1. What is heat treatment?
It is a process method that heats solid metal or alloy in an appropriate way, keeps it warm for a certain period of time, and cools it at a certain cooling rate to change its structure and obtain the required performance.
2. What is the purpose of heat treatment?
The internal structure of steel is changed through appropriate heat treatment process to control the degree of organizational transformation and the morphology of transformation products during phase change, thereby improving the performance of steel.
3. What are the conditions for heat treatment?
Alloys that must undergo solid phase change can be heat treated.
4. What is the process of heat treatment?
(1) Heating: critical point + △T value
(2) Insulation
(3) Cooling: critical point - △T value certain cooling rate
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5. What are the main parameters?
(1) Heating temperature T
(2) Insulation time t
(3) Cooling rate V, the cooling medium determines the cooling rate, such as: water, salt water, alkaline water, air
6. According to the treatment stage and purpose, it can be divided into which types
(1) Pretreatment
The purpose is to eliminate segregation and internal stress, and obtain a balanced structure for the final heat treatment or subsequent processing.
(2) Final treatment
As the last step of workpiece processing, the final structure is obtained.
7. What types of heat treatment can be classified according to process parameters?
(1) Ordinary heat treatment
This is the most commonly used heat treatment process in production, such as annealing, normalizing, quenching, tempering, etc. This type of heat treatment generally does not add other elements, and mainly obtains the required performance through the transformation of its own structure.
(2) Chemical heat treatment
This type of heat treatment is often used on wear-resistant parts such as gears and shafts. When the workpiece is subjected to chemical heat treatment, other elements will be infiltrated into the surface layer, which will not have any effect on the composition of the core. Generally, what elements are infiltrated is called infiltration treatment, such as surface infiltration of C, infiltration of N, C, N co-infiltration, etc.
(3) Surface heat treatment
It combines the characteristics of the above two types of heat treatment, that is, no other elements are added during heat treatment, and it is only a heat treatment for the surface, which does not affect the structure of the core, such as surface quenching, but it requires the workpiece to have a higher carbon content.
8. What is annealing?
Annealing is a heat treatment process in which metals and alloys are heated to an appropriate temperature, maintained for a certain period of time, and then slowly cooled. After annealing, the structure of hypoeutectoid steel is ferrite plus lamellar pearlite; eutectoid steel or hypereutectoid steel is granular pearlite. In short, the annealed structure is a structure close to the equilibrium state.
9. What is the purpose of annealing?
(1) Reduce the hardness of steel and improve plasticity to facilitate cutting and cold deformation processing.
(2) Refine the grains, eliminate the structural defects caused by casting, forging and welding, make the structure and composition of steel uniform, improve the performance of steel or prepare the structure for subsequent heat treatment.
(3) Eliminate internal stress in steel to prevent deformation and cracking.
10. What are the types of annealing processes?
Mainly homogenizing annealing, complete annealing, incomplete annealing, isothermal annealing, spheroidizing annealing, recrystallization annealing, and stress relief annealing.
11. What is homogenizing annealing?
Homogenizing annealing is an annealing process for the purpose of reducing the segregation of chemical composition and structural inhomogeneity of metal ingots, castings or forgings by heating them to high temperatures, maintaining them for a long time, and then slowly cooling them to homogenize the chemical composition and structure.
The heating temperature of homogenizing annealing is generally Ac3+ (150-200℃), that is, 1050-1150℃, and the holding time is generally 10-15h to ensure that the diffusion is fully carried out and the purpose of eliminating or reducing the uneven composition or structure is achieved. Since the heating temperature of diffusion annealing is high, the time is long, and the grains are coarse, for this reason, full annealing or normalizing is carried out after diffusion annealing to refine the structure again.
12. What is full annealing? Full annealing is also called recrystallization annealing. It is an annealing process that completely austenitizes the iron-carbon alloy and then slowly cools it to obtain an annealing process close to the equilibrium state. Full annealing is mainly used for hypoeutectoid steel, generally medium carbon steel and low and medium carbon alloy structural steel forgings, castings and hot-rolled profiles, and sometimes also for their welded components. Full annealing is not suitable for hypereutectoid steel, because the full annealing of hypereutectoid steel needs to be heated to above Acm. When slowly cooled, cementite will precipitate along the austenite grain boundaries and be distributed in a network, resulting in increased brittleness of the material, leaving hidden dangers for the final heat treatment. The heating temperature for full annealing is generally Ac3+ (30-50℃) for carbon steel; Ac3+ (500-70℃) for alloy steel; the holding time is determined based on various factors such as the type of steel, the size of the workpiece, the amount of furnace loading, and the selected equipment model. In order to ensure that the supercooled austenite completely transforms into pearlite, the cooling of the complete annealing must be slow, and the furnace is cooled to about 500℃ and then air-cooled.
13. What is incomplete annealing?
Incomplete annealing is an annealing process in which the iron-carbon alloy is heated to a temperature between Ac1 and Ac3 to achieve incomplete austenitization, followed by slow cooling.
Incomplete annealing is mainly applicable to medium and high carbon steel and low alloy steel forgings, etc., and its purpose is to refine the structure and reduce the hardness. The heating temperature is Ac1+(40~60)℃, and it is slowly cooled after insulation.
14. What is isothermal annealing?
Isothermal annealing is an annealing process in which the steel or blank is heated to a temperature higher than Ac3 (or Ac1), maintained for an appropriate time, and then quickly cooled to a certain temperature in the pearlite temperature range and maintained isothermally, so that the austenite is transformed into a pearlite structure, and then cooled in the air.
Isothermal annealing process is applied to medium carbon alloy steel and low alloy steel, and its purpose is to refine the structure and reduce the hardness. The heating temperature of hypoeutectoid steel is Ac3+(30～50)℃, and the heating temperature of hypereutectoid steel is Ac3+(20～40)℃. They are kept for a certain time, and then cooled to a temperature slightly lower than Ar3 for isothermal transformation, and then air-cooled. The isothermal annealing structure and hardness are more uniform than those of complete annealing.
15. What is spheroidizing annealing
Spheroidizing annealing is an annealing process for spheroidizing carbides in steel. The steel is heated to 20～30℃ above Ac1, kept warm for a period of time, and then slowly cooled to obtain a spherical or granular carbide structure evenly distributed on the ferrite matrix.
Spheroidizing annealing is mainly suitable for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. These steels are air-cooled after rolling and forging, and the resulting structure is lamellar pearlite and network cementite. This structure is hard and brittle, not only difficult to cut, but also easy to deform and crack during the subsequent quenching process. The spheroidizing annealing results in a spheroidal pearlite structure, in which cementite is in the form of spherical particles dispersed in the ferrite matrix. Compared with the lamellar pearlite, it not only has a low hardness and is easy to cut, but also the austenite grains are not easy to grow during quenching and heating, and the workpiece has a small tendency to deform and crack during cooling. In addition, spheroidizing annealing can sometimes be used for some hypoeutectoid steels that need to improve cold plastic deformation (such as stamping, cold heading, etc.).
The spheroidizing annealing heating temperature is Ac1+(20～40)℃ or Acm-(20～30)℃, and isothermal cooling or direct slow cooling is performed after heat preservation. During spheroidizing annealing, austenitization is "incomplete", only the lamellar pearlite is transformed into austenite, and a small amount of excess carbides are dissolved. Therefore, it is impossible to eliminate the network carbides. If there are network carbides in hypereutectoid steel, normalizing must be performed before spheroidizing annealing to eliminate them in order to ensure the normal progress of spheroidizing annealing.
16. What is recrystallization annealing?
Recrystallization annealing is a heat treatment process in which the metal after cold deformation is heated to above the recrystallization temperature and maintained for an appropriate time to allow the deformed grains to recrystallize into uniform equiaxed grains 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 processing, welding, etc. and existing in castings.
There are internal stresses inside the workpiece after forging, casting, welding and cutting. If not eliminated in time, the workpiece will deform during processing and use, affecting the accuracy of the workpiece. It is very important to use stress relief annealing to eliminate the internal stress generated during the processing.
The heating temperature of stress relief annealing is lower than the phase change temperature A1, so no structural transformation occurs during the entire heat treatment process. Internal stress is mainly eliminated by the workpiece during 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, the furnace is entered at a low temperature, and then heated to the specified temperature at a heating rate of about 100℃/h. The heating temperature of the welded parts should be slightly higher than 600℃. The holding time depends on the situation and is usually 2 to 4 hours. The holding time of the stress relief annealing of the casting is taken as the upper limit, and the cooling rate is controlled at (20 to 50)℃/h. It can only be taken out of the furnace and air-cooled when it is cooled to below 300℃.
18. What is tempering?
It is a metal heat treatment that reheats the quenched workpiece to an appropriate temperature below the lower critical temperature, keeps it warm for a period of time, and then cools it in air or water, oil and other media.
19. What is the purpose of tempering?
(1) Reduce brittleness and eliminate or reduce internal stress. After quenching, steel parts have great internal stress and brittleness. If they are not tempered in time, they will often deform or even crack.
(2) Obtain the mechanical properties required by the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted through appropriate tempering, the brittleness can be reduced, and the required toughness and plasticity can be obtained.
(3) Stabilize the size of the workpiece. (4) For some alloy steels that are difficult to soften by annealing, high-temperature tempering is often used after quenching (or normalizing) to properly aggregate the carbides in the steel and reduce the hardness to facilitate cutting.
20. What are the types of tempering?
Low-temperature tempering, medium-temperature tempering or high-temperature tempering can be used according to different requirements. Usually, as the tempering temperature increases, the hardness and strength decrease, and the ductility or toughness gradually increases.
