1. What is heat treatment?
A process in which a solid metal or alloy is heated in a suitable manner for a certain period of time and cooled at a certain cooling rate to change its structure to obtain desired properties.
2. What is the purpose of heat treatment?
The internal structure of the steel is changed by a suitable heat treatment process to control the degree of structural transformation and the morphology of the transformed product during the phase change, thereby improving the properties of the steel.
3. What are the conditions for heat treatment?
An alloy that must have a solid phase transition can be heat treated.
4. What is the process of heat treatment?
(1) Heating: critical point + ΔT value
(2) insulation
(3) Cooling: critical point - â–³T value is a certain cooling rate
5. What are the main parameters?
(1) Heating temperature T
(2) Holding time t
(3) cooling rate V, the cooling medium determines the cooling rate, such as: water, brine, alkaline water, air
6, according to the processing stage and purpose can be divided into several
(1) Pretreatment
The purpose is to eliminate segregation, internal stress, and obtain a balanced structure for the final heat treatment or subsequent processing.
(2) Final treatment
As the final step of the workpiece processing, the final tissue is obtained.
7, according to the heat treatment process parameters can be divided into several
(1) ordinary heat treatment
This is the most commonly used heat treatment process in production, such as annealing, normalizing, quenching, tempering, etc. Heat treatment of this type generally does not add additional elements, primarily through self-organizational transformation to achieve the desired properties.
(2) Chemical heat treatment
This type of heat treatment is often used on wear parts such as gears and shafts. When the workpiece is subjected to chemical heat treatment, other elements are infiltrated into the surface layer, and the influence on the components of the core is not affected. What elements are generally infiltrated, we call it osmosis × treatment, such as surface infiltration C, infiltration N, C, N co-infiltration.
(3) Surface heat treatment
The characteristics of the above two types of heat treatment are combined, that is, no other elements are added during the heat treatment, and only the heat treatment for the surface is performed, and the structure of the core is not affected, such as surface quenching, but the workpiece is required to have a 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, the sub-eutectoid steel is ferrite plus flaky pearlite; the eutectoid steel or hypereutectoid steel is granular pearlite. In summary, the annealed tissue is a tissue that is close to equilibrium.
9. What is the purpose of annealing?
(1) Reduce the hardness of steel and improve the plasticity to facilitate cutting and cold deformation processing.
(2) Refine the crystal grains, eliminate the structural defects caused by casting, forging and welding, uniform the structure and composition of the steel, improve the properties of the steel or prepare the structure for the subsequent heat treatment.
(3) Eliminate internal stress in 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, stress relief annealing.
11. What is homogenization annealing?
Homogenization annealing is to reduce the segregation and structural non-uniformity of the chemical composition of metal ingots, castings or forgings, heat them to high temperatures, hold them for a long time, and then slowly cool them for the purpose of chemical composition and homogenization of the structure. Annealing process.
The heating temperature of the homogenization annealing is generally Ac3+ (150 to 200 ° C), that is, 1050 to 1150 ° C, and the holding time is generally 10 to 15 h to ensure that the diffusion is sufficiently performed, and the avenue eliminates or reduces the unevenness of components or tissues. Since the heating temperature of the diffusion annealing is high, the time is long, and the crystal grains are coarse, for this reason, after the diffusion annealing, the entire annealing or normalizing is performed to refine the structure.
12. What is complete annealing?
Complete annealing, also known as recrystallization annealing, is an annealing process in which the iron-carbon alloy is completely austenitized and then slowly cooled to obtain a structure close to equilibrium.
Full annealing is mainly used for hypoeutectoid steels, generally medium carbon steel and low and medium carbon alloy structural steel forgings, castings and hot rolled profiles, and sometimes used for their welded components. Full annealing is not suitable for hypereutectoid steel, because the hypereutectoid steel needs to be heated to above Acm when it is completely annealed. When it is slowly cooled, cementite will precipitate along the austenite grain boundary and be distributed in a network, resulting in an increase in material brittleness. Leave a hidden danger to the final heat treatment.
The fully annealed heating temperature carbon steel is generally Ac3+ (30～50°C); the alloy steel is Ac3+ (500～70°C); the holding time depends on the type of steel, the size of the workpiece, the amount of furnace installed, and the type of equipment selected. A variety of factors are determined. In order to ensure that the supercooled austenite is completely transformed into pearlite, the cooling of the complete annealing must be slow, and the furnace is cooled to about 500 ° C to be air-cooled.
13. What is incomplete annealing?
Incomplete annealing is an annealing process in which an iron-carbon alloy is heated to a temperature between Ac1 and Ac3 to achieve incomplete austenitization followed by slow cooling.
Incomplete annealing is mainly applied to medium and high carbon steel and low alloy steel forgings. Its purpose is to refine the structure and reduce the hardness. The heating temperature is Ac1+(40~60) °C, and it is slowly cooled after heat preservation.
14. What is isothermal annealing?
Isothermal annealing is to heat the steel or blank to a temperature higher than Ac3 (or Ac1). After a suitable time, it is cooled to a certain temperature in the pearlite temperature range and maintained isothermally, so that the austenite is transformed into a pearlite type. The annealing process of the tissue and then cooling in air.
The isothermal annealing process is applied to medium carbon alloy steels and low alloy steels with the purpose of refining the structure and reducing the hardness. The heating temperature of the hypoeutectoid steel is Ac3+(30～50)°C, and the heating temperature of the hypereutectoid steel is Ac3+(20～40)°C, which is kept for a certain period of time, and then isothermally transformed with the furnace to a temperature slightly lower than Ar3, and then air-cooled. . The isothermally annealed structure is more uniform than the hardness.
15. What is spheroidizing annealing?
Spheroidizing annealing is an annealing process performed by spheroidizing carbides in steel. The steel is heated to a temperature of 20 to 30 ° C above Ac1, kept for a while, and then slowly cooled to obtain a structure of spherical or granular carbide uniformly distributed on the ferrite matrix.
Spheroidizing annealing is mainly applied to eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel and so on. These steels are air-cooled after rolling and forging, and the resulting structure is a lamellar pearlite and a network of cementite. This kind of structure is hard and brittle, which is not only difficult to be machined, but also easily deformed and cracked during the subsequent quenching process. The spheroidized annealing results in a spherical pearlite structure in which the cementite is spherical particles dispersed in the ferrite matrix. Compared with the flaky pearlite, it has low hardness, is easy to be cut, and is quenched. When heated, austenite grains are not easily grown, and the tendency of deformation and cracking of the workpiece during cooling is small. In addition, for some hypoeutectoid steels that need to improve cold plastic deformation (such as stamping, cold heading, etc.), spheroidizing annealing may also be used.
The spheroidizing annealing heating temperature is Ac1+(20-40) °C or Acm-(20-30) °C, and the isothermal cooling or the direct slow cooling after the heat preservation. Austenitization is "incomplete" during spheroidizing annealing, except that the flaky pearlite is transformed into austenite and a small amount of excess carbide is dissolved. Therefore, it is impossible to eliminate reticulated carbides. For example, if hypereutectoid steel has reticulated carbides, normalizing must be carried out before spheroidizing annealing to eliminate spheroidal annealing.
16. What is recrystallization annealing?
Recrystallization annealing is a heat treatment process in which the cold-deformed metal is heated above the recrystallization temperature for a suitable period of time to recrystallize the deformed grains into uniform equiaxed grains to eliminate deformation strengthening and residual stress.
17. What is stress relief annealing?
The stress relief annealing is an annealing process for eliminating residual stress existing in the casting due to plastic deformation processing, welding, or the like.
There are internal stresses inside the workpiece after forging, casting, welding and cutting. If it is not removed in time, the workpiece will be deformed during processing and use, which will affect the accuracy of the workpiece. It is important to use stress relief annealing to eliminate internal stresses generated during processing.
The heating temperature of the stress relief annealing is lower than the phase transition temperature A1, and therefore, no structural transformation occurs throughout the heat treatment. The internal stress is mainly eliminated by the workpiece during the heat preservation and slow cooling process. In order to eliminate the stress in the workpiece more thoroughly, the heating temperature should be controlled during heating. Generally, it is introduced into the furnace at a low temperature, and then heated to a predetermined temperature at a heating rate of about 100 ° C / h. The heating temperature of the welded parts should be slightly higher than 600 °C. The holding time depends on the situation and is usually 2 to 4 hours. The holding time of casting stress relief annealing is upper limit, the cooling speed is controlled at (20 ~ 50) ° C / h, and it can be cooled to 300 ° C to be air cooled.
18. What is tempering?
The quenched workpiece is reheated to a suitable temperature below the lower critical temperature, and the metal cooled in air or water, oil or the like is heat treated for a while.
19. What is the purpose of tempering?
(1) Reducing brittleness, eliminating or reducing internal stress, there is great internal stress and brittleness after quenching of steel parts. If it is not tempered in time, it will cause deformation or even cracking of steel parts.
(2) Obtain the mechanical properties required for 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 by appropriate tempering to reduce the brittleness and obtain the required Resilience, plasticity.
(3) Stabilize the workpiece size.
(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. Generally, as the tempering temperature increases, the hardness and strength decrease, and the ductility or toughness gradually increases.
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