COURSE: The Structure of Metals Recovery, Recrystallization, and Grain Growth
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Recovery, Recrystallization, and Grain Growth OPlastic deformation at room temperature causes distortion of the grains and grain boundaries (leading to anisotropic behavior), a general increase in strength, and a decrease in ductility. These effects can be reversed, and the properties of the metal can be brought back to their original levels, by heating the metal to a specific temperature range for a given period of time-a process called annealing. Three events take place consecutively during the heating process: 1. Recovery 2. Recrystallization 3. Grain growth
Recovery, Recrystallization, and Grain Growth 1. Recovery: During recovery, which occurs at a certain temperature range below the recrystallization temperature of the metal (described next), the stresses in the highly deformed regions of the metal piece are relieved. Sub grain boundaries begin to form (a process called polygonization), with no significant change in mechanical properties such as hardness and strength. 2. Recrystallization: This is the process in which, within a certain temperature range, new equiaxed and strain- free grains are formed, replacing the older grains. The temperature required for recrystallization ranges approximately between 0.3Tm and 0.5 Tm, where Tm is the melting point of the metal on the absolute scale. Generally, the recrystallization temperature is defined as the temperature at which complete recrystallization occurs within approximately one hour. Recrystallization decreases the density of dislocations, lowers the strength, and raises the ductility of the metal. Lead, tin, cadmium, and zinc recrystallize at about room temperature; consequently, they do not work harden when cold worked.
Recovery, Recrystallization, and Grain Growth 3. Grain Growth: If the temperature of the metal is raised further, the grains begin to grow, and their size may eventually exceed the original grain size; called grain growth, this phenomenon adversely affects mechanical properties. Residual stresses Large grains also produce a rough surface appearance on sheet metals, called orange peel, when they are stretched to form a part, or on the surfaces of a piece of metal when subjected to bulk deformation, such as compression in forging Ductility Strength,Strength hardness ductility Hardness and recovered New grains Grain size Grain growth Recovery Recrystal-i lization Temperature
Effects on recrystallization The effects on recrystallization of temperature, time, and plastic deformation by cold working are as follows: a) For a constant amount of deformation by cold working, the time required for recrystallization decreases b) The more the prior cold work, the lower the temperature required for recrystallization; c) The higher the amount of deformation, the smaller the grain size becomes during recrystallization; this with increasing temperature; effect is a commonly used method of converting a coarse-grained structure to one having a finer grain, and thus one with improved properties; d) Some anisotropy due to preferred orientation usually persists after recrystallization; to restore isotropy, a temperature higher than that required for recrystallization may be necessary.
Cold, Warm, and Hot Working Cold working refers to plastic deformation that is usually, but not necessarily, carried out at room temperature When deformation occurs above the recrystallization temperature, it is called hot working "Cold" and "hot" are relative terms, as can be seen from the fact that deforming lead at room temperature is a hot-working process, because the recrystallization temperature of lead is about room temperature. OAs the name implies, warm working is carried out at intermediate temperatures; thus, warm working is a compromise between cold and hot working.
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