TWI Frequently asked questions
Because welding involves highly localised heating of joint edges to fuse the material, non-uniform stresses are set up in the component because of expansion and contraction of the heated material.
Initially, compressive stresses are created in the surrounding cold parent metal when the weld pool is formed due to the thermal expansion of the hot metal (heat affected zone) adjacent to the weld pool. However, tensile stressesoccur 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 metalvolume 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 roomtemperature.
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 componentdimensions and distorts the structure.
Copyright by TWI, 1999
