HD oven for metal heating. HDTV installation - the principle of operation for hardening. HDTV hardening furnaces from SMK CJSC

A domestic metal melting furnace can also be used to quickly heat up metal elements, for example, when they are tinned or shaped. The characteristics of the operation of the presented installations can be adjusted to a specific task by changing the parameters of the inductor and the output signal of the generator sets - this way you can achieve their maximum efficiency.

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

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

High frequency hardening technology

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

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

Temperature selection

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

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

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

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

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

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

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

induction plant

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

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

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

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

The inductor for HDTV hardening has several versions:

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

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

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

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

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

Part cooling

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

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

Advantages and disadvantages

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

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

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

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

Description of the method.

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

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

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

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

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

The inductor gets very hot during operation, as it absorbs its own radiation. In addition, he absorbs thermal radiation from a hot workpiece. They make inductors from copper tubes cooled by water. Water is supplied by suction - this ensures safety in case of a burn or other depressurization of the inductor.

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

Advantages.

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

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

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

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

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

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

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

It is easy to carry out local and selective heating.

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

Easy automation of equipment - heating and cooling cycles, temperature control and holding, feeding and removal of workpieces.

Induction heating units:

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

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

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

Since the inductor operates most efficiently at high frequencies, induction heating received industrial application after the development and start of production of powerful generator lamps. Prior to World War I, induction heating was of limited use. At that time, high-frequency machine generators (works by V.P. Vologdin) or spark discharge installations were used as generators.

The generator circuit can, in principle, be any (multivibrator, RC generator, independently excited generator, various relaxation generators) that operates on a load in the form of an inductor coil and has sufficient power. It is also necessary that the oscillation frequency be sufficiently high.

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

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

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

Disadvantages of three dots:

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

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

When blanks are changed (for example, from smaller to larger ones), the inductance of the inductor-blank system changes, which also leads to a change in the frequency and depth of the skin layer.

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

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

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

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

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

Remarks.

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

Increasing the frequency dramatically reduces the power factor (in proportion to the cube of the frequency).

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

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

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

Ideally, the loop impedance is infinity - the circuit draws no current from the source. When the frequency of the generator changes in any direction from the resonant frequency, the impedance of the circuit decreases and the linear current (Itotal) increases.

Series oscillatory circuit - voltage resonance.

The main feature of a series resonant circuit is that its impedance is minimal at resonance. (ZL + ZC - minimum). When the frequency is tuned to a value above or below the resonant frequency, the impedance increases.
Conclusion:
In a parallel circuit at resonance, the current through the circuit leads is 0, and the voltage is maximum.
In a series circuit, the opposite is true - the voltage tends to zero, and the current is maximum.

The article was taken from the site http://dic.academic.ru/ and reworked into a more understandable text for the reader by the LLC Prominduktor company.