous layers of metal remain under the hardened solid outer surface of the workpiece. Such a structure reduces brittleness, increases the durability and reliability of the entire product, and also reduces energy consumption for heating the entire part.

High frequency hardening technology

HFC surface hardening is a heat treatment process to improve the strength characteristics and hardness of the workpiece.

The main stages of surface hardening of HDTV are induction heating to a high temperature, holding at it, then rapid cooling. Heating during hardening of HDTV is carried out using a special induction unit. Cooling is carried out in a bath with a coolant (water, oil or emulsion) or by spraying it onto the part from special shower installations.

Temperature selection

For the correct passage of the hardening process, the correct selection of temperature is very important, which depends on the material used.

According to the carbon content, steels are divided into hypoeutectoid - less than 0.8% and hypereutectoid - more than 0.8%. Steel with carbon less than 0.4% is not hardened due to the resulting low hardness. Hypoeutectoid steels are heated slightly above the phase transformation temperature of pearlite and ferrite to austenite. This occurs in the range of 800-850°C. Then the workpiece is rapidly cooled. When cooled abruptly, austenite transforms into martensite, which has high hardness and strength. A short holding time makes it possible to obtain fine-grained austenite and fine-acicular martensite, the grains do not have time to grow and remain small. This steel structure has high hardness and at the same time low brittleness.

Hypereutectoid steels are heated slightly lower than hypoeutectoid ones, to a temperature of 750-800 ° C, that is, incomplete hardening is performed. This is due to the fact that when heated to this temperature, in addition to the formation of austenite in the metal melt, a small amount of cementite remains undissolved, which has a higher hardness than that of martensite. After rapid cooling, austenite transforms into martensite, while cementite remains in the form of fine inclusions. Also in this zone, carbon that has not had time to completely dissolve forms solid carbides.

In the transition zone during hardening of high-frequency current, the temperature is close to the transition one, and austenite is formed with residual ferrite. But, since the transition zone does not cool down as quickly as the surface, but cools down slowly, as during normalization. At the same time, the structure improves in this zone, it becomes fine-grained and uniform.

Overheating of the workpiece surface promotes the growth of austenite crystals, which has a detrimental effect on brittleness. Underheating does not allow a completely ferritic-perritic structure to pass into austenite, and unquenched spots can form.

After cooling, high compressive stresses remain on the metal surface, which increase the operational properties of the part. Internal stresses between the surface layer and the middle must be eliminated. This is done using low-temperature tempering - holding at a temperature of about 200 ° C in an oven. To avoid the appearance of microcracks on the surface, it is necessary to minimize the time between quenching and tempering.

It is also possible to carry out the so-called self-tempering - to cool the part not completely, but to a temperature of 200 ° C, while it will remain warm in its core. Further, the part should cool slowly. This will equalize the internal stresses.

induction plant

The HDTV induction heat treatment plant is a high-frequency generator and an inductor for HDTV hardening. The part to be hardened can be located in the inductor or near it. The inductor is made in the form of a coil, a copper tube is wound on it. It can have any shape depending on the shape and dimensions of the part. When an alternating current passes through the inductor, an alternating electromagnetic field appears in it, passing through the part. This electromagnetic field induces eddy currents in the workpiece, known as Foucault currents. Such eddy currents, passing through the metal layers, heat it to a high temperature.

A distinctive feature of induction heating using HDTV is the passage of eddy currents on the surface of the heated part. So only the outer layer of the metal is heated, and the higher the frequency of the current, the smaller the depth of heating, and, accordingly, the depth of hardening of the HDTV. This makes it possible to harden only the surface of the workpiece, leaving the inner layer soft and viscous to avoid excessive brittleness. Moreover, it is possible to adjust the depth of the hardened layer by changing the current parameters.

The increased frequency of the current allows a large amount of heat to be concentrated in a small area, which increases the heating rate to several hundred degrees per second. This high heating rate moves phase transition to the higher temperature zone. In this case, the hardness increases by 2-4 units, up to 58-62 HRC, which cannot be achieved with bulk hardening.

For the correct course of the HDTV hardening process, it is necessary to ensure that the same clearance between the inductor and the workpiece is maintained over the entire hardening surface, it is necessary to exclude mutual touches. This is ensured, if possible, by rotating the workpiece in the centers, which makes it possible to ensure uniform heating, and, as a result, the same structure and hardness of the surface of the hardened workpiece.

The inductor for HDTV hardening has several versions:

single or multi-turn annular - for heating the outer or inner surface of parts in the form of bodies of revolution - shafts, wheels or holes in them;
loop - for heating the working plane of the product, for example, the surface of the bed or the working edge of the tool;
shaped - for heating parts of complex or irregular shape, for example, gear teeth.

Depending on the shape, size and depth of the hardening layer, the following HDTV hardening modes are used:

simultaneous - the entire surface of the workpiece or a certain zone is heated at once, then it is also simultaneously cooled;
continuous-sequential - one zone of the part is heated, then when the inductor or part is displaced, another zone is heated, while the previous one is cooled.

Simultaneous HFC heating of the entire surface requires a lot of power, so it is more profitable to use it for hardening small parts - rolls, bushings, pins, as well as part elements - holes, necks, etc. After heating, the part is completely lowered into a tank with coolant or poured with a stream of water.

Continuous-sequential hardening of high-frequency current makes it possible to harden large-sized parts, for example, gear rims, since this process heats up a small area of the part, which requires less power of the high-frequency generator.

Part cooling

Cooling is the second important stage of the hardening process, the quality and hardness of the entire surface depends on its speed and uniformity. Cooling takes place in coolant or splash tanks. For high-quality hardening, it is necessary to maintain a stable temperature of the coolant, to prevent its overheating. The holes in the sprayer must be of the same diameter and evenly spaced, so that the same structure of the metal on the surface is achieved.

To prevent the inductor from overheating during operation, water constantly circulates through the copper tube. Some inductors are made combined with the workpiece cooling system. Holes are cut in the inductor tube through which cold water enters the hot part and cools it.

Advantages and disadvantages

Hardening parts using HDTV has both advantages and disadvantages. The advantages include the following:

After HFC hardening, the part retains a soft center, which significantly increases its resistance to plastic deformation.
The cost-effectiveness of the hardening process of HDTV parts is due to the fact that only the surface or zone that needs to be hardened is heated, and not the entire part.
In the mass production of parts, it is necessary to set up the process and then it will automatically repeat, ensuring required quality hardening.
The ability to accurately calculate and adjust the depth of the hardened layer.
The continuous-sequential hardening method allows the use of low power equipment.
The short heating and holding time at high temperature contributes to the absence of oxidation, decarburization of the upper layer and the formation of scale on the surface of the part.
Rapid heating and cooling reduces warpage and leash, which reduces the finishing allowance.

But it is economically feasible to use induction installations only in mass production, and for a single production, the purchase or manufacture of an inductor is unprofitable. For some parts of complex shape, the production of an induction installation is very difficult or impossible to obtain a uniform hardened layer. In such cases, other types of surface hardening are used, for example, flame or bulk hardening.

The high-frequency current is generated in the installation due to the inductor and allows heating the product placed in close proximity to the inductor. The induction machine is ideal for hardening metal products. It is in the HDTV installation that you can clearly program: the desired 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 of high-frequency heating, it has been used for steel hardening since 1935. HDTV hardening units are by far the most productive method of heat treatment of metal products.

Why induction is better for hardening

High-frequency hardening of metal parts is carried out 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 during high-frequency hardening remains completely unchanged.
The induction installation has many very important advantages in comparison with alternative types of heating: if earlier HDTV installations were more cumbersome and inconvenient, now this drawback has been corrected, and the equipment has become universal for heat treatment of metal products.

Advantages of induction equipment

One of the disadvantages of the induction hardening machine is the inability to process some products that have 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: used, as a rule, for parts that have a small flat surface. The position of the workpiece and the inductor when using this method of hardening remains unchanged.
Continuous-sequential hardening: used for hardening cylindrical or flat products. With continuous-sequential hardening, the part can move under the inductor, or it keeps its position unchanged.
Tangential hardening of workpieces: excellent for machining small parts that have a cylindrical shape. Tangential continuous-sequential hardening scrolls the product once during the entire heat treatment process.
An HDTV hardening unit is equipment capable of high-quality hardening of a product and at the same time saves production resources.

In hydromechanical systems, devices and assemblies, parts that work on friction, compression, twisting are most often used. That is why the main requirement for them is sufficient hardness of their surface. To obtain the required characteristics of the part, the surface is hardened by high frequency current (HF).

In the process of application, HDTV hardening has proven to be an economical and highly effective method of heat treatment of the surface of metal parts, which gives additional wear resistance and high quality processed items.

Heating by high-frequency currents is based on the phenomenon in which, due to the passage of an alternating high-frequency current through an inductor (a spiral element made of copper tubes), a magnetic field is formed around it, creating eddy currents in a metal part, which cause heating of the hardened product. Being exclusively on the surface of the part, they allow you to heat it to a certain adjustable depth.

HDTV hardening of metal surfaces differs from standard full hardening, which consists in an increased heating temperature. This is due to two factors. The first of them is at high speed heating (when pearlite turns into austenite), the temperature level of the critical points rises. And the second - the faster the temperature transition passes, the faster the transformation of the metal surface takes place, because it must occur in the minimum time.

It is worth saying that, despite the fact that when using high-frequency hardening, heating is caused more than usual, overheating of the metal does not happen. This phenomenon is explained by the fact that the grain in the steel part does not have time to increase, due to the minimum time of high-frequency heating. In addition, due to the fact that the level of heating is higher and the cooling is more intense, the hardness of the workpiece after hardening by HDTV increases by approximately 2-3 HRC. And this guarantees the highest strength and reliability of the surface of the part.

At the same time, there is an additional important factor that provides an increase in the wear resistance of parts during operation. Due to the creation of a martensitic structure, compressive stresses are formed on the upper part of the part. The action of such stresses manifests itself to the highest extent at a small depth of the hardened layer.

Installations, materials and auxiliary means used for HDTV hardening

A fully automatic high-frequency hardening complex includes a hardening machine and high-frequency equipment (fastening systems mechanical type, nodes for turning the part around its axis, movement of the inductor in the direction of the workpiece, pumps that supply and pump out liquid or gas for cooling, electromagnetic valves for switching working liquids or gases (water / emulsion / gas)).

The HDTV machine allows you to move the inductor along the entire height of the workpiece, as well as rotate the workpiece at different speed levels, adjust the output current on the inductor, and this makes it possible to select the correct mode of the hardening process and obtain a uniformly hard surface of the workpiece.

A schematic diagram of an HDTV induction installation for self-assembly was given.

High-frequency induction hardening can be characterized by two main parameters: the degree of hardness and the depth of hardening of the surface. Technical specifications manufactured induction installations are determined by the power and frequency of operation. To create a hardened layer, induction heating devices with a power of 40-300 kVA are used at frequencies of 20-40 kilohertz or 40-70 kilohertz. If it is necessary to harden layers that are deeper, it is worth using frequency indicators from 6 to 20 kilohertz.

The frequency range is selected based on the range of steel grades, as well as the depth level of the hardened surface of the product. There is a huge range of complete sets of induction installations, which helps to choose a rational option for a particular technological process.

The technical parameters of automatic hardening machines are determined overall dimensions used parts for hardening in height (from 50 to 250 centimeters), in diameter (from 1 to 50 centimeters) and weight (up to 0.5 t, up to 1 t, up to 2 t). Complexes for hardening, the height of which is 1500 mm or more, are equipped with an electronic-mechanical system for clamping the part with a certain force.

High-frequency hardening of parts is carried out in two modes. In the first, each device is individually connected by the operator, and in the second, it occurs without his intervention. Water, inert gases, or polymer compositions with thermal conductivity properties close to oil are usually chosen as the quenching medium. The hardening medium is selected depending on the required parameters of the finished product.

HDTV hardening technology

For parts or surfaces of a flat shape of small diameter, stationary type high-frequency hardening is used. For successful operation, the location of the heater and the part does not change.

When using continuous-sequential high-frequency hardening, which is most often used when processing flat or cylindrical parts and surfaces, one of the components of the system must move. In such a case, either the heating device moves towards the workpiece, or the workpiece moves under the heating apparatus.

To heat exclusively cylindrical parts of small size, scrolling once, continuous-sequential high-frequency hardening of the tangential type is used.

The structure of the metal of the gear tooth, after hardening by the HDTV method

After high-frequency heating of the product, its low tempering is performed at a temperature of 160-200°C. This allows to increase the wear resistance of the surface of the product. Holidays are made in electric furnaces. Another option is to take a break. To do this, it is necessary to turn off the device that supplies water a little earlier, which contributes to incomplete cooling. The part retains a high temperature, which heats the hardened layer to a low tempering temperature.

After hardening, electric tempering is also used, in which heating is carried out using an RF installation. To achieve the desired result, heating is carried out at a lower rate and more deeply than with surface hardening. The required heating mode can be determined by the selection method.

To improve the mechanical parameters of the core and the overall wear resistance of the workpiece, it is necessary to carry out normalization and volumetric hardening with high tempering immediately before surface hardening of the HFC.

Scope of hardening HDTV

HDTV hardening is used in a number of technological processes manufacture of the following parts:

shafts, axles and pins;
gears, gear wheels and rims;
teeth or cavities;
cracks and internal parts of parts;
crane wheels and pulleys.

Most often, high-frequency hardening is used for parts that consist of carbon steel containing half a percent carbon. Such products acquire high hardness after hardening. If the presence of carbon is less than the above, such hardness is no longer achievable, and at a higher percentage, cracks are likely to occur when cooling with a water shower.

In most situations, quenching with high-frequency currents makes it possible to replace alloyed steels with more inexpensive carbon steels. This can be explained by the fact that such advantages of steels with alloying additives, such as deep hardenability and less distortion of the surface layer, lose their significance for some products. With high-frequency hardening, the metal becomes stronger, and its wear resistance increases. In the same way as carbon steels, chromium, chromium-nickel, chromium-silicon and many other types of steels with a low percentage of alloying additives are used.

Advantages and disadvantages of the method

Advantages of hardening with high-frequency currents:

fully automatic process;
work with products of any form;
lack of soot;
minimum deformation;
variability of the depth level of the hardened surface;
individually determined parameters of the hardened layer.

Among the disadvantages are:

the need to create a special inductor for different shapes of parts;
difficulties in overlaying the levels of heating and cooling;
high cost of equipment.

The possibility of using high-frequency current hardening in individual production is unlikely, but in mass flow, for example, in the manufacture of crankshafts, gears, bushings, spindles, cold rolling shafts, etc., hardening of HDTV surfaces is becoming increasingly widespread.