Cold working is deformation carried out under conditions where recovery processes are not effective. Hot working is deformation under conditions of temperature and strain rate such that recovery processes take place simultaneously with the deformation.
Structural changes during cold working of polycrystalline metals and alloys
(1) Changes in shape and size of grains: The equiaxed grains on deformation are elongated in the direction of acting force i.e. stretched in the direction of main tensile deformation stress–say, in the direction of rolling or wire drawing.
(2) Changes in orientation of grains: Preferred orientation or texture of is the state of severely cold worked metal in which certain crystallographic planes of the grains orient themselves in a preferred manner with respect to the direction of the stress (or maximum strain).
(3) Changes in internal structure of grains: during cold working around 15% of the work of the deformation gets absorbed in the material (rest is lost as heat). This stored energy is the form of energy of crystal defects. Plastic deformation increases the concentration of point defects. With increase of cold working, the number of stacking-faults increases, thus density of extended dislocations increases. The number of kinks, jogs, dipoles, prismatic loops increase. The most important internal change of structure is increase in density of dislocation from 106 – 108 cm-2 in annealed state to 1010 – 1012 by moderate cold working.
Effect of cold work on properties
Cold working or strain hardening is the increase in the stress required to cause further slip because of previous plastic deformation. This is an important industrial process that is used to harden metals or alloys that do not respond to heat treatment. It changes various mechanical, physical and chemical properties of metals and alloys.
With increase in amount of cold work, Ultimate Tensile Strength, Yield Strength, Hardness increases but ductily (elongation and reduction in area) decreases. Cold worked texture and mechanical fibering leads to Anisotropy in in properties of materials. The ductility and impact toughness is much lower in transverse section rather than in longitudinal section. As the internal energy of cold worked state is high, the chemical reactivity of the material increases i.e. the corrosion resistance decreases, and may cause stress corrosion cracking in certain alloys. The rate of strain hardening (slope of flow curve) is generally lower in HCP metals than cubic metals. High temperatures of deformation also lower the rate of strain-hardening.
Annealing of Cold worked materials
In certain applications materials are used in the cold-worked state to derive benefits of increased hardness and strength. The cold worked dislocation cell structure is mechanically stable, but not thermodynamically stable. It is necessary to restore the ductility to allow further cold deformation or to restore the optimum physical properties such as electrical conductivity essential for applications. The treatment to restore the ductility or electrical conductivity with a simultaneous decrease in hardness and strength is Annealing (or Recrystallization annealing). It is heating cold worked metal to a temperature above recrystallization temperature, holding there for some time and then slow cooling.
The process of Annealing can be divided into three fairly distinct stages (1) Recovery (2) Recrystallization (3) Grain growth. There is no change in composition or crystal structure during annealing. The driving force for recovery and recrystallization is the stored cold-worked energy, whereas for grain growth is the energy stored in grain boundaries.
Recovery It is restoration of the physical properties of the cold worked metal without of any observable change in microstructure. It is the Annihilation and rearrangement of point imperfections and dislocations without the migration of high angle grain boundaries. Recovery is initially very rapid, and more when the annealing temperature is high. Electrical conductivity increases rapidly toward the annealed value and lattice strain measured using XRD is appreciably reduced. Properties those are sensitive to point defects are affected, and strength properties are not affected. With increasing time at constant temperature the recovery becomes slower. The greater the initial cold work, the more rapid is the initial rate of recovery. The rate of recovery of fine grains is higher than that of coarse grains.
Polygonization one of the recovery processes which leads to rearrangement of the dislocations, with a resultant lowering of the lattice strain energy. It is a process of arranging excess edge dislocations in the form of tilt boundaries, and the excess screw dislocations in the form of twist boundaries, with the resultant lowering of the elastic strain energy. Climb and slip of dislocations are essential for polygonization. The presence of solute atoms in a metal reduces the rate of polygonization.
Recrystallization It is nucleation and growth of new strain-free crystals from the cold worked metal. Kinetics of recrystallization resembles a phase transformation. Two distinct nucleation mechanisms have been identified. (1) Strain-induced boundary migration, where a strain-free nucleus is formed when one of the existing grain boundaries into its neighbour, leaving a strain-free recrystallized region. (2) new grains are formed in the regions of sharp lattice curvature through subgrain growth. This seems to predominate at high strains, with nuclei appearing at grain boundaries or at inclusions or second phase particles. Mechanical properties change drastically over a very small temperature range to become typical of the annealed material. Electrical resistivity decrease sharply.
Factors influence recrystallization behavior are (1) Amount of deformation (2) temperature (3) time (4) initial grain size (5) composition (6) amount of recovery or polygonisation (7) Method of deformation. Hence recrystallization temperature is not a fixed temperature in the sense of a melting temperature. It can be defined as the temperature at which a given alloy in a highly cold-worked state completely recrystallizes in 1h. The laws of recrystallization are: (1) a minimum amount of deformation is needed to cause recrystallization. (2) Smaller the degree of deformation, higher the temperature required to cause recrystallization. (3) Recrystallization rate increases exponentially with temperature. Doubling the annealing time is approximately equivalent to increasing the annealing temperature 10°C. (4) Greater degree of deformation and lower annealing temperature, the smaller the recrystallized grains. (5) Larger the original grain size, the greater the amount of cold-work required to produce equivalent recrystallization temperature. (6) The recrystallization temperature decreases with increasing impurity of motel. Alloying always raise recrystallization temperature. (7) The amount of deformation required to produce equivalent recrystallization behavior increases with increased temperature of working.
Solute and Pinning effects The impurity in metal segregate at grain boundary and retard the migrating boundaries during recrystallization. This is known as the solution drag effect. When fine second phase particle (carbides) lies on the migrating boundary, the grain boundary area is reduced by an amount equal to cross sectional area of particle. When the boundary moves further, it has to pull away from the particle and thereby create new boundary are equal to cross sectional area of particle. This increases energy and manifests itself as a pinning acting on the boundary. Consequently the rate of recrystallization decreases.
Grain growth It is uniform increase in the average grain size following recrystallization. The grain size distribution does not change during normal grain growth. During abnormal grain growth called secondary recrystallization because the phenomenon shows kinetics similar to recrystallization, the grain size distribution may radically change i.e. some very large grains present along with the fine grains. The driving force for abnormal growth is decrease in surface energy. Solute drag and pinning action of second phase particles retard movement of a migrating boundary during grain growth as well.
Grain growth It is uniform increase in the average grain size following recrystallization. The grain size distribution does not change during normal grain growth. During abnormal grain growth called secondary recrystallization because the phenomenon shows kinetics similar to recrystallization, the grain size distribution may radically change i.e. some very large grains present along with the fine grains. The driving force for abnormal growth is decrease in surface energy. Solute drag and pinning action of second phase particles retard movement of a migrating boundary during grain growth as well.
comparison of mechanical properties during Recovery, Recrystallization and Grain growth.
