Welding , Hardfacing , Cladding and Cutting of metals: Distortion of Welded structures and its control

DISTORTION IN THE WELDING AND FABRICATION INDUSTRY

In the welding and fabrication industry one of the greatest difficulties confronting the welders and fabricators is that of avoiding distortion of the welded work piece / structure.
Distortion or deformation is caused by the localised heating and cooling of the molten weld pool , the heat being generated by a external heat source . This results in the uneven expansion and contraction of the weld metal and base metal during the heating and cooling cycle. The uneven heating and cooling cycles cause stresses to form in the weld as a result of the changes in volume. If distortion takes place the job may have to be completely scrapped. Depending on the size and complexity of the job it may take a long time, effort and money to repair a distorted work / structure. The smaller the molten weld pool the less the distortion .
The various types of distortion are as listed below

Angular Distortion

Fillet weld distortion

Neutral axis distortion

Two or more types of distortion may occur at the same time.
The customer should check the job drawings to ascertain work piece size (dimensions) and weight, chemical composition of base material and welded material and Filler material expected to be used. Facilities available at the work shop / site for welding, fabrication, machining and other processes should be taken into account. This will help to select the correct welding process and consumables required.
The Physical and chemical properties of the parent material and the inherent stresses induced from previous metal-working processes such as rolling, forming and bending should be investigated before hand . NDT should be carried out to rule out internal defects in the parent material.
The welding designer should ensure that welding should be done only where required and caution against excessive welding build up and uncontrolled heat input. Material to be welded should be under minimum restraint. Exercise care in distributing the welding heat by skip sequence, back stepping etc. to prevent excessive local heating. Preheat prior to welding is sometimes desirable to avoid an excess heat condition particularly on heavy structures and to relieve stresses brought about by rolling or forming.
The factors affecting distortion are

Welds should be designed to minimise or prevent distortion. The sequence of welding should be balanced. Make sure the various parts of the weldment are properly aligned prior to starting welding. Make allowance for shrinkage by pre-setting the parts to be welded out of position. Plan the weld sequence to ensure that shrinkages are counteracted progressively. Minimize welding time and lower heat input to minimum required.
The solutions to avoid distortion are,

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