When a metal is cold worked, i.e. when it is cut, beaten, hammered, bent, twisted or shaped, etc. at normal room temperature, tremendous internal forces are set up within its grain structure and the metal becomes extremely hard and liable to split. The term ‘heat treatment’ is applied to metals that undergo some form of heating process in order to change their properties. Generally, any heating process carried out on a solid metal is referred to as heat treatment. Heat treatments involve processes such as annealing,
normalising, forging, hardening, tempering, etc
If a material has been bent,
hammered or twisted consistently over a period of time the metal will be Work Hardened.
What is meant by this, is, 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. This can be fixed by Annealing the metal.
This process makes the metal as soft as possible to relieve the internal stresses, and make it easier to shape. 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. If soap is applied to Aluminium prior to heating it will turn black when the correct temperature has been reached.
This 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. Dependant on what properties are required of the steel being
tempered will determines what heat it will be heated to. E.g. when it reaches
a dark blue colour it is at 300o C. These colours are known as TEMPERING
Mild steel cannot be hardened and tempered as its carbon content is too low. What can be done is to provide it with a hard outer case. In this process the metal is heated to a bright red heat and then rolled in a carbon rich powder. The carbon is absorbed into the skin of the metal thus making it very hard on the outer skin. This type of metal is ideal for components such as gear wheels which require to be hard wearing.
To enable carbon steel (i.e. tool steel) to be used for the wide variety of tools and articles that are necessary in the school workshop and in industry it must first be hardened, then tempered. Taking a high carbon screwdriver blade for example, this is HARDENED by
heating it slowly to a dull cherry colour and then quenching it in oil or tepid water. When this part of the process has been carried out, it is unusable. Although it is very hard it is also very brittle (i.e. it can break very easily). To make the hardened steel usable it must now be TEMPERED, i.e. given properties such as toughness, elasticity, strength.