Tempering and annealing are both heat treatment processes that alter the physical and chemical properties of metals to prepare them for manufacturing. The difference between the two processes relate to the temperatures and cooling rates, with tempering happening at lower temperatures but with faster cooling times.
Both heat treatments are used for treating steel, although annealing creates a softer steel that is easier to work while tempering produces a less brittle version that is widely used in building and industrial applications.
In order to understand the differences between the processes it is important to first understand the benefits of heating steel as a method of treating metal.
What are Heat Treatments?
Heat treatments are used to change the physical and mechanical properties of metals without altering their shape. Heating the metal increases the desirable characteristics, allowing further processing to take place.
The common reasons for heat treatments include:
- Increased ductility
- Improved elasticity
- Improved formability
- Increased hardness
- Improved machining
- Increased strength
- Improved toughness
There are three factors that determine the effects on heat treated metals, these are:
- The specific temperature the metal is heated to
- The length of time the metal is kept at this temperature
- The cooling process used
Effective heat treatments require all three of these factors to be controlled regardless of the type of metal being treated and the desired results.
What is the Tempering Process?
Tempering is a process whereby a metal is precisely heated to below the critical temperature, often in air, a vacuum, or inert atmospheres. The exact temperature varies according to the amount of hardness that needs to be reduced. High temperatures will reduce hardness and increase elasticity and plasticity but can cause a reduction in yield and tensile strength. Lower temperatures will maintain much of the hardness but will reduce brittleness.
Tempering requires the metal to be heated gradually to prevent cracking. Once the desired temperature has been reached the temperature is held for a fixed period. A rough guideline for this proposes one hour per inch of thickness, although this differs depending on the type of metal being treated. The heat relieves the internal stresses in the metal, after which the metal undergoes rapid cooling in air.
Visual Assessment of Tempering
It is possible to gain a visual indication of the effects of tempering on steel by assessing the colours that appear on the surface of tempered steel. The colours range from a light yellow to varying shades of blue, depending on factors such as carbon contact. These allow the final properties of the steel to be assessed.
Tempering Applications
As mentioned above, tempering is used to increase the toughness of iron alloys, including steel. Tempering is commonly performed after hardening to reduce excess hardness, since untampered steel is very hard yet too brittle for most industrial applications.
Tempering can change ductility, hardness, strength, structural stability and toughness.
What is the Annealing Process?
Annealing involves heating a metal to a specified temperature before cooling the material at a slow and controlled rate. The metal is placed inside an oven that is large enough to allow air to circulate around the workpiece.
The metal is heated to a temperature where recrystallization can occur. This causes any defects caused by deformation or working to be repaired. Once the metal has been held at the required heat for a fixed period it is cooled down very slowly to room temperature. The slow cooling rates maximise softness and produce a refined microstructure. This can be done by simply turning off the oven and leaving the metal inside to cool down naturally or by immersing the heated material on sand, ashes, or another substance with low heat conductivity.
Annealing can be broken down into three stages; recovery, recrystallization and grain growth, as follows:
Recovery
The recovery stage is when the metal is heated so that the internal structures of the material relax.
Recrystallization
As the heat increases the metal will reach a temperature where recrystallization occurs, allowing new grains to develop in the metal’s internal structure without forming stresses. The temperature for this needs to be greater than the recrystallization temperature of the metal but lower than the melting temperature.
Grain Growth
A controlled cooling rate aids the development of the grains that formed during recrystallization, this yields a more ductile, less hard material.
Annealing Applications
Annealing is primarily used to reduce the hardness/ increase the softness of a metal, however it can also be used to increase electrical conductivity. The process allows a metal to be softened enough to allow cold working, improves machinability and restores ductility.
This is important for several applications as, without annealing, cold working can cause cracking. The annealing process releases the mechanical stresses that are produced by machining or grinding, allowing the metal to be worked further.
The process is commonly used for steel but can also be used for metals including aluminium, brass and copper.
The Differences Between Tempered and Annealed Steel
While both processes are heat treatments, they follow different rules to produce different results for different purposes.
Tempered steel is used in applications where strength, toughness and elasticity are paramount. This includes large-scale construction work, industrial machinery and automotive drive trains. Tempering makes these applications possible and reduce any associated danger.
Annealing produces softer metals that can be used for products that do not need to endure substantial stresses. This includes many household items and other everyday products.
Heat Treatments and TWI
TWI’s materials experts can offer advice on heat treatments including annealing and tempering as well as other processes such as case hardening and normalising.
Our in-house capabilities have been expanded through the establishment of a new heat treatment facility at our headquarters near Cambridge, which has been created and operates accordance with our ISO 9001 certification and in line with the requirements of AMS 2750 Rev E.