The chemical composition and internal structure of a material are largely responsible for its mechanical properties. However, these mechanical characteristics can be changed through heat treatment. There are different heat treatment processes available, a common option being annealing.
Understanding the annealing process allows engineers to improve the quality and workability of metal products. This article explores everything you need to know about this process – how it works, its benefits, different annealing types, commonly annealed metals, and more.
What is Annealing？
Annealing is a heat treatment process commonly used to alter the physical and sometimes chemical properties of a material. More specifically, this procedure enhances the ductility of a workpiece while decreasing its hardness. This improves its workability.
The annealing process involves heating a material until it surpasses its recrystallization temperature. The high temperature gives energy to the material atoms allowing them to move within the microstructure. During this migration, dislocations are corrected leading to the restoration of the material’s initial ductility.
This annealing temperature is maintained for a specific duration before the cooling phase begins. The ideal cooling rate will vary depending on the workpiece material. For instance, steel is typically allowed to cool down in still air. On the other hand, copper is either left to cool slowly in the air or the process is expedited using water.
When is Annealing Required and Why is it Important?
Annealing is typically used on metal parts that don’t need to withstand significant stresses. Here are a few scenarios when this heat treatment process is used.
Reverse Work Hardening
One of the primary uses of annealing is to undo hardening and restore a metal to its pre-work state. Manufacturing processes such as bending, grinding, and forming generate internal stresses that increase the hardness of a metal. For example, stamping sheet metal can create a component that is too hard. These worked metals are challenging to handle and susceptible to cracking during further processing procedures. Given that some parts may require subsequent processing, it becomes important to return the metal to a more workable state.
The annealing process increases the ductility of a metal and decreases its hardness creating a more machinable workpiece. Annealed metals can, therefore, withstand further procedures. This is especially beneficial when fabricating complex parts. Since they typically require multiple operations, we utilize the annealing process to make the workpiece more workable after every procedure.
Reduce Weld Solidification
The high temperatures associated with welding can result in a heat-affected zone (HAZ). The HAZ has undesirable properties such as increased hardness, brittleness, and a higher likelihood of corrosion. By annealing metal, we can reestablish some of the original mechanical properties in the weld zone. This reduces the hardness and brittleness induced by welding.
Enhance Electrical Conductivity
The annealing process can enhance the electrical and magnetic properties of a material. By correcting dislocations and helping a material regain its original crystal lattice, this process helps re-establish electrical conductivity in the workpiece.
Remove Residual Stresses
In addition to impacting the workability of a material, mechanical operations can lead to the formation of internal stresses in a material. It’s best practice to relieve these internal stresses whether the workpiece requires further processing or not. If not dealt with, you may experience cracking, distortion, and a host of other issues. Annealed metal is more durable and it offers improved functionality.
How Does an Annealing Furnace Work
The annealing process usually takes place in an annealing furnace, also known as an annealing oven. The specially designed furnace provides a controlled environment that is free from oxidation and other reactions in the environment.
An annealing furnace works by heating a material above its recrystallization temperature but below its melting point. Heating the workpiece facilitates atomic motion and erases dislocations. Cooling and recrystallization will follow after the annealing temperature has been maintained for a suitable duration.
The Stages of Annealing
The annealing process occurs in three distinct phases: recovery stage, recrystallization stage, and gain growth stage.
Let’s take a closer look at each of these phases.
1. Recovery Stage
This is the initial phase of the annealing process. It involves heating the workpiece to a specific, pre-established temperature in the furnace or oven. The high temperatures play an important role in atomic migration and, therefore, decrease the number of dislocations. During the recovery stage, material hardness is reduced and ductility restored. This process also eliminates internal stresses that have built up in the workpiece.
2. Recrystallization Stage
In the recrystallization stage, the workpiece is heated further until we achieve the recrystallization temperature. This promotes the reorganization of the crystal structure and new grain structures start to develop. The resulting grain structure is clear of previous stresses and restores the original material properties before working.
3. Grain Growth Stage
Grain growth is the last stage of the annealing process. As the name suggests, grains formed during the recrystallization stage increase in size as the material cools down. The final size of the grains will depend on the rate of cooling and material grade.
The annealing process can be conducted in a couple of hours or as long as a day, based on several factors. The type of metal being annealed influences how long the recrystallization temperature is maintained and the ideal cooling rates. In turn, these aspects determine the annealing time.
Types of Annealing
There are different types of annealing, each with distinctive features and applications. The ideal annealing process will depend on the material’s initial condition, the workpiece’s intended application, and the altered physical properties. Here are the common annealing processes.
Stress Relief Annealing
The primary objective of stress relief annealing is to clear residual stresses in a metal component. This is achieved by heating the metal to a point where dislocations are eliminated from the crystal lattice. Next, the workpiece is slowly cooled in still air.
Stress relief annealing is done at relatively low temperatures that do not impact the microstructure of the metal substantially. This process is typically applied to blanks and machined parts.
Isothermal Annealing
This type of annealing is conducted to improve machinability. To accomplish this goal, the metal (or alloy) is heated toa temperature over its recrystallization temperature. After a period of heat preservation, the workpiece is moved to another furnace with a lower temperature. The isothermal temperature is maintained for a set duration before the workpiece is finally cooled.
Diffusion Annealing
Also known as homogenization annealing, diffusion annealing re-establishes the homogeneity in the chemical composition of an alloy. This process is characterized by high temperatures and a long heat preservation period. Diffusion annealing is typically used for steel and non-ferrous alloys such as white copper and magnesium alloys.
During the homogenization annealing process, the metal alloy is heated in a furnace to a high temperature over the upper critical temperature. The heat is preserved for a suitable duration, usually a couple of hours. During this period, the individual elements fuse together restoring homogeneity in the alloy mix. Diffusion annealing is often followed up by other annealing processes to develop the desired grains.
Incomplete Annealing
This process is typically used for hyper-eutectic steel and is conducted at the upper basic temperature. The steel workpiece is heated enough, and then the temperature is brought down gradually. Incomplete annealing introduces spherical pearlite within the annealed steel. This enhances machinability and reduces the workpiece hardness.
Complete Annealing
Also known as full annealing, complete annealing produces a microstructure that is softer and more machinable. It involves heating the metal to a temperature above the upper critical point, maintaining the temperature for a period, and then slowly cooling it. Full annealing is an efficient way of refining the grain structure and enhancing ductility.
Spherification Annealing
The key purpose of spherification annealing is to generate spheroid structures in the material, improve its softness, and maximize machinability. This process is applied to high-carbon steels. It involves heating the workpiece to a temperature slightly under the lower critical point and maintaining it for several hours. The cooling rate does not have a significant impact on this process. Spherification annealing creates a microstructure with a spheroidal globular form.
Recrystallization Annealing
Recrystallization annealing is conducted to regenerate the microstructure of metals and replace grains that have been previously deformed due to cold-working. This method involves heating the metal under its critical temperature, holding it there for a long duration, and then cooling it. Recrystallization annealing improves ductility and lowers the hardness of the workpiece.
Which materials can be annealed？
The annealing process is compatible with a wide range of materials. Ferrous metals like iron and alloys such as steel are often annealed to reduce hardness, enhance ductility, and release stress. Different types of steels require different annealing parameters to achieve the desired characteristics.
Non-ferrous metals such as copper, aluminum, and brass may also require annealing to improve softness and machinability. With glass components, the primary focus of annealing is to eliminate internal stresses and lower the risk of breakage.
Advantages of Annealing
Annealing offers a wide range of benefits including;
Enhanced ductility
Annealed metals are more ductile and can be manipulated without breaking. This is beneficial in manufacturing processes that require significant deformation.
Improved machinability
Annealing makes it easier to cut and shape a workpiece by reducing its hardness. This increases production efficiency.
Relief of internal stresses
Annealing relieves stresses that build up in a component after forming or working. This reduces the risk of warping or distortion.
Improves electrical conductivity
By removing dislocations, annealing can enhance the electrical conductivity of a material. This is particularly useful in applications requiring high electrical performance.
Disadvantages of Annealing
Despite its benefits, there are some drawbacks associated with this heat treatment process.
It’s costly and time-consuming. Annealing is an energy-intensive process that involves heating the workpiece to a specific temperature and holding it there for a set duration. The time and cost implications can quickly add up, especially if you’re annealing a large volume of metals.
Annealing is not applicable to all materials. Not all metals and alloys can be annealed. For instance, if you’re using some high-alloy steels you’ll need to use different heat treatment processes.
Can Annealing be Used with Alloys?
Yes, annealing can be conducted on alloys. Non-heat treatable alloys, with the exception of alloys of the 5000 series, can only utilize partial or full annealing. Annealing temperatures for alloys usually lie between 300 and 410°C. The specific temperature will vary from alloy to alloy.
Different alloys will also have varying heating durations, usually between 30 minutes and 3 hours. This parameter is also determined by the size of the workpiece. When annealing alloys, the cooling rate must be at most 20°C per hour until the temperature is dropped to 290°C. Past this point, the cooling rate does not have significant effects.