Distortion

Distortion is a problem that exists in all industrial metal working processes that employ heat none more evident than shipbuilding where complex assemblies are joined together using various welding processes.

There are a number of factors to be considered in dealing with the cause of distortion and its reduction

Plate and section manufacture: Residual stresses and tolerance. Transportation of steel: Loading and unloading. Storing of steel: Inducing sag when placing plates and sections on unevenly spaced batons. Design: Plate thickness (weight restraint) openings and cut-outs Fillet design size. Thermal cutting processes: Induces and affects the distribution of residual stress. Welding Process: Non-uniformed temperature change. Weight and Design: Lifting turning and transportation can and does affect the structure.

Concentrating on the fabrication and welding processes.

Ensure that the joint set-up is acceptable, good fit-up reduces the amount of welding and therefore the heat input into the structure.

Consideration should be given to using block weights prior to welding small panels. Dog down or stud panels where deemed appropriate. Fit ribbon bars to plate edges. Tack welds should be of the appropriate length and the welding current should be properly set.

Fillet welding of panels to commence only when the pre-requisites are in place.

The fillet size is known. The general sequence of welding is clear.

The broad welding sequence requirements are as follows.

Care should be taken to weld those welds which produce the greatest amount of shrinkage first. Spread welding generated heat throughout the panel i.e. avoid the concentration of heat in any

one place. Where possible minimize the number of welders working on a panel at any one time. Flat fillet welding to be completed prior to welding vertical connections where possible. Work from Centre out to free edges and diagonally away from each other when deploying two

welders on a flat panel. Avoid over welding where fit-up allows, a 7mm leg length has almost twice the amount of weld

metal than a 5mm leg which equates to twice the amount of heat input resulting in greater distortion.