Work hardening is the strengthening of a metal by plastic deformation. When metal is formed using cold forming processes, the material is permanently deformed by the forces applied to it. Once the tensile stress being applied to the metal reaches a certain force, the metal will start to become work hardened. As more force is applied, the deformation will eventually cause the metal to fracture.
Before work hardening, the crystal lattice of the metal exhibits a regular, nearly defect-free pattern; with almost no dislocations (line defects in a material's crystal structure). As the metal is worked further, the crystals become increasingly saturated with new dislocations, and so more dislocations are prevented from forming. This will mean that greater forces will have to be applied to deform the metal, hence the increased hardness of the metal. The initial regularly spaced lattice can be created or restored by annealing.
The following is a list of some common cold forming processes:
- Compressing
- Barrel polishing
- Burnishing
- Forging
- Planishing
- Dapping
- Swaging
- Sinking
- Raising
- Riveting
- Rolling
- Stamping
- Die forming
- Bending
- Angle bending
- Flanging
- Roll forming
- Seaming/hemming
- Straightening
- Twisting
- Shearing
- Blanking
- Perforating/notching
- Shaving/trimming
- Drawing
- Chasing
- Repoussé
- Spinning
- Wire drawing
- Tube making
Advantages and disadvantages
Advantages:
- No heating required, hence avoiding oxidation
- Lack of oxidation and use of chemicals can reduce contamination problems
- Good surface finish
- Metal can be formed to exacting measurements
Disadvantages:
- Greater forces are required
- Heavier and stronger tools are required
- Metal surfaces must be clean to avoid impurities being forced into the surface
- Periodic annealing may have to be performed
- Undesirable stresses may be retained in the metal
