Welds going bananas?

Fabricators frequently ask: 'How much should I allow for distortion caused by welding?'. The formulae for calculating the shrinkage that will occur in an arc welded fabrication can be applied only to very simple cases. Distortion of arc welded components is generally caused by two factors: shrinkage of the cooling weld metal and local expansion and contraction of the plate.

Longitudinal shrinkage shortens the weld, transverse shrinkage decreases the width, and angular distortion causes rotation of the plates. Apart from these simple effects of shrinkage, longitudinal contraction of a weld may cause components to bow in a direction depending on the location of the weld in relation to the neutral axis of the component. The middle of a length of weld will bow towards the neutral axis.

Some values for shrinkage quoted in TWI's booklet Control of distortion in welded fabrications (available from Woodhead Publishing) are as follows:

Transverse shrinkage
Fillet welds: 0.8mm per weld where the leg length of the weld does not exceed 0.75 x the plate thickness.
Butt welds: 1.5-3mm per weld for 60° V joints, depending on the number of runs per weld.

Longitudinal shrinkage
Fillet Welds: 0.8mm/3m of weld.
Butt Welds: 3mm/3m of weld.

However, it may not always be necessary to make allowances for shrinkage at the assembly stage. This is particularly true when only one set of dimensions is important, for example, that which relates to the overall length of the fabrication. In this case, it is often possible to leave the fitting of one member, such as an endplate, until the remainder of the fabrication has been welded and most of the shrinkage has taken place. The remaining item is then fitted and welded in its correct position.

The above allowances for distortion apply to welded joints that are free to move; in practice, the restraint built up during the fabrication will determine the distortion. TWI welding engineers have, over the years, built up a wealth of experience available to Industrial Members to help them avoid distortion. Rectification of distortion is possible by the use of mechanical force or judiciously applied heating, but the cost of correction is generally at least ten times that of making the job to the required dimensional tolerances in the first place.

Rules for minimising distortion during welding
1. Design fabrications so that welds are balanced each side of the neutral axis.	11. Use high speed welding processes where possible, eg, iron powder MMA electrodes, MIG welding or mechanised welding.
2. Do not over specify fillet weld sizes.	12. Use frequent tacking.
3. Use double sided welds rather than single sided, and minimum bevel angles, to reduce the amount of weld metal.	13. Balance welding on each side of the neutral axis, i.e. do not weld all one side before starting the other.
4. Use minimum gap sizes.	14. Weld fabrications clamped back to back and preset if possible; alternatively stress relieve before releasing from the clamps.
5. In non-fatigue sensitive areas use intermittent fillet welds where possible.	15. Use block welding to prevent movement.
6. Use double fillet welds where possible, rather than full penetration T butt welds.	16. When block welding thick plate, butter the sides of the preparation and build up the buttering progressively towards the centre of the joint, so that most of the joint can contract transversely before the joint is bridged.
7. Use clamps, strongbacks, jigs or fixtures.	17. Weld first the joints that cause the most contraction.
8. Use welding positioners so that welding can be carried out in the flat or horizontal-vertical positions with high deposition rates.	18. Make use of sub-assemblies.
9. Deposit a few weld runs alternately on each side of the joint in double V butt welds.	19. Make frequent dimensional checks during welding, and if distortion is evident change the welding sequence or the clamping arrangements accordingly.
10. Weld a large construction from the centre outwards.