During the first four processes metal issubjected to large amounts of strain (deformation). But if deformation goes at a high temperature, the metal will recrystallize — that is, new strain-free grains will grow instead of deformed grains. For this reason metals are usually rolled, extruded, drawn, or forged above their recrystallization temperature. This is called hot working. Under these conditions there is no limit to the compressive plastic strain to which the metal can be subjected.
Other processes areperformed below the recrystallization temperature. These are called cold working. Cold workinghardens metal and makes the part stronger. However, there is a limit to the strain before a cold part cracks.
Rolling
Rolling is the most common metalworking process. More than 90 percent of the aluminum, steel and copper produced is rolledat least once in the course of production. The mostcommon rolled product issheet. Rolling can be done either hot or cold. If the rolling is finished cold, the surface will be smoother and the product stronger.