Why is heat treatment of steel needed?

Heat treatment of steel

Heat treatment of steel allows you to give products, parts and workpieces the required qualities and characteristics. Depending on the stage at which heat treatment was carried out in the manufacturing process, the workpieces’ workability increases, residual stresses are removed from the parts, and the parts’ performance qualities increase.

Steel heat treatment technology is a set of processes: heating, holding and cooling with the aim of changing the internal structure of the metal or alloy. In this case, the chemical composition does not change.

Thus, the molecular lattice of carbon steel at a temperature of no more than 910°C is a body-centered cube. When heated above 910°C to 1400°C, the lattice takes the shape of a face-centered cube. Further heating turns the cube into a body-centered one.

Heat treatment of steel

The essence of heat treatment of steels is a change in the grain size of the internal structure of the steel. Strict adherence to temperature conditions, time and speed at all stages, which directly depend on the amount of carbon, alloying elements and impurities that reduce the quality of the material. During heating, structural changes occur, which upon cooling occur in the reverse order. The figure shows what transformations occur during heat treatment.

Change in metal structure during heat treatment

Purpose of heat treatment

Heat treatment of steel is carried out at temperatures close to critical points. Here's what happens:

• secondary crystallization of the alloy;
• transition of gamma iron to the alpha iron state;
• transition of large particles into plates.

The internal structure of a two-phase mixture directly affects performance and ease of processing.

Formation of structures depending on cooling intensity

The main purpose of heat treatment is to give steels:

• In finished products:
  1. strength;
  2. wear resistance;
  3. corrosion resistance;
  4. heat resistance.

• In blanks:
  1. relief of internal stress after
     • casting;
     • stamping (hot, cold);
     • deep drawing;
  2. increased plasticity;
  3. facilitating cutting.

Heat treatment is applied to the following types of steels:

1. Carbon and alloyed.
2. With varying carbon contents, from low carbon 0.25% to high carbon 0.7%.
3. Structural, special, instrumental.
4. Any quality.

Classification and types of heat treatment

The fundamental parameters affecting the quality of heat treatment are:

• heating time (speed);
• heating temperature;
• duration of holding at a given temperature;
• cooling time (intensity).

By changing these modes, you can obtain several types of heat treatment.

Types of heat treatment of steel:

• Annealing
  1. I – kind:
     • homogenization;
     • recrystallization;
     • isothermal;
     • removal of internal and residual stresses;
  2. II – kind:

• Hardening;
• Vacation:

Heating temperature of steel during heat treatment

Vacation

Tempering in mechanical engineering is used to reduce the strength of internal stresses that appear during hardening. High hardness makes products brittle, so tempering is used to increase impact strength and reduce the hardness and brittleness of steel.

Low tempering is characterized by the internal structure of martensite, which, without reducing hardness, increases viscosity. Measuring and cutting tools are subjected to this heat treatment. Processing modes:

• Heating to a temperature of 150°C, but not higher than 250°C;
• holding time - one and a half hours;
• cooling - air, oil.

For medium tempering, transformation of martensite into trostite. Hardness decreases to 400 HB. Viscosity increases. Parts that operate under significant elastic loads are subjected to this tempering. Processing modes:

• heating to a temperature of 340°C, but not higher than 500°C;
• cooling - air.

3. High release

With high tempering, sorbitol crystallizes, which eliminates stress in the crystal lattice. Critical parts are manufactured that have strength, ductility, and toughness.

Annealing steel

Processing modes:

Heating to a temperature of 450°C, but not higher than 650°C.

Annealing

The use of annealing makes it possible to obtain a homogeneous internal structure without stress on the crystal lattice. The process is carried out in the following sequence:

• heating to a temperature slightly above the critical point, depending on the grade of steel;
• holding with constant temperature maintenance;
• slow cooling (usually cooling occurs together with the furnace).

1. Homogenization

Homogenization, otherwise known as diffusion annealing, restores the non-uniform segregation of castings. Processing modes:

• heating to a temperature from 1000°C, but not higher than 1150°C;
• exposure – 8-15 hours;
• cooling:
  • oven – up to 8 hours, temperature reduction to 800°C;
  • air.

Recrystallization, otherwise low annealing, is used after plastic deformation treatment, which causes hardening by changing the grain shape (hardening). Processing modes:

• heating to a temperature above the crystallization point by 100°C-200°C;
• holding – ½ – 2 hours;
• cooling is slow.

3. Isothermal annealing

Alloy steels are subjected to isothermal annealing to cause austenite decomposition. Heat treatment modes:

• heating to a temperature of 20°C - 30°C above the point;
• holding;
• cooling:
  • fast - not lower than 630°C;
  • slow – at positive temperatures.

4. Annealing to eliminate stress

Removal of internal and residual stresses by annealing is used after welding, casting, and machining. With the application of work loads, parts are subject to destruction. Processing modes:

• heating to a temperature of – 727°C;
• holding - up to 20 hours at a temperature of 600°C - 700°C;
• cooling is slow.

5. Complete annealing

Full annealing makes it possible to obtain an internal structure with fine grains, which contains ferrite and pearlite. Full annealing is used for cast, forged and stamped workpieces, which will subsequently be processed by cutting and subjected to hardening.

Complete annealing of steel

Processing modes:

• heating temperature – 30°C-50°C above point ;
• excerpt;
• cooling to 500°C:
  • carbon steel – temperature decrease per hour is no more than 150°C;
  • alloy steel – temperature decrease per hour is no more than 50°C.

6. Incomplete annealing

With incomplete annealing, lamellar or coarse pearlite is transformed into a ferrite-cementite grain structure, which is necessary for welds produced by electric arc welding, as well as tool steels and steel parts subjected to processing methods whose temperature does not provoke grain growth of the internal structure.

Processing modes:

• heating to a temperature above the point or, above 700°C by 40°C - 50°C;
• curing - about 20 hours;
• cooling is slow.

Hardening

Hardening of steels is used for:

• Promotions:
  1. hardness;
  2. strength;
  3. wear resistance;
  4. elastic limit;

• Reductions:
  1. plasticity;
  2. shear modulus;
  3. compression limit.

The essence of hardening is the fastest cooling of a thoroughly heated part in various environments. Heating is performed with and without polymorphic changes. Polymorphic changes are possible only in those steels that contain elements capable of transformation.

Steel hardening

Such an alloy is heated to a temperature at which the crystal lattice of the polymorphic element undergoes changes, due to which the solubility of alloying materials increases. As the temperature decreases, the lattice changes structure due to an excess of alloying element and takes on a needle-like structure.

The impossibility of polymorphic changes during heating is due to the limited solubility of one component in another at a rapid cooling rate. There is little time for diffusion. The result is a solution with an excess of undissolved component (metastable).

To increase the cooling rate of steel, the following media are used:

• water;
• water-based brine solutions;
• technical oil;
• inert gases.

Comparing the rate of cooling of steel products in air, cooling in water from 600°C occurs six times faster, and from 200°C in oil 28 times faster. Dissolved salts increase the hardening ability. The disadvantage of using water is the appearance of cracks in places where martensite forms. Industrial oil is used to harden alloy alloys, but it sticks to the surface.

Metals used in the manufacture of medical products should not have a film of oxides, so cooling occurs in an environment of rarefied air.

To completely get rid of austenite, which causes high brittleness in steel, products are subjected to additional cooling at temperatures from -40°C to -100°C in a special chamber. You can also use carbonic acid mixed with acetone. This processing increases the accuracy of parts, their hardness, and magnetic properties.

If parts do not require volumetric heat treatment, only the surface layer is heated using HDF (high-frequency current) installations. In this case, the depth of heat treatment ranges from 1 mm to 10 mm, and cooling occurs in air. As a result, the surface layer becomes wear-resistant, and the middle is viscous.

The hardening process involves heating and holding steel products at temperatures reaching about 900°C. At this temperature, steels with a carbon content of up to 0.7% have a martensite structure, which, during subsequent heat treatment, will transform into the required structure with the appearance of the desired qualities.

Normalization

Normalization produces a fine grain structure. For low-carbon steels this is a ferrite-pearlite structure, for alloyed steels it is a sorbitol-like structure. The resulting hardness does not exceed 300 HB. Hot-rolled steels are subjected to normalization. At the same time, they increase:

• fracture resistance;
• processing performance;
• strength;
• viscosity.

Steel normalization process

Processing modes:

• heating occurs to a temperature of 30°C-50°C above the point ;
• maintaining in a given temperature range;
• cooling - in the open air.

Benefits of Heat Treatment

Heat treatment of steel is a technological process that has become a mandatory step in obtaining sets of parts made of steel and alloys with specified qualities. This can be achieved by a wide variety of modes and methods of thermal exposure. Heat treatment is used not only for steels, but also for non-ferrous metals and alloys based on them.

Steels without heat treatment are used only for the construction of metal structures and the manufacture of non-critical parts, the service life of which is short. There are no additional requirements for them. Everyday operation, on the contrary, dictates stricter requirements, which is why the use of heat treatment is preferable.

In thermally untreated steels, abrasive wear is high and proportional to its own hardness, which depends on the composition of chemical elements. Thus, non-hardened die matrices are well combined when working with hardened punches.

Source: https://stankiexpert.ru/spravochnik/materialovedenie/termicheskaya-obrabotka-stali.html

6 Heat treatment of steel

6

Heat treatment of steel

Classification of types of heat treatment of steel. Types of heat treatment of steel (annealing, tempering, hardening).

Heat treatment (heat treatment) of steel 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.

What methods of heat treatment of metal exist?

To change the technical characteristics of a metal, you can create an alloy based on it and add other components to it. However, there is another way to change the parameters of a metal product - heat treatment of the metal. With its help, you can influence the structure of the material and change its characteristics.

Features of heat treatment

Heat treatment of metal is a series of processes that allow you to remove residual stress from a part, change the internal structure of the material, and improve performance. The chemical composition of the metal does not change after heating. When the workpiece is uniformly heated, the grain size of the material structure changes.

Story

The technology of heat treatment of metal has been known to mankind since ancient times. During the Middle Ages, blacksmiths heated and cooled sword blanks using water. By the 19th century, people learned to process cast iron. The blacksmith placed the metal in a container full of ice and poured sugar on top. Next, the process of uniform heating begins, lasting 20 hours. After this, the cast iron billet could be forged.

In the mid-19th century, Russian metallurgist D.K. Chernov documented that when a metal is heated, its parameters change. From this scientist came the science of materials science.

Why is heat treatment needed?

Equipment parts and communication units made of metal are often subjected to severe loads. In addition to exposure to pressure, they may be exposed to critical temperatures. To withstand such conditions, the material must be wear-resistant, reliable and durable.

Purchased metal structures are not always able to withstand loads for a long time.
To make them last much longer, metallurgy masters use heat treatment. During and after heating, the chemical composition of the metal remains the same, but the characteristics change. The heat treatment process increases the corrosion resistance, wear resistance and strength of the material. How does it work. Heat treatment

Types of heat treatment of steel

In metallurgy, three types of steel processing are used: technical, thermomechanical and chemical-thermal. Each of the presented methods of heat treatment must be discussed separately.

Thermo-mechanical treatment

This is a typical mode of heat treatment of steels. This technological process uses equipment that creates pressure, heating elements and cooling tanks. At different temperatures, the workpiece is heated, and after this plastic deformation occurs.

Cryogenic treatment

The main difference between heat treatment and cryogenic exposure is that the latter involves cooling the workpiece. At the end of this procedure, the parts become stronger, do not require tempering, and are better ground and polished.

When interacting with cooling media, the temperature drops to minus 195 degrees.
The cooling rate may vary depending on the material. To cool the product to the desired temperature, a processor is used that generates cold. The workpiece is cooled evenly and remains in the chamber for a certain period of time. After that, take it out and allow it to warm up to room temperature on its own. Heat treatment of steel Vanadis 4

Chemical-thermal treatment

Another type of heat treatment, in which the workpiece is heated and exposed to various chemical elements. The surface of the workpiece is cleaned and coated with chemical compounds. This process is carried out before hardening.

The master can saturate the surface of the product with nitrogen. To do this, they are heated to 650 degrees. When heated, the workpiece must be in a cryogenic atmosphere.

Heat treatment of non-ferrous alloys

The presented types of heat treatment of metals are not suitable for various types of alloys and non-ferrous metals. For example, when working with copper, recrystallization annealing is carried out. Bronze heats up to 550 degrees. They work with brass at 200 degrees. Aluminum is initially hardened, then annealed and aged.

Heat treatment of metal is considered a necessary process in the manufacture and further use of structures and parts for industrial equipment, cars, aircraft, ships and other equipment. The material becomes stronger, more durable and more resistant to corrosion processes. The choice of technological process depends on the metal or alloy used.

Source: https://metalloy.ru/obrabotka/termo/vidy

Methods and types of heat treatment of steel

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Heat treatment of steel is carried out with the aim of imparting a certain set of properties to the material by changing its internal structure at the molecular level. The method involves heating or cooling the metal to a certain temperature level and then returning it to its normal state. Multiphase heat treatment is sometimes used, which allows the production of the most hardened steel grades.

The procedure takes place in special ovens or refrigeration units, which make it possible to clearly control the temperature at each stage of the technological process. This is a very important condition for successful hardening, since non-compliance with the technology can, on the contrary, give the metal negative properties.

Heat treatment modes for steel depend on the structural composition of the material. All of them were established experimentally as a result of repeated tests, so modern hardening methods, if all conditions are met, make it possible to obtain high-quality materials with a large margin of safety.

Heat treatment of steels should prepare them for operation in an aggressive environment under the influence of destructive factors.

The following types of heat treatment of steels are distinguished: hardening, tempering, annealing, normalization, exposure to cold and chemical-thermal treatment. 

Steel hardening

Hardening involves heating the metal to a set temperature and maintaining the achieved level for a certain period of time. The time interval is determined by the rate of transformation of the internal structure of the alloy into a stable substance. After this, the steel is quickly cooled in water or oil, since gradual cooling can lead to disruption of the achieved crystal lattice structure.

Hardening makes the material hard but reduces its toughness, making the steel more brittle. This treatment is applied to parts that are intended to be used under static loads without the influence of dynamic vibrations. Some parts are subject to tempering after hardening. Its essence is to reheat the metal to a temperature lower than the quenching temperature. This will again disrupt the achieved intermolecular bonds and lead to their restructuring.

After heating, the metal is removed from the furnace and allowed to cool naturally without the use of coolants. This procedure slightly reduces hardness, but at the same time increases toughness and malleability.

So after quenching and then tempering, the steel will be harder and more ductile than the untreated alloy. Annealing is carried out according to the scheme of heating the metal, followed by slow cooling directly in the furnace without the use of special means.

This removes the heterogeneity of the distribution of elements in the alloy and makes it possible to create a stable compound of iron and carbon at the intermolecular level.

Exposure to cold

Cold exposure is necessary to complete the transformation of austenite to martensite. It gives the metal additional elasticity and prevents the formation of ragged cracks when excessive pressure is applied to the part. This material is well suited for use under high dynamic loads. The required hardness is usually added to it using appropriate additives.

Heat treatment of different types of steel

Heat treatment of alloy steel should be carried out with slow heating to the required temperature, and then with slow cooling of the workpiece. As a result of the addition of alloying additives, steels of this grade have low thermal conductivity, so a sharp change in temperature can lead to warping or cracking. It is also very important that heating occurs evenly over the entire area of ​​the part.

The heat treatment of stainless steel also has its own nuances. After annealing, it must be left in the oven until it cools completely, and then a tempering procedure is carried out to obtain the material of optimal quality. A sharp change in temperature is also undesirable, as it can negatively affect performance properties.

Heat treatment of austenitic steels is carried out in furnaces with uniform heating of the workpiece to a temperature of 1000-1150 degrees Celsius. This is followed by rapid cooling in liquid, which makes it possible to obtain a material with a stable ferritic internal structure. These steels are used for the manufacture of structural materials, and therefore must obtain increased strength when hardened.

Heat treatment of high-speed steel is a labor-intensive process. It belongs to the class of high-alloy alloys, so it does not tolerate sudden temperature changes. This material is hardened using high-precision equipment, which allows precise control of each phase of the technological process. This brand is used to produce cutting tools that, even when heated to 600 degrees, do not lose their original hardness.

Heat treatment of carbon steel is reduced to obtaining a stable bond between iron and carbon atoms in the crystal lattice. The method depends on the need to obtain a specific substance at the end of the process.

Source: https://promplace.ru/obrabotka-metallov-staty/termicheskaya-obrabotka-stali-1555.htm

Heat treatment of steel. Types of heat treatment of metals:

Heat treatment of steel (HT) is a very important final operation in the manufacture of parts and tools. It provides them with the necessary mechanical properties and ensures normal operation.

Definition

Heat treatment of metals is a set of strictly sequential operations of heating, holding and subsequent cooling of blanks or finished products under certain modes to change their structure and provide them with the necessary mechanical, physical, chemical and other properties. The basis of heat treatment is transformations in the internal structure of materials during heating and subsequent cooling.

Types of heat treatment

The determining factors that influence the results of maintenance are the heating rate and temperature, as well as the holding time in the heated state and the cooling rate. Depending on the temperature indicators and the cooling rate of the products, the following stages of heat treatment are distinguished:

• annealing;
• further normalization;
• hardening and tempering of steel.

Diffuse annealing

It is also called homogenization. Used for large steel castings to reduce chemical heterogeneity (liquation). At the first stage, the material being processed is heated to temperatures of 1050-1150°C. After heating, stand for about 10-15 hours and then slowly cool. At the same time, the characteristics of steels are improved.

Full annealing

The technology is used to form a fine-grained structure of steel products made by hot stamping, forging, and casting. After the complete annealing procedure, steels become plastic, soft, without internal stresses. The internal (crystalline) structure becomes homogeneous, fine-grained, and consists of ferrite and pearlite. By complete annealing, the steel is prepared for cutting and subsequent hardening. Hypoeutectoid steels are predominantly processed this way.

Heat treatment of steel is carried out according to the following technical process: products (blanks) are heated to temperatures exceeding the so-called critical upper point (in materials science referred to as Ac3) by 30-50°C, then slowly cooled. Cooling to a temperature of 500-550°C occurs at the following speed:

• for carbon steels - 150-200°C per hour;
• for alloyed ones – 50-75°C per hour.

Partial annealing

This steel heat treatment technology is used for hypoeutectoid and hypereutectoid metals in order to reduce rigidity, relieve internal stresses and obtain a homogeneous structure. The procedure applies to forgings and stampings processed at temperatures that do not cause significant grain growth.

Technical process: steel is heated at a temperature above the lower critical point (denoted on the graphs as Ac1) in the temperature range of 740-750°C, maintained for a certain time at this temperature, and then slowly cooled.

Isothermal annealing

Used for products made of alloy steels when they are heated 20-30°C above Ac3, held and quickly cooled to a temperature of 630-700°C. Blanks (products) are kept until austenite decomposes, then cooled at above-zero temperatures. After isothermal annealing, steels have similar properties to fully annealed metals. Heat treatment of metals using this technical process has an important advantage - reducing processing time.

Annealing on granular pearlite

Widely used before machining of tool eutectoid and hypereutectoid alloy and carbon steels. The material is heated 25-30°C above RT and held for a specified time. The workpieces are cooled very slowly (30°C per hour) to a temperature of 600°C together with the furnace, and then cooled naturally. As a result, the carbides acquire a grainy (rounded) shape, and the hardness decreases, which favors the metal cutting process.

Recrystallization annealing

The second name is low annealing. The process helps to relieve internal stresses and work hardening in products made by cold rolling, cold stamping, drawing and calibration (sheets, rods, tubes, wire). In this case, the material is heated to recrystallization temperatures 50-100°C below the Ac1 point (630-680°C), maintained, then cooled naturally (in air). After recrystallization annealing, a homogeneous structure with low hardness is formed.

Steel tempering

They are used to smooth out the internal stresses of the crystal lattice and reduce the hardness of metals, as well as to increase the impact strength of hardened products. Highlight:

• high;
• average;
• low vacation.

High tempering is carried out at a temperature of 500-650°C with gradual cooling. In this case, the steel acquires a sorbitol structure, which eliminates internal stresses. Structural, carbon and alloy steels, from which shafts, gears and others are made, are subjected to this type of tempering. The characteristics of steels are high strength, ductility and toughness with sufficient hardness.

The average tempering is carried out at a temperature of 350-450°C, held for a certain time and cooled. With this tempering, martensite turns into troostite, the hardness of the steel decreases to approximately 400 HB, and the toughness increases significantly. Tempering is used (after hardening) for processing springs, leaf springs, dies and other products operating under moderate shock loads.

Low tempering is carried out in the temperature range of 150-250°C, maintained and cooled. In this case, a structure of tempered martensite is formed. Therefore, the internal stresses in the product decrease, the viscosity increases slightly, and the heat brittleness disappears, and the hardness remains practically unchanged. They are used for cutting and measuring instruments, which must be hard and not brittle and have high wear resistance, including cemented products.

Conclusion

Heat treatment of steel is an integral stage in the production of most metal products. Thanks to a wide range of technical processes, it is possible to obtain materials with the required characteristics.

Source: https://www.syl.ru/article/203678/new_termoobrabotka-stali-vidyi-termicheskoy-obrabotki-metallov

Application of heat treatment of steel: main types, pros and cons

Heat treatment of metal is an important part of the production process in non-ferrous and ferrous metallurgy. After this procedure, the materials acquire the necessary characteristics. Heat treatment has been used for quite some time, but it was imperfect. Modern methods allow you to achieve better results with less effort and reduce costs.

To impart the desired properties to a metal part, it is subjected to heat treatment. During this process, a structural change occurs in the material .

Metal products used in the household must be resistant to external influences. To achieve this, the metal must be strengthened by exposure to high temperature. This treatment changes the shape of the crystal lattice, minimizes internal stress and improves its properties.

Heat treatment of alloys. Types of heat treatment

Heat treatment of alloys is an integral part of the production process of ferrous and non-ferrous metallurgy. As a result of this procedure, metals are able to change their characteristics to the required values. In this article we will look at the main types of heat treatment used in modern industry.

The essence of heat treatment

During the production process, semi-finished products and metal parts are subjected to heat treatment to give them the desired properties (strength, resistance to corrosion and wear, etc.). Heat treatment of alloys is a set of artificially created processes during which structural and physical-mechanical changes occur in alloys under the influence of high temperatures, but the chemical composition of the substance is preserved.

Purpose of heat treatment

Metal products that are used daily in any sector of the national economy must meet high wear resistance requirements. Metal, as a raw material, needs to enhance the necessary performance properties, which can be achieved by exposing it to high temperatures.

Thermal treatment of alloys at high temperatures changes the original structure of the substance, redistributes its constituent components, and transforms the size and shape of the crystals. All this leads to minimizing the internal stress of the metal and thus increases its physical and mechanical properties.

Aging

Aging is a heat treatment of alloys that causes the decomposition of supersaturated metal after hardening. The result of aging is an increase in the limits of hardness, fluidity and strength of the finished product. Not only cast iron, but also non-ferrous metals, including easily deformable aluminum alloys, undergo aging.

If a metal product subjected to hardening is kept at normal temperature, processes occur in it that lead to a spontaneous increase in strength and a decrease in ductility. This is called natural aging of metal.

If the same manipulation is performed under conditions of elevated temperature, it will be called artificial aging.

Features of heat treatment of cast iron

Cast iron alloys are subjected to heat treatment using a slightly different technology than non-ferrous metal alloys.

Cast iron (gray, high-strength, alloyed) undergoes the following types of heat treatment: annealing (at t 500-650), normalization, hardening (continuous, isothermal, surface), tempering, nitriding (gray cast iron), aluminizing (pearlitic cast iron), chrome plating.

As a result, all these procedures significantly improve the properties of the final cast iron products: they increase the service life, eliminate the possibility of cracks during use of the product, and increase the strength and heat resistance of cast iron.

Summary

Heat treatment of metals and alloys is the main technological process in both ferrous and non-ferrous metallurgy. Modern technologies have a variety of heat treatment methods that make it possible 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.

Source: https://FB.ru/article/306609/termicheskaya-obrabotka-splavov-vidyi-termoobrabotki

Steel normalization - description of the process and its essence

Most of the operations associated with heat treatment involve the same algorithm of actions:

• heating the product to certain temperatures;
• holding under the influence of the set temperature for a specified time;
• cooling, which can be carried out in different environments and at different rates.

Heat treatment of parts can act both as an intermediate technological process and as a finishing one. In the first case, those parts that will still be processed, for example, drills or aircraft turbine blades, pass through it. The second case implies that after heat treatment, the finished part will receive new properties.

Steel normalization is a type of heat treatment of metal followed by cooling in air. The result of this operation is the formation of a normalized steel structure. By the way, this is where the name comes from. The operation is used in relation to forgings, castings, etc. Normalization is used to minimize grains in the steel structure formed by the weld.

The essence of the process

The normalization procedure is as follows. The part is heated to temperatures that exceed the maximum permissible parameters (Ac1, Ac3) by 30 - 50 degrees Celsius, then it is kept at this temperature for some time, after which it is cooled.

The temperature is selected based on the steel grade. Thus, alloys containing 0.8% carbon, so-called hypereutectoid, are processed at temperatures lying between the critical points Ac1 and Ac3.

What are critical points? This is the name given to the temperatures at which phase changes and structure of the alloy occur when it is heated or cooled.

The result of this is that a certain volume of carbon enters the solid solution and austenite is fixed. That is, a structure consisting of martensite and cementite appears. It is cementite that leads to an increase in wear resistance and hardness. Heating high-carbon steel above ac3 leads to an increase in internal stresses. This is due to the fact that the amount of austenite increases due to an increase in carbon concentration.

When heated above the critical point Ac3, steel with a carbon content of less than 0.8% acquires increased viscosity. This happens because in steel of this type austenite (fine-grained) appears, turning into martensite (fine-grained).

Hypoeutectoid steel is not processed at temperatures in the range Ac1 - Ac3. Since in this case ferrite appears, which reduces the hardness parameters.

Time required to complete the operation

It takes some time to obtain a homogeneous structure of the alloy at a certain temperature. This time will be determined as the holding time of steel during normalization. It was experimentally determined that a layer of metal 25 mm thick becomes homogeneous after an hour. Thus. and determine the normalization time.

The final stage is cooling

The cooling rate plays a significant role in the formation of perlite volume and the size of its plates. Numerous studies have shown that high cooling rates increase the amount of perlite and the steel gains increased hardness and strength. Low cooling intensity leads to steel losing hardness and strength.

When processing parts with significant differences in size, for example. shafts, it is advisable to remove stresses arising under the influence of temperature fluctuations. To do this, they are preheated in a container filled with different salts. When the temperature drops, it is possible to speed up this process by placing hot parts in water or specially selected oil.

In other words, steel normalization eliminates stress inside the part and minimizes its structure. That is, it has a direct effect on changes in the microstructure of steel alloys.

Using Normalization

This form of heat treatment is used to achieve different purposes. Thus, the use of normalization can increase or decrease the hardness of a steel alloy, toughness and strength characteristics. This method of heat treatment is used when it is necessary to improve the machinability of steel using different methods - cutting, stamping, etc.

Parts produced by casting undergo normalization in order to obtain a homogenized structure and eliminate internal stresses. The same can be said about parts obtained after forging.

That is, normalization serves to obtain a homogeneous metal structure and eliminate internal stresses. In addition, this process can be used as a replacement for hardening products with complex profiles.

In addition to the mentioned results of the normalization process, you can add such results as minimizing grains in the alloy structure, removing secondary cementite, and increasing the machinability of steel.

Essentially similar heat treatment processes

In addition to normalization, the following operations can be added to the list of heat treatment of steels:

• annealing;
• vacation;
• hardening;
• cryogenic treatment and several others.

The annealing operation provides a high-quality, finer structure of pearlite; this occurs because furnaces are used to cool the parts. The purpose of this operation is to reduce the heterogeneity of the structure, remove stress, and increase machinability.

The principles underlying the hardening operation are identical to those of normalization, but there are some differences. For example, when hardening, much higher temperatures and high cooling rates are used. Hardening leads to improved strength characteristics, hardness, etc. But often workpieces that have undergone hardening are characterized by reduced viscosity and high fragility.

Tempering of parts is used after the hardening operation. Tempering reduces brittleness and internal stress. In this case, the temperature range is lower than that used in normalization. The parts are cooled in air. As the temperature increases, the tensile strength and hardness decrease and at the same time the impact strength increases.

Cryogenic treatment of steel results in a uniform metal structure and increased hardness. This processing technology is used for hardened carbon steel.

Normalization and its application in practice

When assigning a heat treatment method, the technologist must take into account the carbon concentration. Steels in which the carbon content does not exceed 0.4% can be processed by both normalization and annealing. Normalization minimizes the grain size in the structure and increases strength characteristics.

Comparing the time spent between normalization and other methods, we can conclude that processing by other methods takes longer.

Due to the speed of the operation, the coverage of a large number of steels, the quality of the resulting parameters (hardness, strength, etc.), this is why normalization is widely used in mechanical engineering.

Source: https://prompriem.ru/stati/normalizaciya-stali.html