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Springback
Springback is also present in cold working operations. In the elastic region, the strained material returns to its original size and shape. But removal of load in the plastic region, decreases the strain from x3 to x2 as shown in the figure 37.1 for metal A i. The decrease in strain, x3 − x2 is elastic springback.
Thus, in cold working the deformation must be carried out beyond the desired point by an amount equal to the springback.
The various cold working processes are squeezing, bending, shearing and drawing
Annealing
Plastic deformation of polycrystalline material in cold working produces microstructural and property changes that include (a) change in grain shape, (b) strain hardening, (c) increase in dislocation density.
Appropriate heat treatment such as annealing reverts back to the pre-cold worked states. The purpose of annealing may involve one or more of the following aims:
- To soften the steel and to improve machinability
- To relieve internal stresses induced by rolling, forging etc.
- To remove coarseness of grains
The annealing consists of
- Heating the steel to a certain temperature
- Soaking at this temperature
- Cooling at a predetermined rate
Such restoration results from recovery,recrystallization, which may be followed by grain growth.
During recovery some of the stored internal strain energy is relieved by virtue of dislocation motion due to atomic diffusion.
Even after recovery is complete, the grains are still in a relatively high strain energy state. Recrystallization is the formation of a new set of strain-free and equixed grains (having approximately equal dimensions in all directions). Strength and hardness decrease, but ductility increases.
After recrystallization is complete, the strain-free grains will continue to grow, if the metal is left at the elevated temperature.
References:
E.P. DeGarmo: Materials and processes in manufacturing
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