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Annealing is a heat treatment used to eliminate part or all of the effects of cold working. Annealing is a process by which the effects of strengthening caused by cold working are eliminated or modified by heat treatment.
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When a material is cold-worked or hot-worked, residual stresses are built in, and, in addition, the material usually has a higher hardness as a result of these working operations. These operations change the structure of the material so that it is no longer represented by the equilibrium diagram. Full annealing and normalizing is a heating oepration that permits the material to transform according to the equilibrium diagram. The material to be annealed is heated to a temperature that is approximately 100°F above the critical temperature. It is held at this temperature for a time that is sufficient for the carbon to become dissolved and diffused through the material. The object being treated is then allowed to cool slowly, usually in the furnace in which it was treated. If the transformation is complete, then it is said to have a full anneal. Annealing is used to soften a material and make it more ductile, to relieve residual stresses, and to refine the grain structure.
The term annealing includes the process called normalizing. Parts to be normalized may be heated to a slightly higher temperature than in full annealing. This produces a coarser grain structure, which is more easily machined if the material is a low-carbon steel. In the normalizing process the part is cooled in still air at room temperature. Since this cooling is more rapid than the slow cooling used in full annealing, less time is available for equilibrium, and the material is harder than fully annealed steel. Normalizing is often used as the final treating operation for steel. The cooling in still air amounts to a slow quench.
Annealing, pronounced uh NEEL ihng, is a process of heating metals, glass, or other materials and then cooling them. The cooling is usually done slowly. Annealing produces various changes in the composition and properties of metals. Most of these changes are desirable. For example, it makes metals softer so that they can be machined more easily. It also can reduce internal pressures called stresses from metals. Stresses may result during the manufacture of metal products or during their use.
The purpose of annealing glass is always to remove internal stresses that might later cause sudden cracking or breakage. Stresses are likely to be present because of unequal temperature distribution in the glass article while it is being cooled from its molten state. Glass articles are annealed by placing them on a metal belt that travels slowly through a long, heated enclosure called a lehr. Thin articles can be annealed in 30 minutes or less.
Annealing may be used to completely eliminate strain hardening achieved during cold working; the final part is soft and ductile but still has a good surface finish and dimensional accuracy. Or, after annealing, additional cold work could be done, since the ductility is restored. By combining repeated cycles of cold working and annealing, large total deformations may be achieved. Finally, annealing at a low temperature may be used to eliminate the residual stresses produced during cold working without affecting the mechanical properties of the finished part. There are three stages in the annealing process.
Recovery
The original cold-worked microstructure is composed of deformed grains containing a large number of tangled dislocations. When the metal is first heated, the additional thermal energy permits the dislocation to move and form the boundaries of a polygonized subgrain structure. The dislocation density, however, is virtually unchanged. This low-temperature treatment is called recovery.
Because the number of dislocations is not reduced during recovery, the mechanical properties of the metal are relatively unchanged. However, residual stresses are reduced or even eliminated when the dislocations are rearranged; recovery is often called a stress relief anneal. In addition, recovery restores high electrical conductivity to the metal, permitting the production of copper or aluminum wire for transmission of electrical power that is strong yet still has high conductivity. Finally, recovery often improves the resistance of the material to corrosion.
Recrystallization
Recrystallization occurs by the nucleation and growth of new grains containing few dislocations. When the metal is heated above the recrystallization temperature, rapid recovery eliminates residual stresses and produces the polygonized dislocation structure. New small grains then nucleate at the cell boundaries of the polyganized structure, eliminating most of the dislocations. Because the number of dislocations is greatly reduced, the recrystallized metal has low strength but high ductility.
Grain Growth
At still higher annealing temperatures, both recovery and recrystallization occur rapidly, producing a fine recrystallized grain structure. The grains begin to grow, however, with favored grains consuming the smaller grains. This phenomenon is called grain growth, and is almost always undesireable.