While the chemical composition of a metal determines much of the mechanical properties, many metals can have their mechanical properties altered through heat treatment. There are many different types of heat treatment used today, and one of the most popular methods is annealing.
What Is Annealing?
Annealing is a heat treatment process used mostly to increase the 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 performed after a material has undergone a hardening or cold working process to prevent it from brittle failure or to make it more formable for subsequent operations.
Why Is Metal Annealed?
As mentioned above, annealing is used to reduce hardness and increase ductility. Changing these mechanical properties through annealing is significant for many reasons:
- Annealing improves the formability of a material. Hard, brittle materials can be difficult to bend or press without creating a material fracture. Annealing helps eliminate this risk.
- Annealing can also improve machinability. A material that is extremely brittle can cause excessive tool wear. Reducing the hardness of a material via annealing can reduce the wear on the tool being used.
- Annealing removes residual stresses. Residual stresses can create cracks and other mechanical complications, and it is often best to eliminate them whenever possible.
What Metals Can Be Annealed?
To perform an annealing process, a material that can be altered by heat treatment must be used. Examples include many types of steel and cast iron. Some types of aluminium, copper, brass and other materials may also respond to an annealing process.
The Annealing Process
There are three main stages to an annealing process.
- Recovery stage.
- Recrystallization stage
- Grain growth stage
During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved.
During the recrystallization stage, the material is heated above its recrystallization temperature, but below its melting temperature. This causes new grains without preexisting stresses to form.
Grain Growth Stage
During the grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool at a specified rate. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent operations that can further alter mechanical properties are sometimes carried out after the annealing process.
When Are Annealed Metals Used?
Common applications for annealed metals include:
- Work-hardened materials such as sheet metal that has undergone a stamping process or cold drawn bar stock.
- Metal wire that has been drawn from one size to a smaller size may also undergo an annealing process.
- Machining operations that create high amounts of heat or material displacement may also warrant an annealing process afterward.
- Welded components can create residual stresses in the area of the material exposed to elevated temperatures; to recreate uniform physical properties, annealing is often used.
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