Cold, Warm, and Hot Forming

Metal forming is the process of shaping metal parts by applying temperature and pressure. The choice of forming method depends on the desired material properties, such as elasticity, strength, and hardness. Different forming methods can affect these properties in different ways. Forming can also involve multiple steps to achieve the required tolerance and desired properties. In addition, secondary machining operations are often used to refine the shape and surface of the formed parts.

Bulk metal forming is a type of forming that involves large deformations of metal. It can be classified into three categories: cold work, warm work, and hot work.

Cold work is performed at room temperature or below the recrystallization temperature of the metal, typically less than 30% of melting temperature.  It has several advantages, such as producing a good surface finish, achieving tight dimensional tolerance, and inducing strain hardening in the material. Strain hardening increases the strength and hardness of the metal, but also reduces its ductility and formability. Cold work also requires high forming force, which can increase the wear and tear of the tools and equipment. Moreover, cold work has limitations on the cross-section size of the metal, as larger sections are more difficult to deform at low temperatures. Cold work is suitable for producing parts with high accuracy and surface finish, but it requires high forces and may cause cracking or fracture.

Warm work is performed above the recrystallization temperature but below the melting point of the metal, typically between 30%-60% of melting temperature. It has some benefits, such as providing a medium surface finish, maintaining medium dimensional tolerance, and causing some strain hardening in the material. Strain hardening in warm work is less than in cold work, as some of the dislocations in the metal are annealed during the process. Warm work also requires medium forming force, which can reduce the energy consumption and tool wear compared to cold work. However, warm work still has some drawbacks, such as possible oxidation and scaling of the metal surface, and potential warping and distortion of the final product. Warm work is suitable for producing parts with moderate accuracy and surface finish, but it requires lower forces and less energy than cold work. Warm work also reduces the residual stresses and improves the microstructure of the metal.

Hot work is performed above the recrystallization temperature and close to the melting point of the metal, typically between 60%-90% of melting temperature. It reduces the strength and hardness of the metal significantly but increases its ductility and formability greatly. It produces a poor surface finish and looser dimensional tolerance than warm and cold work. However, strain hardening in hot work is negligible, as the metal recrystallizes continuously during the process. Hot work also requires low forming force, which can enable large deformations and complex shapes. Furthermore, hot work has no size limitations on the cross section of the metal, as any section can be deformed at high temperatures. Hot work is suitable for producing parts with complex shapes and large deformations, but it requires high temperatures and energy, and may cause oxidation or scaling. Hot work also reduces the residual stresses and improves the microstructure of the metal.

Watch Professor Cummings explain how work affects the microstructure and material properties.

Tolerance Specification

Suppliers of stock material generated through bulk forming process often provide tolerances which are based on ASTM (American Society of Testing and Materials) standards. It is based on both the geometry of the part and the material. For example, see OnlineMetals tolerance charts for stock material: Material Dimensional Tolerances | OnlineMetals.com®