Based on the temperature of the billet in processing, forging can be divided into: cold forging and hot forging.

Cold forging is usually done at room temperature, while hot forging is performed at a temperature higher than the recrystallization temperature of the metal of the billet.

Sometimes, forging at a heated temperature but not higher than the recrystallization temperature is called warm forging. This classification is not very standardized in production.

 

Cold forging: A forging technique where metallic materials are processed at room temperature by the action of pressure.

 

Hot forging: hot forging refers to such a forging technique in which metallic materials must be heated above their recrystallization temperatures but below solidus line then subjected to a pressure processing method.

 

Recrystallization temperature: an approximate value for it is 0.4 Tm, here Tm indicates the melting point of the metal.

 

In general, for forged products, cold forging produces better flatness, hot forging produces the poorest flatness, and warm forging (in which the temperature of the metal is between cold and hot forging) produces medium flatness.

 

Advantages and Disadvantages of Hot Forging

Advantages:

After processing, if the temperature of the forged material remains much higher than the recrystallization temperature, the grain structure has more time to grow. As a result, the coarser the grains are, the better the fracture toughness will be with lower crack propagation rates.

Disadvantages:

• High-temperature operations pose a greater safety risk to personnel and materials.
• At high temperatures, materials are more likely to oxidize and develop oxide scales that deteriorate the quality of the surface, making it less smooth and less flat.
• On cooling, the material may shrink after hot forging, which affects the accuracy of the dimensions of the forged part.
• High-temperature operations involve many expensive equipment and maintenance costs.

 

Advantages and Disadvantages of Cold Forging

Advantages:

• The forged material is less likely to form oxide scales hence better surface smoothness.
• The dimensional accuracy of the forged part is higher.
• Cold forging causes work hardening (strain hardening) in processing, hence improving strength and hardness.
• It aids in attaining specific directional flow of metal fibers in the forged part.
• No heating treatment is required, hence minimizing contamination-related problems.
• No heating is used, hence reducing the cost of production.

Disadvantages:

• The plasticity and toughness of the forged material are poorer, which may have an adverse effect on subsequent processing.
• The required deformation force is larger than in hot forging.
• Residual stresses can be created in the material.
• Grains of metal can distort or fragment.
• The directional properties developed during processing may not be beneficial to the usage of the final product.