Within the name of forging, a lot of different processes are referred to, so that a classification must be endeavored. As for other metal-forming processes, the deformation temperature can be the first criterion, and we shall therefore distinguish between the usual three cases.

In hot forging the part is heated above half the fusion absolute temperature to reduce the forging force, take advantage of the higher material ductility, and allow large deformation. However, the tools are submitted to high thermal stresses as the temperature difference between the part and the tool is high (even when the tools are preheated). For example, steel is forged at about 1100–1200 °C, while the tools have a temperature ranging from room temperature to 250 °C. For simplicity we shall introduce here free forging, in which the shape of the tools is rather simple, and which is generally used for a small series of large or very large parts; open-die forging, in which the preform is squeezed between two dies to give a prescribed shape; and closed-die forging (see Fig. 10.1).

Cold forging is performed at room temperature, but the dissipated plastic work may raise the temperature of the part up to about 250 °C. Cold forging is used to achieve better tolerance, higher mechanical properties, and a better surface aspect that can avoid further machining. But as cold metals are less ductile in general than hot ones, cold forging allows moderate deformation, unless a heat treatment is introduced to restore a deformation capability.