HDTV heat treatment technology. Hardening of metals with high frequency currents

A high-frequency current is generated in the installation thanks to an inductor and makes it possible to heat a product placed in close proximity to the inductor. The induction installation 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 received from V.P. Volodin in 1923. After much trial and testing of high-frequency heating, it began to be used for hardening steel in 1935. High-frequency hardening installations 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 top layer of the product to mechanical damage, while the center of the workpiece has increased viscosity. It is important to note that the core of the product remains completely unchanged during high-frequency hardening.
The induction installation has many very important advantages compared to alternative types heating: if previously HDTV installations were more bulky 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 an induction hardening unit is the inability to process some products with complex shapes.

Types 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.
The following types of hardening exist:

• Stationary hardening: used, as a rule, for parts with a small flat surface. The position of the part and the inductor when using this hardening method remains unchanged.
• Continuous-sequential hardening: used for hardening cylindrical or flat products. During continuous-sequential hardening, the part can move under the inductor, or maintain its position unchanged.
• Tangential hardening of products: excellent for processing small parts that have a cylindrical shape. Tangential continuous-sequential hardening turns the product once during the entire heat treatment process.
• A high-frequency hardening installation is equipment that can produce high-quality hardening of a product and at the same time save production resources.

In hydromechanical systems, devices and assemblies, parts that operate on friction, compression, and torsion 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 with high frequency current (HFC).

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

Heating with HF 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 the metal part, which cause heating of the hardened product. Being located exclusively on the surface of the part, they allow it to be heated to a certain adjustable depth.

High-frequency 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 pearlite transforms into austenite), the temperature level of the critical points increases. And the second is that the faster the temperature transition occurs, the faster the transformation of the metal surface occurs, because it must occur in a 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 occur. 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 heating level is higher and the cooling is more intense, the hardness of the workpiece after its high-frequency quenching 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 increases 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 effect of such stresses is most pronounced at a small depth of the hardened layer.

High-frequency installations, materials and auxiliaries used for hardening

A fully automatic high-frequency hardening complex includes a hardening machine and high-frequency installations (mechanical fastening systems, units for rotating the part around its axis, movement of the inductor in the direction of the workpiece, pumps supplying and pumping 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 over 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 uniform hard surface blanks.

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

Induction high-frequency hardening can be characterized by two main parameters: the degree of hardness and the depth of surface hardening. Technical Parameters Induction units produced in 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 a frequency 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 level of depth 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 specific technological process.

The technical parameters of automatic hardening machines are determined overall dimensions used parts for hardening in height (from 50 to 250 centimeters), in diameter (from 1 to 50 centimeters) and weight (up to 0.5 t, up to 1 t, up to 2 t). Hardening complexes, 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. The quenching medium is usually water, inert gases or polymer compounds that have thermal conductivity properties similar to oil. 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 with a small diameter, high-frequency hardening of a stationary type 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 this case, either the heating device moves towards the part, or the part moves under the heating device.

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

Metal structure of a gear tooth after hardening by the high-frequency method

After high-frequency heating of the product, it is subjected to low tempering at a temperature of 160-200°C. This allows you to increase the wear resistance of the product surface. Temperings are carried out in electric furnaces. Another option is to take a self-vacation. To do this, you need to turn off the water supply device a little earlier, which contributes to incomplete cooling. Item saves 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 a high-frequency installation. To achieve the desired result, heating is carried out at a lower speed and more deeply than with surface hardening. The required heating mode can be determined by selection.

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 with high-frequency heat.

Areas of application of HDTV hardening

High-frequency hardening is used in a number of technological processes for the manufacture of the following parts:

• shafts, axles and pins;
• gears, cogwheels and rims;
• teeth or grooves;
• 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 with a higher percentage, cracks are likely to occur when cooled with a water shower.

In most situations, hardening with high-frequency currents makes it possible to replace alloyed steels with more inexpensive ones - carbon ones. 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. Just like 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 HF currents:

• fully automatic process;
• work with products of any shape;
• no soot;
• minimal deformation;
• variability in the level of depth 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 adjusting heating and cooling levels;
• high cost of equipment.

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

Metal smelting by induction is widely used in various industries: metallurgy, mechanical engineering, jewelry. You can assemble a simple induction furnace for melting metal at home with your own hands.

Heating and melting of metals in induction furnaces occurs 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 eddy currents to flow through the molten metal, it is placed in the zone of action of the electromagnetic field of an inductor - a coil. It can be in the shape 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.

Species

Eddy currents are closed along a contour limited magnetic field inductor. Therefore, heating of the conductive elements is possible both inside the coil and on its outside.

Therefore, induction furnaces come in two types:
• channel, in which the container for melting metals is channels located around the inductor, and a core is located inside it;
• crucible, they use a special container - a crucible made of heat-resistant material, usually removable.

Channel furnace too large and designed for industrial volumes of metal smelting. It is used in the smelting of cast iron, aluminum and other non-ferrous metals.
Crucible furnace It is quite compact, it is used by jewelers and radio amateurs; such a stove can be assembled with your own hands and used at home.

Device

A homemade furnace for melting metals has a fairly simple design and consists of three main blocks placed in a common body:
• high frequency alternating current generator;
• inductor - spiral winding made of copper wire or pipes made by yourself;
• 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 appears around it. In a magnetic field, eddy currents arise, directed perpendicular to its vector and passing along a closed loop inside the winding. They pass through the metal placed in the crucible, heating it to the melting point.

Advantages of an induction furnace:

• fast and uniform heating of the metal immediately after turning on the installation;
• direction of heating - only the metal is heated, and not the entire installation;
• high melting speed and melt homogeneity;
• there is no evaporation of metal alloying components;
• The installation is environmentally friendly and safe.

A welding inverter can be used as a generator for an induction furnace for melting metal. You can also assemble a generator using the diagrams below with your own hands.

Furnace for melting metal using a welding inverter

This design is simple and safe, since all inverters are equipped with internal protections from overloads. 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 thin-walled copper tube with a diameter of 8-10 mm. It is bent according to a template of the required 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 as not to cause overcurrent in the inverter, otherwise it will be turned off by the internal protection.

The inductor can be fixed in a housing made of graphite or textolite and a crucible can be installed inside. You can simply place the inductor on a heat-resistant surface. The housing must not conduct current, otherwise eddy currents will pass through it and the power of the installation will decrease. For the same reason, it is not recommended to place foreign objects in the melting zone.