The process of heating solid metal or alloy in a proper way, keeping the temperature for a certain time, and cooling at a certain cooling rate to change its structure, so as to obtain the required performance.

2. What is the purpose of heat treatment?

By changing the internal structure of steel through proper heat treatment process, the degree of microstructure transformation and the shape of transformation products can be controlled during phase transformation, thus improving the properties of steel.

3. What are the conditions of heat treatment?

Heat treatment requires alloys with solid phase transformation.

4. What is the technological process of heat treatment?

(1) heating: critical point +△T value

(2) heat preservation

(3) Cooling: the critical point -△T has a certain cooling rate.

5. What are the main parameters?

(1) heating temperature t

(2) heat preservation time t

(3) cooling speed v, the cooling medium determines the cooling speed, such as water, salt water, alkaline water and air.

6, according to the processing stage and purpose can be divided into what kinds?

(1) pretreatment

The purpose is to eliminate segregation and internal stress and obtain a balanced structure for final heat treatment or subsequent processing.

(2) Final treatment

As the last working procedure of workpiece processing, the final structure is obtained.

7, according to the heat treatment process parameters can be divided into what kinds?

(1) ordinary heat treatment

This is the most commonly used heat treatment process in production, such as annealing, normalizing, quenching and tempering. This kind of heat treatment generally does not add additional elements, and the required properties are obtained mainly through the transformation of its own structure.

(2) chemical heat treatment

This heat treatment is often used for wear-resistant parts such as gears and shafts. When the workpiece is subjected to chemical heat treatment, other elements will penetrate into the surface layer, but the composition of the core will not be affected. Generally, what elements are infiltrated is called infiltration treatment, such as surface infiltration of C, N, C, N*** and so on.

(3) surface heat treatment

The characteristics of the above two kinds of heat treatment are integrated, that is, no other elements are added during heat treatment, and the heat treatment is only aimed at the surface and does not affect the structure of the core, such as surface quenching, but the workpiece is required to have high carbon content.

8. What is annealing

Annealing is a heat treatment process in which metals and alloys are heated to a suitable temperature for a certain period of time and then slowly cooled. After annealing, sub-steel is ferrite and flaky pearlite; * * * steel precipitation or more than * * * steel precipitation is granular pearlite. In a word, the structure after annealing is close to equilibrium.

9. What is the purpose of annealing?

(1) reduces the hardness of steel, improves the plasticity of steel, and is beneficial to cutting and cold deformation.

(2) Grain refinement, elimination of structural defects caused by casting, forging and welding, homogenization of steel structure and composition, improvement of steel properties or preparation of structure for future heat treatment.

(3) Eliminate internal stress of steel to prevent deformation and cracking.

10, what are the types of annealing processes?

There are mainly homogenization annealing, complete annealing, incomplete annealing, isothermal annealing, spheroidizing annealing, recrystallization annealing and stress relieving annealing.

1 1. What is homogenization annealing?

Homogenization annealing is an annealing process that aims at reducing the chemical composition segregation and microstructure inhomogeneity of metal ingots, castings or forging blanks, heating to a high temperature, keeping it for a long time, and then slowly cooling to homogenize the chemical composition and microstructure.

Generally, the heating temperature of homogenization annealing is AC3+( 150 ~ 200℃), that is, 1050 ~ 150℃, and the holding time is 10 ~ 15h to ensure full diffusion and eliminate or reduce components or structures. Because of the high heating temperature, long time and coarse grains, diffusion annealing is followed by complete annealing or normalizing to refine the structure.

12, what is complete annealing?

Complete annealing, also known as recrystallization annealing, is an annealing process that completely austenitizes the iron-carbon alloy and then slowly cools it to obtain a nearly balanced structure.

Complete annealing is mainly used for sub-* * steel, generally medium carbon steel and low-carbon alloy structural steel forgings, castings and hot-rolled profiles, and sometimes used for its welded components. Complete annealing is not suitable for overalloyed steel, because it needs to be heated above Acm. When cooling slowly, carburizing will precipitate along austenite grain boundaries and distribute in a network, which will increase the brittleness of the material and leave hidden dangers for the final heat treatment.

Generally, the heating temperature of fully annealed carbon steel is AC3+(30 ~ 50℃); The alloy steel is AC3+(500 ~ 70℃); The holding time should be determined according to many factors, such as the type of steel, the size of the workpiece, the charging amount and the selected equipment model. In order to ensure the complete pearlite transformation of undercooled austenite, the cooling of complete annealing must be slow, and it must be cooled to about 500℃ with the furnace, and then discharged from the furnace for air cooling.

13, what is incomplete annealing?

Incomplete annealing is an annealing process in which the iron-carbon alloy is heated to a temperature between AC 1 ~ AC 3 to realize incomplete austenitization and then slowly cooled.

Incomplete annealing is mainly suitable for forgings of medium and high carbon steel and low alloy steel, and its purpose is to refine the structure and reduce the hardness. The heating temperature is AC 1+(40 ~ 60)℃, and it is cooled slowly after heat preservation.

14, what is isothermal annealing?

Isothermal annealing is an annealing process in which steel or blank is heated to a temperature higher than Ac3 (or Ac 1), then cooled to a certain temperature within the pearlite temperature range and kept isothermal, so that austenite is transformed into pearlite structure, and then cooled in air.

Isothermal annealing process is suitable for medium carbon alloy steel and low alloy steel, and its purpose is to refine the structure and reduce the hardness. The tapping heating temperature is AC3+(30 ~ 50)℃, the tapping heating temperature is AC3+(20 ~ 40)℃, the temperature is kept for a certain time, and it is cooled with the furnace to a temperature slightly lower than Ar3 for isothermal transformation, and then it is discharged for air cooling. The microstructure and hardness of isothermal annealing are more uniform than that of complete annealing.

15, what is spheroidizing annealing?

Spheroidizing annealing is an annealing process that spheroidizes carbides in steel. Heating the steel to 20 ~ 30℃ above Ac 1, keeping the temperature for a period of time, and then slowly cooling to obtain spherical or granular carbide structures uniformly distributed on ferrite matrix.

Spheroidizing annealing is mainly suitable for steels above * * * *, such as carbon tool steel, alloy tool steel and bearing steel. After rolling and forging, these steels are air-cooled, and the microstructure obtained is flaky pearlite and reticular cementite. This structure is hard and brittle, which is not only difficult to cut, but also easy to deform and crack in the later quenching process. Spherical pearlite structure was obtained by spheroidizing annealing, in which cementite was spherical particles and dispersed on ferrite matrix. Compared with flaky pearlite, it is not only low in hardness, but also easy to cut. Austenite grains are not easy to grow when quenching and heating, and the workpiece is not easy to deform and crack when cooling. In addition, spheroidizing annealing can sometimes be used for some sub-* * steels (such as stamping and cold heading) that need to improve cold plastic deformation.

The heating temperature of spheroidizing annealing is AC 1+(20 ~ 40)℃. During spheroidizing annealing, austenite is "incomplete", only flaky pearlite is transformed into austenite, and a small amount of excess carbide is dissolved. Therefore, it is impossible to eliminate the network carbide. If there are reticular carbides in the steel after precipitation, it must be normalized before spheroidizing annealing to ensure normal spheroidizing annealing.

16 what is recrystallization annealing?

Recrystallization annealing is a heat treatment process that heats the cold deformed metal above the recrystallization temperature and keeps it for a proper time to recrystallize the deformed grains into uniform equiaxed grains, so as to eliminate deformation strengthening and residual stress.

17, what is stress relief annealing?

Stress relief annealing is an annealing process to eliminate residual stress caused by plastic deformation and welding. And is present in the casting.

After forging, casting, welding and cutting, there is internal stress in the workpiece. If it is not eliminated in time, the workpiece will be deformed during processing and use, which will affect the accuracy of the workpiece. It is very important to use stress relief annealing to eliminate the internal stress produced during machining.

The heating temperature of stress relief annealing is lower than the transformation temperature A 1, so the structural transformation will not occur during the whole heat treatment. The internal stress is mainly eliminated by the workpiece in the process of heat preservation and slow cooling. In order to eliminate the internal stress of the workpiece more thoroughly, the heating temperature should be controlled during heating. Generally, it is put into the furnace at low temperature, and then heated to the specified temperature at a heating rate of about100℃/h. The heating temperature of the weldment should be slightly higher than 600℃. The heat preservation time depends on the situation, generally 2 ~ 4h. The upper limit of stress-relieving annealing and heat preservation time of castings should be taken, and the cooling rate should be controlled at (20 ~ 50)℃/h, and only when it is cooled below 300℃ can it be discharged for air cooling.

18, what is tempering?

Reheat the quenched workpiece to an appropriate temperature below the lower critical temperature, keep the temperature for a period of time, and then cool it in air, water, oil and other media for metal heat treatment.

19, what is the purpose of tempering?

(1) Reduce brittleness and eliminate or reduce internal stress. After quenching, the steel parts have large internal stress and brittleness. If not tempered in time, steel parts often deform or even crack.

(2) Obtain the required mechanical properties of the workpiece. After quenching, the workpiece has high hardness and brittleness. In order to meet the requirements of different properties of various workpieces, the hardness can be adjusted by appropriate tempering to reduce brittleness and obtain the required toughness and plasticity.

(3) Stabilize the workpiece size.

(4) For some alloy steels that are difficult to soften after annealing, high temperature tempering is often used after quenching (or normalizing) to make carbides in the steel gather properly and reduce the hardness, which is beneficial to cutting.

20. What are the types of tempering?

According to different requirements, low temperature tempering, medium temperature tempering or high temperature tempering can be adopted. Usually, with the increase of tempering temperature, hardness and strength decrease, and ductility or toughness gradually increase.