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What is the Difference Between Tempering and Annealing?

Published by Lucy Thurston on

Tempering and annealing are both commonly used methods of heating metal in order to change the physical properties of the material. There are a number of ways heat treatment can distort the chemical composition and physical properties of metal to achieve desired properties such as increased ductility, machinability, and strength. 

Depending on the properties you’re looking for, you may choose to either anneal or temper the metal. Tempering and annealing, although both heat treatments, are very different processes that achieve different results. If you’re unsure what the difference between tempering and annealing is, keep reading as we explain everything you need to know. 

Annealing

Annealing is used to make metals less hard and more malleable (soft). It can also improve electrical conductivity and make metal softer and more suitable for cold working. 

How does annealing work?

There are several types of annealing, but the general process remains the same: heating the metal up to a point beyond the recrystallisation temperature, allowing the atoms inside the crystal lattice structure to move. It’s kept at this temperature for a set period of time before being slowly cooled down to room temperature. 

There are two ways a metal can be cooled down during the annealing process; either by allowing the metal to cool down naturally inside the furnace once it’s been switched off, or by submerging the metal into a substance with a low heat conductivity, such as ash or sand. 

What metals can be annealed?

There are some metals that simply don’t respond to annealing, but some of the most commonly used metals do. Steel, copper, brass, and aluminium can all be annealed, but it’s worth noting that the method used to cool the metal down will differ in line with the metal type. 

What are the benefits of annealing?

The main reason annealing is used is to make metals less hard. When they’re too hard they become brittle, meaning they can’t withstand cold working without breaking. This is not helpful when you want to be able to drill or cut the metal, and that’s why annealing is used. Through this process, metals become more machinable and are able to be worked to a greater degree without the risk of breaking or wearing down quickly. 

Another benefit of annealing is that it allows you to repurpose metals that have already been worked, closing the production loop and enabling you to get the most out of each material. When metals have been drawn, bent, or cold formed, they become harder. This means they’re more likely to break due to their brittleness. Annealing effectively reverses the effects of working, giving you metal as it was before it was worked. 

What are the disadvantages of annealing? 

Despite being an incredibly useful and widely used process, annealing has one major disadvantage in that it takes a very long time. For metals to reach the recrystallisation temperature, they need to be heated to extremely high temperatures in professional furnaces. Annealing requires the metal to be cooled down slowly so that it can become softer, and as mentioned, this is typically done within the furnace. It takes a long time for a furnace to cool down after reaching such high temperatures, making it a lengthy process. 

Tempering 

Tempering is another widely used heat treatment for metal, but it has the opposite goal to annealing. When metals are tempered, they become stronger and less brittle, whereas when metals are annealed, they become softer and less brittle. Tempering is ideal when you need to improve the toughness of a metal and reduce the hardness. 

How does tempering work?

When metals are tempered, they are heated to the lower critical temperature in a furnace where there is a vacuum or inert gas to reduce the risk of oxidisation. The temperature will vary depending on the metal and its composition, but when it has been reached, it is held there for a set period of time. 

Typically speaking, the lower the temperature, the less brittle yet more hard the metal will become. This increases the strength but means the metal is less likely to break under pressure, making tempered steel ideal for high-stress applications. 

The metal is then cooled very slowly. If it’s cooled too quickly, it may be at risk of cracking.

What metals can be tempered?

Tempering is most often used on steel and other iron alloys that can be too hard and brittle. 

What are the benefits of tempering?

There are several benefits to tempering, with the most obvious one being that metals have an increased strength without being too hard. If they’re too hard they become brittle and can break, making them unusable for rigorous applications. Tempering can prevent this, effectively giving you the best of both worlds through improved machinability without compromising on tensile strength. 

What are the disadvantages of tempering?

The main drawback of tempering metal is that it doesn’t always provide uniform results, and tempering can also change the colour of the metal, so depending on the application, this may be problematic. 

Summary 

Tempering and annealing both have their unique uses, but they’re very different treatments. If you want to make a metal softer, anneal it. On the other hand, if you want to make it stronger, temper it. For more information on metal processes and our range of metals to buy online, please contact us

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