Annealing process
The heat treatment process of heating a metal or alloy to an appropriate temperature, holding it for a certain period of time, and then cooling it slowly (generally cooling with the furnace).
It is called annealing.
The essence of annealing is:
To heat the steel to austenite for pearlite transformation, and the microstructure after annealing is close to equilibrium.
Purpose of annealing:
(1). Reduce the hardness of steel, improve the plasticity, and facilitate machining and cold deformation processing;
(2). Uniform the chemical composition and structure of the steel, refine the grain, improve the properties of the steel or make the microstructure for quenching preparation;
(3). Eliminate internal stress and work hardening to prevent deformation and cracking.
Annealing and normalizing are mainly used for preliminary heat treatment.
For parts with small stress and low performance requirements, annealing and normalizing can also be used as final heat treatment.
Classification of annealing methods
Common annealing methods are divided into:
Phase transformation recrystallization annealing above critical temperature (Ac1 or Ac3):
- Complete annealing
- Diffusion annealing
- Incomplete annealing
- Spheroidizing annealing
Annealing below the critical temperature (Ac1 or Ac3):
- Recrystallization annealing
- Stress relief annealing
Types of annealing methods
1. Complete annealing
Process: heat the steel to 20 ~ 30 ℃ above Ac3 for a period of time, and then cool it slowly (with the furnace) to obtain a heat treatment process close to equilibrium structure (complete austenitization).
Complete annealing is mainly used for hypoeutectoid steel (WC = 0.3 ~ 0.6%), generally medium carbon steel and low and medium carbon alloy steel castings, forgings and hot rolled profiles, and sometimes for their weldments.
The hardness of low carbon steel after complete annealing is low, which is not conducive to machining;
When the hypereutectoid steel is heated to the austenite state above Accm and slowly cooled and annealed, fe3cII will precipitate along the grain boundary in a network, which will significantly reduce the strength, hardness, plasticity and toughness of the steel, leaving hidden dangers for the final heat treatment.
Objective: to refine grain, uniform structure, eliminate internal stress, reduce hardness and improve the machinability of steel. The microstructure of hypoeutectoid steel after complete annealing is F + P.
In actual production, in order to improve productivity, annealing cooling to about 500 ℃ is discharged for air cooling.
2. Isothermal annealing
Complete annealing takes a long time, especially for alloy steel with stable undercooling austenitization.
If the austenitized steel is quickly cooled to a temperature slightly lower than Ar1, A is transformed into P, and then air cooled to room temperature, the annealing time can be greatly shortened. This annealing method is called isothermal annealing.
Process: heat the steel to a temperature higher than Ac3 (or Ac1), hold it for an appropriate time, quickly cool it to a certain temperature in the pearlite area, keep it isothermal, transform austenite into pearlite, and then air cool it to room temperature.
Objective: as with complete annealing, the transformation is easy to control.
Applicable to relatively stable steel: high carbon steel (WC > 0.6%), alloy tool steel and high alloy steel (total amount of alloy elements > 10%).
Isothermal annealing is also beneficial to obtain uniform microstructure and properties.
However, it is not suitable for large section steel parts and large quantities of furnace charge, because isothermal annealing is not easy to make the interior of the workpiece or batch of workpieces reach isothermal temperature.
3. Incomplete annealing
Process: heat the steel to AC1 ~ AC3 (hypoeutectoid steel) or Ac1 ~ Accm(hypereutectoid steel) and cool it slowly after heat preservation to obtain a heat treatment process close to equilibrium structure.
It is mainly used to obtain the spherical pearlite structure of hypereutectoid steel to eliminate internal stress, reduce hardness and improve machinability.
Spheroidizing annealing is a kind of incomplete annealing.
4. Spheroidizing annealing
A heat treatment process for spheroidizing carbides in steel to obtain granular pearlite.
Process: when heated to the temperature of 20 ~ 30 ℃ above Ac1, the holding time should not be too long, generally 2 ~ 4H.
The cooling method is usually furnace cooling, or isothermal at about 20 ℃ below Ar1 for a long time.
It is mainly used for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc.
After rolling and forging, the air-cooled structure of hypereutectoid steel is lamellar pearlite and network 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.
Spheroidal pearlite is obtained by spheroidizing annealing. In the spherical pearlite, cementite is spherical fine particles dispersed on the ferrite matrix.
Compared with flake pearlite, spherical pearlite not only has low hardness and is convenient for cutting, but also austenite grains are not easy to be coarse during quenching and heating, and has little tendency of deformation and cracking during cooling.
If there is network cementite in hypereutectoid steel, it must be eliminated by the normalizing process before spheroidizing annealing in order to ensure the normal spheroidizing annealing.
Objective: To reduce the hardness, homogenize the structure, improve the machinability and prepare the structure for quenching. There are many spheroidizing annealing processes, mainly including:
A). Primary spheroidizing annealing process:
Heat the steel to 20 ~ 30 ℃ above Ac1 for appropriate time, and then cool it slowly with the furnace.
It is required that the original structure is fine flake pearlite before annealing, and cementite mesh is not allowed.
Isothermal spheroidizing annealing process:
After heating and holding the steel, it is cooled to a temperature slightly lower than Ar1 for isothermal annealing (generally 10 ~ 30 ℃ below Ar1).
After isothermal cooling, the furnace will be slowly cooled to about 500 ℃ and then discharged for air cooling.
It has the advantages of short cycle, uniform spheroidized structure and easy quality control.
Reciprocating spheroidizing annealing process.
5. Diffusion annealing (homogenization annealing)
Process: the heat treatment process of heating ingot, casting or forging stock to a temperature slightly lower than the solidus for a long time, and then cooling slowly to eliminate the non-uniformity of chemical composition.
Objective: to eliminate dendrite segregation and regional segregation during solidification of ingot and homogenize composition and structure.
The heating temperature of diffusion annealing is very high, usually 100 ~ 200 ℃ above Ac3 or Accm The specific temperature depends on the degree of segregation and steel grade,
The holding time is generally 10 ~ 15 hours. After diffusion annealing, complete annealing and normalizing are required to refine the structure.
It is applied to some high-quality alloy steel and alloy steel castings and ingots with serious segregation.
6. Stress relief annealing
Process: heat the steel to a temperature lower than AC1 (generally 500 ~ 650 ℃), keep it warm, and then cool it with the furnace.
The stress relief annealing temperature is lower than A1, so the stress relief annealing does not cause structural changes.
Objective: to eliminate residual internal stress.
7. Recrystallization annealing
Recrystallization annealing, also known as intermediate annealing, is a heat treatment process that heats the cold deformed metal above the recrystallization temperature for an appropriate time to transform the deformed grains into uniform equiaxed grains and eliminate work hardening and residual stress.
In order to produce recrystallization, first there must be a certain amount of cold plastic deformation.
Then it must be heated to a certain temperature.
The lowest temperature at which recrystallization occurs is called the lowest recrystallization temperature.
The minimum recrystallization temperature of general metal materials is:
TRecrystallization=0.4Tmelt
The heating temperature of recrystallization annealing shall be 100 ~ 200 ℃ higher than the minimum recrystallization temperature (the minimum recrystallization temperature of steel is about 450 ℃), and it shall be cooled slowly after proper insulation.
Selection of annealing method
The selection of annealing method generally has the following principles:
(1) All kinds of steels with hypoeutectoid structure generally adopt complete annealing. In order
to shorten the annealing time, isothermal annealing can be selected;
(2) Spheroidizing annealing is generally used for hypereutectoid steel.
Incomplete annealing can be used when the requirements are not high. Spheroidizing annealing is often used for tool steel and bearing steel. Spheroidizing annealing is sometimes used for cold extrusion and cold heading of low or medium carbon steel;
(3) In order to eliminate work hardening, recrystallization annealing can be selected;
(4) In order to eliminate the internal stress caused by various machining processes, stress relief annealing can be selected; Z diffusion annealing is often used for large steel castings of high-quality alloy steel in order to improve the heterogeneity of microstructure and chemical composition.