Types of heat treatment annealing normalization hardening tempering

The basic properties and qualities of a metal are determined by its structure. Heat treatment is the most common method of influencing a material, which is used to change its structure and, consequently, properties. How is heat treatment of steel and metals carried out - the main types of technological process, and for what purposes is this type of treatment used? All this knowledge can be obtained by familiarizing yourself with the basics of metal technology - a branch of science that studies the techniques and methods of creating and processing metal materials.

Types of annealing

The essence of the process is to heat the metal product and then slowly cool it. As a result, the viscosity index improves and chemical and structural homogeneity is achieved. Heat treatment by annealing has a negative effect on the hardness of steel.

Depending on the required qualities of the product, the following types of annealing are performed:

Diffusion. The purpose of processing is to reduce the chemical heterogeneity of the composition. First, the steel is heated to a temperature of +1150°C and the workpiece remains in this state for 10-15 hours. Then slow (natural) cooling is performed. Full. It is performed for stamping products or blanks made by casting or forging. The goal is to form a fine-grained structure. The steel is heated to a temperature exceeding the critical upper point by +50°C. Then slow cooling occurs at a rate of no more than 75°C (for alloyed grades) or +200°C (for carbon grades) per hour. Incomplete annealing. Heat treatment is used to reduce stiffness and relieve structural stress. The technology is similar to that described above, with the exception of the maximum temperature value. It should not exceed +750°C. Isothermal. It is relevant only for alloy steels. The exposure temperature is 20-30% higher than the critical point. Differences from full annealing are rapid cooling to +600°C. The technique is used for rapid processing of steel workpieces.

To perform these procedures, special equipment is required. The quality of processing depends on the requirements being met. If the technology is not followed, there is a high probability of defects – burnout.

Heat treatment: hardening, tempering, normalization, annealing

Metal products used in any sector of the economy must meet wear resistance requirements. For this purpose, exposure to high temperatures is used, as a result of which the desired performance properties are enhanced. This process is called heat treatment.

Heat treatment is a set of operations of heating, cooling and holding metal hard alloys to obtain the required properties by changing the structure and internal structure. Heat treatment is used as an intermediate operation in order to improve machinability by cutting, pressure, or as the final operation of the technological process, which provides the required level of properties of the part.

Various hardening methods have been used for a long time: craftsmen immersed a heated metal strip in wine, oil, or water. For cooling, blacksmiths sometimes used quite interesting methods, for example, they mounted a horse and raced, cooling the product in the air.

Depending on the method of implementation, heat treatment can be of the following types:

-Thermal (normalization, hardening, tempering, annealing, aging, cryogenic treatment).

-Thermo-mechanical. Involves processing at high temperatures in combination with mechanical stress on the alloy.

-Chemical-thermal. It involves heat treatment of metal followed by enrichment of the surface of the product with chemical elements (carbon, nitrogen, chromium, etc.).

Main types of heat treatment:

1. Hardening . It is a type of heat treatment of various materials (metals, glass), consisting of heating them above a critical temperature with rapid subsequent cooling. Performed to obtain nonequilibrium structures with an increased cooling rate. Hardening can be either with a polymorphic transformation or without a polymorphic transformation.

2. Tempering is a technological process, the essence of which is the heat treatment of a metal or alloy hardened to martensite, the main processes in which are the decomposition of martensite, recrystallization and polygonization. It is carried out with the aim of relieving internal stresses and giving the material the necessary operational and mechanical properties.

3. Normalization. In this case, the product is heated to an austenitic state and then cooled in still air. As a result of normalization, internal stresses are reduced and steel recrystallizes. Compared to annealing, normalization is a shorter and more productive process.

4. Annealing . It is a heat treatment operation consisting of heating steel, holding it at a given temperature and then slowly cooling it along with the furnace. As a result of annealing, a stable structure is formed, free from residual stresses. Annealing is one of the most important mass heat treatment operations of steel.

Annealing purpose:

1) Reducing hardness and increasing ductility to facilitate metal cutting;

2) Reduction of internal stress arising after pressure treatment (forging, stamping), machining, etc.;

3) Removing brittleness and increasing impact resistance;

4) Elimination of structural heterogeneity in the composition of the material that occurs during solidification of the casting as a result of segregation.

For non-ferrous alloys (aluminum, copper, titanium), heat treatment is also widely used. Non-ferrous alloys are subjected to both softening and strengthening heat treatment, depending on the required properties and scope of application.

Heat treatment of metals and alloys is the main technological process in ferrous and non-ferrous metallurgy. At the moment, technical specialists have at their disposal many heat treatment methods that allow them to achieve the desired properties of each type of processed alloys. Each metal has its own critical temperature, which means that heat treatment must be carried out taking into account the structural and physicochemical characteristics of the substance. Ultimately, this will allow not only to achieve the desired results, but also to significantly streamline production processes.

Hardening

The technique is relevant for creating an uneven structure in a steel workpiece. This increases hardness, but also increases the fragility of the structure. The choice of exposure temperature depends on the chemical composition. Also important are the cooling rate and frequency of repetition of the procedure.

When choosing a hardening technology, the following factors are taken into account:

Processing temperature. If it does not exceed the critical value, the hardening is classified as incomplete. To process the entire structure of the workpiece, the thermal effect must be 30-40° above the Acz point. Cooling. It can be done quickly or slowly. In the first case, the hardness is uneven, closer to the surface. With slow cooling, the stress of the structure is equalized. Selection of quenching medium. Most often, a salt bath or oil with the addition of special substances is used. Periodicity. It affects the distribution of rigidity in the steel structure.

Hardening methods are calculated individually for each type of product. Read about how to harden and temper metal at home here.

Vacation

To normalize the characteristics of steel workpieces after hardening, it is recommended to temper it. Its essence lies in thermal exposure to temperatures at which phase transformation does not occur. The result of this operation will be the homogeneity of the steel structure.

Types of tempering for metal blanks:

Short. Used for carbon steel grades. The maximum exposure temperature is +200°C. As a result, the fragility index decreases and the tension in the structure decreases. Average. Heat treatment occurs at +400°C. Technology is needed to remove excess carbon. In this case, the crystal lattice becomes cubic. High. Processing temperature – up to +650°С. It is used to achieve optimal characteristics of strength, viscosity and ductility.

The defining indicator for this process is temper brittleness. It indicates the degree to which impact strength drops during sudden temperature changes.

What are the advantages of heat treatment?

When carrying out heat treatment, the properties of the metal are improved, which is very valuable on the scale of modern industrial production. The main advantages of heat treatment include:

• increasing wear resistance, which means extending the shelf life of processed metal products;
• significant reduction in the percentage of defective products;
• saving money and resources in production as a result of increasing the strength and improving the quality characteristics of industrial equipment parts.

The essence of heat treatment is to follow a certain sequence of technological operations for heating, holding and cooling the metal.

Due to this, materials acquire different physical and mathematical properties due to the effects of temperatures and changes in the structure of the metal.

Normalization of metal blanks

The technology is similar to steel annealing. The difference lies in the method of cooling the workpiece. This does not happen in the oven, as in the first case, but in the air. As a result, the structure of the crystal lattice is normalized, and strength and toughness indicators increase.

When performing this process, the following indicators are taken into account:

Excerpt. It characterizes the degree of uniform thermal impact on all layers of the steel billet. Cooling rate. Affects the thickness of pearlite plates. Staged cooling. In some cases, after reaching a certain level of temperature reduction, the part is placed in oil for rapid cooling.

To achieve the desired properties of a steel billet, several types of heat treatment can be performed.

Heat treatment (heat treatment) of steel, non-ferrous metals is the process of changing the structure of steel, non-ferrous metals, alloys during heating and subsequent cooling at a certain speed. Heat treatment (heat treatment) leads to significant changes in the properties of steel, non-ferrous metals, and alloys. The chemical composition of the metal does not change.

A little history

Even in ancient times, blacksmith masters used the most primitive hardening methods. To do this, a red-hot piece of iron was immersed in water, oil or wine. But time passed, and along with experience, methods of hardening metal also developed.

At the beginning of the 19th century, brittle cast iron was placed in a container with ice and covered with sugar. After the heating process lasted for 20 hours, the cast iron became soft and easy to forge.

The middle of the 19th century is significant in that the Russian inventor, metallurgist D.K. Chernov, made an outstanding discovery. He found that when the temperature changes, the metal changes its properties.

Dmitry Konstantinovich Chernov became the founder of the science that studies the properties of metals - materials science.

Types of heat treatment of steel

Annealing

Annealing is a thermal treatment (heat treatment) of a metal that involves heating the metal and then slowly cooling it. This heat treatment (i.e. annealing) comes in different types (the type of annealing depends on the heating temperature and the cooling rate of the metal).

Hardening

Hardening is a heat treatment (heat treatment) of steel and alloys, based on the recrystallization of steel (alloys) when heated to a temperature above critical; After sufficient exposure to the critical temperature to complete the heat treatment, rapid cooling follows. Hardened steel (alloy) has a nonequilibrium structure, so another type of heat treatment is applicable - tempering.

Vacation

Tempering is a heat treatment (heat treatment) of steel and alloys, carried out after hardening to reduce or relieve residual stresses in steel and alloys, increasing toughness, reducing the hardness and brittleness of the metal.

Heat treatment of steels and metals, what is it - the main types of heat treatment of alloys

Varieties of metallic substances have varying degrees of strength, susceptibility to corrosion and other chemical reactions.

Using heating, you can achieve the required properties from the workpiece, improve wear resistance, and prepare it for further procedures during metalworking.

In this article we will talk about heat treatment of steel parts - what it is, what are the main types of heat treatment of metals.

Purpose of the technological process

You can work with both blanks and finished products. For the former, internal stress is relieved after various types of casting and stamping, the material becomes more plastic, and it is much easier to work with, especially cutting it. If a whole part is processed, then the following goals are pursued:

• increased strength;
• protection against premature rusting;
• increased resistance to temperature changes, the upper and lower temperature thresholds at which the item can be used become larger;
• extension of potential service life.

Benefits of technology

This process is used universally in many enterprises - every second production of metal products requires thermal exposure. This is due to the advantages:

• You can work with steel, non-ferrous metals and alloys - a wide range.
• Increased product shelf life.
• Reduced level of abrasive wear.
• The percentage of defects in production shops is becoming much lower.
• Cost savings, since it is easier to carry out a number of manipulations with a heat-treated workpiece.

Processing principles

The main rule is that the time spent on one part is equal to the duration of heating of the material, depending on its maximum temperature, holding period and cooling. The total count allows you to calculate the total time value. Each of these points depends on:

• workpiece dimensions;
• type of metal subjected to heat treatment;
• oven power.

All this determines how quickly the transformations will take place.

Classification

All varieties are used for different purposes, with different materials. For this, the technology remains the same - heating, holding, cooling, but the time of each stage changes. Features are presented in the video:

Short

The main task is to increase viscosity at the same hardness. This is achieved by giving the internal microstructure a needle or plate type. Often used for heat treatment of cutting parts and medical instruments. The workpiece is heated within 150-250 degrees. Leave for at least one and a half hours, and then cool with air or oil.

Average

Here, martensite (the type of structure described above) is transformed into trustite, which is characteristic of cast iron. Feature – high dispersion. At the same high viscosity, the hardness also increases. This is very important for elements that will be subject to large elastic loads. Temperature limits – from 340 to 500, air cooling.

High

Crystallization occurs with the appearance of sorbitol. Thanks to it, the stress inside the alloy is completely eliminated. This method is used for structures that are very important - in aircraft construction, in the construction of space objects. Heating temperature – from 450 to 650 degrees.

General definition and types

During casting or other primary processing processes, in addition to stress, defects appear. You can remove these changes and achieve a homogeneous structure of the crystal lattice using the following algorithm of actions:

• heating - it is necessary to slightly exceed the critical level for this type of steel;
• a certain period is required to maintain a stable temperature;
• The workpiece should be cooled slowly along with the oven.

Annealing has the following varieties.

Homogenization

Refers to the first kind, when changes are considered minor. The purpose of such manipulation is to remove the heterogeneity of the structure and bring it to uniformity. In this case, the product should be heated at a temperature range of 1000 to 1150 degrees, then held for about 8-15 hours and gradually reduced heating, cooling the workpiece with oxygen.

Recrystallization

Also a type of annealing phase 1. The purpose of the procedure is to bring all the crystals into a single form, as well as relieve the internal stress of the metal. There are two subspecies:

• softening - usually used as a final treatment, implies an improvement in plastic characteristics;
• strengthening – increases elasticity, especially important for hardening springs.

The temperature is selected depending on the alloy, usually 100-200 degrees higher than the recrystallization point. It is necessary to maintain the temperature for an hour or two, then allow it to cool slowly.

Isothermal annealing

The goal is to achieve high-temperature face-centered modification of iron (decomposition of austenite) to soften it. This results in a more uniform structure of the product. More often, this type of metalworking is used for small stampings, because they can be subjected to rapid cooling without problems. Process:

• heating is 20-30 degrees more than the material limit;
• short aging;
• rapid cooling is an advantage over other subspecies.

To eliminate stress

This is an operation of removing, removing the negative internal state of excessive hardness, due to which the metal becomes brittle and short-lived. It quickly deforms from external physical influences. The process involves temperatures from 700 to 750, then a slight cooling to 600 and holding for up to 20 hours, then slowly cooling under the influence of air.

Complete annealing

Used to create a plastic, uniform fine-grained structure. The most typical method of intermediate impact on rolled metal is after casting, forging, stamping and before cutting by any method. Stages:

• heating is 30-50 more than the steel limit;
• holding;
• very slow cooling with the oven - no more than 50-150 degrees in 60 minutes.

Incomplete

There are no significant transformations at the level of the crystal lattice, but hardness is imparted to previously plastic materials. This is especially necessary for structures formed by welded joints, as well as tools that require special strength. The method involves a temperature of about 700, and after 20 hours gradual cooling.

Hardening as the main type of heat treatment of steel

A very common method of heat treatment, as it allows you to make the product less susceptible to compression, shear, and also give it strength and durability, immunity to external physical influences. This occurs by imparting a needle-like structure to the metal. The substance hardens with “needles” due to a lack of alloying materials.

The workpiece is heated up strongly, and then cooled as quickly as possible, using external sources - water, oil, a solution with added salt. Due to the speed, diffusion processes do not have time to occur in the semi-molten alloy. It is cheapest to create water baths, but cracks may appear on the surface; an oil medium is the most preferable.

Cryogenic heat treatment

Another thermal method of exposure, but without heating. The product is placed in a refrigeration unit, sometimes an entire workshop is used for large-sized structures. Low temperatures and subsequent warming reduces the risk of corrosion, extends service life, increasing strength.

Thermo-mechanical effect

TMO was used by blacksmiths in ancient times. These are any plastic deformations (impacts, compressions) produced by heating the entire product or element. It is usually combined with hardening, that is, after deformation it is quickly cooled.

Hardenability and hardenability of steel

These indicators determine the results of all of the above procedures. The first term is hardness, which is directly related to the amount of carbon, and the second is the depth of hardening, that is, which top layer has been subjected to changes.

Heating the workpiece

Heating the workpiece is a critical operation. The quality of the product and labor productivity depend on the correctness of its implementation. You need to know that during the heating process the metal changes its structure, properties and characteristics of the surface layer and as a result of the interaction of the metal with atmospheric air, scale is formed on the surface; the thickness of the scale layer depends on the temperature and duration of heating, the chemical composition of the metal. Steels oxidize most intensively when heated above 900°C; when heated to 1000°C, oxidation increases 2 times, and at 1200°C - 5 times.

Chrome-nickel steels are called heat-resistant because they practically do not oxidize.

Alloy steels form a dense, but not thick layer of scale, which protects the metal from further oxidation and does not crack during forging.

When heated, carbon steels lose carbon from a surface layer of 2-4 mm. This threatens the metal with a decrease in the strength and hardness of the steel and hardening deteriorates. Decarburization is especially harmful for small-sized forgings followed by hardening.

Carbon steel blanks with a cross-section of up to 100 mm can be quickly heated and therefore they are placed cold, without preheating, in a furnace where the temperature is 1300°C. To avoid cracks, high-alloy and high-carbon steels must be heated slowly.

When overheated, the metal acquires a coarse-grained structure and its ductility decreases. Therefore, it is necessary to refer to the iron-carbon diagram, which defines the temperatures for the start and end of forging. However, overheating of the workpiece can, if necessary, be corrected by heat treatment, but this requires additional time and energy. Heating the metal to an even higher temperature leads to burnout, which disrupts the bonds between grains and such metal is completely destroyed during forging.

Burnout

Burnout is an irreparable marriage. When forging products from low-carbon steels, less heating is required than when forging a similar product from high-carbon or alloy steel.

When heating metal, it is necessary to monitor the heating temperature, heating time and temperature at the end of heating. As the heating time increases, the scale layer grows, and with intense, rapid heating, cracks may appear. It is known from experience that on charcoal a workpiece 10-20 mm in diameter is heated to forging temperature in 3-4 minutes, and workpieces with a diameter of 40-50 mm are heated for 15-25 minutes, monitoring the color of the heat.

Chemical-thermal treatment

Chemical thermal treatment (CHT) of steel is a set of heat treatment operations involving saturation of the surface of the product with various elements (carbon, nitrogen, aluminum, silicon, chromium, etc.) at high temperatures.

Surface saturation of steel with metals (chromium, aluminum, silicon, etc.), which form substitutional solid solutions with iron, is more energy-intensive and longer lasting than saturation with nitrogen and carbon, which form interstitial solid solutions with iron. In this case, the diffusion of elements proceeds more easily in the alpha-iron lattice than in the more densely packed gamma-iron lattice.

Chemical-thermal treatment increases hardness, wear resistance, cavitation, and corrosion resistance. Chemical-thermal treatment, creating favorable residual compressive stresses on the surface of products, increases reliability and durability.

Steel Cementation

Cementation of steel - chemical-thermal treatment of low-carbon steel by surface saturation (from Table 1

A thin film of iron oxides that gives the metal a variety of rapidly changing colors, from light yellow to gray. Such a film appears if a steel product cleaned of scale is heated to 220°C; As the heating time increases or the temperature rises, the oxide film thickens and its color changes. Tarnish colors appear equally on both raw and hardened steel.

With low tempering (heating to a temperature of 200-300°), martensite mainly remains in the steel structure, which, however, changes the lattice. In addition, the separation of iron carbides from the solid solution of carbon in alpha iron begins and their initial accumulation in small groups. This entails a slight decrease in hardness and an increase in the plastic and viscous properties of steel, as well as a decrease in internal stresses in parts.

For low tempering, parts are kept for a certain time, usually in oil or salt baths. If for low tempering parts are heated in air, then tarnish colors appearing on the surface of the part are often used to control the temperature.

table 1

Hardening defects

Hardening defects include:

• cracks,
• leashes or warping,
• decarbonization.

The main reason for cracks and cracks is an uneven change in the volume of the part when heated and, especially, during sudden cooling. Another reason is the increase in volume during martensite quenching.

Cracks arise because stresses due to uneven changes in volume in individual places of the part exceed the strength of the metal in these places.

The best way to reduce stress is to cool slowly around the martensitic transformation temperature. When designing parts, it is necessary to take into account that the presence of sharp corners and sudden changes in cross-section increases internal stress during hardening.

Warping (or warping) also arises from stress as a result of uneven cooling and manifests itself in the curvature of parts. If these distortions are small, they can be corrected, for example, by grinding. Cracks and warping can be prevented by preliminary annealing of parts, uniform and gradual heating of them, as well as the use of stepwise and isothermal hardening.

Decarburization of steel from the surface is the result of carbon burnout during high and prolonged heating of the part in an oxidizing environment. To prevent decarburization, parts are heated in a reducing or neutral environment (reducing flame, muffle furnaces, heating in liquid media).

The formation of scale on the surface of the product leads to waste of the metal and deformation. This reduces thermal conductivity and, therefore, reduces the rate of heating of the product in the furnace and complicates mechanical processing. Scale is removed either mechanically or chemically (etching).

Carbon burned from the surface of the metal makes the product decarbonized with reduced strength characteristics and difficult machining. The intensity with which oxidation and decarburization occurs depends on the heating temperature, i.e., the greater the heating, the faster the processes occur.

The formation of scale during heating can be avoided if a paste is used for hardening, consisting of liquid glass - 100 g, refractory clay - 75 g, graphite - 25 g, borax - 14 g, carborundum - 30 g, water - 100 g. The paste is applied to the product and allow it to dry, then heat the product in the usual way. After hardening, it is washed in a hot soda solution. To prevent the formation of scale on high-speed steel tools, a borax coating is used. To do this, the instrument heated to 850°C is immersed in a saturated aqueous solution or borax powder.

HEAT TREATMENT OF CARBON STEEL

Carbon steels are subjected to annealing, normalization, hardening and tempering.

Annealing is a thermal operation consisting of heating steel 20-30° above the GS or SE' line (Fig. 61), holding at this temperature and then slowly cooling. Annealing is used to level the structure of steel after casting, eliminate internal stresses in steel after forging or casting, obtain a fine-grained structure of steel, reduce the hardness of steel and improve its cold workability.

The heating temperature during annealing of carbon steels is selected depending on the carbon content in them.

Annealing is carried out in furnaces and forges. For annealing in a forge, the parts are placed in an iron box, sprinkled with fine sand or charcoal. In this case, the heating temperature of the parts is determined by the glowing color of the walls of the metal box, which must be evenly heated and free of dark spots.

The holding time during annealing is set depending on the size of the parts. On average, the heating time for every 25 mm of the largest cross-sectional thickness of a part is taken to be 45 minutes.

Cooling of the part after annealing should be slow. The cooling rate of carbon steel during annealing is 2-3° per minute or 120-150° per hour. Slow cooling during annealing is achieved by the fact that metal boxes with parts

previously sprinkled with fine sand or charcoal, remain in the forge and cool with it. After annealing, steel becomes softer and easier to process.

Normalization

is a thermal operation consisting of heating steel 50-60° above the GSE' line (Fig. 61), holding at this temperature and then cooling in still air. Normalization is used to obtain a fine-grained structure and improve the mechanical properties of steel.

Rice. 61. Carbon steel heating diagram for various heat treatment operations

After normalization, the steel acquires a more homogeneous and fine-grained structure compared to the initial state. The mechanical properties of steel after normalization are slightly higher than after annealing.

Hardening

called a thermal operation consisting of heating steel 20-30° above the GS or SE' line (Fig. 61), followed by rapid cooling in water or oil. After hardening, carbon steel gains increased hardness and strength, and its plastic properties decrease.

Successful hardening depends on proper heating and cooling of the steel. Steel is heated in a forge, in furnaces or in molten salts (salt baths). When heated in a forge, charcoal is the best fuel, since it does not contain sulfur, which can turn into steel. Carbon steel containing up to 0.9% carbon should be heated 20-30° above the GS line to ensure the transition of pearlite to auetenite and the transformation of ferrite into auetenite. If heating the steel does not ensure the formation of a homogeneous austenitic structure in it, then the hardening will not be complete.

Carbon steel with a carbon content of 0.9% and above should be heated to 730-750° to ensure the transition of all pearlite to auetenite while preserving cementite.

In a steady-state heating mode, you should always keep in mind that the more carbon and special impurities in the steel, the greater the mass of the heated parts and the more complex their shape, the slower the heating should be. This is necessary so that the parts have time to heat up evenly in all parts in order to avoid the appearance of large internal stresses. But it should also be taken into account that too slow heating in an oxidizing atmosphere causes the appearance of scale, and therefore spoils the surface of the parts.

Determination of the cooling rate for different steels is ensured by selecting the appropriate hardening medium. Cooling media according to their hardening ability are distributed as follows:

environments that provide strong hardening: water at 15-20°, water slightly acidified with hydrochloric or sulfuric acid, aqueous solutions of table salt, etc.;

environments that provide moderate hardening: water coated with a thin layer of oil (-20-40 mm), oil, fuel oil, soapy water, liquid mineral oil, vegetable oil, boiling water, etc.;

environments that give weak hardening: a stream of air, molten lead and its alloys at a temperature of 350-500 °, etc.

Hardening sometimes causes the following defects: 1) cracks formed as a result of internal stresses arising during sudden cooling of the steel; 2) warping, which occurs due to the non-uniform structure of the steel, uneven cooling, improper immersion in the cooling medium; 3) incomplete hardening, which occurs as a result of insufficient heating for hardening.

Vacation

is a thermal operation consisting of heating steel to a temperature not higher than the KSE' line, holding at this temperature, followed by rapid or slow cooling. As a result of tempering, the steel becomes softer and more ductile, and harmful internal stresses that arise during hardening are reduced.

Depending on the heating temperature, the following types of tempering are distinguished:

low tempering is carried out at a temperature of 150-260°,

causes minor changes in the structure of hardened steel and relieves internal stresses that arise during hardening;

moderate tempering is carried out at a temperature of 300 to 400°, relieves internal stresses that arise during hardening, slightly reduces the hardness and increases the toughness of the steel;

high tempering is carried out at a temperature of 500-650 °, significantly reduces the hardness and increases the toughness of steel.

Heated steel is cooled in a stream of air, water and oil.

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