When a weld pool is first formed, thermal expansion of the hot metal in the heat affected zone adjacent to the weld pool creates compressive stresses in the surrounding cold parent metal. However, tensile stresses occur on cooling when the contraction of the weld metal and immediate heat affected zone is resisted by the bulk of the cold parent metal.
The magnitude of thermal stresses induced into the material can be seen by the volume change in the weld area on solidification and subsequent cooling to room temperature. For example, when welding C-Mn steel, the molten weld metal volume will be reduced by approximately 3% on solidification and the volume of the solidified weld metal/heat affected zone will be reduced by a further 7% as its temperature falls from the melting point of steel to room temperature.
If the stresses generated from thermal expansion/contraction exceed the yield strength of the parent metal, localised plastic deformation of the metal occurs. Plastic deformation causes a permanent reduction in the component dimensions and distorts the structure.