What is Work Hardening?
Work hardening, also known as strain hardening, is a process that leads to the strengthening of material through plastic deformation.
A great example is a paper clip. If you bend a paper clip, it becomes harder to bend back. The process continues until the paper clip suffers a sudden failure. The strengthening of the paper clip is work hardening!
Work hardening falls into two categories, desirable and undesirable work hardening.
How Does Work Hardening Strengthen a Metal?
For the process to occur, the material must undergo plastic deformation. During plastic deformation, new dislocations form within the metals lattice structure. As plastic deformation continues, more dislocations form and they start to interact with each other. During this interaction, they hinder each other’s movement and act as obstacles.
As a result of the dislocations hindered movement, plastic deformation can no longer occur at the normal stress.
Types of Work Hardening
Undesirable
Undesirable work hardening occurs during the cutting process on machinery. During the early stages of the process, the cutter or tip unintentionally word hardens the surface of the workpiece.
As the process continues, the workpiece damages the cutter.
If you have been exposed to cutting/machining, you will have come across this. During many cutting processes, the tips of the cutter need changing when a high degree of accuracy is required. The change is necessary because the cutter/tip can no longer provide the accuracy required due to the hardened surface.
To achieve the desired precise dimension a new tip is required, which is made of a stronger material such as titanium.
Desirable
Desirable work hardening occurs during manufacturing in cold working processes. Cold working is a metalworking process that shapes a metal below its recrystallisation temperature, commonly occurring at room temperature.
Types Of Cold Working
- Rolling
- Bending
- Piercing
- Thread rolling
- And many more
Cold working metals increases their yield strength by the process discussed above (interaction of dislocations). In addition to this, the metals hardness and tensile strength also decrease, although its ductility decreases.
Cold working / Work Hardening / Strain Hardening and Ductility
Ductility is a crucially important mechanical property of many metals in various applications. Therefore we must maintain a level of ductility of the metal. As discussed above, the process reduced the ductility of the metal.
When selecting metals for applications, a metal that meets the ductility requirements and the strength requirements should be selected.
Additionally, suitable testing is necessary on the component/product post-manufacturing. If cold working processes have been used to create the final product, the ductility will vary compared to the sheet of metal it was manufactured from.