Structural Repairs - minimising distortion

Distortion

Distortion occurs during electric arc welding process due to localised heating of the material and by the hot infill material cooling and shrinking.

These processes are well knwn and proper selection of preparation and weld technique have largely eliminated this as an area of concern. However faults can still occur and this is due mainly to poor workmnship.

This shows typical angular distortion of a plate. Distortion may be transverse or longitudinal and may also cause bowing or dishing or some bomination of all three

Weld Proceedures

There has to be a balance with the desire for a perfect repair against the practical and commercial viabiity of achieving this.

Distortion can be minimised by the use of double sided welds. However this may prove difficult to achieve in practice. instead single sided welds may be preferred with the use of backing strips.

It should be noted that many yards now prefer MIG welding due to its speed, reduced cleaning requirements and simplicity. The down side of MIG is lack of penetration and great care must be taken to ensure that welds using this techniques have been properly prepared.

Repair of Distortion

Should distortion occur there are two methods of restoring the shape.

Mechanical - This involves the use of direct force on the material. Typically this will take the form of strong back and jack. As these are considered point loads there is rela risk of further damage and car should be taken.

Thermal - This involves the application f localised heat. The amount of heat is governed by the material but generally the temperature should not exceed 650'C although this may be increased to 1000'C for lw carbon and high tensile steels. Care should be taken with water quenching not to change the structure of the material

The plate is bent by applying localised heat, this causes a hot spot were the material expands but is constricted by the cold surrounding material. When the heated material cools it tends t bunch up pulling the plate inwards towards the original heat source

Where there is a large area area of distortion suggest as plate dishing, then straightening should be my several point heating rather than a single large one