The Cold Forging Process: Part One

概要:

Cold forging is a process by which a metal material is plastically deformed at room temperature under a massive application of pressure.
As well as improved overall material properties, some other key advantages of the cold forming process can include a higher dimensional accuracy than with forged parts, excellent surface quality, and no need to apply additional energy into the process such as extreme temperatures.

The cold forging process can be described as the process by which a metal is plastically deformed at room temperature with application of huge pressure. The process not only changes the shape but also improves the properties of the forged parts due to grain size refinement.

Throughout history, cold forging or cold forming as a production process has seen intensifying interest and become one of the most widely used methods of making parts. Work hardening, also called strain hardening, occurs during cold forming due to dislocations in the metallic crystal structure. The materials yield strength is not decreased, its ductility is not increased, and work hardening occurs quickly. All these factors make cold forging extremely difficult.

A large amount of force is needed and multiple operations are sometimes required to achieve more complex shapes. With proper lubrication, however, tool life is greatly increased when compared to hot or warm forging. The grain structure is stronger and many repetitions of hardening by heat treatment is not necessary due to the work hardening that occurs during the forming process.

Both ferrous and non-ferrous metals can be cold formed. The ability to forge these metals and the amount of possible deformation depends greatly on the chemical composition and annealed properties of grade. Properties such as hardness and ductility are critical properties in determining the formability of a metal. It is important to know that the mechanical properties of materials are greatly improved after cold forming.

Sometimes this improvement is so great that grades which would have been considered unsuitable if machined, warm forged, or hot forged, could have suitable mechanical properties for a new application after cold forming. The improvement in mechanical properties of a grade depends partly on the amount and type of deformation taking place. Portions of the forging that see little work will not see as great an improvement as the locations of greater deformation.

Materials that can be cold formed include, but are not limited to:

  • Carbon steels
  • Brass
  • Lead
  • Stainless steel
  • Copper
  • Alloy steels
  • Aluminum
  • Bronze
  • Nickel alloys
  • Precious metals
Advantages Difficulties
Near-net-shape forming Extensive treatment of the work piece
Higher dimensional accuracy than with forged parts Less degree of forming than with hot forming
Very high degree of material utilization Complex forms difficult to realize
No scaling Higher tool expenditure
High surface quality  
High work piece strength through strain hardening  
Expedient grain flow as with hot forming  
No heating necessary  

Table 1: Advantages and associated difficulties of the cold forging process

Applications for cold formed parts:
Automotive: brake parts, ball joints and steering parts, starter pinions, oxygen sensors, constant velocity joints, manifold bolts, engine valves;
Appliance industry: gears, fasteners for assembly;
Aerospace: rivets, fuselage, engine bolts, fasteners – landing gear, interior;
Construction, off-road equipment: bolts, nuts, screws-tapping, window, roofing, deck, transmission gears, similar parts for automotive.







Table 2: Chemical compositions of typical cold forging quality grades: 1. Carbon steels 2. Boron steels 3.Alloy steels

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