Welding distortions arise:
- Because of a shrinkage of melted filler material
- Because of an upsetting of material due to heating
Plastic upsetting is a highly non-linear material behavior. Inconsistent thermal expansion of the heat affected zone leads to non-homogeneous plastic deformation through the cross section of a weld. The mechanism behind this effect is used by flame straightening. Thermal expansion and shrinkage lead to residual stresses that cause further distortions and stress redistribution through the cross section. The component takes its initial shape if no plastic deformations are present during this process. Due to changed material properties, such as yield strength, high temperatures and even small stresses lead to plastic deformations. Additional restraints by clamping or the shape of a component itself are able to increase the residual stresses.
As a result of the temperature field, there is asymmetric behavior during heating and cooling triggering the development of plastic deformations. Three main mechanisms are to be noted:
- Plastic deformations during heating are of a local nature. During cooling, the surrounding material experiences effects of heating by heat conduction from the weld pool, generates pressure towards shrinking material, and permits a partial regeneration of plastic deformations
- The elasticity of surrounding material is reduced
- Plastic deformations during heating appear at positions where tensile strength is reduced and thermal expansion is increased. Compressive stresses are reduced during cooling, which leads to plastic deformation of highly shrunk material at lower temperatures and higher tensile strength. After heating and cooling, if there are plastic deformations observable, residual stresses remain in the structure. Those residual stresses lead to reduced fatigue strength.