Why Strength and hardness of material are increased in cold working?

Answer 1

The reason that the hardness increases as the cold working increases is because of defects. Before the reduction, or cold working, the lattice of the material has a virtually defect free, or dislocation free, pattern. The cold working increases the amount of dislocations, and because of this, the material becomes more resistant to additional deformations. This manifests itself, most noticeably, as resistance to plastic deformations.

Microscopically, this explained by the two types of dislocation shifts. Dislocation boundaries can move within the plane of dislocation relatively easily; this is known as a glide. However, when a dislocation boundary is moved up or down, out of the plane of dislocation, it takes much more energy. When a material becomes saturated with dislocations, any new dislocation will inevitably have to shift some dislocation boundaries out of their plane. This is known as a climb. Since climbs require more energy, more force is needed to plastically deform the material, i.e. the material becomes harder. The more dislocations a material has, the more climbs are done to achieve this new dislocation, making it harder to plastically deform a material. So the more cold worked the material is, the more climbs are needed, meaning that more energy is needed to deform the material.

I'm pretty sure this is right

-SMG