During hot forging, the temperature reaches above the recrystallization point of the formed metal. As the step of plastically deforming the metal above the recrystallization temperature, these high temperatures are required in order to avoid strain hardening during deformation. This process typically involves heating the metal (above its recrystallization point) and then comminuting it into a mold that can also be heated as needed. Because the metal is hot, it is easy to “move” and enables manufacturers to make more complex shapes than cold forging.
For superalloys, which have low malleability, processes such as isothermal forging (deformation in a controlled atmosphere) are used to avoid oxidation. Isothermal forging, also known as hot forging, is a thermal processing process that keeps a workpiece at its maximum temperature throughout the forming process.
Maintaining this temperature is done by heating the mold – it will be at an elevated or slightly lower temperature of the workpiece. The force applied by the mold forms the workpiece, and because the mold is also at an elevated temperature, the cooling of the workpiece between the mold working interfaces is eliminated. This in turn leads to an improvement in the flow properties of the metal (work piece).