What is annealing?
Annealing is a metal heat treatment process that improves material properties by slowly heating the metal to a certain temperature, holding it for an appropriate period, and then cooling it at an appropriate rate. Depending on the purpose and characteristics of the material, annealing techniques can be categorized into several types.
The purpose of annealing is to transform pearlite into steel after heating it to the austenitizing temperature.
After the annealing process, the material structure is close to the equilibrium state.
Common types of annealing include:
| Type of annealing | Description | Forms |
|---|---|---|
| Complete Annealing | Mainly used for hypoeutectoid steels such as medium carbon steel and low to medium carbon alloy structural steel, castings and hot rolled sections. The purpose of full annealing is to refine the structure and reduce hardness. | – Medium carbon steel – Forged from low to medium carbon alloy structural steel – Foundries – Hot rolled sections |
| Incomplete Annealing | Suitable for forged and rolled parts of medium to high carbon steel and low alloy steel. The degree of refinement of the structure is lower than that of full annealing. | – Forged and rolled parts made of medium to high carbon steel – Forged and rolled low alloy steel parts |
| Isothermal Annealing | Suitable for situations where it is necessary to maintain a specific temperature over a period of time to obtain the desired effect. | – Specific applications that require maintenance at a certain temperature |
| Spheroidizing Annealing | Mainly used to improve the machinability and extend the service life of steel, suitable for bearing steel, etc. | – Bearing steel – Improving machinability and service life |
| Stress Relief Annealing | Used to eliminate internal stresses after cold deformation, maintaining the effect of cold work hardening. | – Elimination of internal stresses after cold deformation – Retention of the hardening effect by cold working |
| Diffusion Annealing | Used to homogenize the chemical composition of alloy castings and improve their performance. | – Homogenization of the chemical composition of alloy castings – Improving performance |
| Laser annealing, traditional furnace annealing, etc. | These are more modern or application specific annealing methods used to reduce hardness, improve machinability, eliminate residual stresses, stabilize dimensions, reduce warping and cracking tendencies, refine grains, adjust structure, and eliminate structural defects. | – Reducing hardness – Improving machinability – Elimination of residual stresses – Stabilizing dimensions – Reduction of deformations and cracking tendencies – Grain refining – Adjusting the structure – Elimination of structural defects |
| Induction annealing and large current annealing contact electric brush transmission | These are methods that use principles of electromagnetic induction or electrical contact brush transmission of large currents to achieve annealing purposes. | – Specific applications requiring large current induction or annealing |
| Thermal annealing, optical annealing, electron beam annealing, laser annealing, etc. | These are commonly used annealing processes for semiconductor materials, among which thermal annealing is the most commonly used. | – Semiconductor materials |
Types of Annealing Process
1. Complete annealing
Process:
Heat the steel above Ac3 by 20 to 30 degrees Celsius, maintain the temperature for a specified period of time, and then slowly cool it (along with the furnace) to reach a near-equilibrium state in the heat treatment process ( full austenitization).
Full annealing is mainly used for subeutectic steels (0.3 to 0.6% carbon content), such as medium carbon steel, low to medium carbon alloy steel castings, forgings and hot rolled profiles, and, sometimes for their welds.
Low carbon steel has low hardness and is not suitable for machining.
When hypereutectoid steel is heated above Accm to the austenitic state and annealed through slow cooling, Fe3CⅡ precipitates in a mesh pattern along the grain boundaries, significantly reducing the strength, hardness, plasticity and toughness of the steel. steel, which represents a potential risk for the final heat treatment.
Goal:
To obtain fine grain size, uniform structure, eliminate internal stress, reduce hardness and improve machinability of steel.
The structure after complete annealing of hypoeutectic steel is F + P.
To increase efficiency in actual production, parts are removed from the furnace for air cooling when the annealing temperature drops to about 500 degrees Celsius.
2. Isothermal annealing
Complete annealing can take a long time, especially when dealing with highly stable austenitic steel.
If austenitized steel is cooled to a temperature slightly lower than Ar1, resulting in a transformation of austenite to pearlite, followed by cooling to room temperature, it can greatly reduce the annealing time.
This annealing method is called isothermal annealing.
Process:
Heat the steel to a temperature higher than Ac3 (or Ac1). After a certain period of heat treatment, it can be cooled to a specific temperature within the range of pearlite, causing the austenitic structure to transform into pearlite, followed by cooling to room temperature.
Goal:
Similar to complete annealing, with greater ease of control of the transformation process.
Suitable for steels with a more stable austenitic structure: high-carbon steels (carbon content greater than 0.6%), alloy tool steels, high-alloy steels (with more than 10% alloying elements).
Isothermal annealing can also help achieve uniform organization and performance.
However, it is not suitable for large-section steel workpieces or large-batch furnace materials because it is difficult to maintain isothermal temperature throughout the interior or batch of workpieces.
3. Incomplete annealing
The spherification annealing process involves heating the steel to a temperature between Ac1 and Ac3 (for hypoeutectic steel) or between Ac1 and Accm (for hypereutectic steel).
After keeping the steel at the appropriate temperature for a set period of time, it is then slowly cooled to complete the heat treatment process.
This annealing method is mainly used in hypereutectic steels to obtain a spherical pearlite structure in order to reduce internal stress, decrease hardness and improve machinability. It is considered a type of incomplete annealing.
4. Spherification Annealing
Heat treatment process for spheroidization of carbides in steel to obtain granular pearlite.
Process:
The steel is heated to a temperature 20-30°C higher than Ac1, with a retention time of 2 to 4 hours. Cooling is usually done by furnace or isothermal method at a temperature slightly lower than Ar1 for a long period of time.
This process is mainly used for eutectoid and hypereutectoid steels such as carbon tool steel, alloy tool steel and bearing steel.
After rolling or forging, hypereutectoid steel forms lamellar pearlite and cross-linked cementite that are hard and brittle, making them difficult to cut and prone to warping and cracking during the quenching process.
Spheroidizing annealing forms globular pearlite in which the carbides appear as spherical particles dispersed in the ferrite matrix. This structure has low hardness and is easier to machine.
Additionally, austenite grains are less likely to thicken during heating and are less likely to deform and crack during cooling.
It is important to normalize eutectic steel before spheroidizing annealing if it contains cross-linked cementite to ensure the spheroidizing process is successful.
Goal:
The purpose of spheroidizing annealing is to reduce hardness, improve uniformity of structure, and improve machinability in preparation for quenching.
There are three main spheroidization annealing methods:
A) One-step spheroidization annealing process:
The steel is heated to more than 20~30℃ above Ac1 and held for the appropriate time, then slowly cooled in the furnace. This process requires the original fabric to be finely laminated pearlite, without any carburized networks.
B) Isothermal spheroidization annealing process:
The steel is heated and insulated, then cooled to a temperature slightly below Ar1 and held isothermally (generally 10~30℃ below Ar1) before being slowly cooled in the furnace to about 500℃ and then removed for air cooling. This method has the advantages of short duration, uniform spheroidization and easy quality control.
C) Alternative spheroidization annealing process.
5. Diffusion annealing (uniform annealing)
Process:
Ingots, castings, or forged billets are heated to a temperature slightly below the solid phase line for an extended period of time and then cooled slowly to eliminate irregularities in the chemical composition.
Goal:
Eliminate dendritic segregation and regional segregation that occur during the solidification process, resulting in homogenization of composition and structure.
Diffusion annealing is conducted at very high temperatures, typically 100-200°C above Ac3 or Accm, with the exact temperature depending on the severity of segregation and the type of steel. Waiting time is typically 10 to 15 hours.
After diffusion annealing, the material must undergo complete annealing and normalization to refine its structure. This process is applied to high-quality alloy steel and to alloy steel castings and ingots with serious segregation problems.
6. Stress relief annealing
Process:
Heat the steel to a temperature below Ac1 (generally 500 to 650°C), hold it at that temperature, and then cool it in the furnace.
The stress annealing temperature is lower than A1, therefore it does not cause changes in the microstructure of the steel.
Goal:
To eliminate residual internal stresses.
7. Recrystallization Annealing
Recrystallization annealing, also known as intermediate annealing, is a heat treatment process applied to metals that have undergone cold plastic deformation.
The objective of this process is to transform the deformation grain into uniform and equal axial grains, which eliminates process hardening and residual stresses.
For recrystallization to occur, the metal must first undergo a certain amount of cold plastic deformation and then must be heated above a certain temperature known as the lowest recrystallization temperature.
The lowest recrystallization temperature for metallic materials in general is given below.
T recrystallization = 0.4T melt
The recrystallization annealing temperature should be heated to a temperature 100 to 200°C higher than the minimum recrystallization temperature (for steel, the minimum recrystallization temperature is approximately 450°C).
Annealing must be followed by adequate heat preservation and a slow cooling process.
How to choose the annealing method?
The following are the principles for selecting the annealing method:
- For hypoeutectoid steel structures, full annealing is generally selected. If the objective is to reduce annealing time, isothermal annealing can be used.
- Spheroidizing annealing is typically used for hypereutectic steel. If the requirements are not high, you can choose not to use full annealing. Tool steel and bearing steel often use spheroidizing annealing. In some cases, spheroidization annealing is also used for cold extruded or cold cut parts of low or medium carbon steel.
- To eliminate process hardening, recrystallization annealing can be used.
- To eliminate internal stress caused by various processing, stress annealing can be used.
- To improve the inhomogeneity of the structure and chemical composition of high-quality alloy steel, diffusion annealing is often used.
Purpose of Annealing
(1) Decrease the hardness of steel, increase its plasticity and facilitate cold machining and deformation processing;
(2) Evenly distribute the chemical composition and structure of steel, refine the grain size and improve its performance or prepare it for quenching;
(3) Eliminate internal stresses and reverse the hardening effect caused by processing, avoiding deformations and cracks.
Both annealing and normalizing are mainly used as a preparatory step for heat treatment.
For parts with low stress and low performance requirements, annealing and normalizing can also serve as final heat treatment.
Types of Annealing Materials
When discussing annealing, it is essential to explore the materials that can be annealed, both metals and nonmetals. This section will focus on the various materials that are commonly annealed.
Metals and Alloys
Annealing plays a significant role in the processing of various metals and their alloys . Some of the widely used annealed metals include:
- Steel : Annealing is crucial for various types of steel such as carbon steel, low carbon steel, and tool steel. This process can increase the ductility of steel and facilitate shaping and machining.
- Copper : Annealing copper helps increase its ductility and relieve internal stresses. This allows it to be shaped more effectively and reduces the risk of cracking during bending.
- Brass : Similar to copper, annealed brass increases its ductility and workability, which is essential for manufacturing processes like forming and machining.
- Aluminum : This lightweight, versatile metal is annealed to improve its overall formability and create more uniform properties throughout the material.
- Silver : Annealing is a critical step in the silver jewelry making process as it softens the metal and makes it easier to work with.
- Cast iron : Annealing cast iron restores its ductility, making it less brittle and more suitable for applications where it needs to be machined or shaped.
- Ferrous metals : Annealing is beneficial for ferrous metals like steel and iron as it helps improve their machinability and improve their mechanical properties.
A commonly used method for annealing these materials is the use of car bottom furnaces that provide uniform heating and slow cooling, essential to the annealing process.
Non-metals
Annealing is also suitable for various non-metallic materials, such as:
- Glass : Annealing glass involves heating it to a specific temperature and then gradually cooling it. This controlled process relieves internal stresses created during the glass forming process.
- Carbon : Annealing carbon materials such as diamond and graphite helps modify their properties to better suit specific applications. This may include modifications such as improving electrical conductivity or structural adjustment.
In conclusion, annealing is a vital process for a wide range of materials, including metals and non-metals. By understanding the importance of annealing in different materials, we can better appreciate the role it plays in various industries.
Classification of Annealing Methods
According to the temperature used during heating, the commonly used annealing methods are categorized into:
Phase change recrystallization annealing above critical temperature (Ac1 or Ac3):
- Complete annealing
- Diffusion Annealing
- Incomplete annealing
- Spherification Annealing
Annealing below critical temperature (Ac1 or Ac3):
- Recrystallization Annealing
- Stress Annealing
What are the specific differences and application scenarios between complete annealing and incomplete annealing during the annealing process?
Complete annealing and incomplete annealing are two different heat treatment processes, differing in heating temperatures, structural transformations, grain refinement effects and application scenarios.
Firstly, in terms of heating temperature, complete annealing typically heats the material above the critical temperature (Ac1 or Ac3) to promote phase change and recrystallization, while incomplete annealing involves heating in the two-phase region, preventing complete recrystallization. This implies that complete annealing can refine the material grains to a certain extent, but due to temperature restrictions, the grain refinement effect of incomplete annealing is not as good as that of complete annealing.
Secondly, in terms of structural transformation, full annealing can achieve a nearly balanced structure, mainly used for medium-carbon steel, etc., with the aim of refining grains, homogenizing structures, eliminating internal stresses, reducing hardness, and so on. In contrast, incomplete annealing is mainly used for hypoeutectoid steel to obtain a spherical pearlite structure, achieving a near-equilibrium structure through slow cooling.
Regarding the effects of grain refinement, due to the lower heating temperature of incomplete annealing, the shape, size and distribution of ferrite cannot change, and the effect of grain refinement is not as good as that of complete annealing.
Finally, in terms of application scenarios, complete annealing is suitable for situations that require grain refinement, structure homogenization, elimination of internal stresses and hardness reduction, as is the case with medium carbon steel. Incomplete annealing, on the other hand, is mainly used for hypoeutectoid steels, especially when the grains are not coarse, spherical pearlite structures can be obtained through incomplete annealing.
What are the effects and limitations of isothermal annealing on different materials?
Isothermal annealing is a heat treatment process that involves heating the material above its critical temperature and holding it for a certain period, then cooling it or holding it at another temperature. This process aims to refine the microstructure, reduce hardness and improve the material’s properties. The effects and limitations of this process vary between different materials.
For medium-carbon alloy steels and low-alloy steels, the purpose of isothermal annealing is to refine the structure and reduce hardness. The heating temperature for hypoeutectoid steel is Ac3+(30~50)℃, and for hypereutectoid steel it is Ac3+(20~40)℃. This indicates that isothermal annealing is suitable for these types of steel, effectively improving their mechanical properties.
However, isothermal annealing is not suitable for all situations. Sometimes the availability of appropriate annealing equipment or the quality requirements of annealed steel parts make slow, continuous cooling the only viable option. This means that in some cases, isothermal annealing may not meet specific heat treatment requirements.
Furthermore, research with amorphous alloy Cu56 Zr44 indicates that isothermal annealing can be used for the crystallization process, changing the microstructure of the material. This suggests that isothermal annealing is also applicable to certain special materials, such as amorphous alloys. By properly controlling the temperature and retention time, the expected crystallization effect can be achieved.
How does stress-relieved annealing eliminate internal stress after cold deformation and what advantages does it have compared to traditional annealing methods?
Stress relief annealing is a technique that eliminates residual internal stresses in workpieces through a process of heating, isolating, and slowly cooling. This method is mainly used to relieve internal stresses generated during welding, casting and machining processes.
Specifically, the stress relief annealing process involves heating the part to a lower temperature (e.g., gray cast iron at 500-550°C, steel at 500-650°C) and holding it for a set period of time. and then slowly cooling until the development of new residual stresses is avoided. Although this treatment cannot completely eliminate residual stresses in the workpiece, it can significantly reduce their impact.
Compared with traditional annealing methods, stress-relieved annealing has several advantages.
Firstly, it targets residual stresses generated specifically by certain manufacturing processes (such as welding, casting and machining), rather than applying broadly to all types of metallic materials, as traditional annealing does.
Second, stress relieving annealing is typically performed at lower temperatures, which means it has less impact on the material, especially those sensitive to high temperatures.
Furthermore, since the primary purpose of stress relieving annealing is to eliminate residual stress rather than just reducing hardness or improving ductility, it can effectively reduce dimensional changes and cracking tendencies during the manufacturing process. without significantly altering other physical properties of the material.
What are the comparative studies between laser annealing and traditional baking annealing in reducing hardness and improving machinability?
Comparative studies between laser annealing and traditional baking annealing in reducing hardness and improving machinability are mainly reflected in the following aspects:
Heating speed and control accuracy: Laser annealing technology has the characteristics of rapid heating and sensitive control, which allows it to reach the desired annealing temperature in a short time and accurately control temperature changes during the heating process annealing. In contrast, traditional bake annealing requires the entire part to be placed in a vacuum oven and held in a certain temperature range for a certain time, and the temperature control of this process is not as precise as laser annealing.
Heat conduction depth and energy production: Laser annealing technology can achieve localized and controllable depth annealing treatment, which means it can precisely heat treat specific areas as needed without affecting other areas. This localized heat treatment capability is very useful for improving the local performance of materials. Traditional bakery annealing is difficult to achieve this localized heat treatment.
Grain refinement and microstructure adjustment: Laser annealing can cause atoms to reorganize due to high temperatures and thermal stress, making the crystal structure more ordered, which helps to increase the grain size and adjust the microstructure. This is beneficial to improve the machinability of materials and reduce hardness. Although traditional cooking annealing can also refine grains and adjust microstructure through the heating and cooling process, its process is relatively simple and straightforward and may not be able to accurately control grain refinement and microstructure adjustment like the laser annealing.
For example, in the preparation of stoichiometric Bi2Te3 thin films, the laser annealing method has a higher Seebeck coefficient than the traditional thermal annealing method, proving its superiority in the preparation of high-quality thin films. This indicates that laser annealing technology can provide better performance in specific application fields (such as the preparation of high-performance thin films).
