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What are the main types of distortion?

 
  • Longitudinal shrinkage
  • Transverse shrinkage
  • Angular distortion
  • Bowing and dishing
  • Buckling
  • Twisting

Contraction of the weld area on cooling results in both transverse and longitudinal shrinkage.

Non-uniform contraction (through thickness) produces angular distortion as well as longitudinal and transverse shrinking.

For example, in a single V butt weld, the first weld run produces longitudinal and transverse shrinkage and rotation. The second run causes the plates to rotate using the first weld deposit as a fulcrum. Therefore balanced welding in a double side V butt joint can be used to produce uniform contraction and prevent angular distortion.

Similarly, in a single sided fillet weld, non-uniform contraction will produce angular distortion of the upstanding leg. Double sided fillet welds can therefore be used to control distortion in the upstanding fillet but because the weld is only deposited on one side of the base plate, angular distortion will now be produced in the plate.

Longitudinal bowing in welded plates happens when the weld centre is not coincident with the neutral axis of the section so that longitudinal shrinkage in the welds bends the section into a curved shape. Clad plate tends to bow in two directions due to longitudinal and transverse shrinkage of the cladding. This produces a dished shape.

Dishing is also produced in stiffened plating. Plates usually dish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds.

In plating, long range compressive stresses can cause elastic buckling in thin plates, resulting in dishing, bowing or rippling.

Distortion due to elastic buckling is unstable; if you attempt to flatten a buckled plate, it will probably snap through and dish out in the opposite direction.

Twisting in a box section is caused by shear deformation at the corner joints. This is caused by unequal longitudinal thermal expansion of the abutting edges. Increasing the number of tack welds to prevent shear deformation often reduces the amount of twisting.

Increasing the leg length of fillet welds, in particular, increases the shrinkage.

For further information on types and causes of distortion see Job Knowledge for Welders 33.

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