In this article we will discuss about the definition and effects of recrystallization of metals.
Definition of Recrystallization:
The property of any material is directly related to its structure (arrangement of atoms within metals). A knowledge of structure guides in controlling and predicting the behaviour and performance of metals in various manufacturing processes.
Metals are composed of crystals (grains), three basic structures being—body centred cubic, face-centred cubic and hexagonal close-packed. These crystals are not perfect and contain various defects and imperfections, such as dislocation, vacancies, impurities, inclusions, and grain boundaries. The behaviour and properties of metals depend on their crystal structure, grain size, grain boundaries, and the nature and extent of defects.
Plastic deformation in metals takes place by means of a slip mechanism. The stress required to continue deformation increases with deformation and is known as work hardening or strain hardening. It results from the entanglement of dislocations with each other and with impurities and grain boundaries.
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In addition to increasing strength, plastic deformation at room temperature also causes anisotropy (different properties in different directions). Both of these effects can be eliminated by recrystallization of metal at elevated temperatures. Anisotropy could be of two forms, viz. preferred orientation and mechanical fibering.
Recovery of effects of plastic deformation occurs at a certain temperature range below the recrystallization temperature of the metals and the stresses in the highly deformed region are relieved.
The process in which, at a certain temperature range, new equiaxed and strain-free grains are formed, replacing the older grains, is called recrystallization.
Effects of Recrystallization:
The effects on recrystallization of temperature, time, and reduction in the thickness by cold working are as follows:
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i. For a constant amount of deformation by cold working, the time required for recrystallization decreases with increasing temperature.
ii. The greater the prior cold work done, the lower the temperature required for recrystallization.
iii. The higher the amount of deformation, the smaller the grain size becomes during recrystallization.
iv. Anisotropy due to preferred orientation usually persists after recrystallization. To restore isotropy, a temperature higher than that required for recrystallization may be necessary.
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If temperature is raised further, the grains begin to grow (grain growth) and it affects the mechanical properties slightly. Large grains produce a rough surface appearance on sheet metal, called orange peel.
Plastic deformation at room temperature is called cold working and when done above recrystallization temperature it is called hot working. Warm working is done at temperature above room temperature but lower than hot working temperature. Warm working is a compromise between cold and hot working.