Forging covers a range of techniques, all of which involve the shaping and displacement metal to a desired shape. You can typically classify forging by the equipment used in the process (hammer or press), the type of tooling employed (open die versus impression), and the temperature at which the forging is carried out.
This blog explores the differences between hot and cold forging, including their benefits, limitations, and use cases.
Hot Forging
Hot forging refers to shaping a metal blank or billet that has been heated above its recrystallization temperature, or the temperature at which new grains form within the metal and consume the old grains, which varies based on the material used in the forging process.
At this temperature, the metal has lost much of its strength, so it takes relatively little force for the dies to shape it to the required geometry. Additionally, grain flow, which gives forged parts superior mechanical properties compared to cast or machined billet parts, is retained.
During hot forging, the workpiece loses heat. However, hot forging can also be performed as an isothermal process, in which the dies are heated to ensure that each forging step (busting, blocking, and finishing) is carried out at the same temperature.
Advantages
- Grain flow throughout the billet ensures high strength.
- Strength characteristics can be tailored by engineering the direction of grain flow.
- Can form complex geometries and fine detail (lettering, fins, ribs).
- Avoids work hardening, so hot forged parts are tougher.
- Compaction and recrystallization eliminate internal defects and improve grain size uniformity.
Limitations
- The metal surface oxidizes at high temperatures, forming scale that usually needs to be removed in a secondary operation.
- Achievable dimensional tolerances are widened by thermal expansion and contraction.
- Thermal cycling shortens die life.
Best Applications
Hot forging is commonly used to create critical components, including:
- Gas turbine rotors
- Automotive crankshafts
- Connecting rods for internal combustion engines
- Gear blanks and other transmission components
- Functional components like pins, bushings, and linkage arms for heavy-duty machinery
- Valve bodies and related parts for high fluid pressure applications
Cold Forging
Cold forging is performed at ambient temperature. Lubricant is usually added to the die to aid metal deformation, but the forces needed are still higher than in hot forging. Some steels can be cold forged if they have sufficient ductility, although work hardening can be a problem. In general, cold forging is used with metals such as aluminum and magnesium, which are more ductile.
Advantages
- Produces good surface finishes.
- No post-processing needed to remove surface scale/oxidation.
- Capable of maintaining close tolerances.
- Produces hard, high-strength parts.
Limitations
- Cannot produce the extensive metal flow needed to form fine detail.
- Requires ductile workpiece materials.
- Work hardening lowers toughness.
- Limited to smaller parts due to the high forces needed and the capacity of available machinery.
Best Applications
- Smaller parts with simple geometry, such as fasteners, rivets, and pins
- Bearing components
- Hinges, locks, and other consumer hardware
- Strengthening plates and discs
Learn More From Trenton Forging
Trenton Forging is an innovative American forging company committed to satisfying customers with high-quality and rapid turnaround. We specialize in impression die forging, producing millions of parts each year for customers in industries from automotive and railroad to transportation and defense.
Beyond forging, our additional in-house capabilities include 3D laser scanning for reverse engineering and rapid prototyping, post-forging machining, and tooling maintenance.
Visit our website for more resources, or contact us today if you’d like to discuss a project.
