What is heat treatment of metal?

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.

Types of heat treatment

Heat treatment of metal alloys comes down to three simple processes: heating the raw material (semi-finished product) to the required temperature, maintaining it in the specified conditions for the required time and rapid cooling. In modern production, several types of heat treatment are used, differing in some technological features, but the process algorithm generally remains the same everywhere.

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

• Thermal (hardening, tempering, annealing, aging, cryogenic treatment).
• Thermo-mechanical involves processing at high temperatures in combination with mechanical stress on the alloy.
• Chemical-thermal involves heat treatment of metal with subsequent enrichment of the surface of the product with chemical elements (carbon, nitrogen, chromium, etc.).

Annealing

Annealing is a production process in which metals and alloys are heated to a given temperature, and then, together with the furnace in which the procedure took place, they cool very slowly naturally.

As a result of annealing, it is possible to eliminate inhomogeneities in the chemical composition of the substance, relieve internal stress, achieve a grain structure and improve it as such, as well as reduce the hardness of the alloy to facilitate its further processing.

There are two types of annealing: annealing of the first and second kind.

Annealing of the first kind involves heat treatment, as a result of which changes in the phase state of the alloy are insignificant or absent altogether. It also has its own varieties: homogenized - the annealing temperature is 1100-1200, under such conditions the alloys are kept for 8-15 hours, recrystallization (at t 100-200) annealing is used for riveted steel, that is, deformed when it is already cold.

Second-order annealing leads to significant phase changes in the alloy. It also has several varieties:

• Full annealing is heating the alloy 30-50 above the critical temperature characteristic of a given substance and cooling at a specified rate (200 / hour - carbon steels, 100 / hour and 50 / hour - low-alloy and high-alloy steels, respectively).
• Incomplete - heating to a critical point and slow cooling.
• Diffusion – annealing temperature 1100-1200.
• Isothermal - heating occurs in the same way as with full annealing, but after this it is rapidly cooled to a temperature slightly below critical and left to cool in air.
• Normalized - complete annealing followed by cooling of the metal in air rather than in a furnace.

Hardening

Hardening is a manipulation with an alloy, the purpose of which is to achieve a martensitic transformation of the metal, which reduces the ductility of the product and increases its strength. Hardening, as well as annealing, involves heating the metal in a furnace above a critical temperature to the hardening temperature; the difference is a higher cooling rate, which occurs in a bath of liquid. Depending on the metal and even its shape, different types of hardening are used:

• Quenching in one environment, that is, in one bath with liquid (water for large parts, oil for small parts).
• Intermittent quenching - cooling takes place in two successive stages: first in a liquid (a sharper coolant) to a temperature of approximately 300, then in air or in another bath of oil.
• Stepped - when the product reaches the hardening temperature, it is cooled for some time in molten salts, followed by cooling in air.
• Isothermal - the technology is very similar to step hardening, differing only in the exposure time of the product at the martensitic transformation temperature.
• Quenching with self-tempering differs from other types in that the heated metal is not completely cooled, leaving a warm area in the middle of the part. As a result of this manipulation, the product acquires properties of increased strength on the surface and high viscosity in the middle. This combination is extremely necessary for percussion instruments (hammers, chisels, etc.)

Vacation

Tempering is the final stage of heat treatment of alloys, determining the final structure of the metal. The main purpose of tempering is to reduce the fragility of the metal product. The principle is to heat the part to a temperature below critical and cool it. Since heat treatment modes and cooling rates of metal products for various purposes may differ, there are three types of tempering:

• High - heating temperature from 350-600 to a value below critical. This procedure is most often used for metal structures.
• Medium – heat treatment at t 350-500, common for spring products and leaf springs.
• Low - the heating temperature of the product is not higher than 250, which allows you to achieve high strength and wear resistance of parts.

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.

Cryogenic treatment

Changes in the structure of alloys, and therefore their properties, can be achieved not only at high, but also at extremely low temperatures. The heat treatment of alloys at temperatures below zero is called cryogenic. This technology is widely used in a variety of sectors of the national economy as a complement to high-temperature heat treatments, since it can significantly reduce the costs of thermal hardening of products.

Cryogenic processing of alloys is carried out at t -196 in a special cryogenic processor. This technology can significantly increase the service life of the treated part and anti-corrosion properties, as well as eliminate the need for repeated treatments.

Thermo-mechanical treatment

A new method of processing alloys combines the processing of metals at high temperatures with mechanical deformation of products in a plastic state. Thermomechanical treatment (TMT) can be of three types according to the method of implementation:

• Low-temperature TMT consists of two stages: plastic deformation followed by hardening and tempering of the part. The main difference from other types of TMT is the heating temperature to the austenitic state of the alloy.
• High-temperature TMT involves heating the alloy to a martensitic state in combination with plastic deformation.
• Preliminary - deformation is carried out at t 20 followed by hardening and tempering of the metal.

Chemical-thermal treatment

It is also possible to change the structure and properties of alloys using chemical-thermal treatment, which combines thermal and chemical effects on metals. The ultimate goal of this procedure, in addition to imparting increased strength, hardness, and wear resistance to the product, is also to give the part acid resistance and fire resistance. This group includes the following types of heat treatment:

• Cementation is carried out to give the surface of the product additional strength. The essence of the procedure is to saturate the metal with carbon. Cementation can be performed in two ways: solid and gas carburization. In the first case, the material being processed, together with coal and its activator, is placed in a furnace and heated to a certain temperature, followed by keeping it in this environment and cooling. In the case of gas carburization, the product is heated in a furnace to 900 under a continuous stream of carbon-containing gas.
• Nitriding is a chemical-thermal treatment of metal products by saturating their surface in nitrogen environments. The result of this procedure is an increase in the tensile strength of the part and an increase in its corrosion resistance.
• Cyanidation is the saturation of a metal with both nitrogen and carbon. The medium can be liquid (molten carbon- and nitrogen-containing salts) and gaseous.
• Diffusion metallization is a modern method of imparting heat resistance, acid resistance and wear resistance to metal products. The surface of such alloys is saturated with various metals (aluminum, chromium) and metalloids (silicon, boron).

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.

Heat treatment of non-ferrous alloys

Non-ferrous metals and alloys have different properties and therefore are processed using different methods. Thus, copper alloys undergo recrystallization annealing to equalize the chemical composition. For brass, low-temperature annealing technology (200-300 °C) is provided, since this alloy is prone to spontaneous cracking in a humid environment.

Bronze is subjected to homogenization and annealing at temperatures up to 550. Magnesium is annealed, hardened and subjected to artificial aging (natural aging does not occur for hardened magnesium). Aluminum, like magnesium, is subjected to three heat treatment methods: annealing, hardening and aging, after which wrought aluminum alloys significantly increase their strength.

The processing of titanium alloys includes: recrystallization annealing, hardening, aging, nitriding and carburization.

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

Heat treatment of metals: what are the methods and technologies for steel

Heat treatment is a fundamental chemical process performed when working with alloys. In ferrous and non-ferrous metallurgy, the technique is taken as a basis and has a huge number of different variations. The chemical, technical and mechanical properties of the metal depend on the correct operation. All types of heat treatment of steel are divided into certain groups, which allows you to select rational variations.

Main types of heat treatment

In industrial enterprises, all processes are automated and humans take only indirect participation in them. All technologies are almost identical, but differ in temperature conditions and other factors.

First of all, the alloy is heated to a certain temperature, then it is maintained in these temperature conditions. At the last stage, instant cooling occurs.

Thus, heat-treated steel will have unique technical characteristics. Main types of technologies:

1. Thermal effects include hardening, aging, tempering, and cryogenic heating.
2. Thermo-mechanical techniques. They are accompanied not only by heating, but also by mechanical stress.
3. Thermochemical technologies. After exposure to temperature, treatment with various types of liquids or gases occurs, which can strengthen the alloy.

Any method implies obtaining the required conditions, so if difficulties arise, secondary processing will be unacceptable. Each technology is unique in its own way, but is based on heating metals.

Therefore, a more thorough understanding of the differences and other factors is required. This will allow you to obtain more specific information about all aspects of interest.

Annealing metals in a furnace

A standard technique in which the workpieces are sent to the oven and heated. Subsequently, cooling occurs not in separate chambers, but in the same oven. Thus, the natural cooling process begins due to the ambient temperature. If we consider the types of heat treatment of metals, the presented technology is one of the simplest. The technology allows you to obtain the following properties:

1. The hardness decreases, making it easier to process the alloys in the future.
2. The granularity of the structure increases.
3. Heterogeneous segments disappear.
4. Internal tension disappears.

Currently, the presented technology is implemented in several different variations. As the technology guide indicates, optimal conditions are created for various needs. At industrial enterprises, this work must be carried out in special furnaces. Today, annealing of steel blanks is used to produce high-quality steel. Such techniques are very important for the industry and the development of the industry in this segment.

Hardening technology

One of the most common heat treatment methods is hardening. The technology involves thermal manipulation of metals and heating them to critical temperatures.

The result of the technology is an increase in the ductility and strength of alloys. The difference between hardening and annealing is fairly rapid cooling. For these purposes, water baths are used, which significantly speeds up the processes.

From a technical point of view, this is a unique technique. There are several main types of hardening:

1. A technology that uses only one type of cooling fluid.
2. Intermittent technique. First, the metal is heated to a critical temperature and dipped into water. After cooling to a temperature of 300 degrees, leave in air or oil.
3. Stepped. In this case, the cooling method is used in water, then in special salts and at the last stage it is left to cool in air. Thus, at each stage the metal acquires more unique technical characteristics.
4. Isothermal - almost identical to step hardening.
5. Partial hardening. Cooling occurs only at the edges of the metal; it remains hot in the middle. This technique is used in the manufacture of fender tools, since the alloy is tough in the middle and strong at the edges.

Hardening technology is very often used in forges as the main heat treatment method. Its effectiveness has been proven over many years of use and shows incredible benefits. Currently, at every stage of the technological process it is necessary to monitor indicators. This will allow you to obtain metal with the required characteristics.

Tempering and aging of alloys

If there is no information about which processing of steel products is characterized by improved technical performance, then you can choose any technique. Everything is due to the fact that each technology has certain advantages and advantages.

Tempering is a technique used in the final stage of metal processing, thereby imparting different physical properties to the final format. To do this, the metal part is heated to a temperature that should be below critical, and cooling is carried out.

Currently, several main types of vacation are known:

• high;
• average;
• short.

The aging process is used to process cast iron and various types of non-ferrous metals. The technology is very widespread, as it allows one to increase the yield limits and strength of metals. Aging is carried out after annealing at normal temperature, this allows you to achieve the desired effect without any third-party technologies.

The peculiarity of any type of heat treatment lies in the professionalism of the performers. Each specialist who works with metal has his own secrets that he puts into practice. It is possible to obtain metal with unique technical characteristics. In a factory environment, you need to adhere to technical regulations, so the metal is always the same format, which is sometimes a big problem. Technical standards remain constant.

Cryogenic exposure

Currently, technology and engineering are constantly evolving, and new options for influencing alloys are emerging. Today you can use not only high temperatures, but also low ones. That is, cold also improves the quality of materials. There are special cryogenic chambers in which technological procedures are carried out. The temperature at which parts and workpieces are located is -196 degrees Celsius. The advantage of the technology is that re-processing is not required.

Of course, technology is not always suitable and has many different nuances. It is recommended to use technological regulations, which will significantly improve the quality of the product. This type of processing also reduces costs significantly. It is enough to use a refrigerator; at high temperatures, third-party resources are needed to heat the oven, and so on.

Thermo-mechanical effect

Of all the listed technologies, the presented technique has been used on an industrial scale for a long time. The essence is to preheat the metal to a plastic state and then apply mechanical action. Thermo-mechanical treatment can be of several types:

1. Low temperature processing. Its difference lies in the fact that the metal is heated to an austenitic state. The technology includes plastic deformation, hardening and tempering. Everything is done in accordance with technical regulations.
2. High temperature processing. The metal is heated to the martensitic state, and plastic deformation is carried out.
3. Preliminary processing.

Practice and the goals you are pursuing allow you to choose the right method. From a technological point of view, each method of any type of heat treatment is suitable only for certain metals and alloys. It is this factor that determines diversity. That is, under no circumstances should steel be subjected to a certain type of exposure if it is not suitable. This will lead to deterioration in the quality of materials.

Chemical treatment

Chemical reactions with metals in combination with thermal effects lead to increased wear resistance and resistance to acids and alkalis. Currently, there are specialized industrial conditions for carrying out a large number of processes. It is important to distinguish between techniques and use them at the right time. Types of thermochemical reactions:

1. Cyanidation - metal is subjected to simultaneous exposure to carbon and nitrogen. The basis of the technique is to saturate the alloy with these elements.
2. Nitriding is a technology that makes it possible to increase the corrosion resistance of metals to maximum levels, and also increases strength. To do this, the alloys are immersed in a nitrogen environment.
3. Diffusion metallization is a very complex technology, but similar to the previous ones. Thanks to its implementation, the metal becomes more durable, wear-resistant and is not exposed to aggressive agents. To do this, the surface of the alloys is treated with bromine, chromium, and aluminum.
4. Cementation is a technique that increases the strength of metal. To do this, carbon is used, which in a gaseous state is continuously fed to the metal in a furnace.

In each individual case, it is important to follow all the rules of technological support. If exposed incorrectly, the alloy may lose its technical characteristics and will be sent for additional remelting. In such situations, control and measuring instruments are used to prevent violation of technology.

Non-ferrous alloys

Each individual non-ferrous metal or alloy differs from others in physical and chemical properties, which cannot be said about ferrous metals. Therefore, it is recommended to select your own methods for each individual case so as not to lose quality. Recrystallization annealing is carried out on copper, which greatly improves the quality, and heat strengthening occurs. The following features are distinguished:

1. Brass should never be heated too much, the limit is 250-300 degrees Celsius. If improperly processed or at high temperatures, cracking of the structure occurs.
2. Bronze must be homogenized and subsequently heated to 600 degrees Celsius.
3. Magnesium can be processed by various methods: aging, annealing, and so on.
4. Titanium alloys can be hardened, annealed, aged, or carburized.

Currently, there are special reference books and technical manuals that allow you to select appropriate methods for increasing the technical properties of metals. Specialists working at industrial enterprises act according to pre-established plans and technical documents. Thus, each technique is unique in its own way and makes metals and alloys of higher quality and suitable for technical and industrial needs.

Industrial companies use almost all existing methods, which makes it possible to obtain alloys of various formats. It is very important to adhere to GOST regulations and standards. Each heat treatment considered has its own standards and technical regulations. Any deviation will result in poor quality material and hence there will be a defect.

Source: https://obrabotkametalla.info/stal/raznovidnosti-termoobrabotki-metallov

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

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.

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.

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

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.

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.

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.

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.

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