Annealing of steel - heating temperature, properties of steel after heat treatment

Annealing is one of the main heat treatment operations designed to obtain certain properties of steel. It can serve as an intermediate step or perform the functions of the final technological process. Goals achieved using various types of annealing: reduce hardness, obtain a homogeneous structure convenient for subsequent machining operations, relieve internal stresses. Depending on the heating temperature, time and holding conditions, two main types of annealing are distinguished – types I and II, which, in turn, are divided into subtypes.

Annealing of steels of the first type - purpose, types, heating temperatures

Depending on the heating temperatures and the initial state of the alloy, during various types of annealing of the first type, processes of homogenization, recrystallization, elimination of residual stresses, and reduction of hardness occur. All these processes take place when alloys are heated both above and below the temperatures at which phase transformations occur. The main goals achieved with this type of heat treatment are the elimination of chemical and physical heterogeneity that occurs after welding, cutting, pressure treatment, and hardening.

Homogenization (diffusion) annealing

This type of heat treatment is used for alloyed ingots. It allows you to reduce dendritic or intracrystalline heterogeneity, which increases the susceptibility of the metal to negative phenomena during pressure treatment, including:

brittle fracture;
uneven properties in different directions;
layered fracture;
cracking;
decrease in plasticity and viscosity.

Diffusion process mode:

heating to high temperatures (up to +1200°C), at which the characteristics of the alloy structure are leveled in all directions;
exposure – 15-20 hours;
rapid cooling of the workpiece to 800-820°C, and then slower cooling in air.

As a result of the homogenization thermal process, coarse grains are obtained, which are crushed by further pressure treatment or heat treatment.

Recrystallization annealing of steel

This type of heat treatment is used on steel blanks or semi-finished products after cold working or between such operations. It consists of heating to temperatures exceeding the temperatures of recrystallization processes, holding and cooling. The operating temperature is determined by the carbon content in the alloy:

0.08-0.2% C–+680…700°C. Such steels are subjected to stamping, rolling, and drawing.
High carbon alloy steel – +680…740°C. Usually these are calibrated rods made of chromium-containing nickel-free and chromium-nickel grades. Exposure – 0.5-1.5 hours.

To relieve stress

This type of heat treatment is used for castings, welded products, and workpieces after cutting, in which residual stresses appear as a result of non-uniform cooling and plastic deformations. Residual stresses provoke a number of negative consequences, including changes in dimensional parameters and deformation processes during storage, transportation and operation of products.

The stress relieving operation is carried out in the following temperature ranges:

Lead screws, gears, worms: +570-600°C, holding for 2-3 hours after the main machining, +160…+180°C, holding for 2-2.5 hours after finishing measures carried out to relieve stress after grinding.
Weld stress relief treatment: +650-700°C.

Residual stresses also decrease during recrystallization annealing, during which phase transformations occur.

Annealing to relieve residual stress

It is used for castings, welded joints, parts after cutting, etc., in which, during previous technological operations, due to uneven cooling, non-uniform plastic deformation, etc. residual stresses have arisen. They can cause dimensional changes, warping and cracks in parts during processing, operation and storage. Annealing is carried out at a temperature of 160...700 0C followed by slow cooling.

After the main machining, high-precision parts (lead screws, high-stress gears, worms, etc.) are subjected to annealing at 570...600 0C for 2...3 hours, and after final machining to relieve grinding stresses at a temperature of 160...180 0C 2…2.5 hours. Annealing to relieve welding stress is carried out at 650...700 0C.

Annealing of the second type > Continue >

Annealing of the second kind – processes with phase recrystallization

Annealing of the second type is carried out only at temperatures above the threshold for the onset of phase transformations. Varieties - complete, isothermal, incomplete.

Full

Complete annealing consists of heating above the critical temperature A3 (the end of recrystallization), holding until phase transformations are completely completed, and slow cooling. When heated to temperatures exceeding point A3 by 30-50°C, the steel, after complete annealing, acquires a single-phase austenitic structure with fine grains, providing increased toughness and ductility. At higher temperatures, the austenite grain increases in size, which reduces the characteristics of the semi-finished product.

The heating temperature and holding time in high-temperature conditions are determined by the type of workpieces, the method of placing them in the oven, and the height of the charge. To protect steel from oxidation and decarburization, annealing is carried out in protective atmospheres.

The cooling rate is determined by the chemical composition of the steel. The greater the stability of supercooled perlite the metal exhibits, the slower it needs to be cooled. Therefore, carbon steels are cooled at a rate of 100-150 degrees per hour, and alloy steels are cooled much more slowly - at a rate of 40-60 degrees per hour. After the decomposition of austenite in the ferritic region, cooling can be more intense. It can be implemented even in air. If the purpose of this type of heating is to relieve stress in parts of a complex configuration, then slow cooling in the furnace is carried out until normal temperatures are reached.

Full annealing is usually used for long products, shaped castings, and forgings made of medium-carbon steels.

Isothermal annealing

With this type of heat treatment, heating is carried out as for complete annealing. The difference between the process is rapid cooling to temperatures below the critical point A1, usually +660...680°C. At the temperature to which the steel was quickly cooled, isothermal exposure is carried out - up to 6 hours, during which the austenitic structure completely disintegrates. At the next stage, the semi-finished products are cooled in air.

The advantage of the isothermal process compared to the full one is the reduction in the operation period. This is especially true for alloyed grades. Another advantage is obtaining the most uniform structure over the entire cross-section of the workpiece. Workpieces that are planned to be processed by cutting are annealed at temperatures of 930-950°C, which ensure a slight coarsening of the grain and improved processing by the cutting tool.

Most often, isothermal annealing is applied to: forgings and small-sized rolled products made from alloy grades. For large cages (from 20 tons), isothermal annealing is not used, since in certain areas of the cage transformations are carried out under different temperature conditions.

For medium carbon spring steel with a carbon content of 0.6-0.9% C, a specialized isothermal treatment called patenting is used. This process serves to prepare the wire for multi-stage reduction during cold drawing.

The first stage is heating the workpieces to temperatures at which complete austenitization of the structure occurs (approximately +900°C), the second is immersion in salts with temperatures in the range of +450...+600°C.

The sorbitol or thin-plate troostite structures formed after such treatment provide:

the possibility of significant compression during broaching;
absence of breaks during cold deformations;
high strength after finishing drawing.

Partial annealing

With incomplete annealing, metal products are heated slightly above the critical temperature A1. This type of heat treatment improves the cutting processing of semi-finished products from hypereutectoid (with a carbon content of more than 0.8%) alloy and carbon steels.

Stages of partial annealing in hypereutectoid steels:

Heating to temperatures above point A1 by 10-30°C (usually +750...770°C). Provides almost complete recrystallization of the structure. During this process, the lamellar ferrite becomes spheroidal in shape. Therefore, this operation is often called spheroidization.
Cooling to 600°C at speeds up to 60°C/hour. The more alloying additives in the steel, the slower the cooling should be.
Cooling in air from +600°C to normal temperature.

Normalization annealing

Normalization (normalization annealing) is considered an intermediate process between hardening and annealing, since it allows one to obtain less fragility of the metal than with hardening and greater hardness than with other types of annealing. Therefore, normalization is a process widely used for the manufacture of mechanical engineering parts.

Normalization is often performed with rolling heating. Heating temperatures:

hypoeutectoid steels - up to temperatures exceeding A3 by 40-50°C;
hypereutectoid steels – 40-50°C above the Am point.

Next, a short holding period is carried out, during which phase transformations are completed, and cooling is done in air.

Normalization is accompanied by complete recrystallization and refinement of the structure formed after casting, forging, rolling, and stamping. For low-carbon steels, normalization is in demand instead of annealing in order to obtain increased hardness, improve cutting performance, and surface quality. For some alloy grades, normalization with air cooling replaces the quenching process. Heating for normalization of hot-rolled bars is often carried out with high-frequency currents.

Annealing of the first and second kind.

Annealing of the 1st kind is possible for any Me and alloys. Its implementation is not due to phase transformations in the solid state. Heating, during annealing of the first kind, increases the mobility of atoms, partially or completely eliminates chemical inhomogeneity, and reduces internal stresses. The heating t and holding time are of primary importance. Characteristic is slow cooling

1. Diff (homogenizing) annealing

.
Used to eliminate segregation (inconsistency of chemical composition according to V), leveling the chemical composition of the alloy. It is based on diffusion. As a result of heating, the composition is equalized and excess carbides are dissolved. Used for alloy steels. Heating temperature depends on melting temperature, ТН = 0.8 Тmelt
. Duration of exposure: 8-20 hours.

2. Recrystallization annealing

wire to relieve stress after cold plastic deformation.
Heating temperature is related to melting temperature: ТН = 0.4 Тmelt
. The duration depends on the dimensions of the product.

3. Annealing to relieve stress after hot processing (casting, welding, cutting, when high dimensional accuracy is required). The heating temperature is selected depending on the purpose, it is in a wide range: TN = 160......700oC.

The duration depends on the dimensions of the product. Parts of precision machine tools (lead screws, highly loaded gears, worms) are annealed after the main machining at t
570...600oC
for
2-3
hours, and after the final machining, to relieve grinding stresses - at t
160...180oC
for
2-2.5
hours.

Annealing of the 2nd kind - to change the phase composition. Heating temperature and holding time provide the necessary structural transformations. The cooling rate must be such that reverse differential phase transformations have time to occur. Being a preparatory operation, castings, forgings, and rentals are subjected to this. Annealing reduces hardness and strength, improves cutting performance of medium and high carbon steels. By refining the grain, reducing internal stresses and reducing structural heterogeneity, it helps to increase plasticity and viscosity. Depending on the heating temperature, annealing is distinguished: 1. full,

with a heating t of
30...50 oC
above the critical t
A3
. This is carried out for hypoeutectoid steels to correct the structure. With such a heating t, A becomes fine-grained, and after cooling the steel also has a fine-grained structure.

2. incomplete,

with heating t
30...50oC
above critical t A1

Used for hypereutectoid steels. With such heating, C2 is retained in the structure, and as a result of annealing, C acquires a spherical shape (spheroidization). The production of granular carbon is facilitated by hot plastic deflation prior to annealing, during which the carbon mesh is crushed. Pages with granular carbon are better processed and have a better structure after quenching. Incomplete annealing is mandatory for tool steels. Sometimes incomplete annealing is used for hypoeutectoid steels, if correction of the structure is not required (fine-grained steel), but it is only necessary to reduce the hardness to improve cutting processing.

3. cyclic or pendulum annealing

used if, after incomplete annealing, the center remains lamellar.
In this case, after heating above t A1, cooling to 680 o C
, then heating again to
750 ... 760
o
C
and cooling. As a result, granular C is obtained.

4. isothermal annealing –

after heating to the required t, the product is quickly cooled to a temperature
50...100oC
below the critical t
A1
and maintained until A is completely converted into P, then cooled in still air. The temperature of isothermal holding is close to the t min stability A. As a result, a more homogeneous structure is obtained, since the transformation occurs at the same degree of supercooling. Used for alloy steels.

5. Normalization.

– a type of annealing.TO, during which the product is heated to an austenitic state,
30...50 oC
above
A3
or
Ast
, followed by cooling in air.

or As a result of these norms, a finer structure of the eutectoid (thin P or sorbitol) is obtained, internal stress is reduced, and defects obtained during the previous processing are eliminated. Hardness and strength are higher than after annealing. In hypereutectoid steels, normalization eliminates the coarse mesh Ts2. For low-carbon steels, normalization is used instead of annealing. For medium-carbon steels, normalization or normalization with high tempering is used instead of quenching with high tempering. In this case, the fur is lower, but the product is subject to less deformation, and cracks are eliminated.

1. Diff (homogenizing) annealing

2. Recrystallization annealing

wire to relieve stress after cold plastic deformation.
Heating temperature is related to melting temperature: ТН = 0.4 Тmelt
. The duration depends on the dimensions of the product.

2. incomplete,

with heating t
30...50oC
above critical t A1

3. cyclic or pendulum annealing

4. isothermal annealing –

5. Normalization.

– a type of annealing.TO, during which the product is heated to an austenitic state,
30...50 oC
above
A3
or
Ast
, followed by cooling in air.

Annealing on granular pearlite

To obtain the structure of granular pearlite, pendulum annealing is carried out, after which eutectoid and hypereutectoid steels provide good machinability, the speed of the cutting process increases and the surface quality improves. This type of T/O is suitable for thin sheets before cold stamping and rods before cold drawing. The result is improved plastic properties.

The pendulum annealing mode consists of several heating cycles above the critical point A3 with slow cooling to +670...+700°C. Three such cycles make it possible to obtain a structure with 100% granular perlite. Final cooling is in air.

Homogenization annealing

Homogenization annealing is used for ingots and castings in which, under real crystallization conditions, a chemically inhomogeneous structure has formed, including dendritic segregation and the presence of nonequilibrium eutectics or other structural components (see lecture 3).

The purpose of homogenization annealing is to eliminate chemical inhomogeneities in the structure and, on this basis, increase the plasticity of the casting material, improve the manufacturability of ingots during pressure processing, increase the homogeneity of the structure of finished products and improve the complex of their properties.

Temperature

heating during homogenization annealing should be extremely high, close to the melting temperature.
This will allow you to minimize the exposure time. However, the upper limit of the annealing temperature regime is limited by the development of possible undesirable phenomena, such as excessive growth of grain size ( overheating
) or melting of grain boundaries enriched with impurities, which is accompanied by gas saturation, the formation of gas and shrinkage porosity, oxidation and the occurrence of cracks (
burnout
).

Practice shows that in most cases the annealing temperature can be preliminarily determined as (0.90 - 0.95) from the melting temperature in Kelvin. Then, for each specific alloy, it is refined on the basis of ongoing studies of the structure and properties of blanks and finished products. The optimal temperature is considered to be the one that, at minimal cost (short annealing time), ensures sufficient manufacturability of the workpiece material during pressure processing (pressing, rolling, etc.) and a given level of properties of the finished products.

Exposure duration

during homogenization annealing depends on the type of alloy, the technology for its production, the size of the workpieces and the amount of charge in the heating device (furnace). It consists of the duration of heating of the product along the thickness or the heating time across the cross-section of a large cage; the time required for the dissolution of nonequilibrium structural components in the alloy structure; and, finally, the time required to eliminate dendritic liquation.

Warm-up duration

is determined by thermotechnical calculations based on solving differential equations of thermal conductivity or by experimental methods by thermometering the charge in a furnace.

Time required for dissolution of nonequilibrium structural components

, can be determined by the empirical expression

t = a× mb

where a

and
b
are constants for a specific alloy and its production technology;
m
is the thickness of dissolving particles of nonequilibrium structural components.
The value of a
is largely determined by the size of the workpieces and products being processed, and the value of the exponent
b
is determined by the stability of nonequilibrium phases and the diffusion characteristics of the system.
For aluminum alloys, for example, the value of b
ranges from 1.2 to 2.5, with average values equal to 2, which indicates predominantly diffusion kinetics of dissolution of nonequilibrium structural components in such alloys.

Third

The component of the holding time during homogenization annealing is determined by the time spent
on homogenization of the solid solution
, which in the initial state or after the end of dissolution is characterized by a nonequilibrium structural component of chemical heterogeneity over the cross section of each grain (intracrystalline or dendritic liquation).

Heating rate and cooling rate -

additional technological parameters of homogenization annealing. Ingots and shaped castings, especially complex shapes, should be heated slowly, usually with a furnace, to avoid the occurrence of thermal stresses that can lead to cracking or warping of the products.

For the same reason, cooling is often carried out slowly (together with the furnace). When assigning cooling modes, the greatest importance is given to taking into account the phase transformations developing during cooling. In this case, methods and cooling rates are used in which, as a result of the development of phase transformations, the plasticity of the alloys further increases. For example, after annealing steel ingots, cooling is usually carried out slowly (together with the furnace) at a rate of several degrees to several tens of degrees per hour. The resulting pearlite structures are characterized by a rather coarse lamellar structure with low strength but high plastic characteristics.

On the contrary, it is recommended to cool ingots made of thermally hardenable aluminum alloys after homogenization annealing at an accelerated rate (for example, in air), thereby preventing secondary precipitation of usually brittle phase components along the grain boundaries of the solid solution.

After a long homogenizing soak, castings from cast aluminum alloys are cooled very quickly - in water, which completely prevents the release of excess phase. In addition, this combination of homogenization annealing and rapid “quenching” cooling eliminates the need for new heating for quenching, reducing the overall duration of the casting production cycle.

Sometimes they use placing hot ingots in an annealing furnace that are not completely cooled in a crystallizer or mold, feeding ingots from an annealing furnace to a hot rolling operation, excluding cooling operations after annealing the ingots and reheating them for pressure treatment. This reduces the overall duration of annealing and increases the overall efficiency of the processes for obtaining products.

Alloy steels containing

chromium, molybdenum, vanadium, tungsten, titanium, etc., are annealed at temperatures of 1050 - 1250 ° C in large cages with exposure from 8 to 20 hours. Heating and cooling are very slow (up to 10 - 20 ° / h). The total cycle duration reaches 160 - 180 hours.

Aluminum alloys

annealed at temperatures from 440 to 640 °C depending on the chemical composition of the alloys. Mostly this temperature is 5 - 40 °C lower than the nonequilibrium solidus temperature of a particular alloy. Thus, for duralumins D1 and D16, whose nonequilibrium solidus temperatures are 509 and 508 °C, the homogenization annealing temperature ranges are respectively: 470 - 500 °C and 470 - 495 °C. For high-strength alloy V95 with a nonequilibrium solidus temperature of 475 °C, the annealing temperature is 440 - 470 °C. For an alloy of the aluminum-magnesium system, grade AMg6, which has a nonequilibrium solidus temperature of 460 °C, the annealing temperature is very close to the solidus - 450 - 460 °C. For a low-alloy AMts alloy, 650 and 600 - 640 °C, respectively. The holding time during annealing ranges from several hours to several tens of hours. For alloys of the duralumin type this time is from 8 to 36 hours, for alloys of the Al - Mg systems up to 48 hours.

Cooling of ingots or billets made of aluminum alloys is usually carried out in air. When using continuous furnaces, ingot billets are fed directly to rolling mills for hot plastic deformation, cooled from the annealing temperature to the deformation temperature. Castings from aluminum alloys are cooled after homogenization in water, combining annealing with hardening.

Magnesium alloys

homogenize at temperatures of 390 - 415 °C. The holding time is 18 - 24 hours. As for aluminum alloys, a combination of homogenization annealing with heating under pressure treatment (for ingots) and hardening (for castings) is often used. A feature of magnesium alloys is their high chemical activity in contact with atmospheric oxygen, and therefore there is always a danger of self-ignition. Therefore, it is advisable to heat ingots or castings to annealing temperatures in protective environments, the simplest of which is a mixture of air and sulfur dioxide.

For both aluminum and magnesium alloys, high-temperature homogenization is sometimes used (at temperatures several degrees higher than the nonequilibrium solidus temperature), which sharply increases the degree of homogenization of ingots and castings and increases the ductility of the alloys by at least 1.5-3 times.

Temperature

Exposure duration

Warm-up duration

is determined by thermotechnical calculations based on solving differential equations of thermal conductivity or by experimental methods by thermometering the charge in a furnace.

Time required for dissolution of nonequilibrium structural components

, can be determined by the empirical expression

t = a× mb

where a

Third

Heating rate and cooling rate -

Alloy steels containing

Aluminum alloys

Magnesium alloys

Below are the sources used in compiling the summary on the topic “Annealing”

Lectures on the course “Materials Science”. Lecture 13. Fundamentals of the theory of heat treatment of steel (continued). Technological features and capabilities of annealing and normalization.

Annealing, reducing hardness and increasing plasticity and toughness by obtaining an equilibrium fine-grained structure, allows you to: - improve the workpiece machinability by pressure and cutting; — correct the structure of welds overheated during pressure treatment and steel casting; — prepare the structure for further heat treatment.

Characteristic is slow cooling at a rate of 30...100°C/h.

Annealing of the first kind

1. Diffusion (homogenizing). It is used to eliminate segregation and equalize the chemical composition. It is based on diffusion. As a result of heating, the composition is leveled and excess carbides are dissolved. Mainly used for alloy steels.

2. Recrystallization annealing is carried out to relieve stress after cold plastic deformation. The duration depends on the dimensions of the product.

Annealing of the second kind

Designed to change the phase composition.

It is a preparatory operation to which castings, forgings, and rolled products are subjected. Annealing reduces hardness and strength, improves machinability in cutting medium and high carbon steels.

Depending on the heating temperature, annealing is distinguished: 1. complete, with a heating temperature 30...50 ° C above the critical temperature A3. Тк=А3+(30…50)°С

Carried out for hypoeutectoid steels to correct the structure.

At this heating temperature, the austenite is fine-grained, and after cooling the steel has the same fine-grained structure.

Normalization is a type of annealing.

Heat treatment, in which the product is heated to an austenitic state, 30...50°C above A3 or AST, followed by cooling in air. Тк=А3+(30…50)°С Or Тк=AST+(30…50)°С

Normalization is more often used as an intermediate operation that improves the structure.

For low-carbon steels, normalization is used instead of annealing. For medium-carbon steels, normalization or high-temper normalization is used instead of high-temper quenching. In this case, the mechanical properties are somewhat lower, but the product is subject to less deformation, and cracks are eliminated.

2. incomplete, with a heating temperature 30...50°C higher than the critical temperature A1. Тк=А1+(30…50)°С

Used for hypereutectoid steels.

Partial annealing is mandatory for tool steels.

Bogodukhov S.I., Grebenyuk V.F., Sinyukhin A.V. Materials science course in questions and answers: textbook. 2nd ed., rev. and additional.. – M.: Publishing house “Machine Building”, 2005. – 288 p.

Annealing consists of heating the metal, holding it, and then slowly cooling it (together with the furnace). Annealing brings the metal closer to equilibrium.

First-order annealing is carried out to obtain a structure that is more equilibrium than the initial one, without relating this goal to the presence or absence of phase recrystallization. Examples of first-order annealing are recrystallization annealing and diffusion annealing.

During recrystallization annealing, deformation-hardened metal is heated slightly above the recrystallization temperature threshold. As a result of annealing, the material acquires the same mechanical properties as it had before deformation.

Diffusion (homogenizing) annealing is carried out by heating to high temperatures (in relation to steels - much higher than Ac3 or Ast), which implies intense diffusion of atoms. For example, castings are subjected to such annealing to eliminate dendritic segregation (homogenization of the alloy). During annealing of the second kind, at least partial phase recrystallization certainly occurs. Annealing of the second type includes incomplete annealing and complete annealing.

In case of incomplete annealing, heating is carried out to temperature Ac1 (below Ac3 or Ast). Partial recrystallization of the alloy occurs (the pearlite component changes). More often, partial annealing is used for hypereutectoid steels (spheroidizing annealing).

When fully annealed, the steel is heated to Ac3 or Ast. Complete recrystallization of the alloy occurs.

If, during complete quenching (full annealing), the heated workpiece is cooled in still air, then such heat treatment is called normalization.