Surface hardening (SHF). Surface hardening

The strength of elements in particularly 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, durability or viscosity, heat treatment operations are carried out. Strengthen the surface of parts various methods. One of them is hardening with high frequency currents, that is, HDTV. It is one of the most common and very productive methods during large-scale production of various structural elements.

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

The technology is used to strengthen components of technological equipment and transport, as well as for hardening various tools.

The essence of technology

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

Experiments have proven that increasing the frequency of the current helps to reduce the depth of penetration. This fact opens up opportunities for regulation and production of parts with a minimal hardened layer.

Heat treatment of HDTV is carried out in special installations - generators, multipliers, frequency converters, which allow adjustment in the required range. In addition to 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 has also been revealed - the smaller the product and the simpler its shape, the better the hardening process. This also reduces the overall energy consumption of the installation.

The inductor is copper. There are often additional holes on the inner surface designed to supply water during cooling. In this case, the process is accompanied by primary heating and subsequent cooling without current supply. The 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 quenching tank.

The main requirement for the high-frequency hardening process is to maintain a constant gap between the inductor and the product. When maintaining a given interval, the quality of hardening becomes the highest.

Hardening can be done in one of the following ways::

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

Features of the induction installation

The installation for high-frequency hardening is a high-frequency generator together with an inductor. The workpiece is located both in the inductor itself and next to it. It consists of a coil on which a copper tube is wound.

Variable electric current when passing through the inductor, it 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 surface structure metal

Increasing the frequency opens up the possibility of concentrating heat on 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 volumetric hardening.

Induction heating - characteristics

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

The heating time is characterized by the total volume of heat expended and the temperature developed. Frequency, as mentioned above, determines the depth of penetration of currents and the hardened layer formed. These characteristics have an inverse relationship. As the frequency increases, the volumetric mass of the heated metal decreases.

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

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

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

Surface hardening HDTV accompanied by high 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 ensure the completion of the transformation of pearlite into austenite in a short time.

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 granular elements in the steel structure do not have time to grow in a minimum time. In addition, volumetric hardening has a lower strength of up to 2-3 units. After high-frequency hardening, the part has greater wear resistance and hardness.

How is the temperature selected?

Compliance with technology must 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 just above that required to convert pearlite and ferrite to austenite. Range from 800 to 850 degrees. After this, the part is cooled at high speed. After rapid cooling, austenite is transformed into martensite, which has high hardness and strength. With a short holding time, austenite with a fine-grained structure, as well as fine-needle martensite, is obtained. Steel gains high hardness and low 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 retain in the structure a certain volume of cementite, which has a higher hardness compared to martensite. Upon rapid cooling, austenite is transformed into martensite. Cementite is preserved by small inclusions. The zone also retains carbon that has not fully dissolved and has turned into solid carbide.

Advantages of technology

• Controlling modes;
• Replacing alloy steel with carbon steel;
• Uniform heating process of the product;
• The ability not to heat the entire part completely. Reduced energy consumption;
• High resulting strength of the processed workpiece;
• There is no oxidation process, no carbon is burned;
• No microcracks;
• There are no warped points;
• Heating and hardening of certain areas of products;
• Reducing the time spent on the procedure;
• Introduction of high-frequency installations into production lines during the manufacture of parts.

Flaws

The main disadvantage of the technology under consideration is the significant installation price. It is for this reason that the feasibility of use is justified only in large-scale production and excludes the possibility of doing the work yourself at home.

Study the operation and principle of operation of the installation in more detail in the presented videos.

Steel is hardened to make the metal more durable. Not all products are subject to hardening, but only those that are often abraded and damaged from the outside. After hardening, the top layer of the product becomes very durable and protected from corrosion formations and mechanical damage. Hardening with high frequency currents makes it possible to achieve exactly the result that the manufacturer needs.

Why HDTV hardening?

When given a choice, the question “why?” very often arises. Why should you choose HDTV hardening if there are other methods of hardening metal, for example, using hot oil?
High-frequency hardening has many advantages, which is why it has become actively used recently.

1. Under the influence of high-frequency currents, heating is uniform over the entire surface of the product.
2. The induction machine's software can fully control the hardening process for more accurate results.
3. High-frequency hardening makes it possible to heat the product to the required depth.
4. The induction installation allows you to reduce the number of defects in production. If, when using hot oils, scale often forms on the product, then heating the HDTV completely eliminates this. High-frequency hardening reduces the number of defective products.
5. Induction hardening reliably protects the product and makes it possible to increase productivity in the enterprise.

Induction heating has many advantages. There is also one disadvantage - in induction equipment it is very difficult to harden a product that has a complex shape (polyhedrons).

HDTV hardening equipment

Modern induction equipment is used for hardening HDTV. The induction unit is compact and allows you to process a significant number of products in a short period of time. If the enterprise constantly needs to harden products, then it is best to purchase a hardening complex.
The hardening complex includes: a hardening machine, an induction installation, a manipulator, a cooling module, and if necessary, a set of inductors for hardening products can be added different shapes and sizes.
HDTV hardening equipment is an excellent solution for high-quality hardening of metal products and obtaining accurate results in the process of metal transformation.

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.
An induction installation has many very important advantages in comparison with alternative types of 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.

Hardening of steels with high frequency currents (HFC) is one of the most common methods of surface heat treatment, which allows increasing the hardness of the surface of workpieces. Used for parts made of carbon and structural steels or cast iron. High-frequency induction hardening is one of the most economical and technologically advanced methods of hardening. It makes it possible to harden the entire surface of a part or its individual elements or zones that experience the main load.

In this case, under the hardened hard outer surface of the workpiece, unhardened viscous layers of metal remain. This structure reduces fragility, increases the durability and reliability of the entire product, and also reduces energy consumption for heating the entire part.

High frequency hardening technology

HDTV surface hardening is a heat treatment process to increase 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 installation. 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 completion of the hardening process, the correct selection of temperature, which depends on the material used, is very important.

Steels based on carbon content 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 temperature of the phase transformation of pearlite and ferrite to austenite. This occurs in the range of 800-850°C. Then the workpiece is quickly cooled. When cooled sharply, austenite transforms into martensite, which has high hardness and strength. A short holding time makes it possible to obtain fine-grained austenite and fine-needle 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 steels, to a temperature of 750-800°C, that is, incomplete hardening is performed. This is due to the fact that when heated to this temperature, in addition to the formation of austenite in the metal melt, undissolved large number cementite, which has a higher hardness than martensite. After rapid cooling, austenite transforms into martensite, and 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 high-frequency quenching, the temperature is close to the transition temperature, and austenite with ferrite residues is formed. 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 in this zone improves, it becomes fine-grained and uniform.

Overheating the surface of the workpiece promotes the growth of austenite crystals, which has a detrimental effect on brittleness. Underheating prevents the complete ferrite-perrite structure from transforming into austenite, and unhardened spots may form.

After cooling, high compressive stresses remain on the metal surface, which increase the performance 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 hardening and tempering.

You can also carry out the so-called self-tempering - cool the part not completely, but to a temperature of 200 ° C, while heat will remain in its core. Then the part should cool slowly. This will equalize internal stresses.

Induction installation

The HDTV induction heat treatment unit is a high-frequency generator and inductor for HDTV hardening. The part to be hardened can be located in or near the inductor. The inductor is made in the form of a coil, with a copper tube wound on it. It can have any shape depending on the shape and size of the part. When alternating current passes through the inductor, an alternating electromagnetic field appears in it, passing through the part. This electromagnetic field causes eddy currents known as Foucault currents to occur in the workpiece. Such eddy currents, passing through layers of metal, 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. In this way, only the outer layer of the metal is heated, and the higher the current frequency, the smaller the heating depth, and, accordingly, the depth of HDTV hardening. This makes it possible to harden only the surface of the workpiece, leaving inner layer soft and viscous to avoid excessive fragility. Moreover, you can adjust the depth of the hardened layer by changing the current parameters.

The increased frequency of the current allows you to concentrate a large amount of heat 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 higher temperature zone. In this case, the hardness increases by 2-4 units, up to 58-62 HRC, which cannot be achieved with volumetric hardening.

For the correct implementation of the HDTV hardening process, it is necessary to ensure that the same clearance is maintained between the inductor and the workpiece over the entire hardening surface, and mutual touching must be avoided. This is ensured, if possible, by rotating the workpiece in the centers, which allows for uniform heating, and, as a consequence, the same structure and hardness of the surface of the hardened workpiece.

The inductor for hardening HDTV has several versions:

• single- or multi-turn annular - for heating the outer or inner surface of parts in the form of bodies of rotation - 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 also cooled simultaneously;
• continuous-sequential - one zone of a part is heated, then when the inductor or part is displaced, another zone is heated, while the previous one is cooled.

Simultaneous high-frequency heating of the entire surface requires large amounts 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 sprayed with a stream of water.

Continuous-sequential hardening of high-frequency frequencies allows you to harden large-sized parts, for example, the crowns of gear wheels, since during this process a small zone of the part is heated, which requires less power of the high-frequency generator.

Cooling parts

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 occurs in coolant tanks or by spray. For high-quality hardening, it is necessary to maintain a stable temperature of the coolant and prevent it from overheating. The holes in the sprayer must be of the same diameter and spaced evenly, this way the same metal structure on the surface is achieved.

To prevent the inductor from overheating during operation, water is constantly circulated through the copper tube. Some inductors are made combined with a 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 of parts using HDTV has both advantages and disadvantages. The advantages include the following:

• After high-frequency quenching, the part retains a soft center, which significantly increases its resistance to plastic deformation.
• The cost-effectiveness of the process of hardening 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.
• During mass production of parts, it is necessary to set up the process and then it will be automatically repeated, ensuring the required quality of 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 time high temperature contributes to the absence of oxidation, decarburization of the top layer and the formation of scale on the surface of the part.
• Rapid heating and cooling does not result in large warpage and distortion, which allows for a reduction in finishing allowance.

But induction installations are economically feasible to use only in mass production, and for single production buying or making an inductor is unprofitable. For some parts with complex shapes, induction production is very difficult or impossible to obtain a uniform hardened layer. In such cases, other types of surface hardening are used, for example, gas-flame or volumetric hardening.

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 change crystal lattice 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 molten metal, it is placed in the zone of action of the electromagnetic field of the inductor - the 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 - a spiral winding made of copper wire or tube, made by hand;
• 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 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 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.