What types of heat treatment of steel are there?

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 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

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.

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.

Types of heat treatment of steel

Heat treatment of steel comes down to three stages: heating, holding and rapid cooling. There are several types of this process, but the main stages remain the same.

The following types of heat treatment are distinguished:

• Technical (tempering, hardening, cryogenic treatment, aging).
• Thermo-mechanical, which uses not only high temperature, but also physical impact on the metal.
• Chemical-thermal involves heat treatment of the metal followed by exposure of the surface to nitrogen, chromium or carbon.

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

Methods and types of heat treatment of steel

​

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

Steel tempering: types and characteristics, technology features and temper brittleness, heat treatment of alloys - Machine

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

Types of heat treatment

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.

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).