While cold forging compresses metal at room temperature, hot forging requires high heat. A primary differentiator of cold and hot forging is that the high heat of hot forging allows the metal to take on more elaborate and complex forms than cold forging.

HOT FORGING

The hot forging temperature varies depending on the type of metal. Hot forging starts with heating the dies to prevent any loss of temperature during the process and ensure crystallization doesn’t occur until forming is complete. Heating causes the metal to become more ductile. When the pressure of the dies squeezes the hot metal, the structure transforms into a more refined grain that results in increased yield strength and ductility.

Factors to consider when hot forging include:

• Cooling. If the metal cools to a temperature lower than the minimum threshold, it completes the forging. The metal must be reheated if this occurs before achieving the final shape.
• Tolerances. Dimensional tolerances with hot forging are less precise than cold forging.
• Dies. Hot forging dies are custom-made to the customer’s part specifications.

COLD FORGING

Cold forging uses a displacement process to shape the material into the desired shape. Compressive force squeezes the metal between a punch and die at room temperature until the material conforms to the die’s contours. Cold forging techniques include rolling, pressing, drawing, spinning, heading, and extruding.

Factors to consider when cold forging include:

• Material volume. Careful control over the material volume prevents stress and damage, particularly in closed forging, as the excess has nowhere to escape.
• Bonderizing. This coating process improves material flow during the process to reduce force, stress, and friction while improving surface quality.
• Annealing. Annealing softens the metal, improving the material flow. It can be applied as an intermediate process when work hardening occurs before the forging process is completed.
• Lubrication. Lubrication is critical during cold forging. High-viscosity oil protects against metal-on-metal friction and applying thin oil dissipates heat.

ADVANTAGES & DISADVANTAGES OF HOT AND COLD FORGING

Advantages of hot and cold forging include:

• Hot forging. Increased ductility for more complex parts and allows for more options for customization
• Cold forging. Creates no waste, requires little to no finishing work, maintains dimensional accuracy, and results in high surface quality.

Disadvantages of hot and cold forging include:

• Hot forging. Additional cost for heat treatment, less precise dimensional tolerance, risk of warping
• Cold forging. Few options for customization, risk of residual stress, may require heat treatment