The annealing process is so simple
Today we will talk about the annealing process, the classification of annealing methods, and the selection of methods.
The heat treatment process of heating metal or alloy to appropriate temperature, holding for a certain period of time, and then slowly cooling (generally with furnace cooling) is called annealing.
The essence of annealing is to heat the steel to austenite and then carry out pearlite transformation. The structure after annealing is close to the equilibrium structure.
The purpose of annealing:
- (1) The hardness of the steel is reduced and the plasticity is improved, which is convenient for machining and cold deformation.
- (2) Homogenize the chemical composition and structure of the steel, refine the grain, improve the properties of the steel or prepare the structure for quenching.
- (3) Eliminate internal stress and work hardening to prevent deformation and cracking.
Annealing and normalizing are mainly used for preparatory heat treatment. For steel parts with little stress and low performance requirements, annealing and normalizing can also be used as final heat treatment.
Classification of annealing methods
According to the heating temperature, the common annealing methods are as follows:
- Phase transformation recrystallization annealing above critical temperature (AC1 or AC3): complete annealing, diffusion annealing, incomplete annealing, spheroidization annealing.
- Annealing below the critical temperature (AC1 or AC3): recrystallization annealing, stress relief annealing.
Seven kinds of annealing methods
1. Complete annealing
Process: heating the steel to 20 ~ 30 ℃ above AC3, holding for a period of time, then slowly cooling (with furnace) to obtain the heat treatment process (complete austenitization) which is close to the equilibrium structure.
Complete annealing is mainly used for hypoeutectoid steel (WC = 0.3% ~ 0.6%), generally for 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 cutting; When hypereutectoid steel is heated to austenite state above ACCM and annealed slowly, Fe3C Ⅱ will precipitate along grain boundary in network form, which will significantly reduce the strength, hardness, plasticity and toughness of the steel and leave hidden trouble for final heat treatment.
Objective: to refine grain, uniform structure, eliminate internal stress, reduce hardness and improve machinability of steel. The microstructure of hypoeutectoid steel after complete annealing is F + P.
In actual production, in order to improve productivity, annealing and cooling to 500 ℃ or so is out of furnace air cooling.
2. Isothermal annealing
Complete annealing takes a long time, especially for the alloy steel with stable undercooling austenization. If the austenitized steel is rapidly cooled to a temperature slightly lower than AR1, then 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: heating the steel to a temperature higher than AC3 (or AC1), holding for an appropriate time, cooling quickly to a certain temperature in pearlite area, and holding isothermal to make austenite transform into pearlite, and then air cooling to room temperature.
Objective: as the same as complete annealing, the transformation is easy to control.
Suitable for a stable steel: high carbon steel (WC > 0.6%), alloy tool steel, high alloy steel (total alloy elements > 10%). Isothermal annealing is also beneficial to obtain uniform microstructure and properties. But it is not suitable for large cross-section steel parts and large batch of furnace charge, because isothermal annealing is not easy to make the internal parts or batch parts reach isothermal temperature.
3. Incomplete annealing
Process: the steel is heated to AC1 ~ AC3 (hypoeutectoid steel) or AC1 ~ ACCM (hypereutectoid steel) and cooled slowly after heat preservation to obtain nearly balanced microstructure.
It is mainly used for obtaining spherical pearlite structure in hypereutectoid steel to eliminate internal stress, reduce hardness and improve machinability. Spheroidizing annealing is a kind of incomplete annealing.
4. Spheroidizing annealing
It is a heat treatment process to spheroidize carbide in steel and obtain granular pearlite.
Process: heating to 20 ~ 30 ℃ above AC1, holding time should not be too long, generally 2 ~ 4H is appropriate, the cooling method is usually furnace cooling, or isothermal at 20 ℃ below AR1 for a long time.
Mainly used for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. The air cooled structure of hypereutectoid steel after rolling and forging is lamellar pearlite and network cementite, which is hard and brittle. It 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 spheroidal pearlite, cementite is spherical and fine particles, which are dispersed on ferrite matrix. Compared with flake pearlite, spherical pearlite not only has low hardness and is easy to cut, but also has less deformation and cracking tendency during quenching and heating. If there is network cementite in hypereutectoid steel, normalizing process must be used before spheroidizing annealing to ensure the normal spheroidizing annealing.
Objective: to reduce hardness, uniform structure, improve machinability and prepare for quenching.
There are many spheroidizing annealing processes:
- a) One time spheroidizing annealing process: the steel is heated to 20 ~ 30 ℃ above AC1 for appropriate time, and then cooled slowly with the furnace. Before annealing, the original structure should be fine pearlite, and cementite net is not allowed.
- b) Isothermal spheroidizing annealing process: after heating and holding, the steel is cooled to a temperature slightly lower than AR1 for isothermal treatment (generally 10 ~ 30 ℃ below AR1). At the end of isothermal cooling, the furnace is slowly cooled to about 500 ℃ and then discharged for air cooling. It has the advantages of short cycle, uniform spheroidizing structure and easy quality control.
- c) Reciprocating spheroidizing annealing process.
5. Diffusion annealing (homogenization annealing)
Process: the ingot, casting or forging billet is heated to a temperature slightly lower than the solidus for a long time, and then cooled slowly to eliminate the uneven chemical composition.
Objective: to eliminate dendrite segregation and regional segregation during solidification of ingot, and to 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 heat preservation time is generally 10 ~ 15 hours. After diffusion annealing, complete annealing and normalizing are needed to refine the microstructure.
It is applied to some high quality alloy steel castings and ingots with severe segregation.
6. Stress relief annealing
Process: the steel parts are heated to a temperature lower than AC1 (generally 500 ~ 650 ℃), kept warm, and then cooled with the furnace.
The temperature of stress relief annealing is lower than that of A1, so there is no change in microstructure.
Objective: to eliminate the 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 make the deformed grains re transform into uniform equiaxed grains and eliminate work hardening and residual stress.
In order to produce recrystallization, firstly, a certain amount of cold plastic deformation is necessary, secondly, it must be heated above a certain temperature. The lowest recrystallization temperature is called the lowest recrystallization temperature. The minimum recrystallization temperature of general metal materials is t = 0.4t.
The heating temperature of recrystallization annealing should be 100 ~ 200 ℃ higher than the minimum recrystallization temperature (the minimum recrystallization temperature of steel is about 450 ℃), and slow cooling after proper heat preservation.
Selection of annealing method
The selection of annealing method generally has the following principles:
- (1) All kinds of steels with hypoeutectoid structure are generally annealed completely. In order to shorten the annealing time, isothermal annealing can be used;
- (2) Spheroidizing annealing is generally used for hypereutectoid steel, and incomplete annealing can be used when the requirement is not high. Spheroidizing annealing is often used for tool steel and bearing steel. Low carbon steel or medium carbon steel cold extrusion parts and cold upsetting parts sometimes use spheroidizing annealing;
- (3) In order to eliminate work hardening, recrystallization annealing can be used;
- (4) In order to eliminate the internal stress caused by various processes, stress relief annealing can be used; In order to improve the inhomogeneity of microstructure and chemical composition, diffusion annealing is often used in some high quality alloy steel castings.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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