|Picture 2.4 A bar of hot steel passes between heavy, shaping rolls.|| |
| ||When a metal is hot worked, it is shaped while it is above its re-crystallisation temperature. In these circumstances, annealing takes place while the metal is worked rather than being a separate process. The metal can therefore be worked without it becoming work hardened. Hot working is usually carried out with the metal at a temperature of about 0.6 of its melting point. |
In hot rolling, the metal is forced between two rolls which have a narrow gap between them.
In forging, the metal is pounded by hammers or squeezed between a pair of shaped dies.
| ||Quenching describes the sudden immersion of a heated metal into cold water or oil. It is used to make the metal very hard.
|When is quenching used?|
| ||Quenching is usually used with metals that are alloyed with small amounts of other metals. At high temperature the alloying metals are dissolved in the base metal. If the material is cooled slowly, the alloy elements have time to precipitate out separately. If the metal is quenched, however, the alloying metals are trapped within the crystal grains which makes them harder. The precipitates also reduce the movement of dislocations which contributes to the hardness of the material. |
Quenching is an important process that is used in the production of steel cutting tools. Steel used for this purpose contains nearly 1% carbon. (This is a high carbon steel).
|Picture 2.5 Photomicrographs of martensite, showiung the grain structure. It is very difficult for dislocations to move through this.|| |
|What happens in quenching?|
| ||At a temperature of around 750 °C, iron has a body centred cubic structure. This type of iron is called ferrite. The carbon atoms can easily be held within this less tightly packed structure in what is called a solid solution. |
If the steel is cooled slowly, the iron ions rearrange into a face centred cubic structure called austenite. The iron ions are more tightly packed in this arrangement and can't hold as many carbon atoms within the structure. The remaining carbon forms a compound with iron called iron carbide or cementite. Some regions of the material are therefore made up of layers of ferrite and cementite. These regions are known as pearlite.
If the same steel is quenched rather than being cooled slowly, the carbon atoms do not have time to form cementite. They are trapped within a frozen austenite structure in an arrangement called martensite. Movement of dislocations is very difficult in this structure so the metal becomes very hard and brittle.
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