~ ~ Weld residual stress and the need to eliminate the need for ~ ~
After the welding deformation and residual stress has a great relationship; in the welding process, in addition to thermal stress, because the heat caused by deformation is also an unavoidable phenomenon, and the main reason for the deformation is that when the thermal stress exceeds the drop strength, The material begins to deform and release the stress beyond the yield strength, and the material under high temperature conditions, the drop strength will be reduced, so that the material susceptible to residual stress and deformation.
Welding deformation caused by residual stress:
(A) Transverse Shrinkage: Shrinkage in the direction of the bead.
(B) Longitudinal Shrinkage: Shrinkage parallel to the direction of the bead.
(C) Angular Distortion: Due to the uneven distribution of temperature along the thickness direction
Caused by the angle of change.
(D) Rotational Distortion (Rotational Distortion): due to the role of thermal expansion caused
(E). Bending Distortion: When the bead position is not neutral to the base metal
Axis, due to heat shrinkage uneven formation in the direction of the bend along the bend changes.
(F). Buckling Distortion (Buckling Distortion): When welding sheet, due to hot compressive stress
The role of the wave caused by deformation.
The following describes the residual stress on the weld of other hazards:
1. Residual stress caused by the impact of welding cracks:
Welding residual stress is one of the main causes of welding cracks, especially for the more brittle materials, welding cracks is a difficult to avoid the problem. Longitudinal residual stress causes transverse cracks to occur, and lateral residual stresses cause longitudinal cracking.
2. Impact of weldment and breaking strength:
When the material itself has a better ductility, the residual stress will cause plastic deformation of the weld bead. If the material itself is poor ductility, the residual stress will make the whole material easier to break the fracture, because there is a tensile stress in the material, as long as a small load, you can reach the yield strength and began to deformation; but because of the material itself ductility , The load continues to increase easily lead to material breakage.
3. Effects of residual stress on fatigue strength:
Residual stress on the fatigue strength of the weldment, with the stress distribution and bear the stress. In general, when the direction of external stress and residual stress are the same, it will cause the fatigue strength to decrease, but when the external stress direction and the residual stress direction is opposite, it will help the fatigue strength.
4. Effect of Residual Stress on Machining Accuracy:
After the welding work is over, the workpiece is often subject to subsequent processing. After the removal of the welding parts, the stress at the same time is released, the welding residual stress in order to achieve a balanced state, must be redistributed, at the same time the shape and size of welding parts will also change, processing accuracy is not easy to control.
5. Effect of Residual Stress on Stress Corrosion:
Corrosion creates a chemical effect on the metal and the surrounding environment and causes rupture, and when corrosion occurs, there is an additional residual stress that accelerates corrosion damage, which is stress corrosion cracking.
In general, the industry encountered the quality of these five kinds of weldments and safety problems, are subject to the impact of residual stress caused by the residual stress to eliminate the weld is very important. The elimination of residual stress is currently only in the form of hot or mechanical force, the way the heat is the most commonly used, but not all people or things are suitable, then you can have another choice. Like when your material will heat after the heat (like stainless steel relaxation after annealing will be sensitized and will start to rust), will become soft (hardened steel relaxation force will be Ma Tian iron will disappear after the disappearance of hardness ), Will be deformed (the workpiece after the relaxation of the stress will be due to the release of a large number of stress deformation), will be oxidized (due to high temperature materials easily combined with oxygen), or your workpiece is too large and so are not applicable, the other heat treatment Of the equipment is expensive (especially with the vacuum or protective atmosphere function), the processing time is long (about four times more than the vibration time), the processing site is limited (can not bring the stove to the construction site), then vibration stress elimination Is the best choice!