Heat treatment of steel. (Heat treatment of metal). Hardening and HDTV. Installation of HDTV - the principle of work for hardening. Lamp induction oven

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 to pass through the product as the inductor current.

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 workpiece. 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, heating duration and 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 during heating 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 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; when overheated, coarse-acicular martensite occurs. The signs of defects when the HDTV is heated are the same as when classical technologies heat treatment.

During surface hardening with HFC, heating is carried out to a higher temperature than during conventional bulk hardening. This is due to two reasons. Firstly, at a very high heating rate, the temperatures of the critical points at which the transition of pearlite to austenite occurs increase, and secondly, it is necessary that this transformation has 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 dross
• 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 vacuum tubes that convert direct current into alternating current of increased frequency - tube generators.
2. Electric machine 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 directed 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 heating. 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 hardening medium. Shower cooling can be carried out both in inductors-sprayers and in special quenching chambers. Cooling by a flow allows creating an excess pressure 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.

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 necessary equipment, for which qualified specialists work. We carry out all orders with high quality and on time. We also accept and carry out orders for 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

Annealing of the first kind:

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 stress

Annealing of the second kind, spheroidizing - 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 (normalizing 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 by 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 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 a 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 a 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. These 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 rapid 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 more easily etched 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 substantially 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 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 "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 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 and 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 steps, 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 exceeding the stress relief annealing temperature and held for recrystallization. Due to rapid diffusion at this temperature, a completely new structure arises due to atomic rearrangement. New grains begin to grow inside the grain structure of the deformed material, which, over time, completely replace it. First, small new grains are formed in 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. Generally speaking, the physical properties of a grain 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 size grain 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 formed 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 heterogeneity 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 of heat treatment.
Man has been using heat treatment of metals since ancient times. Even in the Eneolithic 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 plunges a red-hot ax or an ax into cold water, and iron hisses with a gurgle, 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 needs 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 point 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 a different basis (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 on 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, to obtain a layer with a thickness of 0.2 0.4 mm it takes 20 50 hours. An increase in temperature accelerates the process, but reduces the hardness of the layer. 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, slide 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.

For the first time, V.P. Volodin. It was almost a century ago - in 1923. And in 1935 this type of heat treatment began to be used for steel hardening. The popularity of hardening today is difficult to overestimate - it is actively used in almost all branches of mechanical engineering, and HFC installations for hardening are also in great demand.

To increase the hardness of the hardened layer and increase the toughness in the center of the steel part, it is necessary to use surface HDTV hardening. In this case, the upper layer of the part is heated to the hardening temperature and abruptly cooled. It is important that the properties of the core of the part remain unchanged. As the center of the part retains its toughness, the part itself becomes stronger.

With the help of HFC quenching, it is possible to strengthen the inner layer of the alloyed part; it is used for medium-carbon steels (0.4-0.45% C).

Advantages of HDTV hardening:

1. With induction heating, only the required part of the part changes, this method is more economical than conventional heating. In addition, HDTV hardening takes less time;
2. With high-frequency current hardening of steel, it is possible to avoid the appearance of cracks, as well as to reduce the risks of rejects due to warpage;
3. During HFC heating, carbon burnout and scale formation do not occur;
4. If necessary, changes in the depth of the hardened layer are possible;
5. Using HFC quenching, it is possible to improve the mechanical properties of steel;
6. When using induction heating, it is possible to avoid the appearance of deformations;
7. The automation and mechanization of the entire heating process is at a high level.

However, HDTV hardening also has disadvantages. So, some complex parts are very problematic to process, and in some cases induction heating is completely unacceptable.

HFC steel hardening - varieties:

Stationary HDTV hardening. It is used for hardening small flat parts (surfaces). In this case, the position of the part and the heater is constantly maintained.

Continuous sequential HDTV hardening... When this type of hardening is carried out, the part either moves under the heater or remains in place. In the latter case, the heater itself moves in the direction of the part. Such HFC hardening is suitable for processing flat and cylindrical parts and surfaces.

Tangential continuous-sequential HDTV hardening... It is used when heating extremely small cylindrical parts that scroll once.

Are you looking for quality hardening equipment? Then contact the research and production company "Ambit". We guarantee that every HDTV hardening unit we produce is reliable and high-tech.

Induction heating of various cutters before brazing, quenching,
induction heating unit IHM 15-8-50

Induction brazing, hardening (repair) of circular saw blades,
induction heating unit IHM 15-8-50

Induction heating of various cutters before brazing, quenching

The strength of elements in particularly critical steel structures largely depends on the state of the nodes. The surface of the parts plays an important role. To give it the required hardness, durability or toughness, heat treatment operations are carried out. The surface of the parts is hardened by various methods. One of them is hardening with high frequency currents, that is, high frequency current. It is one of the most common and highly productive way during the high-volume production of various structural elements.

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

The technology is used to strengthen the nodes of technological equipment and transport, as well as when hardening various tools.

The essence of technology

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

Experiments have proven that an increase in the frequency of the current contributes to a decrease in the penetration depth... This fact opens up possibilities for regulating and obtaining parts with a minimum hardened layer.

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

The following regularity was also revealed - the smaller the product and the simpler its shape, the better the hardening process goes. This also reduces the overall power consumption of the installation.

Copper inductor. There are often additional holes on the inner surface for water supply during cooling. In this case, the process is accompanied by primary heating and subsequent cooling without power supply. The configurations of the inductors are different. The selected device is directly dependent on the workpiece being processed. Some units are missing holes. In such a situation, the part is cooled in a special quenching tank.

The main requirement for the HFC hardening process is to maintain a constant gap between the inductor and the product. While maintaining the specified interval, the quality of hardening becomes the highest.

Strengthening can be done in one of the ways:

• Continuous-sequential: the part is stationary, and the inductor moves along its axis.
• Simultaneous: the product is moving, and the inductor is vice versa.
• Sequential: the different parts are processed in sequence.

Features of the induction installation

The HDTV hardening unit is a high-frequency generator together with an inductor. The workpiece to be processed is located both in the inductor itself and next to it. It is a coil on which a copper tube is wound.

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

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

Increasing the frequency opens up opportunities for concentrating heat in a small area of ​​the part. This increases the rate of temperature rise and can reach up to 100 - 200 degrees / sec. The degree of hardness increases to 4 units, which is excluded during bulk hardening.

Induction heating - characteristics

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

The heating time is characterized by the total amount of consumed heat and the developed temperature. Frequency, as mentioned above, determines the depth of penetration of currents and the formed hardenable layer. These characteristics are inversely related. As the frequency increases, the bulk density of the heated metal decreases.

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

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

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

HFC surface hardening accompanied by great heating compared to the conventional process. This is explained as follows. First of all, the high rate of temperature rise tends to increase the critical points. Secondly, it is necessary in short term to ensure the completion of the transformation of pearlite to austenite.

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

How is the temperature chosen?

Compliance with the technology must be accompanied by the correct selection of the temperature range. In the main, everything will depend on the metal being processed.

Steel is classified into several types:

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

Hypoeutectoid steel is heated to a value slightly higher than necessary to convert pearlite and ferrite to austenite. Range from 800 to 850 degrees. Then the part with high speed cooled down. After abrupt cooling, austenite transforms into martensite, which has high hardness and strength. With a short exposure time, austenite of a fine-grained structure is obtained, as well as fine-acicular martensite. The steel gets high hardness and low brittleness.

Hypereutectoid steel heats up less. Range is 750 to 800 degrees. In this case, incomplete hardening is performed. This is explained by the fact that such a temperature allows maintaining a certain volume of cementite in the structure, which has a higher hardness in comparison with martensite. Upon rapid cooling, austenite transforms into martensite. Cementite is preserved by small inclusions. The zone also retains non-fully dissolved carbon, which has turned into solid carbide.

Technology advantages

• Controlling modes;
• Replacement of alloy steel with carbon steel;
• Uniform process of warming up the product;
• The ability not to heat the entire part completely. Reduced energy consumption;
• High obtained strength of the processed workpiece;
• The oxidation process does not occur, carbon is not burned;
• No microcracks;
• There are no warped points;
• Heating and hardening of certain areas of products;
• Reducing the time spent on the procedure;
• Implementation in the manufacture of parts for HFC installations in technological lines.

disadvantages

The main disadvantage of this technology is the significant cost of the installation. It is for this reason that the expediency of application is justified only in large-scale production and excludes the possibility of doing work with your own hands at home.

Learn more about the operation and principle of operation of the installation in the presented videos.