The strength of elements in especially critical steel structures largely depends on the condition of the nodes. The surface of the parts plays an important role. To give it the necessary hardness, resistance or viscosity, heat treatment operations are carried out. Strengthen the surface of parts by various methods. One of them is hardening with high-frequency currents, that is, HDTV. It belongs to the most common and very productive method during large-scale production of various structural elements.
Such heat treatment is applied both to the whole parts and to their individual sections. In this case, the goal is to achieve certain levels of strength, thereby increasing the life and performance.
Technology used to strengthen knots technological equipment and transport, as well as when hardening various tools.
Essence of technology
HDTV hardening is an improvement in the strength characteristics of a part due to the ability electric current(with variable amplitude) to penetrate the surface of the part, exposing it to heat. The depth of penetration due to the magnetic field can be different. Simultaneously with surface heating and hardening, the core of the node may not be heated at all or only slightly increase its temperature. The surface layer of the workpiece forms the necessary 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 opportunities for regulation and production of parts with a minimum hardened layer.
HDTV heat treatment is carried out in special installations - generators, multipliers, frequency converters, which allow adjustment in the required range. In addition to the frequency characteristics, the final hardening is influenced by the dimensions and shape of the part, the material of manufacture and the inductor used.
The following pattern was also revealed - the smaller the product and the simpler its shape, the better the hardening process goes. This also reduces the overall energy consumption of the installation.
copper inductor. On the inner surface there are often additional holes designed to supply water during cooling. In this case, the process is accompanied by primary heating and subsequent cooling without current supply. Inductor configurations are different. The selected device directly depends on the workpiece being processed. Some devices do not have holes. In such a situation, the part is cooled in a special hardening tank.
The main requirement for the process of HD hardening is to maintain a constant gap between the inductor and the workpiece. While maintaining the specified interval, the quality of hardening becomes the highest.
Strengthening can be done in one of the ways:
- Continuous-series: the part is stationary, and the inductor moves along its axis.
- Simultaneous: the product is moving, and the inductor is vice versa.
- Sequential: Processing the various parts one by one.
Features of the induction installation
Installation for HDTV hardening is a high-frequency generator together with an inductor. The workpiece is located both in the inductor itself and next to it. It is a coil on which a copper tube is wound.
Alternating electric current when passing through the inductor creates an electromagnetic field that penetrates the workpiece. It provokes the development of eddy currents (Foucault currents), which pass into the structure of the part and increase its temperature.
The main feature of the technology– penetration of eddy current into the surface structure of the metal.
Increasing the frequency opens up the possibility of concentrating heat 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, electric current frequency. Power determines the time spent on heating the part. Accordingly, with a larger value of time, less time is spent.
The heating time is characterized by the total amount of heat expended 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 volumetric mass of the heated metal decreases.
It is these 3 parameters that make it possible to regulate the degree of hardness and layer depth, as well as the volume of heating, in a wide range.
Practice shows that the characteristics of the generator set (voltage, power and current values), as well as the heating time, are controlled. The degree of heating of the part can be controlled using a pyrometer. However, in general, continuous temperature control is not required, as there are optimal HDTV heating modes that ensure stable quality. The appropriate mode is selected taking into account the changed electrical characteristics.
After hardening, the product is sent to the laboratory for analysis. The hardness, structure, depth and plane of the distributed hardened layer are studied.
Surface hardening HDTV accompanied by a lot of heat compared to the conventional process. This is explained as follows. First of all, a high rate of temperature increase contributes to an increase in critical points. Secondly, it is necessary to short term to ensure the completion of the transformation of pearlite into austenite.
High-frequency hardening, in comparison with the conventional process, is accompanied by higher heating. However, the metal does not overheat. This is explained by the fact that the granular elements in the steel structure do not have time to grow in a minimum time. In addition, bulk hardening has a lower strength of up to 2-3 units. After HFC hardening, the part has greater wear resistance and hardness.
How is the temperature chosen?
Compliance with technology should be accompanied the right choice temperature range. Basically, everything will depend on the metal being processed.
Steel is classified into several types:
- Hypoeutectoid - carbon content up to 0.8%;
- Hypereutectoid - more than 0.8%.
Hypoeutectoid steel is heated to a value slightly higher than necessary to convert pearlite and ferrite to austenite. Range from 800 to 850 degrees. After that, the detail high speed cools down. After rapid cooling, austenite transforms into martensite, which has high hardness and strength. With a short holding time, fine-grained austenite is obtained, as well as finely acicular martensite. Steel gets high hardness and little brittleness.
Hypereutectoid steel heats up less. Range from 750 to 800 degrees. In this case, incomplete hardening is performed. This is explained by the fact that such a temperature makes it possible to preserve in the structure a certain volume of cementite, 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 fully dissolved carbon, which has turned into solid carbide.
Advantages of technology
- Mode control;
- Replacement of alloy steel with carbon steel;
- Uniform heating process of the product;
- Possibility not to heat the whole part completely. Reduced energy consumption;
- High resulting strength of the processed workpiece;
- There is no oxidation process, carbon is not burned;
- No microcracks;
- There are no warped points;
- Heating and hardening of certain sections of products;
- Reducing the time spent on the procedure;
- Implementation in the manufacture of parts for high-frequency installations in production lines.
Flaws
The main disadvantage of the technology under consideration is the significant installation cost. It is for this reason that the expediency of the application is justified only in large-scale production and excludes the possibility of doing the work yourself at home.
Learn more about the operation and principle of operation of the installation on the presented videos.
Tool soldering
Aluminum soldering
heat treatment
CJSC "Modern Machine-Building Company", official representative CIEA (Italy) brings to your attention induction heating generators (HDTV units) for heat treatment of metal products.
HDTV hardening furnaces
Since its inception, in the late 60s, CEIA has been developing and manufacturing industrial equipment based on the application of the electromagnetic field effect. In the late 1980s, CEIA introduced the first solid state induction heater to the specialty soldering equipment market. In 1995, CEIA introduces another innovation with the "Power Cube Family" range of induction heating appliances, which includes:
- generators (power from 2.8 kW to 100 kW and operating frequencies from 25 kHz to 1800 kHz) and heating heads;
- control devices (controller, master controller, special programmer) that ensure operation in automatic or semi-automatic mode;
- optical pyrometers with a measurement range from 80 to 2000 ºС;
- supports for heating heads, pyrometers and solder feeders.
CIEA carries out all stages of production from the development of devices and electronic boards to the assembly of generators. The production employs highly qualified personnel. Each device undergoes mandatory electromagnetic testing.
HDTV hardening furnaces from SMK CJSC
The modular design of HDTV induction heating installations allows you to assemble workstations with different characteristics, corresponding to the technical and economic needs of the customer. It also makes it possible to change the original configuration (when changing the model of the generator or controller).
CJSC "Modern Machine-Building Company" has experience in automating heat treatment processes according to the conditions terms of reference Customer.
Principle of operation:
Induction heating is carried out due to the energy of the electromagnetic field. An inductor loop of the required size is brought to the workpiece. The medium and high frequency alternating current (HF) passing through the loop creates eddy currents on the surface of the workpiece, the magnitude of which can be controlled and programmed. Induction heating takes place without direct contact, and only the metal parts are heat treated. Induction heating is characterized by high energy transfer efficiency without heat loss. The depth of penetration of induced currents directly depends on the operating frequency of the generator (HDTV induction heating installation) - the higher the frequency, the greater the current density on the surface of the workpiece. By lowering the operating frequency, it is possible to increase the HDTV penetration depth, i.e. heating depth.
Advantages:
Generators (HDTV induction heating units) CEIA have the following advantages:
- high efficiency;
- small dimensions and the possibility of embedding in automated lines;
- localization of the heating area (thanks to a precisely selected inductor);
- a microprocessor that ensures the repeatability of the work cycle;
- self-diagnosis system that gives a signal and turns off the unit in case of a malfunction;
- the possibility of moving only the heating head with an inductor into the working area (connecting cable up to 4 m long);
- equipment complies with the requirements of electrical safety and has ISO certificate 9001.
Application:
Generators (HDTV induction heating units) CIEA is applied for various kinds heat treatment of all conductive products (metal alloys, non-ferrous metals, carbon and silicon compounds):
- heating;
- hardening;
- annealing;
- soldering tools, including diamond or carbide;
- soldering microcircuits, connectors, cables;
- aluminum brazing.
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. Hardness decrease, machinability improvement, removal internal stresses in hypoeutectoid and eutectoid steels before hardening (see note to 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 hypereutectoid 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 ductility, 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 ensures the tempering of the hardened outer layer Local hardening of a striking tool of a simple configuration made of carbon tool steel, as well as during induction heating
Hardening with cold treatment - Deep cooling after hardening to a temperature of 253-193 K. 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. Saturation of the surface layer of steel with carbon and nitrogen (0.15-0.3 mm layer) occurs. Low-carbon steels undergo cyanidation, as a result of which, along with a solid surface, the products have a viscous core. 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. A Change Is Coming internal structure 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 be caused.
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 the grain boundaries are easier to etch 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 of aluminum alloys melting at low temperatures can in some cases be carried 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 divide 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 "festoons" 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 correspond 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. Consequently, the composition of the solid that appeared at the initial moment of solidification will be different than at the end of solidification, and this leads to spatial inhomogeneity of the composition on a microscopic scale. 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 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, differing in composition and structure from the original grains and usually scattered between them in the form of individual particles. Such particles of the second phase can have strong influence on the physical properties of the material, which depends on their size, shape and distribution. These factors can be changed by heat treatment (heat treatment).
Heat treatment - 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 was faced with 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 Homer's "Odyssey" (8-7th centuries BC) there are such lines: "How a blacksmith plunges a red-hot ax or an ax into cold water, and the iron hisses with a gurgle, stronger than iron is, hardening in fire and water." In the 5th c. BC e. the Etruscans tempered mirrors made of high-tin bronze in water (most likely to improve the gloss when polished). 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 the barrels of guns 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 duralumin, which he invented, discovered aging after hardening (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 appeared thermomechanical treatment of aging copper alloys, and in the 50s, 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. The actual heat treatment includes the following types: annealing of the 1st kind, annealing of the 2nd kind, hardening without polymorphic transformation and with polymorphic transformation, aging and tempering.
Nitriding is the saturation of the surface of metal parts with nitrogen in order to increase hardness, wear resistance, fatigue limit and corrosion resistance. Nitriding is applied to steel, titanium, some alloys, most often alloyed steels, especially chromium-aluminum, 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 maintained upon 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 hardened and hardened steels Nitriding is a long process, it takes 20-50 hours to obtain a layer 0.2-0.4 mm thick Raising the temperature speeds up the process, but reduces the hardness of the layer To protect places, do 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 is formed and solid solution nitrogen in ±-titanium. 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.
For the first time, hardening of parts using induction heating was proposed by V.P. Volodin. It was almost a century ago - in 1923. And in 1935, this type of heat treatment began to be used for hardening steel. It is difficult to overestimate the popularity of hardening today - it is actively used in almost all branches of engineering, and HDTV hardening installations 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 HDTV surface 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. Since the center of the part retains its toughness, the part itself becomes stronger.
With the help of high-frequency hardening, 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:
- With induction heating, only the desired part of the part is changed, this method is more economical than conventional heating. In addition, HDTV hardening takes less time;
- With high-frequency hardening of steel, it is possible to avoid the appearance of cracks, as well as reduce the risk of warping defects;
- During the heating of HDTV, carbon burnout and scale formation do not occur;
- If necessary, changes in the depth of the hardened layer are possible;
- Using HDTV hardening, it is possible to improve the mechanical properties of steel;
- When using induction heating, it is possible to avoid the appearance of deformations;
- Automation and mechanization of the entire heating process is at a high level.
However, HDTV hardening also has disadvantages. So, it is very problematic to process some complex parts, and in some cases, induction heating is completely unacceptable.
HDTV steel hardening - varieties:
Stationary HDTV hardening. It is used for hardening small flat parts (surfaces). In this case, the position of the workpiece and the heater is constantly maintained.
Continuous-sequential HDTV hardening. When performing this type of hardening, 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 high-frequency hardening is suitable for processing flat and cylindrical parts, surfaces.
Tangential continuous-sequential HDTV hardening. It is used when heating only small cylindrical parts that scroll once.
Do you want to purchase quality equipment for hardening? Then contact the research and production company "Ambit". We guarantee that each issued by us HDTV installation for hardening - reliable and high-tech.
Induction heating of various cutters before soldering, hardening,
induction heating unit IHM 15-8-50
Induction soldering, hardening (repair) of saw blades,
induction heating unit IHM 15-8-50
Induction heating of various cutters before soldering, hardening
Many critical parts work for abrasion and are simultaneously subjected to impact loads. Such parts must have a high surface hardness, good wear resistance and at the same time not be brittle, i.e., not break down under impact.
High surface hardness of parts while maintaining a tough and strong core is achieved by surface hardening.
From modern methods Surface hardening is most widely used in mechanical engineering for the following: hardening when heated high frequency currents (TVCh); flame hardening and hardening in an electrolyte.
The choice of one or another method of surface hardening is determined by technological and economic feasibility.
Hardening when heated by high-frequency currents. This method is one of the most efficient methods of surface hardening of metals. The discovery of this method and the development of its technological foundations belongs to the talented Russian scientist V.P. Vologdin.
High frequency heating is based on the following phenomenon. When an alternating electric current of high frequency passes through a copper inductor, a magnetic field is formed around the latter, which penetrates into the steel part located in the inductor and induces Foucault eddy currents in it. These currents cause the metal to heat up.
heating feature HDTV is that the eddy currents induced in steel are not distributed uniformly over the section of the part, but are pushed to the surface. The uneven distribution of eddy currents leads to its uneven heating: the surface layers heat up very quickly to high temperatures, and the core either does not heat up at all or heats up slightly due to the thermal conductivity of steel. The thickness of the layer through which the current passes is called the penetration depth and is denoted by the letter δ.
The thickness of the layer mainly depends on the frequency of the alternating current, the resistivity of the metal and the magnetic permeability. This dependence is determined by the formula
δ \u003d 5.03-10 4 root of (ρ / μν) mm,
where ρ is the electrical resistivity, ohm mm 2 /m;
μ, - magnetic permeability, gs/e;
v - frequency, Hz.
It can be seen from the formula that with increasing frequency, the depth of penetration of induction currents decreases. High frequency current for induction heating of parts is obtained from generators.
When choosing the current frequency, in addition to the heated layer, it is necessary to take into account the shape and dimensions of the part in order to obtain a high quality surface hardening and economically use electrical energy high frequency installations.
Copper inductors are of great importance for high-quality heating of parts.
The most common inductors have a system of small holes on the inside through which cooling water is supplied. Such an inductor is both a heating and cooling device. As soon as the part placed in the inductor heats up to the set temperature, the current will automatically turn off and water will flow from the holes of the inductor and cool the surface of the part with a sprayer (water shower).
Parts can also be heated in inductors that do not have choking devices. In such inductors, the parts after heating are dumped into the hardening tank.
Hardening of HDTV is mainly carried out by simultaneous and continuous-sequential methods. With the simultaneous method, the hardened part rotates inside a fixed inductor, the width of which is equal to the hardened section. When the set heating time expires, the time relay cuts off the current from the generator, and another relay, interlocked with the first one, turns on the water supply, which bursts out of the inductor holes in small but strong jets and cools the part.
With the continuous-series method, the part is stationary, and the inductor moves along it. In this case, sequential heating of the hardened section of the part, after which the section falls under the water jet of a showering device located at some distance from the inductor.
Flat parts are hardened in loop and zigzag inductors, and gear wheels with a small module are simultaneously hardened in ring inductors. Macrostructure of the hardened layer of a fine-modulus car gear made of steel grade PPZ-55 (low hardenability steel). The microstructure of the hardened layer is finely acicular martensite.
The hardness of the surface layer of parts hardened by heating with high-frequency current is obtained by 3-4 units HRC higher than the hardness of conventional bulk hardening.
To increase the strength of the core, the parts are improved or normalized before hardening.
The use of HDTV heating for surface hardening of machine parts and tools makes it possible to drastically reduce the duration technological process heat treatment. In addition, this method makes it possible to manufacture mechanized and automated units for hardening parts, which are installed in the general flow of machining shops. As a result, there is no need to transport parts to special thermal shops and the rhythmic operation of production lines and assembly lines is ensured.
Flame surface hardening. This method consists in heating the surface of steel parts with an oxy-acetylene flame to a temperature that is 50-60 ° C higher than the upper critical point A C 3 , followed by rapid cooling with a water shower.
The essence of the flame hardening process is that the heat supplied by the gas flame from the burner to the hardened part is concentrated on its surface and significantly exceeds the amount of heat distributed into the depth of the metal. As a result of such a temperature field, the surface of the part first quickly heats up to the hardening temperature, then cools down, and the core of the part practically remains unhardened and does not change its structure and hardness after cooling.
Flame hardening is used to harden and increase the wear resistance of large and heavy steel parts such as crankshafts of mechanical presses, large-modulus gears, excavator bucket teeth, etc. In addition to steel parts, parts made of gray and pearlitic cast iron are subjected to flame hardening, for example guides of the beds of metal-cutting machines.
Flame hardening is divided into four types:
a) sequential, when the hardening torch with the coolant moves along the surface of the fixed part being processed;
b) hardening with rotation, in which the burner with the coolant remains stationary, and the part to be hardened rotates;
c) sequential with the rotation of the part, when the part continuously rotates and a hardening burner with a coolant moves along it;
d) local, in which the fixed part is heated to a given quenching temperature by a fixed burner, after which it is cooled by a jet of water.
A method of flame hardening a roller that rotates at a certain speed while the burner remains stationary. The heating temperature is controlled by a milliscope.
Depending on the purpose of the part, the depth of the hardened layer is usually taken equal to 2.5-4.5 mm.
The main factors affecting the depth of hardening and the structure of the hardened steel are: the speed of movement of the hardening torch relative to the hardened part or part relative to the burner; gas flow rate and flame temperature.
The choice of hardening machines depends on the shape of the parts, the hardening method and the required number of parts. If you need to harden parts of various shapes and sizes and in small quantities, then it is more expedient to use universal hardening machines. In factories, special installations and lathes are usually used.
For hardening, two types of burners are used: modular with a module from M10 to M30 and multi-flame with replaceable tips having a flame width of 25 to 85 mm. Structurally, the burners are arranged in such a way that the holes for the gas flame and cooling water are arranged in one row, in parallel. Water is supplied to the burners from the water supply network and serves simultaneously for hardening parts and cooling the mouthpiece.
Acetylene and oxygen are used as combustible gases.
After flame hardening, the microstructure in different zones of the part is different. The hardened layer gets a high hardness and remains clean, without traces of oxidation and decarburization.
The transition of the structure from the surface of the part to the core occurs smoothly, which is of great importance for increasing the operational stability of parts and completely eliminates harmful phenomena - cracking and delamination of hardened metal layers.
The hardness changes according to the structure of the hardened layer. On the surface of the part, it is equal to 56-57 HRC, and then lowered to the hardness that the part had before surface hardening. To provide High Quality hardening, obtaining uniform hardness and increased core strength, cast and forged parts are annealed or normalized in accordance with ordinary conditions before flame hardening.
Surface forcalc in the electrolyte. The essence of this phenomenon is that if a constant electric current is passed through the electrolyte, then a thin layer is formed on the cathode, consisting of the smallest hydrogen bubbles. Due to the poor electrical conductivity of hydrogen, the resistance to the passage of electric current increases greatly and the cathode (part) heats up to high temperature, after which it hardens. As an electrolyte, an aqueous 5-10% solution of soda ash is usually used.
The hardening process is simple and consists in the following. The part to be hardened is lowered into the electrolyte and connected to the negative pole of a DC generator with a voltage of 200-220 in and density 3-4 a / cm 2, as a result of which it becomes the cathode. Depending on which part of the part is subjected to surface hardening, the part is immersed to a certain depth. The part heats up in a few seconds, and the current is turned off. The cooling medium is the same electrolyte. So, the electrolyte bath serves as both a heating furnace and a quenching tank.
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