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After the plastic deformation of the metal after cold processing, because of its inner grain boundary fragmentation is the location of the nucleation, by crushing the grain boundary to produce the nucleus, the lattice distortion produces the slip surface fragment, in the room temperature time-varying to the original shape is produces the crystal nucleus the place, therefore exists the internal stress, therefore is unstable, it has restores to the original stable state the spontaneous tendency. But at room temperature, the activity of the atoms is very weak and the recovery process is difficult.
The cold deformed metal is heated so that the active power of the atom is enhanced, which causes the crystal nucleus to grow into grains, causing the change of microstructure and properties, and this process has the following three stages:
1, the recovery phase when the heating temperature is not high (below the minimum recrystallization temperature) atomic activity is still low, although there are small movement, but can not cause obvious changes in the organization. Because the atom has been able to do the short distance movement, the lattice distortion degree greatly reduces, thus causes the internal stress to descend greatly.
But there is no obvious change in the metal tissue, so the mechanical properties change little, this stage is called the recovery phase, also known as stress annealing.
2, recrystallization cold deformation of metal heating to a higher temperature, due to the increase in atomic activity, the formation of some lattice bearings and deformed grains, less internal defects of the isometric (all sides to the same diameter) small grains.
These small grains continue to grow in the surrounding deformation tissue until all the cold deformed tissues of the metal disappear, deform to equiaxed crystals, and reduce their stress, a process called recrystallization of metals.
Cold deformed metal after recrystallization, will be due to cold deformation and other defects such as lattice distortion and internal stress completely eliminated, so strength, hardness decreased, conductivity increased, plasticity and toughness greatly improved, cold working hardening state completely eliminated. 3, the agglomeration recrystallization cold deformed metal in the newly completed recrystallization process, generally can obtain small and uniform new equiaxed grains.
With the increase in heat too much, or prolonged insulation time, recrystallization grains after the merger and growth, so that the grains become coarse, mechanical properties are also deteriorating, this process is called agglomeration recrystallization. The mechanical properties of this coarse grained metal also become worse. So too much heating temperature or too long heat preservation time can cause metal "over burning" or "overheating." Causes the strength, especially the plasticity and the impact toughness to reduce, causes brittle break.