Introduction

• With increasing stress on a material, by applying load, there are possibilities that a material may fail before reaching the desired stress value.
• To improve the hardness of a substance so that it  is able to sustain more load in the elastic region process of strain hardening is done.
• Here ductility is compromised to get hardness and strength.

Principle of Strain Hardening

• If the concentration of the dislocation increases, the material resists further outflow by resisting further deformation or becoming more harder.
• The ability of a metal to plasticity deform depends on the ability of dislocation to move.

Theory of Work Hardening

• Work hardening is the strengthening of a metal by plastic deformation.
• Work hardening occurs because of dislocation movements and dislocation generation within the crystal structure of the material.
• Work hardening also known as strain hardening or cold hardening.
• As the material is work hardened it becomes increasingly saturated with new dislocations, and more dislocations are prevented from nucleating.
• Work hardening however, reduce ductility and plasticity.
• Before work hardening, the lattice of the material exhibits a regular, nearly defect-free pattern.
• This resistance to dislocation-formation manifests itself as a resistance to plastic deformation, hence the observed strengthening.

Principle of Work Hardening

• When loaded, the strain increase with stress and the curve reaches the point A in the plastic range.
• If at this stage, the specimen is unloaded, the strain does not recover along the original path AO, but moves along AB.
• If the specimen is reloaded immediately, the curve again rises from B to A, but via another path, and reaches the point C, after which it will follow the curvature, if loading is continued.
• A comparison of paths ACD and AD shows that the cold working has increased the yield strength and ultimate strength of the metal.

Stages of Work Hardening

• A typical shear stress-shear strain curve for a single crystal shows three stages of work hardening.
• STAGE 1- Easy Glide Region.
• STAGE 2- Linear Hardening Region.
• STAGE 3- Parabolic Hardening Region.

Easy Glide Region

• Very low work hardening rate.
• BCC system do not exhibit an easy glide.
• Shear stress is almost constant.

Parabolic Hardening Region

• Increase in degree of cross slip.
• Shape is parabolic.
• Low hardening rate.

Liner Hardening Region

• Hardening rate is high as well as constant.

Advantages

• No heating required.
• Better surface finish.
• Better reproducibility and interchange ability.
• Directional properties can be imparted into the metal.
• Superior dimensional control.

Disadvantages

• Metal is less ductile.
• Greater forces are required.
• Undesirable residual stress may be produced.
• Heavier and more powerful equipment and stronger tooling are required.
• Intermediate anneals may be required to compensate for loss of ductility that accompanies strain hardening.

Industrial Applications of Strain Hardening

• Construction Material- High strength reduces the need for material thickness which generally saves weight and cost.
• Knife blades- a high hardness blade keeps a sharp edge.
• Anti-fatigue- Hardening can drastically improve the service life of mechanical components with repeated loading/unloading, such as axles and cogs.
• Machine cutting tools needs be much harder than the material they are operating on in order to be effective.