What is Cold Working? (Examples, Advantages & Disadvantages)

Cold working of metals

What is Cold Working?
Definition: Cold working (also known as cold forming) is defined as a metal forming process in which the plastic deformation of metals takes place below their recrystallization temperature.

Now the question is; 
What is recrystallization temperature? Recrystallization temperature is a temperature in metal forming operations at which the old grain structure are destroyed and new grain crystals are formed by applications of mechanical pressure on the metals.

In cold working of metals, the metals do not reach this temperature. Usually cold working is carried at or near to room temperature.

Sometimes fractures may occur in metals before reaching a desired shape. This fracture occurs due to excessive cold working on the metals.

To avoid this defect, cold working is generally carried out in a few steps.

During the cold working operation, the intermediate annealing operation is done on the metals workpiece. By doing this, the cold worked metal remains soft and the ductility of the metal is restored.

Such a sequential cycle of cold working and annealing is also known as cold work annealing cycle.

cold work annealing cycle

Above graph shows you how the strength and ductility varies in the cold work annealing cycle.

At a lower temperature range in cold working, the yield point of the metal is higher.

So in cold working operations, a high amount of force is required for metal forming.

Also the ductility of the metal is less at lower temperature range, so only limited shape change is possible by cold forming operation.

Examples of cold working

Examples of cold working are listed below.

  1. Cold shearing
  2. Cold spinning
  3. Cold drawing
  4. Cold bending
  5. Cold piercing
  6. Cold rolling

#1) Cold shearing

cold shearing
Lever shear machine

Cold shearing is a cold working operation in which the metal pieces (usually metal rods) are sheared at a room temperature.

#2) Cold spinning

cold spinning

Cold spinning operation is a sheet metal forming operation in which the sheet metal is revolved at a high speed. Then it is pressed against the headstock spindle in order to give the desired shape.

#3) Cold drawing

cold drawing

In cold drawing, the wires, tubes, rods and other sections are produced by pulling it through a die.

(Note: Drawing is generally a cold working operation, but for large diameter tubes or wires, hot working is required which helps in reducing the forces.)

#4) Cold bending

cold bending

Cold bending is a process of sheet metal forming in which a force is applied to a piece of sheet metal which ultimately causes it to bend at an angle and form a desired shape.

#5) Cold piercing

cold piercing

Cold piercing is a type of punching process in which a cylindrical punch pierces a hole into the sheet.

#6) Cold rolling

cold rolling

Cold rolling is a process in which metal (whose temperature is either at a room temperature or near to the room temperature) gets plastically deformed when it is allowed to pass in between the two opposing spinning rolls. 

Advantages of cold working

Advantages of cold working are mentioned below.

  • Heating of the workpiece is not required in cold forming.
  • Surface finish obtained by cold working is better than that obtained with hot working.
  • Secondary machining is not required in most of the cases as higher dimensional accuracy is obtained using cold working.
  • Cold working leads to better mechanical properties like strength, wear, fatigue, etc.
  • There is negligible problem of contamination and good surface finish can be achieved.
  • In cold working, the strain hardening takes place which increases the mechanical properties like strength and hardness.
  • In cold working, the material developed anisotropy which helps in getting deeper draws.

Disadvantages of cold working

Disadvantages of cold working are mentioned below.

  • Very high mechanical forces are required for cold working operation.
  • Heavy, powerful and robust equipment is required to perform cold working.
  • The surface of the metal should be clean and free of scale or oxides.
  • During the cold working, the strain hardening takes place and the process may require intermediate annealing.
  • Residual stresses are higher.
  • Durable machines are required to perform cold working operations, so the cost of machines may increase.
  • The ductility of the metal decreases due to strain hardening.

External links:
Sheet bending machine: Image credits indiamart
Lever shear: Image by Audrius Meskauskas (Audriusa), CC BY-SA 3.0, via Wikimedia Commons

Leave a Comment