Stress relief is a process of heat treatment in which a metal is subjected to a constant temperature below its critical temperature and then controlled cooling is carried out.
Drawing, forming and processing cause stress in materials. A stress relief process is typically used to remove internal (residual) stresses that have accumulated in the material. These stresses can cause loss of tolerance, cracking and deterioration, and can contribute to malfunctions during operation. For these reasons, stress relief is often necessary and even mandatory.
Stress relief is performed by heating a furnace to a temperature below the critical temperature (different materials have different temperatures) and holding it at that temperature long enough to achieve the desired reduction in residual stresses. The temperature, duration and time kept at temperature are the basic processcycles, if necessary, stress relief can be carried out in a protective gas furnace to protect the surfaces from oxidation. In extreme conditions, vacuum ovens can be used.
This process is used for steels with a carbon content of less than 0.4%. This type of steel cannot be hardened as desired, but because they are relatively ductile, they are usually cold formed and formed and hardened. In other words, the crystal structures are deformed to the degree of sec according to normal equilibrium conditions. The recrystallization temperature starts at 500oC. In practice, the annealing temperature is usually carried out at 630oC. Grain formation is accelerated by accelerating heat treatment. Hot forming processes, such as hot rolling, are performed again above the crystallization temperature (at this temperature, the crystals take proper shape again after being deformed). The deteriorated state in the crystal structure is removed as fast as possible.
The cold forming process is carried out below the crystallization temperature again, resulting in excessive deterioration in the structure of the steel. In these conditions, steel is extremely tense, hard and brittle. The success of stress relief annealingis attributed to the initiation of crystallization and the confinement of stresses by the formation of cores at subcritical temperatures. The core crystal forms a structure at each deformed stress condensation point. The development of these core crystals or nuclei continues.
If the temperature can be maintained normally, the balanced grain structure is restored. When continued heating, the grain structure develops in undesirable properties.
Benefits of cold-formed low-carbon full annealing processes:
1.Energy consumption is low because low temperature is required.
2.Maintenance costs of furnace linings and heating elements are reduced due to low temperatures.
3.Due to the low oxidation of steel at low temperatures, cheaper “closed annealing” is applied instead of the expensive method of controlling the atmosphere. In indoor annealing, the steel plates are placed in the oven and only the two outer plate surfaces change color. Such sheet plates are generally called plates obtained by cold rolled closed annealing.
4.This process is faster than full annealing, grain formation is less, resulting in more advanced mechanical properties.
Advantages of Stress Relief:
Relieves internal stresses without causing any color change.
Returns the material to a strength level approximately equivalent to that it was in before forming.
Increases the mechanical strength of the material.
Welded structures can be de-stressed by stress relief.