While the chemical composition of a metal determines a large part of the mechanical properties, the mechanical properties of many metals can be changed by heat treatment. There are many different types of heat treatment available today, and one of the most popular methods is annealing.
What Is Annealing?
Annealing is a heat treatment process used mostly to increase ductility and reduce the hardness of a material. This change in hardness and ductility is a result of the reduction of dislocations in the crystal structure of the material being annealed.
Annealing is often done after the material has undergone a hardening or cold working process to prevent brittleness from failing or to make it more malleable for subsequent operations.
Why Is Metal Annealed?
As mentioned above, annealing is used to decrease hardness and increase ductility. Changing these mechanical properties through annealing is important for many reasons:
- Annealing improves the formability of a material. Hard, brittle materials can be difficult to bend or press without breaking the material. Annealing eliminates this risk.
- Annealing can also improve machinability. A material that is extremely brittle can cause excessive tool wear. By reducing the hardness of a material by annealing, the wear and tear of the tool used can be reduced.
- The annealing removes residual stresses. Residual stresses can cause cracks and other mechanical complications, and it is often best to eliminate them whenever possible.
For more information, see the Mechanical properties of a material to learn more.
What Metals Can Be Annealed?
In order to carry out an annealing process, a material must be used that can be modified by heat treatment. Examples are many types of steel and cast iron. Some types of aluminum, copper, brass, and other materials can also respond to an annealing process.
The Annealing Process
Annealing is a heat treatment process that changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable.
An annealing furnace works by heating a material above the recrystallization temperature and then cooling the material once it has been held at the desired temperature for a suitable length of time. The material recrystallizes when it cools, as soon as the heating process has led to the atomic movement being redistributed and dislocations in the workpiece is removed.
There are three main stages to an annealing process:
- Recovery stage.
- Recrystallization stage
- Grain growth stage
Recovery Stage
During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature at which its internal stresses are relieved.
Recrystallization Stage
During the recrystallization stage, the material is heated above its recrystallization temperature but below its melting temperature. This causes new grains to form without any pre-existing tension.
Grain Growth Stage
As the grain grows, the new grains develop fully. This growth is controlled by letting the material cool down at a certain rate. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent processes, which can further change the mechanical properties, are sometimes carried out after the annealing process.
When is Annealing Required and Why is it Important?
Annealing is used to reverse the effects of work hardening that can occur in processes such as bending, cold forming, or drawing. If the material gets too hard, it can make it impossible to work or cause cracks.
Heating the material above the recrystallization temperature makes it more ductile and therefore workable again. The annealing also removes stresses that can occur when the welds solidify. Hot rolled steel is also shaped and shaped by heating it above the recrystallization temperature. While annealing steel and alloy steel is common, other metals such as aluminum, brass, and copper can also benefit from the process.
Metal processors use annealing to make complex parts and keep the material workable by bringing them back near their pre-machined state. The process is important to maintain ductility and reduce hardness after cold working. In addition, some metals are annealed to increase their electrical conductivity.
Can Annealing be Used with Alloys?
Annealing can be performed on alloys, with partial or full annealing being the only method used for non-heat-treatable alloys. The alloys of the 5000 series, which can be stabilized at low temperatures, are an exception.
Alloys are annealed at temperatures between 300 and 410 ° C, depending on the alloy, the heating times being between 0.5 and 3 hours depending on the size of the workpiece and the type of alloy. Alloys must be cooled at a maximum rate of 20 ° C per hour until the temperature is reduced to 290 ° C. After that, the cooling rate is no longer important.
Advantages of Annealing
The main benefits of annealing are how the process improves the workability of a material, increases toughness, decreases hardness, and increases the ductility and machinability of a metal.
The heating and cooling process also reduces the brittleness of metals while improving their magnetic properties and electrical conductivity.
Disadvantages of Annealing
The main disadvantage of annealing is that it can be a time-consuming process, depending on which materials are being annealed. Materials with high-temperature requirements can take a long time to cool sufficiently, especially if they naturally cool in an annealing furnace.
Applications of Annealing
Annealing is used in a wide variety of industries where metals have to be processed into complex structures or processed several times.
One of the main applications of annealing is reversing the effects of work hardening. In a similar manner, annealing is utilized to remove the internal stresses which occur when welds solidify. Besides steels, other metals may also benefit from annealings such as copper, aluminum, and brass.
