Annealing and tempering are terms used commonly in the steel industry – but what exactly do they mean? These terms refer to two different types of heat treatment intentionally applied to steel for a specific purpose or application.
If you’re looking to purchase steel, or maybe you’re curious about what these different terms refer to, continue reading our in-depth guide into annealing vs tempering. This article will cover heat treatments, the processes involved in annealing and tempering, and the purposes and applications of the two.
What are Heat Treatments?
Heat treatments are different processes that involve heating and cooling steel to alter specific physical or chemical properties. For example, some techniques are designed to make steel tougher and increase the hardness. In contrast, other heat treatments may alter the conductivity or malleability of steel, depending on the material’s intended purpose.
There are a range of heat treatments, including:
- Hardening
- Ageing
- Stress Relieving
- Annealing
- Tempering
- Normalising
Each of these methods uses an alternative approach in order to achieve the intended result. For example, they may use higher or lower temperatures, or the time it takes to heat or cool the steel will vary. These methods will affect various properties, including strength, hardness, ductility, machinability, formability and malleability.
Why do we heat treat steel?
Steel is one of the most widely used materials across the engineering and construction industries. Steel is used for such a vast range of purposes that we need to alter its properties to accomplish different objectives.
Heat treating steel can make it more durable, make it easier to weld, more flexible, or increase wear resistance. These changes can drastically alter the usage of the material.
For example, if you were looking to make a knife, you would want a hard steel and won’t lose its shape, something that holds a sharp edge, and can resist wear and tear.
Whereas, if you were looking to create an art installation with complex features, you would want a steel that’s highly malleable and ductile, and you wouldn’t need to worry about the wear resistance or hardness of your material.
So, how are these changes made to steel?
The Annealing Process
In order to anneal steel, the material must be heated to above its recrystallisation temperature. This is the temperature at which the crystal lattice structure of the metal begins to change, making it more workable. For steel, the temperature needed to reach this stage is high; depending on the type of steel being annealed, temperatures will range from 400°C to 700°C!
Once heated for a specific amount of time, the steel then needs to be cooled. The rate at which this cooling occurs will depend on the type of steel used, but typically steel is left to cool to room temperature at a steady rate – compared to some non-ferrous metals that need to be cooled instantly using quenching.
The annealing of steel occurs in three stages: the recovery stage, the recrystallisation stage and the grain growth stage.
- Recovery – The recovery stage is essentially the heating phase of the process. The steel is placed into an annealing furnace to heat the material to high temperatures of up to 700°C.
- Recrystallisation – The stage of the annealing process involves keeping the steel above its recrystallisation temperature but still below its melting point for a designated period. The chemical structure begins to transform at this stage, and new grains begin to form without any residual stress.
- Grain Growth – During this final stage of the annealing process, the steel is cooled at a specific rate; this rate is determined by the intended properties needed in the final material. It is during this phase where new grains begin to develop, creating new and altered properties.
The Tempering Process
Tempering steel is relatively straightforward in theory; however, the practice itself is complex and incredibly precise. All variables in the process are carefully measured to guarantee the intended result, including the exact temperature the steel is heated to and the amount of time it’s heated or cooled.
The tempering of steel usually occurs after a hardening method – typically oil hardening or air hardening. The process involves heating the steel to a temperature just below its lower critical temperature and keeping the material at this temperature for a specified time, depending on the desired mechanical properties and the type of steel used.
When this time runs out, the furnace or alternative heating method is turned off, and the steel is allowed to cool at a predetermined rate. Again, the cooling rate will also affect the resulting properties of the steel and will be carefully measured to ensure the desired outcome.
The Differences Between Annealed & Tempered Steel
The processes for both annealing and tempering are somewhat similar; both involve the heating and cooling of steel, even though the timings and temperatures differ. The reasons behind utilising these processes on steel are comparable; however, the intended applications and uses of the final products vary.
Purpose of Annealed Steel
The primary purpose of annealing steel is to achieve a good balance between hardness and ductility. When steel is hardened, it can lose its ductility; the harder the steel, the more brittle it will be.
Annealing helps the crystallisation structure of steel realign in a less brittle way while still keeping its strength. This process makes the steel much more durable, as it can be bent and absorb impact without cracking or breaking.
As well as this, annealing can also help to reduce the internal stresses caused by most hardening processes. For example, steel can often contain internal stresses in the crystal lattice structure when heated and formed, making the material more likely to break. Annealing reorders the grains in the crystal structure, allowing them to realign in a way that removes the pre-existing stresses naturally.
Purpose of Tempered Steel
The purpose of tempering steel is to achieve a similar effect to that of annealing steel; as previously mentioned, hardened steel is often more brittle than is useable.
Hardening processes are needed to make the material strong enough for particular uses, but it can cause stress in the structure that results in brittleness. Therefore, tempering reduces tensions in the steel, allowing for the optimum balance between hardness and strength.
Tempered steel is less brittle, with increased ductility and machinability. It is also much easier to weld without the internal stresses in the lattice structure.
Application of Annealed Steel
Steel is annealed in order to achieve a stronger and more ductile final product. These qualities make annealed steels a prominent material used across the manufacturing industries.
As well as this, annealed steels can be used in tool-making and machines as they are ductile enough to be moulded into shape but strong enough to resist wear and tear that comes with use.
Cold-rolled sheet steel and ground flat stock are often annealed as the process of cold rolling can make the steel too hard and therefore brittle. By annealing the steel, we restore the material’s ductility, allowing for bending, cutting, and punching without the risk of cracking the steel.
Application of Tempered Steel
The overall purpose of tempering steel is to make it more wear-resistant, increase ductility, and make it less brittle. Tempering steel also improves the material’s weldability, which is helpful in many industries.
Common uses for tempered steel include bridge construction, gears and mechanical parts, deflector plates, and storage tanks.
Tempering is also an important process in the knife-making industry. The high ductility and wear resistance make tempered steel ideal for knives and swords. In addition, traditional Japanese katanas were created using tempering!
At Sheffield Gauge Plate, we have been manufacturing the highest quality ground flat stock for over 40 years. If you’re looking for steel, you’re in the right place! View our helpful guide on how to choose the correct tool steel or speak to our friendly team via telephone on (+44)114 233 5291, or email at sales@sgpltd.co.uk for all queries answered.