1、What is heat treatment?
A process in which solid metal or alloy is heated in an appropriate manner, kept warm for a certain period of time, and cooled at a certain cooling rate to change its structure and obtain the required properties.
2, what is the purpose of heat treatment?
By changing the internal structure of steel through appropriate heat treatment processes, we can control the degree of organizational transformation and the form of the transformation products during the phase transformation process, thereby improving the properties of steel.
3, what are the conditions for heat treatment?
Alloys that must undergo a solid phase transformation 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
5, what are the main parameters?
(1) Heating temperature T
(2) Keeping time t
(3) Cooling speed V, the cooling medium determines the cooling speed, such as: water, salt water, alkaline water, air
6, which types can be divided according to processing stages and purposes?
(1) Preprocessing
The purpose is to eliminate segregation and internal stress and obtain a balanced structure for final heat treatment or subsequent processing.
(2) Final processing
As the final step of workpiece processing, the final structure is obtained.
7, which types can be divided according to the heat treatment process parameters?
(1) General 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 additional elements, but mainly obtains the required performance through its own organizational transformation.
(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 penetrate into the surface layer, but will not have any impact on the composition of the core. What elements are generally infiltrated, we call it infiltration × treatment, such as surface infiltration of C, N infiltration, 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 the heat treatment is only performed on the surface and does not affect the structure of the core, such as surface quenching, but it requires a higher carbon content in the workpiece.
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 cooled slowly. The structure of hypoeutectoid steel after annealing is ferrite plus lamellar pearlite; eutectoid steel or hypereutectoid steel is granular pearlite. In short, the annealed tissue is close to the equilibrium state.
9, what is the purpose of annealing?
(1) Reduces the hardness of steel and improves plasticity to facilitate cutting and cold deformation processing.
(2) Refine grains, eliminate structural defects caused by casting, forging, and welding, uniformize the structure and composition of steel, improve the performance of steel or prepare the structure for subsequent heat treatment.
(3) Eliminates 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 and stress relief annealing.
11, what is homogenization annealing
Homogenization annealing is an annealing process that aims to reduce the segregation of the chemical composition and the non-uniformity of the structure of the metal ingot, casting or forging by heating it to a high temperature, holding it for a long time, and then slowly cooling it to homogenize the chemical composition and structure.
The heating temperature of homogenization annealing is generally Ac3+ (150～200℃), that is, 1050～1150℃, and the holding time is generally 10～15h to ensure that diffusion is fully carried out and to eliminate or reduce uneven composition or tissue. Due to the high heating temperature and long time of diffusion annealing, the grains are coarse. Therefore, complete annealing or normalizing is performed after diffusion annealing to refine the structure again.
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 structure close to an equilibrium state.
Complete 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 for their welded components. Complete annealing is not suitable for hypereutectoid steel, because complete annealing of hypereutectoid steel requires heating to above Acm. During slow cooling, 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 complete annealing is generally Ac3+ (30~50℃) for carbon steel; Ac3+ (500~70℃) for alloy steel; the holding time depends on the type of steel, the size of the workpiece, the amount of furnace charging, the selected equipment model and other factors. In order to ensure that the supercooled austenite undergoes complete pearlite transformation, the cooling of complete annealing must be slow, and the furnace is cooled to about 500°C and air-cooled out of the furnace.
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 suitable for 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)℃, and it is slowly cooled after insulation.
14, what is isothermal annealing
Isothermal annealing is an annealing process that heats steel parts or blank parts to a temperature higher than Ac3 (or Ac1), maintains it for an appropriate time, and then cools it quickly to a certain temperature in the pearlite temperature range and maintains it isothermally, so that the austenite transforms into a pearlite structure, and then cools in the air.
The 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)℃. Keep it for a certain period of time, and then it will undergo isothermal transformation as the furnace is cooled to slightly lower than the Ar3 temperature, and then it will be air-cooled out of the furnace. The structure 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. The steel is heated to a temperature above Ac1 20 to 30°C, kept warm for a period of time, and then slowly cooled to obtain a structure of spherical or granular carbides evenly distributed on the ferrite matrix.
Spheroidizing annealing is mainly suitable for eutectoid and hypereutectoid steels, such as carbon tool steels, alloy tool steels, and bearing steels. When these steels are air-cooled after rolling or forging, the resulting structure consists of lamellar pearlite and a network of cementite. This structure is hard and brittle, making machining difficult and increasing susceptibility to deformation and cracking during subsequent quenching. Spheroidizing annealing produces a spheroidal pearlite structure in which cementite occurs as spherical particles dispersed in a ferrite matrix. Compared with lamellar pearlite, this structure has lower hardness and is easier to machine. Austenite grains are also less likely to grow during heating for quenching, and the workpiece is less prone to deformation and cracking 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 heating temperature for spheroidizing annealing is Ac1+(20~40)℃ or Acm-(20~30)℃. After heat preservation, it is cooled isothermally or directly cooled slowly. Austenization is "incomplete" during spheroidizing annealing, with only the flaky pearlite transforming into austenite and a small amount of excess carbide dissolving. Therefore, it is impossible to eliminate reticular carbides. If reticular carbides exist in hypereutectoid steel, normalizing must be performed before spheroidizing annealing to eliminate them to ensure normal spheroidizing annealing.
16, what is recrystallization annealing?
Recrystallization annealing is a heat treatment process in which cold-deformed metal is heated above the recrystallization temperature and maintained for an appropriate time to recrystallize the deformed grains into uniform equiaxed grains to eliminate deformation strengthening and residual stress.
17, what is stress relief annealing?
Stress relief annealing is an annealing process performed 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 be deformed 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 processing.
The heating temperature of stress relief annealing is lower than the phase transformation temperature A1. Therefore, no structural transformation occurs during the entire heat treatment process. Internal stress is mainly eliminated through the process of heat preservation and slow cooling of the workpiece. In order to eliminate the internal stress of the workpiece more completely, 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 speed of about 100°C/h. The heating temperature of the welded parts should be slightly higher than 600℃. The holding time depends on the situation, usually 2~4h.
The holding time for stress relief annealing of castings is set to the upper limit, and the cooling rate is controlled at (20~50) ℃/h. It can be air-cooled only when it is cooled to 300 ℃ or below.
18, what is tempering
Metal heat treatment in which the quenched workpiece is reheated to an appropriate temperature below the lower critical temperature, kept warm for a period of time and then cooled in air, water, oil and other media.
What is the purpose of 19 and tempering?
(1) Reduce brittleness and eliminate or reduce internal stress. After quenching, steel parts will have great internal stress and brittleness. If they are not tempered in time, the steel parts will often deform or even crack.
(2) Obtain the required mechanical properties of 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, reducing the brittleness and obtaining the required toughness and plasticity.
(3) Stable 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 properly gather 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.
(Source: Learn something about heat treatment every day)