Heat Treatments
Metal can be treated to further enhance its mechanical properties. You will need to be aware of the following heat treatments and familiar with how they are performed: Annealing, Hardening and Tempering
Annealing
When metal is worked (bent to and fro) it will be come stiff, this is called work hardening. This happens because the tiny molecules which make up the metal have been pushed and twisted out of their original positions thus making the metal very liable to breaking. You can demonstrate this with a paperclip, if you bend it repeatedly it will stiffen just before it snaps.
When we are working metal we may need to bend it to a great degree but we don't want it to snap! The process that allows us to 'relax' the metal again is annealing.
This process relieves the internal stresses and makes the metal easier to shape again. It is important to note that the process doesn't prevent work hardening, it only returns the metal to its a more malleable state; as such work might need to be repeatedly annealed during the forming process.
The annealing process generally involves heating up to a certain temperature and allowing to cool, either in the air or in water depending on the material being annealed. More details of annealing different metals can be found in the document on the right.
Annealing.pdfHardening
As the name suggests, this process involves making steel harder. This is required so that tools or parts won't wear (e.g. screwdriver tip) or so that they are able to cut other materials (e.g. chisel). High Carbon steel (or Tool Steel as it is sometimes known) is most commonly hardened because it contains sufficient carbon to make the process work. It is not possible to harden mild steel as there is not enough carbon in the material for the process to be effective.
Hardening is achieved by heating the metal slowly to a dull cherry colour and then quenching it (cooling rapidly) in oil or tepid water. Once this is done, the material has become hardened but also brittle (i.e. it can break very easily). To make the hardened steel usable it must now be tempered, this will give it properties such as toughness, elasticity, strength.
The process of Hardening and tempering is explained further here
You don't need to know about case hardening, but if you are interested this video shows you how mild steel can be hardened through adding extra carbon to the surface.
Tempering
This process is used after hardening and makes the metal much tougher, this means that as well as being hard it will be more robust in use. For example, a chisel will be struck repeatedly as it is used and it must not break.
The process involves heating the metal to various temperatures and then immediately quenching it in water. As the metal is being heated it changes colour starting with a pale straw to dark straw to reddish brown to purple then dark blue. Different colours will be selected for different applications as different tools will need different properties. E.g. when it reaches a dark blue colour it is at 300o C. These colours are known as TEMPERING COLOURS
