The metal melting in a small area, leads to the appearance of shrinkage stress. Even if the external fixation of the sample was ensured during the heating and cooling cycle, the tension caused by uneven heating and cooling persists in the cooling part. This voltage is called the residual voltage. Due to the residual stress, the sample remains curved. However, the sample will not continue to bend further, since it has already cooled to room temperature and has acquired sufficient strength to resist residual stress forces. The residual stress remains in the sample if no measures are taken to relieve tension.
In all cases, there are several ways to reduce or eliminate the residual stress. This can be achieved by changing the temperature; The entire part or large section of the part including the cuneiform zone, is evenly heated, after which a certain temperature is maintained for a fixed period of time. When using this method, uniform heating allows us to relax the residual stress due to a decrease in the strength of the metal. With slow uniform cooling to room temperature, the residual stress in the part decreases significantly. Also, there are methods for removing internal stress by vibration or mechanical treatment. Both methods have demonstrated their effectiveness in various applications
The third way to reduce the residual stress, which can be used in welding, is called suture. This method is also a mechanical treatment. When forging to strike the front side of intermediate layers of a multilayered joint, a heavy pneumatic hammer (but not a hammer to remove the slag) is used. Such a shock action deforms the surface, which leads to a decrease in the thickness of the layer. Deformation causes expansion and lengthening of the seam on the front side. As there is a slight expansion of the metal, the residual load decreases.
When carrying out the forging to relieve stress, care must be taken to prevent cracks in the welded part from appearing as a result of applying such a radical treatment method. It is not recommended to layer the layer at the root of the seam, as this can lead to its breakage.
