The effects of welding on a metal plate. It may appear as shrinkage or an angular twisting. Distortion of steel structures caused by welding and other thermal processes can be assigned to three main causes.
The first is the inherent stresses in the steel received from suppliers. These stresses are a result of heating, rolling and cooling of steel plates in the steel mills. Other causes may include the handling and storage of the plates during shipment to the shipyard and also while in storage prior to production. The inherent stresses may be relieved by cold rolling of the plates, but this may not completely eliminate them. In case of plates below 8mm thickness, cold rolling is not very effective.
The second cause is the distortion directly related to the thermal processes. If the plate is heated quickly, as during cutting or welding, the steel expands. When the plate cools slowly, the cooling causes schrinkage.
The third cause of distortion is an outcome of the management of the overall production process. This includes not only strict adherence to the correct welding procedures, but also the correct alignment of structural parts. Furthermore, it is essential to manage the correction of incipient distortion as it occurs during the assembly.