02-09-2017, 01:30 PM
Because welding involves highly localized heating of the edges of the joints to melt the material, non-uniform stresses in the component are established due to the expansion and contraction of the heated material. Initially, compression stresses are created in the surrounding cold parent metal when the weld pool is formed due to thermal expansion of the hot metal (heat affected zone) adjacent to the weld pool. However, tensile stresses occur in cooling when the shrinkage of the weld metal and the area affected by the immediate heat is resisted by the mass of the cold original metal.
The magnitude of the thermal stresses induced in the material can be seen by the change in volume in the welding area in the solidification and the subsequent cooling to room temperature. For example, when CMn steel is welded, the volume of weld molten metal will be reduced by about 3% in solidification and the volume of the welded / heat affected metal (HAZ) solidified zone will be reduced by 7% of the steel at room temperature.
If the stresses generated by the thermal expansion / contraction exceed the resistance to deformation of the parent metal, a localized plastic deformation of the metal occurs. The plastic deformation causes a permanent reduction of the dimensions of the component and distorts the structure.
The magnitude of the thermal stresses induced in the material can be seen by the change in volume in the welding area in the solidification and the subsequent cooling to room temperature. For example, when CMn steel is welded, the volume of weld molten metal will be reduced by about 3% in solidification and the volume of the welded / heat affected metal (HAZ) solidified zone will be reduced by 7% of the steel at room temperature.
If the stresses generated by the thermal expansion / contraction exceed the resistance to deformation of the parent metal, a localized plastic deformation of the metal occurs. The plastic deformation causes a permanent reduction of the dimensions of the component and distorts the structure.