Metals mechanical properties often need manipulating/altering to get them to behave how we want. Changing mechanical properties can be achieved chemically or through heat treatments. There is a wide range of heat treatment processes available however, today, we will cover one of the most popular heat treatment processes, annealing. By the end of the article, you will be able to answer the question: What is the annealing heat treatment process?
What is The Annealing Heat Treatment?
Annealing is a heat treatment process that has the primary goal of increasing the ductility of metals while reducing their hardness. Increasing the metal’s ductility stems from reducing the internal energy within the metals crystal structure. The chemical properties of the metal can also alter as a result. The annealing process generally occurs after the metal/part has been machined or formed because these processes toughen the metal.
How Does Annealing Work?
Annealing occurs when the metal is heated above its recrystallisation temperature, held at an appropriate temperature for a prescribed set of time and then allowed to cool. The primary method utilises an oven because it has accurate control over the temperature be throughout the process. The basis of annealing is diffusion. Atoms diffuse into the metals crystal lattice, affecting the distribution of dislocations and reducing dislocation density. The redistribution and eradication of dislocations make it easier for the metal to deform, making it more ductile.
What Metals Can Be Annealed?
A material that can have its mechanical properties altered via heat treatment can have the annealing process performed on them. The most popular metals used are steel and cast iron. In addition to these, specific types of aluminium, copper and brass can have annealed performed on them.
3 Stages of The Heat Treatment Annealing Process
Recovery Stage
The first of the three stages of annealing is called the recovery stage. In the recovery stage, dislocations are removed from the crystal structure and hence their corresponding stresses are removed, resulting in the structures stored energy reducing. The two predominant methods of eradicating dislocations are annihilation and rearrangement.
Annihilation is when dislocations of opposite signs meet, they effectively cancel each other out. As a result of the metal being heated, the dislocations are more mobile resulting in a greater likelihood of them interacting with each other.
Rearrangement involves the dislocations arranging themselves in such a manner that their stress fields interact with one another therefore minimising their contribution to the stored energy. During this process the size and shape of the grains are not altered unlike the other stages.
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Recrystallisation Stage
The second stage is called the recrystallisation. During this stage the metal is heated above its recrystallisation temperature; for steels, this is typically between 400 and 700 °C. During this stage, all grains that were deformed are replaced with new defect-free grains. After the formation of the new grains, they grow until all deformed grains are no longer present in the structure. During this stage, a reduction in the metals strength and hardness occurs, although its ductility increases.
Grain Growth Stage
Following the recrystallisation process is the final stage, grain growth. As the name suggests, this stage consists of grain growth building from the recrystallisation stage, allowing the grains to grow fully. Fully grown grains further reduce the internal energy by reducing the grain boundary total area. The growth of the grains is controlled by the rate at which the metal is cooled.
Where Is The Heat Treatment Annealing Used?
The most frequent use of annealing is after a metal has undergone work hardening or cold working. During this process, a metals hardness increases as new dislocations form (from the plastic deformation), which interact with each other resulting in the dislocations locked in position. At this point, the metal is hardened and brittle. If any more processes are required on the metal, it may result in crack initiation. Annealing a metal at this stage would increase its workability and allow a continuation of forming processes.
Summary
Annealing increases the ductility of metal while decreasing its strength and hardness, making the metal more workable. Larger workability will allow further forming processes to be carried out on a metal that has already been work hardened or cold worked.
The 3 stages of annealing are recovery, recrystallisation and grain growth. As we travel through the stages, the metals internal energy is further reduced, which is essential for increasing a metal’s ductility.
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