What is steel annealing

Equipment for heat treatment of fasteners, hardware and parts

Complex heat treatment of metals is a 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 ( heat treatment ) of steel and alloys can be of the following types: annealing , normalization , hardening , tempering .

• Annealing is a thermal treatment (heat treatment) of a metal that involves heating the metal and then slowly cooling it. This heat treatment (i.e. annealing) comes in different types (the type of annealing depends on the heating temperature and the cooling rate of the metal).
• Hardening is a heat treatment (heat treatment) of steel and alloys, based on the recrystallization of steel (alloys) when heated to a temperature above critical; After sufficient exposure to the critical temperature to complete the heat treatment, rapid cooling follows. Hardened steel (alloy) has a nonequilibrium structure, so another type of heat treatment is applicable - tempering.
• Tempering is a heat treatment (heat treatment) of steel and alloys, carried out after hardening to reduce or relieve residual stresses in steel and alloys, increasing toughness, reducing the hardness and brittleness of the metal.
• Normalization is a heat treatment (heat treatment) similar to annealing. The differences between these heat treatments (normalization and annealing) are that during normalization the steel is cooled in air (when annealing, it is cooled in a furnace).

ANNEALING OF STEEL

Annealing is a metal heat treatment process that involves heating and then slowly cooling the metal. Transition of a structure from a nonequilibrium state to a more equilibrium one.

Annealing of the first kind , its types: return (also called metal rest), recrystallization annealing (also called recrystallization ), annealing to relieve internal stress , diffusion annealing (also called homogenization ).

Annealing of the second type is a change in the structure of the alloy through recrystallization near critical points in order to obtain equilibrium structures. Annealing of the second kind, its types: complete , incomplete , isothermal annealing . Annealing and its types in relation to steel are discussed below.

• Return (rest) of steel - heating to 200 - 400°C, annealing to reduce or remove hardening. Based on the results of annealing, a decrease in the distortion of crystal lattices in crystallites and a partial restoration of the physicochemical properties of steel are observed.
• Recrystallization annealing of steel (recrystallization) - heating to temperatures of 500 – 550°C; annealing to relieve internal stress – heating to temperatures of 600 – 700°C. These types of annealing relieve internal stresses in the metal of castings due to uneven cooling of their parts, also in workpieces processed by pressure (rolling, drawing, stamping) using temperatures below critical. As a result of recrystallization annealing, new crystals grow from the deformed grains, closer to equilibrium ones, therefore the hardness of the steel decreases, and the ductility and toughness increase. To completely remove the internal stresses of steel, a temperature of at least 600°C is required. Cooling after holding at a given temperature must be quite slow: due to the accelerated cooling of the metal, internal stresses arise again.
• Diffusion annealing of steel (homogenization) is used when the steel has intracrystalline segregation. Leveling the composition in austenite grains is achieved by the diffusion of carbon and other impurities in the solid state, along with the self-diffusion of iron. According to the results of annealing, the steel becomes homogeneous in composition (homogeneous), therefore diffusion annealing is also called homogenization. The homogenization temperature should be high enough, but overburning and melting of the grains should not be allowed. If the burnout is allowed to occur, the oxygen in the air oxidizes the iron, penetrating into its thickness, and crystallites are formed, separated by oxide shells. Overburning cannot be eliminated, therefore overburnt workpieces are a final defect. Diffusion annealing of steel usually results in too much grain coarsening, which should be corrected by subsequent full annealing (to fine grains).
• Complete annealing of steel is associated with phase recrystallization, grain refinement at temperatures of points AC1 and AC2. Its purpose is to improve the structure of steel to facilitate subsequent processing by cutting, stamping or hardening, as well as to obtain a fine-grained equilibrium pearlite structure of the finished part. For complete annealing, the steel is heated 30-50°C above the GSK line temperature and cooled slowly. After annealing, excess cementite (in hypereutectoid steels) and eutectoid cementite have the form of plates, which is why pearlite is called lamellar
• When annealing steel onto lamellar perlite, the workpieces are left in the furnace until cooled, most often with the furnace partially heated with fuel, so that the cooling rate is no more than 10-20°C per hour. Annealing also achieves grain refinement. A coarse-grained structure, for example, of hypoeutectoid steel, is obtained during solidification due to the free growth of grains (if the cooling of the castings is slow), as well as as a result of overheating of the steel. This structure is called Widmanstätten (named after the Austrian astronomer A. Widmanstätten, who discovered such a structure on meteoric iron in 1808). This structure imparts low strength to the workpiece. The structure is characterized by the fact that inclusions of ferrite (light areas) and pearlite (dark areas) are located in the form of elongated plates at different angles to each other. In hypereutectoid steels, the Widmanstätten structure is characterized by a streak-like arrangement of excess cementite. Grain refinement is associated with the recrystallization of alpha iron into gamma iron; Due to cooling and the reverse transition of gamma iron to alpha iron, the fine-grained structure is preserved. Thus, one of the results of annealing on lamellar pearlite is a fine-grained structure.
• Incomplete annealing of steel is associated with phase recrystallization only at point temperature A C1; partial annealing is used after hot pressure treatment, when the workpiece has a fine-grained structure.
• Annealing steel into granular pearlite is usually used for eutectoid and hypereutectoid steels to increase the ductility and toughness of steel and reduce its hardness. To obtain granular pearlite, the steel is heated above the AC1 point, then held for a short time so that the cementite does not completely dissolve in the austenite. Then the steel is cooled to a temperature slightly below Ar1 and maintained at this temperature for several hours. In this case, the particles of the remaining cementite serve as crystallization nuclei for all the released cementite, which grows as rounded (globular) crystallites dispersed in the ferrite. The properties of granular pearlite differ significantly from the properties of lamellar pearlite in the direction of lower hardness, but greater lamellarity and viscosity. This especially applies to hypereutectoid steel, where all cementite (both eutectoid and excess) is obtained in the form of globules.
• Isothermal annealing - after heating and holding, the steel is quickly cooled to a temperature slightly below point A 1, then maintained at this temperature until the austenite completely decomposes into pearlite, after which it is cooled in air. The use of isothermal annealing significantly reduces time and also increases productivity. For example, ordinary annealing of alloy steel lasts 13-15 hours, and isothermal annealing - only 4-7 hours.

HARDENING OF STEEL

A distinction is made between hardening with polymorphic transformation, for steels, and hardening without polymorphic transformation, for most non-ferrous metals. The hardened material acquires greater hardness, but becomes brittle, less ductile and viscous if more repetitions of heating and cooling are performed.

To reduce brittleness and increase ductility and toughness, tempering is used after hardening with a polymorphic transformation. After hardening without polymorphic transformation, aging is applied. During tempering, there is a slight decrease in the hardness and strength of the material.

Source: https://www.metiz.com.tw/heat_information.htm

What you need to know about steel annealing?

In the production of different types of metal, raw materials undergo a number of technological operations. One of them is steel annealing. During this processing stage, the metal acquires certain parameters, without which it cannot proceed to the next technological operations.

What is annealing and why is it needed?

The annealing method is necessary to improve characteristics and change the properties of metals and alloys. Thanks to additional heat treatment, the following goals can be achieved:

1. Reduce hardness. This allows you to spend less effort on further processing of the material and use more tools.
2. Change the structure. The result is a homogeneous microstructure, which improves physical and mechanical characteristics.
3. Using heating, craftsmen reduce the internal stress that arises in the material during the first stages of working with raw materials.

Heat treatment can be complete or incomplete.
Sometimes the second option is enough to change the technical characteristics to the required level. Heat treatment, tempering, annealing, normalization, tarnishing

Kinds

There are two key annealing methods - 1st and 2nd kind. The first option involves heat treatment, after which the structure of the material does not change. However, it acquires the necessary parameters. When carrying out type 2 processing, the structure of the metal changes dramatically. In this case, it is necessary to properly cool it so as not to degrade the performance.

Isothermal

The principle of isothermal annealing is that the raw material is heated to an austenitic state. Next comes the cooling process. The temperature slowly drops to 680 degrees Celsius. The part is kept at a low temperature until pearlite is obtained. Next, the product is allowed to cool at room conditions. This type of processing is used in the production of alloy steels.

Isothermal heating differs from other types by maintaining the same temperature during cooling. This makes it possible to achieve a uniform and complete change in the structure, which has a positive effect on the technical characteristics of alloys and homogeneous metals.

Diffusion

An extreme type of heating of products. Diffusion annealing is carried out at critical rates. After this processing method, the plasticity of materials increases and hardness decreases. You can use more methods for further work with workpieces, spending less energy.

When temperatures rise above a critical level, strict control is required. If the technology is used with errors or deviations, the workpiece can be burned out. To select the correct temperature regime, a guide has been developed. Diffuse heating allows you to achieve the following changes:

• grain enlargement;
• reduction of excess phases;
• normalization of the product structure.

However, there is one drawback. Due to extreme processing, pores become larger, which negatively affects the integrity of the workpiece.

Recrystallization

Recrystallization annealing is a method by which metallurgists get rid of most of the disadvantages of a metal or alloy. The workpiece is heated above the temperature changing the structure by 200 degrees. This is how metal rods, fittings, wire, and sheet metal are processed.

Full

When metal parts are fully heated, their temperature rises to critical levels. After this, the temperature regime is set in one position, the part is maintained for a certain period of time. Next, the workpiece is cooled according to a special schedule.

Incomplete

The process of incomplete heating differs from complete heating in that the temperature of the metal parts does not reach a critical level. Prolonged cooling is also not required.

The technologies are precisely described by GOSTs, which establish a number of rules regarding their implementation. Failure to comply with the requirements can lead to defective products and destruction of equipment.

What equipment is used?

Annealing furnace.
http://www.netmus.ru Various equipment is used to heat homogeneous metals and alloys. This includes:

1. Shaft furnaces. Suitable for various technological processes involving metal workpieces. Can be heated by gas or electric elements.
2. Chamber furnaces.

   Used to heat small workpieces.

3. Ovens with an installed retractable hearth mechanism. Designed for heat treatment of large-sized parts. A beam crane is attached to the top of the structure, with the help of which workpieces are unloaded and new ones are loaded.
4. Vacuum ovens.

   They are used for heat treatment of high-speed steels, refractory metals, titanium, and copper.

Features of annealing various types of steel

When heating different types of steel, it is necessary to take into account the carbon content in their composition. Annealing steel requires knowledge of the composition of the material. The hardness indicator depends on the heating, holding, and cooling temperatures.

The enterprises install two industrial furnaces. In one, the workpiece is heated to critical temperatures or higher. The other is needed for holding or slow cooling. It is easier to work with steels that have less than 0.08% carbon in their composition. To change their characteristics, it is enough to carry out incomplete heating. The heating temperature of the metal does not reach the critical temperature. The cooling rate is set depending on the type of metal.

Full annealing of hypoeutectoid steel is rarely carried out.
Processing carbon and alloy steels is more difficult and energy-consuming. Lecture.
Technology of heat treatment of steel Annealing of steel is a technological process that is carried out in the production of different types of metals and alloys. With its help, the characteristics of workpieces are changed, certain disadvantages are removed, and the structure is changed.

However, it is important to accurately calculate the heating temperature, cooling interval, and material composition.

Source: https://metalloy.ru/obrabotka/termo/otzhig-stali

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

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:

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://regionvtormet.ru/okrashivanie/otpusk-stali-vidy-i-harakteristika-osobennosti-tehnologii-i-otpusknaya-hrupkost-termoobrabotka-splavov.html

Annealing various types of steel: methods, equipment, features

Annealing is a heat treatment that results in an equilibrium structure in the alloy. There are several types of this operation, but they all include heating to a temperature depending on the grade of steel, holding and cooling at a low speed. The purpose of steel annealing is to reduce internal stresses and increase ductility, accompanied by a slight decrease in strength.

What is metal annealing

Annealing of metal is used to obtain an equilibrium and homogeneous structure when preparing a product for subsequent thermal or mechanical treatment, as well as to improve its physical characteristics after cutting, welding, stamping, rolling or hardening operations. The purpose of annealing is to eliminate internal inhomogeneities of steel, improve its grain size and uniformity of the crystal lattice, and also remove residual stress caused by deformation of the product during various types of processing. Features of this technology allow:

• bring the properties of steel to the requirements of subsequent heat treatment;
• improve the characteristics of the workpiece material before cutting or pressure processing;
• prevent deformation and eliminate internal stresses of welded and cast products;
• restore the original quality of steel after unsuccessful hardening.

One of the characteristic features of this heat treatment is that the heated metal cools naturally, without the use of cooling media. And the heating temperature during annealing depends on the composition of the steel and the desired result.

Processes in metal during annealing

As a result of mechanical or thermal treatment of a metal, its internal structure goes into a nonequilibrium state, characterized by a combination of various phase components. At the same time, its chemical composition and crystal structure and, as a consequence, hardness, strength, ductility and internal tension change.

Annealing of a metal is performed to return its microstructure to its original state, which is usually characterized by softness, ductility and the absence of stress. When annealing carbon steels, the product is first heated to a temperature slightly above the austenite point, and then naturally cooled to room temperature.

The result is a steel consisting of a combination of pearlite and ferrite with an ordered crystalline structure. Depending on the composition of the metal and the purposes of processing, annealing of steel can be done without phase transformations (1st kind) or with their use (2nd kind).

The first method is most often used after machining to eliminate cold hardening, and the second method is used before hardening to obtain the original structure of the material.

Source: https://instanko.ru/drugoe/vidy-otzhiga.html

The main types of heat treatment of steel: annealing, hardening, tempering and normalization

To impart special properties to steel blanks, heat treatment is performed. The technology depends on the final parameters and properties of the metal, its quality. The choice of the type of heat treatment is influenced by the grade of steel and the required performance characteristics of the final product.

Types of annealing

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

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

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

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

Normalization of metal blanks

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

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

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

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

Source: https://ismith.ru/metalworking/vidy-termicheskoj-obrabotki-stali/

Annealing steel

The range of metal products is huge and in each case certain, often specific qualities of the material are required. The manufacturer is not able to provide a complete list of brands.

Metallurgical enterprises offer raw materials that meet GOST, which are subsequently refined in manufacturing plants. One of the key operations is steel annealing. At this stage, the metal acquires the necessary technical properties for subsequent processing.

To understand what steel annealing is, you need to understand why it is done and what processes occur during this process.

Annealing steel

Why is heat treatment of metal necessary?

The operation is carried out to improve the technological qualities of raw materials. The key factor is the annealing temperature of the steel, which must be maintained for a certain time. In this case, the following goals are achieved:

1. Reduced hardness. Quality indicators after processing can significantly reduce labor costs and reduce operation time using a wider range of cutting tools.
2. Improvement of microstructure. Under the influence of high temperature, significant changes occur at the molecular level over a certain period of time. The resulting homogeneous steel structure after annealing is optimal for subsequent mechanical and physical operations.
3. To relieve internal stress. During primary processing at metallurgical enterprises, an imbalance in the crystalline structure occurs in the metal. By correctly selecting the types of steel annealing, the required characteristics of the metal for a particular case are achieved.

Sometimes incomplete annealing of steel is sufficient to obtain the required technological conditions. Depending on the desired quality characteristics of the metal, complex and time-consuming modes can be used. Complete annealing of steel can take more than a day for large products. Most of this time is occupied by heating to the desired temperature and slow cooling, regulated by the type of heat treatment at a given standard.

The steel annealing mode is described in detail in specialized literature. Some operations require adherence to a time regime and precise temperature, down to several degrees. If you have a muffle furnace, then the procedure can be performed efficiently. When such equipment is not available, it will be difficult to accurately carry out certain types of heat treatment. You will have to navigate solely by the color of the hot metal.

Heated steel colors

You can anneal steel at home using a simplified procedure. It is definitely not possible to control the temperature of an object heated by a gas burner. It is possible to regulate the heating and cooling modes of the metal only approximately.

When processing steel at home, it is impossible to do a structural analysis. The temperature of incomplete annealing is determined only visually. The goals in domestic conditions are to reduce the strength and increase the workability of the product.

The microstructure of steel changes after annealing and further operations can be carried out.

Types of annealing

It is customary to divide this operation into two main types. Annealing of steel can be of the 1st and 2nd type. In the first case, phase recrystallization does not occur, but the metal acquires the desired qualities. The consequences of mechanical processing of metal in rolling mills and dies are eliminated.

Hardening of the steel surface after physical impact at a metallurgical plant is called cold hardening.

The main purpose of annealing steel of the 1st type is to reduce strength and increase ductility necessary for further processing. Partial recrystallization reduces internal stresses, which makes products more reliable and durable.

Annealing of type 2 steel is characterized by dramatic changes in the structure. Phase recrystallization is achieved by heating the metal above critical points and precisely implementing the cooling regime in temperature and time. These types of annealing and their purpose are determined by production tasks to obtain the necessary qualities of the metal.

Critical temperatures are a serious risk factor. In some cases, for example, when burned, irreversible changes in the structure occur. This metal is sent for smelting.

Heat treatment, annealing and normalization of steels is a complex process that makes it possible to obtain products from initial raw materials that meet the requirements of manufacturers of final products according to the specified characteristics.

Full, partial annealing

Heat treatment is used to achieve the required qualities of the metal. The purpose of steel annealing is defined as obtaining specified technological properties. They can be both general and quite specific.

Thus, incomplete annealing of hypereutectoid steel is acceptable in the manufacture of structural elements, but in the production of parts with specified characteristics it will be insufficient. Changes in the metal structure in both types of processing are different. Not only the annealing time of the steel plays a role, but also the temperature.

An important factor in successfully solving the problem is the cooling mode.

Complete annealing of steel

When steel is not completely annealed, the temperature does not reach the upper critical point. The requirements for cooling time are also less stringent. When performing complete annealing of steels, the metal is heated above the critical point. Then the specified time is maintained and the cooling schedule is accurately followed. When heat treatment and annealing, it is important to take into account the grade of raw materials, hardness, and chemical composition, since the technology and modes are determined by GOST standards.

Isothermal annealing

This type of processing is mainly used for alloy alloys. Isometric annealing of steel involves heating the metal to an austenitic state, followed by accelerated cooling to 660-680° C. Then the workpiece is kept at this temperature until the austenite turns into pearlite. After this, the metal is cooled in air naturally.

This is the fastest and most effective way to increase the ductility of metals with high chromium content.

High-temperature annealing of stainless steel and some other structural and tool alloys is done in this way. This technology makes it possible to reduce the hardness of alloyed materials to a level that allows the workpiece to be effectively processed later on metal-cutting equipment.

Isothermal annealing is characterized by a special cooling method. For a given time, the material is maintained at the temperature specified in the standards at one level, and does not drop gradually, as in other processing options. The formation of a homogeneous structure occurs due to the complete decomposition of austenite and the transformation of ferrites and pearlites. Heat-resistant alloys are processed in this way.

This technique is effective for processing small products, stampings, and tool blanks.

Isothermal annealing has a short technological cycle, but is quite effective for solving many production problems.

Diffusion annealing

According to industry norms, this type of heat treatment can be classified as extreme. The metal is heated to the highest possible temperature, exceeding critical points. The technology is often used for alloys with complex and low-melting compounds.

At the same time, the structure of hypereutectoid steel after annealing becomes less hard and much more ductile, which allows the use of a wide range of techniques for further processing.

The method requires full control and adherence to technology, since there are high risks of overheating and burnout, which can lead to partial or complete loss of the necessary qualities and such metal will be unsuitable for further operations. The exact temperature for complete annealing of hypoeutectoid steel and other grades of metal is available in special reference books.

Diffusion annealing of steel

Properly performed heat treatment allows you to obtain:

• equilibrium chemical composition;
• grain growth;
• dissolution of excess phases;
• education, pore growth.

The last point is a side effect, refers to defects and during production they try to avoid the occurrence of this phenomenon. The technology of steel annealing using this method requires skills and knowledge, understanding the difference between individual types and grades of metal.

Recrystallization annealing

A technique that allows you to get rid of many undesirable qualities of metal. Recrystallization annealing of steel is carried out in order to remove cold hardening and other consequences after certain mechanical operations. The technology is used to process:

• sheet metal;
• wires;
• rods;
• pipes;
• stamping.

After recrystallization annealing of steel, the metal acquires the necessary characteristics to obtain products with the specified qualities.

The choice of technology is determined by the chemical composition. During the procedure, the material is heated to values ​​exceeding the crystallization temperature by at least 100-200 ° C. The necessary properties appear to varying degrees depending on the type of treatment. More often, full annealing is used. At the same time, structural changes are more significant. In some cases, incomplete annealing is sufficient.

Temperature zones for recrystallization annealing

Description of the annealing process of steel and metal, its types, their features and technology

The 21st century is the century of developed technologies, infrastructure and industry. This also applies to the field of metallurgy, which is of utmost importance for construction. With the birth of new opportunities and ideas, the requirements for the quality of materials also increase. Humanity, which has recently mastered the technology of processing and using metal and various alloys, is no longer satisfied with the natural mechanical properties.

From now on, only high-strength and high-quality materials can be used in construction. And it is to change the natural properties of the metal that various heat treatment techniques are used, such as annealing the metal, which can significantly increase its strength and workability.

What is annealing

Annealing is one of the methods of heat treatment of metal and steel. It is based on heating to a very high temperature. That is, the metal is heated to the required temperature depending on the purpose and method, maintained in this state for some time, and then gradually cooled.

Annealing can be carried out in a wide variety of cases. For example, we can consider the most basic ones. It is usually carried out for the following purposes:

• to reduce the internal tension of the metal that may arise as a result of forging, other impact on it, or processing;
• to increase the mechanical properties and strength of the metal;
• to give uniformity to its structure;
• to improve ductility, which is very important during processing;
• to increase the level of resistance and impact strength, etc.

Types of features

Depending on the purpose and purpose, annealing can have the following varieties:

• complete and incomplete;
• recrystallization;
• diffusion;
• isothermal;
• spheroidization;
• normalization, etc.

Let's take a closer look at some of them.

Full Annealing Technology

Full annealing is carried out in order to refine the grain and improve the quality of processing using the cutting tool, as well as to eliminate internal tension. It affects products made from a hypoeutectoid alloy or steel, which contains carbon in an amount not exceeding 0.8%. These products include forged and cast parts.

As for the technology: the product is subjected to heating, which reaches a critical point of approximately 20-50 degrees, designated A3. Then it is kept in this state for as long as necessary and slowly cooled.

The heating temperature is determined depending on the type of steel using the state diagram. For each type of steel, there are certain temperature values ​​at which the required degree of heating is achieved.

These values ​​can be found in the reference tables.

The cooling time is also dictated by the structure and composition of the steel, for example, carbon steel products are cooled at 180-200 degrees per hour, low-alloy steel parts are cooled at 90 degrees per hour, high-alloy steel, if it is fully annealed, is cooled even slower - 50 degrees per hour. hour. Since products made of high-alloy steel are often subjected to another type of heat treatment, isothermal, however, there are exceptions.

As a result of complete annealing, the heterogeneous structure of carbon and hypoeutectoid steel, consisting of large and small grains and often not satisfying in its mechanical properties, becomes homogeneous and pliable for processing. It is for these purposes that complete annealing is carried out.

Features and purpose of incomplete annealing

If complete annealing is intended for products that do not meet any requirements, then incomplete annealing is carried out on the same objects with more or less satisfactory mechanical properties.

That is, as a result of incomplete heat treatment, only the pearlite structure of the metal will change, while the ferrite structure will remain unchanged. “Perlite” translated from French means “pearl”; it is part of the structure of steel, cast iron and other iron-carbon alloys.

Pearlite consists of ferrite and cementite, forming a eutectoid mixture. In other words, the main goal is to make the steel as soft and ductile as possible.

Technologically, the process of incomplete annealing differs in the degree of heating; in this case, it reaches a critical point 30−50 degrees higher to A1. The heating temperature reaches 770 degrees, gradual cooling occurs at a rate of 60 degrees per hour: first in the oven to 600 degrees, and then in the open air.

This heat treatment is also used for hypereutectoid and alloy steels. It heats up to the critical point Ac1, which exceeds it by 10−30 degrees. As a result of such heating, recrystallization of the alloy occurs, which, in turn, contributes to the formation of a spherical pearlite. This process is also called spheroidization.

Recrystallization and diffusion

• Recrystallization annealing is carried out to restore the crystal lattice damaged as a result of steel deformation. Deformation leads to hardening, which is accompanied by a decrease in ductility; the steel becomes very hard, making it impossible to process. The deformed steel is heated to 650-680 degrees, as a result of which ferrite and pearlite grains, which are in an elongated state towards the deformation, are distributed evenly, restoring the crystal lattice and restoring the steel’s ductility and softness.
• Diffusion annealing is carried out in order to level out structural homogeneity at the chemical level, that is, at the atomic level. Such a need may arise during the solidification of cast ingots, otherwise this effect is called dendritic segregation. Homogenization, or diffusion annealing, eliminates dendritic segregation by moving impurity atoms from a part with high accumulation to a part where there is a lack of them, thus leveling the chemical structure.

In order for this process to proceed successfully, heating is carried out at very high temperatures, with longer holding times and slow cooling, unlike the types discussed above. That is, these are temperatures exceeding 1000 degrees, the exposure duration is more than 12 hours.

Purpose of isothermal annealing and normalization

Isothermal annealing is used for high-alloy and high-chromium steels. Its peculiarity lies in heating the metal 30-50 degrees above the critical point Ac3 and in accelerated cooling to a holding temperature below the critical point A1, and then in natural cooling in the open air.

This type provides several visible advantages, the first of which is time, that is, the entire process - from heating, holding to cooling - takes much less time than the stage of cooling the part together with the furnace. The second advantage is that with isothermal holding and sudden cooling, a smoother and more uniform structure across the cross-section of the part is achieved.

• Normalization . The normalization process is carried out as an intermediate before processing and hardening in order to eliminate work hardening and internal stress. Hypoeutectoid steel is heated to the critical point Ac3 30–50 degrees higher, and is gradually cooled in the open air. Moreover, in contrast to annealing, during normalization, supercooling occurs, due to which a more uniform, fine and fine-grained structure is achieved.
• Consequences of normalization . The strength and toughness of steel is significantly increased. Normalization is much faster than annealing, and its performance is much higher. Therefore, it is recommended to normalize steels containing carbon rather than anneal them.

Source: https://tokar.guru/metally/stal/process-otzhiga-stali-i-metalla-vidy-osobennosti-tehnologiya.html

Annealing of steels

According to the book definition, annealing is heating steel to a temperature above the critical temperature, holding it at this temperature and slowly cooling it along with the furnace. In fact, this is a general definition that does not cover all types of annealing. Annealing modes depend primarily on the final requirements for the steel or product, primarily the requirements for the mechanical or technological properties of the metal.

Annealing of the first kind (I-th kind)

Annealing of the first kind is a thermal operation consisting of heating a metal in an unstable state, obtained by previous treatments, to bring the metal into a more stable state.

This type of annealing may include processes of homogenization, recrystallization, hardness reduction and residual stress relief. The peculiarity of this type of annealing is that these processes occur regardless of whether phase transformations occur during heat treatment or not.

There are homogenization (diffusion), recrystallization annealing and annealing, which reduces stress and reduces hardness.

Homogenization annealing

Homogenization annealing is a heat treatment in which the main process is the elimination of the effects of dendritic and intracrystalline segregation in steel ingots. Liquation increases the susceptibility of steel processed by pressure to brittleness, anisotropy of properties and defects such as slate (layered fracture) and flakes.

Elimination of segregation is achieved through diffusion processes. To ensure a high diffusion rate, the steel is heated to high (1000–1200 °C) temperatures in the austenitic region. At these temperatures, long (10–20 hours) holding and slow cooling with a furnace are done. Diffusion processes are most active at the beginning of aging.

Therefore, in order to avoid a large amount of scale, cooling with a furnace is usually carried out to a temperature of 800 - 820 ° C, and then in air. During homogenization annealing, large austenite grains grow. You can get rid of this undesirable phenomenon by subsequent pressure treatment or heat treatment with complete recrystallization of the alloy.

Leveling the composition of steel during homogenization annealing has a positive effect on mechanical properties, especially ductility.

Recrystallization annealing of steel

Recrystallization annealing, used for cold worked steels, is a heat treatment of a deformed metal or alloy. Can be used as a final or intermediate operation between cold forming operations.

The main process of this type of annealing is recovery and recrystallization, respectively. Return refers to all changes in the fine structure that are not accompanied by changes in the microstructure of the deformed metal (the size and shape of the grains do not change). The return of steels occurs at relatively low (300–400°C) temperatures.

During this process, restoration of crystal lattice distortions is observed.

Recrystallization is the nucleation and growth of new grains with fewer defects in the crystal structure. As a result of recrystallization, completely new, most often equiaxed crystals are formed. There is a simple relationship between the temperature threshold of recrystallization and the melting temperature: TR ≈ (0.3–0.4) TPL., which is 670–700°C for carbon steels.

Stress Relief Annealing

Stress relief annealing is a heat treatment in which the main process is complete or partial relaxation of residual stresses.

Such stresses arise during pressure or cutting processing, casting, welding, grinding and other technological processes. Internal stresses remain in parts after the end of the technological process and are called residual.

You can get rid of unwanted stresses by heating steels from 150 to 650°C, depending on the grade of steel and the method of previous processing.

High annealing steel

This operation is often called high tempering. After hot plastic deformation, the steel has a fine grain and a satisfactory microstructure. Steel obtains this state during accelerated cooling after plastic deformation.

However, the structure may contain components: martensite, bainite, troostite, etc. The hardness of the metal can be quite high. To increase ductility and, accordingly, reduce hardness, high annealing is done.

Its temperature is below the critical Ac1 and depends on the requirements for the metal for the next processing operation.

Annealing of the second kind (ΙΙ-th kind)

Annealing of the ΙΙ kind is based on the use of phase transformations of alloys and consists of heating above the transformation temperature, followed by slow cooling to obtain a stable structural state of the alloys.

Full annealing

Full annealing is carried out for hypoeutectoid steels. To do this, the steel part is heated above the critical point A3 by 30–50°C and after heating, slow cooling is carried out. As a rule, parts are cooled together with the furnace at a rate of 30–100°C/hour. The structure of hypoeutectoid steel after annealing consists of excess ferrite and pearlite.

The main goals of full annealing are:

— elimination of structural defects that arose during previous processing (casting, hot deformation, welding, heat treatment), such as coarse grain and Widmanstätten structure;

- softening of steel before cutting - obtaining coarse grain to improve surface quality and greater fragility of low-carbon steel chips;

- reduction of stress.

Partial annealing

Incomplete annealing differs from complete annealing in that heating is carried out 30–50 °C above the critical point A1 (line РСК on the “Iron - cementite” diagram). Incomplete annealing of hypoeutectoid steels is carried out to improve cutting machinability.

With incomplete annealing, partial recrystallization of the steel occurs due to the transition of pearlite to austenite. Excess ferrite is only partially converted to austenite.

Such annealing is carried out at a temperature of 770 - 750 ° C, followed by cooling at a rate of 30 - 60 ° C / s to 600 ° C, then in air.

Partial annealing is widely used for hypereutectoid carbon and alloy steels. Heating these steels by 10 - 30°C above Ac1 causes almost complete recrystallization of the alloy and makes it possible to obtain a granular (spherical) form of pearlite instead of a lamellar one. This annealing is called spheroidization.

Particles of cementite that did not dissolve during heating, or areas of austenite with an increased carbon concentration due to its incomplete homogenization after cementite dissolution, serve as crystallization centers for cementite, which is released upon subsequent cooling to a temperature below A1 and in this case takes on a granular form.

As a result of heating to a temperature well above A1 and the dissolution of most of the cementite and more complete homogenization of austenite, the subsequent precipitation of cementite below A1 occurs in lamellar form.

If excess cementite was in the form of a network, then before this annealing it is necessary to carry out normalization with heating above Acm (preferably with cooling in a directed air flow).

Steels close to the eutectoid composition have a narrow heating temperature range (750 - 760°C) for annealing to granular cementite; for hypereutectoid steels, the range expands to 770 - 790°C. Alloyed hypereutectoid steels can be heated to higher temperatures of 770 - 820°C. Cooling and spheroidization of cementite occurs slowly. Cooling should ensure the decomposition of austenite into a ferrite-carbide structure, spheroidization and coagulation of the resulting carbides to 620 - 680°C.

Annealing on granular pearlite (pendulum annealing)

To obtain granular pearlite, annealing is used with various variations of thermal cycling in the supercritical and intercritical temperature range, pendulum types of annealing with different shutter speeds and number of cycles.

Steel with granular pearlite has lower hardness, tensile strength and, accordingly, higher values ​​of plasticity characteristics. For example, eutectoid steel with lamellar pearlite has a hardness of 228HB, and with granular pearlite it has a hardness of 163HB and, accordingly, a tensile strength of 820 and 630 MPa, a relative elongation of 15 and 20%.

The microstructure of steel after annealing to granular pearlite (GP) looks like this:

After annealing to granular pearlite, steels have the best machinability and a higher surface finish is achieved. In some cases, annealing for granular pearlite is a mandatory preliminary operation. For example, to avoid crack formation when setting bolts and rivets.

Patenting

Patenting is an annealing operation, usually assigned to spring wire, with a carbon content of 0.65 - 0.9%, before drawing.

The process consists of austenitizing the metal and then passing it through molten salts at a temperature of 450 - 550 ° C (in DIPA these are isothermal holding temperatures in the region of minimum stability of austenite).

This leads to the formation of thin-plate troostite or sorbitol, which makes it possible to obtain reduction rates of more than 75% for drawing and a final tensile strength of 2000 - 2250 MPa after CPD.

Normalization annealing (steel normalization)

Normalization annealing or normalization of steel is used as an intermediate operation to soften the steel before cutting and to generally improve its structure before hardening. During normalization, hypoeutectoid steel is heated to temperatures Ac3 + (30–50°C), hypereutectoid steel to Acm + (30–50°C) and after holding it is cooled in still air.

Accelerated cooling compared to annealing causes a slightly greater undercooling of austenite, therefore, upon normalization, a finer eutectoid structure (fine pearlite or sorbitol) and a smaller eutectoid grain are obtained.

The strength of steel after normalization is slightly higher than after annealing. In hypereutectoid steel, normalization eliminates the coarse network of secondary cementite. When heated above the Acm point, secondary cementite dissolves, and with subsequent accelerated cooling in air it does not have time to form a coarse network, which reduces the properties of steel. In hypoeutectoid steel, as mentioned above, normalization makes it possible to eliminate large grains after overheating and Widmanstätt after a violation of the GPD cycle.

Source: https://HeatTreatment.ru/otzhig-stalej

What is the difference between annealing and tempering a metal?

The difference between annealing and tempering a metal comes down to how it is handled. Annealing involves heating the steel to a specific temperature and then cooling it at a very slow and controlled rate, while tempering involves heating the metal to a precise temperature below the critical point and is often carried out in air, vacuum or an inert atmosphere.

Heat treatment

Heat treatment is used to change the physical and mechanical properties of a metal without changing its shape. They are important processes in metal fabrication that enhance the desired characteristics of the metal and allow for further processing.

Various heat treatment processes involve carefully controlled heating and cooling of metal. For example, steel is commonly heat treated for use in a variety of commercial applications.

The general purposes of heat treatment are:

• Increase strength
• Increase hardness
• Improve strength
• Improve processing
• Improve formability
• Increase ductility
• Improve elasticity

The cooling step has different effects depending on the metal and the process. When steel is rapidly cooled, it hardens, whereas the rapid cooling step of solution annealing softens aluminum.

Although there are many types of heat treatments, two important types are annealing and tempering.

annealing

Annealing involves heating the steel to a predetermined temperature and then cooling it at a very slow and controlled rate.

Annealing is commonly used for:

• Soften metal for cold working
• Improve machinability
• Increase electrical conductivity

Annealing also restores ductility. During cold working, the metal can be hardened to such an extent that any additional work will result in cracking. By pre-annealing the metal, cold working can occur without the risk of cracking, since annealing relieves the mechanical stresses encountered during machining or grinding.

Annealing is used for steel, however other metals, including copper, aluminum and brass, can undergo a process called solution annealing.

Large furnaces are used to anneal steel. The inside of the oven must be large enough to allow air to circulate around the metal. For large items, gas conveyor ovens are used, while bottom car ovens are more practical for smaller pieces of metal.

During the annealing process, the metal is heated to a specific temperature at which recrystallization can occur. At this stage, any defects caused by metal deformation are eliminated. The metal is kept at this temperature for a fixed period, then cooled to room temperature.

The cooling process must be done very slowly to achieve a refined microstructure, which maximizes softness. This is often done by immersing the hot steel in sand, ash, or other substances with low thermal conductivity, or by turning off the furnace and cooling the steel in the furnace.