Strain hardening, also referred to as cold working or work hardening, is the process where metals are made stronger and harder through permanent deformation. You cannot discuss strain hardening without mentioning the word “temper”. Tempering is the type and amount of processing done to the metal when it is at the mill, including thermal treatments and cold work. When an alloy is referred to as being full hard, half hard, or having a spring temper etc., the temper is what it is referring to.
How Does Work Hardening Strengthen a Metal?
When a metal is permanently deformed, the dislocations (movement of crystalline imperfections through the grains within the metal) move until they hit something that stops them. One of the most effective ways of stopping dislocations is with more dislocations. Confusing? Let me explain.
When dislocations that are running on different planes intersect, they cannot move past one another. They end up piling up on one another, also becoming intertwined in the process. This is known as dislocation entanglement, and stops more deformation occurring for that particular grain, unless significantly greater energy is used. This increases the strength of the metal under any further loading.
Cold working / Work Hardening / Strain Hardening process
Cold working is a very important way of increasing the strength within metals. Cold rolling is a type of cold work, which involves passing a metal through two rollers that impose a great pressure on the metal. This deforms the metal and elongates the grains within, causing dislocations to pile up and increasing the strength of the metal.
When a high level of cold work is applied to the metal, it becomes quite difficult to form or process any further. If more forming or reduction is needed, annealing (heating and slow cooling of a metal to reduce internal stresses) must be carried out. This causes the internal grains to recrystallize, obliterating old grains and this allows new dislocations to grow within the new grains. Any and all prior work the metal was subjected to disappears and the elongation and strength that the metal had initially are restored. This allows for further cold working to commence.
The last temper on the metal after the final anneal is the actual amount of strain hardening that has occurred. The anneal must have been carried out at a high enough temperature and for enough time to destroy the old grains, but short enough to not allow the new grains from growing too big in size.
Cold working / Work Hardening / Strain Hardening and ductility
There must be a balance struck between cold working (aka work hardening) and loss of ductility (ability to be formed). Spring and superspring are two types of hard temper and have low ductility and high strength. Annealed and ¼ hard are known as softer tempers and have ductability (formability) and low strength.
When choosing a material for an application (don’t choose any of these!), it is advised that you choose the material that meets the ductability requirement and has the highest strength. For instance if you were choosing an alloy for the wheels on your car, you would want the strongest wheels possible, right?
Do you have any experience with cold working (also known as strain hardening or work hardening)? We would love to hear your thoughts in the comments below!
More about metals in manufacturing:
- Why CNC Machining Beats Traditional Machining Every Time
- CNC Fabrication: 3, 4 and 5-Axis CNC Explained
- What is Burnishing and what is it used for?
- What is Thermoforming and what is it used for?
- A Guide to Shot Blasting / Shot Peening
- What is Electroforming and where is it used?
- What is Electropolishing and what is it used for?
- Cold Rolling – process overview
- Circular sawing – process description and design guide
- Electron Beam Melting (EBM): process, materials, uses
- Die Casting – design guide, materials, advantages and disadvantages