News

Recrystallization of metals

After cold working, metal undergoes plastic deformation, and its internal lattice will be destroyed, resulting in displacement, distortion and other phenomena different from the normal crystal structure.

When the plastically deformed metal is heated to a certain temperature, the structure of the distorted lattice can be restored in the deformed structure, and new grains begin to form. Finally, all the structures are replaced by new grains, and the changes caused by cold working are eliminated. This process is called recrystallization. The temperature at which this new fine grain begins to form is called the recrystallization temperature.

The recrystallization temperature of different metals is different. The recrystallization temperature is determined by the degree of deformation of the metal during mechanical processing, the duration of the metal during the processing, and other factors. When the metal is deformed more and the heating duration is longer, the recrystallization temperature decreases.

When the deformed metal is heated to above the recrystallization temperature, the small grains formed by the generation of the crystal nucleus (crystallization center) and the growth of the crystal during the crystallization process gradually increase. The fiber structure obtained by the metal during pressure processing gradually disappears, and round grains are formed again. The interface between grains is far more irregular and loose than the interior of the grains due to the different orientations of adjacent grains, and is prone to fracture.

When metals recrystallize, the changes in mechanical properties are mainly manifested in the decrease of strength and hardness limits, the increase of relative elongation during stretching, and the acceleration of creep. It must be noted that when the temperature is too high and the duration is too long, the grains will become very coarse, causing the metal to lose strength and break easily. For example, the brittleness of incandescent filaments increases after recrystallization. Therefore, by selecting appropriate processing specifications for metals, grains of the required size can be obtained, thereby ensuring ideal metal mechanical properties.

During annealing, the temperature at which the metal is heated is generally slightly higher than the recrystallization temperature, because at the recrystallization temperature, the process of eliminating the effects of cold working is very slow. The recrystallization temperature is not only the main criterion for determining the annealing temperature of metals, but also largely determines the operating temperature of metal parts of electric light sources.

PREVIOUS：Alloys used in electric light sources NEXT：Thermal properties of metals