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What is Annealing?

Published by Rapid Metals on

There are many different types of metal processes, and annealing is one of them. In this article, we will go through what annealing is, how it’s done, why it’s used and how it compares to other processes such as normalising. So, if annealing is something you’re interested in learning more about, keep reading.

What Does Annealing Mean?

Put simply, annealing is the name given to the process that alters the physical properties of metal to make it less hard and more malleable.

By heating the metal beyond its recrystallisation temperature and keeping it there for a certain period of time, the atoms within the crystal lattice of the metal move. When the metal is cooled down it crystallises once more; the migration of the atoms when it was heated resulting in a difference in the ductility and level of hardness of the metal. 

Not all metals respond to annealing, but those that do include steel, brassaluminium and copper – although the cooling down method changes depending on the metal you’re annealing.

Why are Metals Annealed?

There are many reasons you might want to change the physical and chemical properties of a metal, namely to make the metal more malleable and less hard. 

By annealing metal you are able to increase its machinability, making it easier to cut, drill and grind the metal without the risk of it breaking. This is particularly useful for tools since brittle metals or those that are too hard and have no give break and wear easily.

If a metal has been previously worked by bending, drawing or cold forming, it can become additionally hard which impacts its ability to be worked further without it cracking. Annealing counteracts the impact of work hardening, allowing you to work the metal again. It does this by returning the metal to its pre-worked state. 

In addition to this, annealing metal increases its electrical conductivity.

What are the Different Types of Annealing?

There are seven main types of annealing. The most common type is recrystallisation annealing, but other types include: 

  • Complete annealing 
  • Incomplete annealing 
  • Stress relief annealing 
  • Uniform annealing 
  • Spherification annealing 
  • Isothermal annealing 

Despite there being seven different forms, each annealing process shares the same end goal: to make metals more ductile.

What are Annealed Metals Used For?

There is a high demand for annealed metals, particularly within the industrial sector. Annealed steel is commonly used in sheet metal, whereas annealed copper can be found in overhead wire. Welding can change how metals look and so in order to make items look more uniform, annealing is often employed.

What is the Annealing Process?

Although there are several different types of annealing, the end goal is always the same, however; there are different methods used depending on the type of annealing that is used. The most common way of annealing metals is done by heating the metal above its recrystallisation temperature. There are three steps in this annealing process, the first of which is the recovery stage. 

The recovery stage is when heat from a furnace or another source is applied to the metal to increase its temperature. During this stage, the temperature should be raised enough to remove the internal stress of the metal and cause the atoms within the crystal lattices to migrate. 

The second stage is the recrystallisation stage. During this phase, the metal is heated to just below its melting point. In doing so, new grains (crystalline areas) are formed as a result of the previous atoms migrating, except the new grains have no stresses. 

The final stage of the annealing process is the grain growth stage which is when the grains that were formed in the second stage complete their development. This is achieved by controlling the speed at which the metal is left to cool down.

What are the Drawbacks of Annealing?

The main issue with annealing is that it’s a lengthy process. This is because more often than not, metals are left to cool naturally – usually, inside the furnace, it has been annealed in. Due to the high temperatures required for annealing, this can take a long period of time.

What is the Difference Between Tempering and Annealing?

Tempering and annealing are opposite processes. Whilst annealing reduces the hardness of the metal to make it softer, tempering metal reduces the hardness to increase the strength. 

In both instances, the metal is heated extensively to change the physical properties. When annealing is used it’s because the metal is too hard and doesn’t have enough give in it for the desired application. When it is annealed, it becomes softer and easier to manoeuvre.

On the other hand, when metal is tempered, it is because the metal is too hard. When a metal is too hard, it becomes brittle and is likely to crack or break, meaning it is not strong enough for the desired application. By tempering the metal, the hardness is reduced and the strength is increased, making it more durable and suitable for heavy-duty uses such as construction. 

Annealing:

  • Reduces hardness 
  • Increases malleability 
  • Increases ductility 
  • Increases magnetism 
  • Enhances electrical conductivity 

Tempering:

  • Reduces hardness 
  • Increases strength 
  • Increases abrasion resistance 
  • Makes metal easier to weld 

Annealing involves heating the metal to the recrystallisation point, whereas when metal is tempered, it is heated to just below the critical point.

Got a Question About Annealing?

If you have a question about the annealing process, the tempering process or any other metal-related treatment, contact our team to find out more.

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