Hot Working

Hot working refers to the plastic deformation of metals at temperatures above the recrystallisation temperature. Materials become more malleable when exposed to this process. Therefore, they can be remodelled with very little force. Forging, in particular, refers to the hot working of iron and steel. Hot working takes place in different steps that include resting processes, which are to reduce the hardness of the material.

The temperature, in which hot working is performed, is determined by the respective metal. An approximate temperature is between 40 and 60% of the melting point.

Results of Hot Working

Hot working serves multiple purposes. The most obvious one is the deformation of materials with relatively little effort. Hardening, however, is not one of those purposes as hot working does not increase yield strength nor hardness. Due to the constant recrystallisation, the crystal structure can not solidify. Furthermore, yield stress can increase when the velocity of deformation is larger than the velocity of the recrystallisation process.

Areas of application

One of the most widely spread procedures for hot working is still forging. Other notable treatments are hot rolling and extrusion molding. Particularly materials that are subject to a lot of strain and stress undergo hot working. Examples of these types of components are cylinders and gears. While the ultimate tensile strength decreases, ductility increases and the components can withstand more.

Hot working is performed on ferritic metals, such as steel. Tool steels (such as manganese-bor alloys) are hot worked because they are relatively hard and brittle in their untreated form. By hot working them, their formability increases, while their hardness decreases.

Distinction from Cold Working

  • Temperature: The most obvious difference between warm and cold working is, of course, the temperature. While cold working operates at temperatures far below recrystallisation temperature, how working operates at temperatures slightly above.
  • Shape change: Due to the high temperature, materials can be made more malleable when it comes to hot working. Cold working, on the other hand, only allows small shape changes. However, the primary intent of cold working is the change of material property, not shape.
  • Size tolerance: Size tolerances are smaller for materials that are cold worked, which is why they can be processed more precisely.
  • Hardening: The main objective of cold working is the hardening of materials. In contrast, hardness is removed when it comes to hot working.