Q. What is Recovery and Recrystallization?
Ans. Due to the cold working of metals there arise a large number of dislocations and distortion of planes. Cold working of metals or alloy increase hardness, yield strength, ultimate strength and electrical resistance whereas ductility and plasticity are decreased.
Plastic deformation which distorts the crystal lattice and breaks up the blocks of initial equated grains to produce fibrous structure of thin plates, increase the energy level of metals. Now let, an example of wire drawing process. In this process the induces work hardening of metals that cause deformation. For breakup of wire resistance do deformation should be equal the resistance to fracture and demonstration has reached the maximum stress limit.
These properties and structures may revert back to the precold worked states by appropriate heat treatment i.e. treatment is called annealing; such restriction results from two different processes that occurs at elevated temperatures are recovery, Recrystallization which may be followed by grain growth.
RECOVERY: Recovery is a process that involves that involves that heating of metal to about 10% of melting temperature of metal in the absolute scale of temperature. During recovery some of stored internal energy is relived by virtue of dislocation motion as a result of enhanced atomic diffusion at the elevated temperature.
Recovery processes produce movement of dislocation due to vacancy diffusion of atoms that forms uniform walls of dislocation one above the other which is known as polyconensation. However the majority of the dislocations and the bulk of strain energy are not removed by the usual recovery treatment, the noticeable changes are in electrical and thermal conductivities.
The main salient features of recovery are:
(a) It occurs below the crystallization temperature
(b) It does not affect grain growth
(c) No change in microstructure at this stage
(d) Recovery soften the material by reducing the dislocation density i.e. little hardness and strength and decrease and ductility increases
(e) Recovery process is used to relieve the residual stresses
(f) During recovery low grain boundaries are formed
At a given temperature the rate of recovery is fastest initially drops longer times Recovery processes are applied on those equipment which are made up of forging , welding and fabrication process so that their internal stress can be removed and there is no decrement in strength during cold working process .
Recrystallization: Even after recovery complete, the grains are still in a relative high strain energy state. Recrystallization is the formation of a new set of strain free and equipped grains when the metal is heated.
Grain boundaries are adjust if heating is continue, grain shape and size is not increase now. During this Recrystallization new structure will not produce but formation of new grains taken place. This process can be seen by metallographic methods and recognized by decrease in hardness or strength and increase in ductility. After Recrystallization process density of dislocation will decrease and finished all effect of strain hardening. Continuous heating of metals will increase grain size increment is due to decrease in free energy.
Recrystallization is also an annealing process which proceeds at higher temperature. In pure metal Recrystallization occurs at about 30% of melting temperature and in alloy about 50% of melting temperature Recrystallization is depends on the following factors:
(a) Nature of metal
(b) Purity of metal
(c) Grain size
(d) Annealing time
(f) Composition of metal
The Recrystallization does not produce new structure but produces new grains and this process lowers energy of a solid by eliminating dislocation.
A crystal which has been severely work hardened has a high dislocation density and therefore releases more energy during Recrystallization.
Recrystallization temperature: The temperature at which crystallization takes place called Recrystallization temperature and at this temperature 50% of metal crystallize in one hour. Generally Recrystallization temperature is 30-50% of melting temperature of metals and it varies metal to metal. For a particular metal this temperature may be varies according to conditions.
The most common factors which affect the Recrystallization temperature are:
(1) Time of heating: greater the period of annealing of metal lowers the Recrystallization temperature.
(2) Temperature: If temperature of cold working is higher strain energy, stored in material is less and hence Recrystallization temperature is high.
(3) Grain size: If grain size is fine crystallization temperature is less.
(4) The degree of deformation: Higher the degree of deformation the Recrystallization temperature is low.
(5) Purity of Metal: During Recrystallization, the impurity atoms segregated at the grain boundaries and retired their motion and obstruct the process of nucleation and growth. Thus Recrystallization temperature is more required for impure materials.
(6) If the level of deformation is less, Recrystallization temperature will be high.
(7) For given reduction in cross sectional areas, different deformations are achieved. Hence Recrystallization behaviours are different.