Analysis of the three stages of annealing using medium frequency induction heating power supply

Annealing is a heat treatment process that eliminates work hardening of metal materials and restores the original plasticity index of the material. The annealing process has a wide range of applications.

First of all, annealing can completely eliminate the stress and strain of the material during cold working and restore its plasticity, while maintaining good workpiece surface and dimensional accuracy. Secondly, after annealing, the plasticity of the metal material is restored, that is, it can be cold worked. After repeated cold working and annealing cycles, the total strain of the workpiece is large. Finally, annealing using a medium-frequency induction heating power supply at low temperatures can eliminate the residual stress formed during cold working without affecting the mechanical properties of the finished parts.

Reply: Internal stress elimination annealing is a low-temperature heat treatment process that can reduce residual stress inside the metal. Its structure consists of deformed grains, and the interior of the grains contains a large number of disordered network dislocations. When we use a medium frequency induction heating power source to heat the metal to a slightly higher temperature, the dislocations begin to move and rearrange, while the residual stress decreases until it is finally eliminated.

However, the number of dislocations is not significantly reduced, but is rearranged into a polygonal structure, which is a subgrain structure of deformed grains, with dislocations located at subgrain boundaries. Since the number of dislocations is not reduced, the mechanical properties of the metal remain relatively unchanged.

The recovery process restores the high electrical conductivity of cold-worked copper or aluminum wires for carrying electrical current. After cold processing, the wire has high strength and can accurately construct wires between far apart electrodes. At the same time, the wire has excellent electrical conductivity.

Recrystallization: The recrystallization process includes two stages: the nucleation and growth of new grains, which also contain a very small amount of dislocations. When the metal is heated to above the recrystallization temperature by the medium-frequency induction heating power supply, which is approximately 0.4 times the melting point of the metal, the rapid recovery process can eliminate residual stress and produce a polygonalized dislocation structure. New grains nucleate on the grain boundaries of the polygonal structure, eliminating most of the dislocations. Since the number of dislocations is greatly reduced, the strength of the metal is reduced and the plasticity is high.

Crystal nucleus growth: At higher annealing temperatures, after rapid recovery and recrystallization stages, a finer recrystallized grain structure is formed. However, the energy contained in numerous grain boundary regions makes the recrystallized grain structure unstable at high temperatures. In order to reduce the energy and make the structure more stable, the grains begin to grow, and the dominant coarse grains replace the smaller grains.