The high-frequency current is generated in the installation due to the inductor and allows heating the product placed in close proximity to the inductor. The induction machine is ideal for hardening metal products. It is in the HDTV installation that you can clearly program: the desired depth of heat penetration, hardening time, heating temperature and cooling process.
For the first time, induction equipment was used for hardening after a proposal from V.P. Volodin in 1923. After long trials and testing of high-frequency heating, it has been used for steel hardening since 1935. HDTV hardening units are by far the most productive method of heat treatment of metal products.
Why induction is better for hardening
High-frequency hardening of metal parts is carried out to increase the resistance of the upper layer of the product to mechanical damage, while the center of the workpiece has an increased viscosity. It is important to note that the core of the product during high-frequency hardening remains completely unchanged.
The induction installation has many very important advantages in comparison with alternative types of heating: if earlier HDTV installations were more cumbersome and inconvenient, now this drawback has been corrected, and the equipment has become universal for heat treatment of metal products.
Advantages of induction equipment
One of the disadvantages of the induction hardening machine is the inability to process some products that have a complex shape.
Varieties of metal hardening
There are several types of metal hardening. For some products, it is enough to heat the metal and immediately cool it, while for others it is necessary to hold it at a certain temperature.
There are the following types of hardening:
- Stationary hardening: used, as a rule, for parts that have a small flat surface. The position of the workpiece and the inductor when using this method of hardening remains unchanged.
- Continuous-sequential hardening: used for hardening cylindrical or flat products. With continuous-sequential hardening, the part can move under the inductor, or it keeps its position unchanged.
- Tangential hardening of workpieces: excellent for machining small parts that have a cylindrical shape. Tangential continuous-sequential hardening scrolls the product once during the entire heat treatment process.
- An HDTV hardening unit is equipment capable of high-quality hardening of a product and at the same time saves production resources.
In hydromechanical systems, devices and assemblies, parts that work on friction, compression, twisting are most often used. That is why the main requirement for them is sufficient hardness of their surface. To obtain the required characteristics of the part, the surface is hardened by high frequency current (HF).
In the process of application, HDTV hardening has proven to be an economical and highly efficient method of heat treatment of the surface of metal parts, which gives additional wear resistance and high quality to the treated elements.
Heating by high-frequency currents is based on the phenomenon in which, due to the passage of an alternating high-frequency current through an inductor (a spiral element made of copper tubes), a magnetic field is formed around it, creating eddy currents in a metal part, which cause heating of the hardened product. Being exclusively on the surface of the part, they allow you to heat it to a certain adjustable depth.
HDTV hardening of metal surfaces differs from standard full hardening, which consists in an increased heating temperature. This is due to two factors. The first of them is that at a high heating rate (when perlite turns into austenite), the temperature level of critical points increases. And the second - the faster the temperature transition passes, the faster the transformation of the metal surface takes place, because it must occur in the minimum time.
It is worth saying that, despite the fact that when using high-frequency hardening, heating is caused more than usual, overheating of the metal does not happen. This phenomenon is explained by the fact that the grain in the steel part does not have time to increase, due to the minimum time of high-frequency heating. In addition, due to the fact that the level of heating is higher and the cooling is more intense, the hardness of the workpiece after hardening by HDTV increases by approximately 2-3 HRC. And this guarantees the highest strength and reliability of the surface of the part.
At the same time, there is an additional important factor that provides an increase in the wear resistance of parts during operation. Due to the creation of a martensitic structure, compressive stresses are formed on the upper part of the part. The action of such stresses manifests itself to the highest extent at a small depth of the hardened layer.
Installations, materials and auxiliary means used for HDTV hardening
A fully automatic high-frequency hardening complex includes a hardening machine and high-frequency equipment (mechanical-type fastening systems, units for turning a part around its axis, movement of the inductor in the direction of the workpiece, pumps that supply and pump out liquid or gas for cooling, electromagnetic valves for switching working liquids or gases (water/emulsion/gas)).
The HDTV machine allows you to move the inductor along the entire height of the workpiece, as well as rotate the workpiece at different speed levels, adjust the output current on the inductor, and this makes it possible to select the correct mode of the hardening process and obtain a uniformly hard surface of the workpiece.
A schematic diagram of an HDTV induction installation for self-assembly was given.
High-frequency induction hardening can be characterized by two main parameters: the degree of hardness and the depth of hardening of the surface. The technical parameters of induction installations produced in the production are determined by the power and frequency of operation. To create a hardened layer, induction heating devices with a power of 40-300 kVA are used at frequencies of 20-40 kilohertz or 40-70 kilohertz. If it is necessary to harden layers that are deeper, it is worth using frequency indicators from 6 to 20 kilohertz.
The frequency range is selected based on the range of steel grades, as well as the depth level of the hardened surface of the product. There is a huge range of complete sets of induction installations, which helps to choose a rational option for a particular technological process.
The technical parameters of automatic hardening machines are determined by the overall dimensions of the parts used for hardening in height (from 50 to 250 centimeters), in diameter (from 1 to 50 centimeters) and weight (up to 0.5 tons, up to 1 ton, up to 2 tons). Complexes for hardening, the height of which is 1500 mm or more, are equipped with an electronic-mechanical system for clamping the part with a certain force.
High-frequency hardening of parts is carried out in two modes. In the first, each device is individually connected by the operator, and in the second, it occurs without his intervention. Water, inert gases, or polymer compositions with thermal conductivity properties close to oil are usually chosen as the quenching medium. The hardening medium is selected depending on the required parameters of the finished product.
HDTV hardening technology
For parts or surfaces of a flat shape of small diameter, stationary type high-frequency hardening is used. For successful operation, the location of the heater and the part does not change.
When using continuous-sequential high-frequency hardening, which is most often used when processing flat or cylindrical parts and surfaces, one of the components of the system must move. In such a case, either the heating device moves towards the workpiece, or the workpiece moves under the heating apparatus.
To heat exclusively cylindrical parts of small size, scrolling once, continuous-sequential high-frequency hardening of the tangential type is used.
The structure of the metal of the gear tooth, after hardening by the HDTV method
After high-frequency heating of the product, its low tempering is performed at a temperature of 160-200°C. This allows to increase the wear resistance of the surface of the product. Holidays are made in electric furnaces. Another option is to take a break. To do this, it is necessary to turn off the device that supplies water a little earlier, which contributes to incomplete cooling. The part retains a high temperature, which heats the hardened layer to a low tempering temperature.
After hardening, electric tempering is also used, in which heating is carried out using an RF installation. To achieve the desired result, heating is carried out at a lower rate and more deeply than with surface hardening. The required heating mode can be determined by the selection method.
To improve the mechanical parameters of the core and the overall wear resistance of the workpiece, it is necessary to carry out normalization and volumetric hardening with high tempering immediately before surface hardening of the HFC.
Scope of hardening HDTV
HDTV hardening is used in a number of technological processes for the manufacture of the following parts:
- shafts, axles and pins;
- gears, gear wheels and rims;
- teeth or cavities;
- cracks and internal parts of parts;
- crane wheels and pulleys.
Most often, high-frequency hardening is used for parts that consist of carbon steel containing half a percent carbon. Such products acquire high hardness after hardening. If the presence of carbon is less than the above, such hardness is no longer achievable, and at a higher percentage, cracks are likely to occur when cooling with a water shower.
In most situations, quenching with high-frequency currents makes it possible to replace alloyed steels with more inexpensive carbon steels. This can be explained by the fact that such advantages of steels with alloying additives, such as deep hardenability and less distortion of the surface layer, lose their significance for some products. With high-frequency hardening, the metal becomes stronger, and its wear resistance increases. In the same way as carbon steels, chromium, chromium-nickel, chromium-silicon and many other types of steels with a low percentage of alloying additives are used.
Advantages and disadvantages of the method
Advantages of hardening with high-frequency currents:
- fully automatic process;
- work with products of any form;
- lack of soot;
- minimum deformation;
- variability of the depth level of the hardened surface;
- individually determined parameters of the hardened layer.
Among the disadvantages are:
- the need to create a special inductor for different shapes of parts;
- difficulties in overlaying the levels of heating and cooling;
- high cost of equipment.
The possibility of using hardening with high-frequency currents in individual production is unlikely, but in the mass flow, for example, in the manufacture of crankshafts, gears, bushings, spindles, cold rolling shafts, etc., hardening of high-frequency currents is becoming more and more widely used.
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.
Of the modern methods of surface hardening, the following are most widely used in mechanical engineering: 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 of surface hardening and economically use the electrical energy of 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 of the heat treatment process. 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, while 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 service life 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 ensure 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) is heated to a high temperature, after which it is hardened. 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.
Melting metal by induction is widely used in various industries: metallurgy, engineering, jewelry. A simple induction type furnace for melting metal at home can be assembled with your own hands.
Heating and melting of metals in induction furnaces occur due to internal heating and changes in the crystal lattice of the metal when high-frequency eddy currents pass through them. This process is based on the phenomenon of resonance, in which eddy currents have a maximum value.
To cause the flow of eddy currents through the melted metal, it is placed in the zone of action of the electromagnetic field of the inductor - the coil. It can be in the form of a spiral, figure eight or trefoil. The shape of the inductor depends on the size and shape of the heated workpiece.
The inductor coil is connected to an alternating current source. In industrial melting furnaces, industrial frequency currents of 50 Hz are used; for melting small volumes of metals in jewelry, high-frequency generators are used, as they are more efficient.
Kinds
Eddy currents are closed along a circuit limited by the magnetic field of the inductor. Therefore, heating of conductive elements is possible both inside the coil and from its outer side.
- Therefore, induction furnaces are of two types:
- channel, in which the channels located around the inductor are the container for melting metals, and the core is located inside it;
- crucible, they use a special container - a crucible made of heat-resistant material, usually removable.
channel furnace too overall and designed for industrial volumes of metal melting. It is used in the smelting of cast iron, aluminum and other non-ferrous metals.
crucible furnace quite compact, it is used by jewelers, radio amateurs, such an oven can be assembled with your own hands and used at home.
Device
- A home-made furnace for melting metals has a fairly simple design and consists of three main blocks placed in a common housing:
- high frequency alternator;
- inductor - do-it-yourself spiral winding of copper wire or tube;
- crucible.
The crucible is placed in an inductor, the ends of the winding are connected to a current source. When current flows through the winding, an electromagnetic field with a variable vector arises around it. In a magnetic field, eddy currents arise, directed perpendicular to its vector and passing through a closed loop inside the winding. They pass through the metal placed in the crucible, while heating it to the melting point.
Advantages of the induction furnace:
- fast and uniform heating of the metal immediately after switching on the installation;
- directivity of heating - only the metal is heated, and not the entire installation;
- high melting rate and homogeneity of the melt;
- there is no evaporation of the alloying components of the metal;
- the installation is environmentally friendly and safe.
A welding inverter can be used as a generator of an induction furnace for melting metal. You can also assemble the generator according to the diagrams below with your own hands.
Furnace for melting metal on a welding inverter
This design is simple and safe as all inverters are equipped with internal overload protection. The entire assembly of the furnace in this case comes down to making an inductor with your own hands.
It is usually performed in the form of a spiral from a copper thin-walled tube with a diameter of 8-10 mm. It is bent according to a template of the desired diameter, placing the turns at a distance of 5-8 mm. The number of turns is from 7 to 12, depending on the diameter and characteristics of the inverter. The total resistance of the inductor must be such that it does not cause an overcurrent in the inverter, otherwise it will be tripped by the internal protection.
The inductor can be mounted in a housing made of graphite or textolite and a crucible can be installed inside. You can simply put the inductor on a heat-resistant surface. The housing must not conduct current, otherwise the eddy current circuit will pass through it and the power of the installation will be reduced. For the same reason, it is not recommended to place foreign objects in the melting zone.
When working from a welding inverter, its housing must be grounded! The socket and wiring must be rated for the current drawn by the inverter.
you will find recommendations for choosing a solid fuel boiler, and in the following you will get acquainted with the types and rules:
Transistor induction furnace: circuit
There are many different ways to assemble an induction heater with your own hands. A fairly simple and proven scheme of a furnace for melting metal is shown in the figure:
- To assemble the installation with your own hands, you will need the following parts and materials:
- two field-effect transistors of the IRFZ44V type;
- two diodes UF4007 (you can also use UF4001);
- resistor 470 Ohm, 1 W (you can take two series-connected 0.5 W each);
- film capacitors for 250 V: 3 pieces with a capacity of 1 microfarad; 4 pieces - 220 nF; 1 piece - 470 nF; 1 piece - 330 nF;
- copper winding wire in enamel insulation Ø1.2 mm;
- copper winding wire in enamel insulation Ø2 mm;
- two rings from chokes taken from a computer power supply.
Do-it-yourself assembly sequence:
- Field-effect transistors are mounted on radiators. Since the circuit gets very hot during operation, the radiator must be large enough. You can also install them on one radiator, but then you need to isolate the transistors from the metal using gaskets and washers made of rubber and plastic. The pinout of field effect transistors is shown in the figure.
- It is necessary to make two chokes. For their manufacture, copper wire with a diameter of 1.2 mm is wound around rings taken from the power supply of any computer. These rings are made of powdered ferromagnetic iron. They need to be wound from 7 to 15 turns of wire, trying to maintain the distance between the turns.
- The capacitors listed above are assembled into a battery with a total capacity of 4.7 microfarads. Connection of capacitors - parallel.
- The inductor winding is made of copper wire with a diameter of 2 mm. 7-8 turns of winding are wound on a cylindrical object suitable for the diameter of the crucible, leaving long enough ends to connect to the circuit.
- Connect the elements on the board in accordance with the diagram. A 12 V, 7.2 A/h battery is used as a power source. The current consumed in operation is about 10 A, the battery capacity in this case is enough for about 40 minutes. If necessary, the furnace body is made of heat-resistant material, for example, textolite. The power of the device can be changed by changing the number of turns of the inductor winding and their diameter.
Induction heater for melting metal: video
Lamp induction oven
A more powerful induction furnace for melting metals can be assembled by hand on vacuum tubes. The diagram of the device is shown in the figure. 
To generate high-frequency current, 4 beam lamps connected in parallel are used. A copper tube with a diameter of 10 mm is used as an inductor. The unit is equipped with a trimmer capacitor for power adjustment. The output frequency is 27.12 MHz.
To assemble the circuit you need:
- 4 vacuum tubes - tetrodes, you can use 6L6, 6P3 or G807;
- 4 chokes for 100 ... 1000 μH;
- 4 capacitors at 0.01 uF;
- neon indicator lamp;
- tuning capacitor.
Assembling the device with your own hands:
- An inductor is made from a copper tube, bending it in the form of a spiral. The diameter of the turns is 8-15 cm, the distance between the turns is at least 5 mm. The ends are tinned for soldering to the circuit. The diameter of the inductor must be 10 mm larger than the diameter of the crucible placed inside.
- Place the inductor in the housing. It can be made from a heat-resistant non-conductive material, or from metal, providing thermal and electrical insulation from the circuit elements.
- Cascades of lamps are assembled according to the scheme with capacitors and chokes. Cascades are connected in parallel.
- Connect a neon indicator lamp - it will signal the readiness of the circuit for operation. The lamp is brought to the installation housing.
- A tuning capacitor of variable capacitance is included in the circuit, its handle is also displayed on the case.
Circuit cooling
Industrial melting plants are equipped with a forced cooling system using water or antifreeze. Water cooling at home will require additional costs, comparable in price to the cost of the metal melting plant itself.
Air-cooling with a fan is possible provided that the fan is sufficiently remote. Otherwise, the metal winding and other elements of the fan will serve as an additional circuit for closing eddy currents, which will reduce the efficiency of the installation.
Elements of the electronic and lamp circuits are also able to actively heat up. For their cooling, heat-removing radiators are provided.Work Safety Measures
- The main danger during operation is the risk of burns from the heated elements of the installation and molten metal.
- The lamp circuit includes elements with high voltage, so it must be placed in a closed case, eliminating accidental contact with the elements.
- The electromagnetic field can affect objects that are outside the device case. Therefore, before work, it is better to put on clothes without metal elements, remove complex devices from the coverage area: phones, digital cameras.
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 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.
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