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.Quenching can be performed with forced air convection, oil, fresh water, salt water and special purpose polymers.