DieselShip

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.

Typical causes of defects are;

• Welders deviating from proceedures
• Poor edge prepation
• Excessive, insufficient uneven gap
• Environmental conditions- high wind, wet surfaces, low steel temperature.

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

The amount of shrinkage is dependent on the following factors;

• Plate thickness Shrinkage will reduce with larger plate thickness
• Gap Shrinkage will increase with increased gap due to the increased amount of hot filler material required
• Joint Fit Tack welding should be used to both set and maintain the gap of butt joins. There is a tendency for gaps to close as the weld progresses
• Parent Metal Properties The higher the thermal coefficient of expansion the more the metal will expand during the welding process. This hot meterial cannot expand freely but is constrained by the surrounding cool material. The lower heat conductivity of the material the more localised the heat is and the greater the possibiity of distortion. Both of the above factors are important in the welding of stainless steels
• Joint restraint To maintain alignement strongbacks may be used to lock plates. The disadvantage of this is that the plates can no longer stress relieve.
  
  For some materials this may require the use of a heat treatment process.



• Joint Design Double sided welding will reduce significantly distortion.

Although there is a risk of cracking due to the locked in stresses the reality is that this is very rare. A greater risk comes from the removal of the strongbacks after completion. These should be ground off and not 'hammered' as this tends to tear the parent material. Should a tear occur this has to be ground out, filled with weld and ground back.

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.

To reduce distortion the folowing may be adopted;

• Reduce the amount of welded joints - by using extruded and preformed sections such as bulb plate stifferners
• Minimise weld volumes - The more hot weld filler material is in the weld the greate will be distortion
  
  The Cap on the weld is weld filler that does not contribute to the load bearing structure of the weld.

The use of double sided welding can reduce the amount of weld used however this is at the cost of time. Proper edge preparation can help to reduce the weld material required especially for single sided welding.

Bevelling one edge of a butt weld can help with compensating for single sided welds

• limit weld runs the more weld runs, the more the distortion. Generally a single large run will create less distortion than severl smaller runs.
• Tack Welds This should maintain the gap

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.