What Is Strain Hardening?

What Is Strain Hardening?

Strain hardening, also known as work hardening, is a process in materials science where the strength of a metal or polymer is increased due to plastic deformation. The outcome of work hardening can be positive, negative, or have no significant impact, depending on the context.

The strengthening is a result of the movement and creation of dislocations within the material’s crystal structure. Many non-brittle metals with high melting points and some polymers can be strengthened through work hardening. Alloys that cannot be heat treated, such as low-carbon steel, are often improved through this process.

However, work hardening can also have negative effects during machining, where early passes with a cutter can work-harden the surface and cause damage to the cutter later on. Superalloys, such as Inconel, are more susceptible to this, and require special machining strategies.

On the other hand, metals used in items like springs that are meant to flex are often made of specialized alloys to avoid work hardening and metal fatigue, which requires specific heat treatments.

Work hardening can also be desirable in metalworking processes where plastic deformation is intentionally induced to change the shape of the metal.

This is known as cold working or cold forming, and involves shaping the metal at a temperature below its recrystallization temperature, usually at room temperature.

Cold forming techniques include squeezing, bending, drawing, and shearing, and are used in applications such as the heading of bolts and cap screws and the finishing of cold-rolled steel. Cold working the metal increases its hardness, yield strength, and tensile strength.

Theory Of Strain Hardening?

Materials have a regular, nearly defect-free lattice structure before work hardening. This structure can be restored at any time through annealing. During work hardening, the lattice becomes saturated with new dislocations, making it more difficult for additional dislocations to form, which results in the material becoming stronger but less ductile.

This is because dislocations serve as pinning points or obstacles, impeding their motion and increasing the yield strength of the material. Cold working increases the concentration of dislocations, which can lead to an increase in yield strength and a decrease in ductility.

The effects of cold working can be reversed through high-temperature annealing, which reduces the dislocation density through recovery and recrystallization. The work hardenability of a material can be predicted through stress-strain analysis or hardness testing before and after a process.

How Does Work Hardening Strengthen A Metal?

Permanent deformation of a metal occurs when dislocations move until they’re blocked. Dislocations are movements of imperfections in the metal’s grain. The most effective way to stop them is by intersecting dislocations from different planes, which pile up and entangle, preventing further deformation. This increases the metal’s strength under added pressure, unless enough energy is applied.

Strain Hardening Process

Cold working is a crucial method to increase metal strength. One type of cold work is cold rolling, which involves subjecting metal to high pressure by passing it through rollers. This process causes metal deformation and grain elongation, resulting in an accumulation of dislocations, thereby increasing metal strength.

However, excessive cold work makes it difficult to form or process the metal further. In this case, annealing, or heating and cooling the metal gradually to relieve internal stress, must be performed. This recrystallizes the metal’s internal grains, allowing new dislocations to form and restoring its initial elongation and strength, thus enabling further cold working.

The last temper on the metal after the final anneal determines the strain hardening achieved. The annealing temperature and duration must be carefully controlled to eliminate old grains without allowing new grains to grow too large.

Elastic And Plastic Deformation

strain hardening is a change in material shape that results from plastic deformation, which is permanent and distinct from elastic deformation, which is temporary and reversible. This behavior is commonly observed in metals, particularly steels.

Ductile materials, which can undergo plastic deformations before breaking, are more prone to work hardening. The tensile test is a popular way to study deformation mechanisms in materials, as it is difficult to examine the intermediate processes in a compressive test.

Materials that are subjected to small forces will deform elastically, meaning they will return to their original shape once the force is removed. This is described by Hooke’s Law.

However, when the deforming force exceeds the elastic limit or yield stress, the material will undergo plastic deformation and remain permanently deformed after the force is removed. For example, stretching a coil spring beyond its elastic limit will result in a permanent deformation.

Elastic deformation occurs when the bonds between atoms are stretched without breaking, while plastic deformation involves breaking the inter-atomic bonds and rearranging the atoms in the material.