Weld Preparation

Angle of bevel

The angle at which the edge of a component is prepared for making a weld

For an MMA weld on carbon steel plates, the angle is:

25-30 Deg. for a V preparation

8-12 Deg. for a U preparation

40-50 Deg. for a single bevel preparation

10-20 Deg. for a J preparation

Included angle

The angle between the planes of the fusion faces of parts to be welded. In the case of single V or U and double V or U this angle is twice the bevel angle. In the case of single or double bevel, single or double J bevel, the included angle is equal to the bevel angle.

Root face

The portion of a fusion face at the root that is not beveled or grooved. Its value depends on the welding process used, parent material to be welded and application; for a full penetration weld on carbon steel plates, it has a value between 1-2mm (for the common welding processes).

Gap

The minimum distance at any cross section between edges, ends or surfaces to be joined. Its value depends on the welding process used and application; for a full penetration weld on carbon steel plates, it has a value between 1-4mm.

Root radius

The radius of the curved portion of the fusion face in a component prepared for a single J or U, double J or U weld. In case of MMA, MIG/MAG and oxy-fuel gas welding on carbon steel plates, the root radius has a value of 6mm for single and double U preparations and 8mm for single and double J preparations.

Land

The straight portion of a fusion face between the root face and the curved part of a J or U preparation can be 0. Usually present in weld preparations for MIG welding of aluminium alloys.

Types of preparation

Open square butt preparation

This preparation is used for welding thin components, either from one or both sides. If the root gap is zero (i.e. if components are in contact), this preparation becomes a closed square butt preparation (not recommended due to the lack of penetration problems)!

Single V preparation

The V preparation is one of the most common preparations used in welding; it can be produced using flame or plasma cutting (cheap and fast). For thicker plates a double V preparation is preferred since it requires less filler material to complete the joint and the residual stresses can be balanced on both sides of the joint resulting in lower angular distortion.

Double V preparation

The depth of preparation can be the same on both sides (symmetric double V preparation) or can be deeper on one side compared with the opposite side (asymmetric double V preparation). Usually, in this situation the depth of preparation is distributed as 2/3 of the thickness of the plate on the first side with the remaining 1/3 on the backside. This asymmetric preparation allows for a balanced welding sequence with root back gouging, giving lower angular distortions. Whilst single V preparation allows welding from one side, double V preparation requires access to both sides (the same applies for all double side preparations).

Single U preparation

U preparation can be produced only by machining (slow and expensive). However, tighter tolerances obtained in this case provide for a better fit-up than in the case of V preparations. Usually it is applied to thicker plates compared with single V preparation as it requires less filler material to complete the joint and this leads to lower residual stresses and distortions. Similar to the V preparation, in the case of very thick sections a double U preparation can be used.

Double U preparation

Usually this type of preparation does not require a land (exception: aluminium alloys).

Single V preparation with backing strip

Backing strips allow the production of full penetration welds with increased current and hence increased deposition rates/productivity without the danger of burn-through. Backing strips can be permanent or temporary. Permanent types are made of the same material as being joined and are tack welded in place. The main problems related to this type of weld are poor fatigue resistance and the probability of crevice corrosion between the parent metal and the backing strip. It is also difficult to examine by NDT due to the built-in crevice at the root of the joint. Temporary types include copper strips, ceramic tiles and fluxes.

All these preparations (single/double bevel and J) can be used on T joints as well. Double preparations are recommended in the case of thick sections. The main advantage of these preparations is that only one component is prepared (cheap, can allow for small misalignments).

For further details regarding weld preparations, please refer to Standard BS EN ISO 9692.