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Although hot forging has been around for centuries, the term warm forging originated sometime around 1950. Although they are different types of forging, they both involve shaping metal by compressive force. Forging parts at elevated temperatures reduces the tonnage necessary to deform the material. The necessity to decrease tonnage may be because of size, material hardness, or complexity of shape. Parts made by warm forging have better tolerances and surface finish than hot forgings, but not as good as cold forgings. Both processes offer a part with increased strength and durability but each method has distinct differences.
Working metal above its re-crystallization temperature (1900-2300F) is called hot forging. Work hardening, also called strain hardening, in metal is caused by dislocation of the metallic crystal structure. Because hot forging takes place above the temperature at which the crystallization occurs, the grains deform to follow the shape of the parts but work hardening does not occur. Therefore, a greater amount of deformation is possible.Warm forging occurs between room temperature and the re-crystallization temperature of the material (800-1800F). Although this allows work hardening to occur, the degree at which it happens is decreased. In addition, the materials yield strength is reduced and ductility increased, allowing for a greater amount of deformation to take place. Working the metal at a lower temperature than hot forging is beneficial to tooling life as well as part tolerances and surface finishes.