What is Butt Welding ? & What are the Various Types of Butt Welds ?

A butt weld is one of the simplest and versatile types of weld joint designs. Butt welding can also be done with brazing for copper pieces. It is a weld made between two plates so as to give continuity of section. The joint is formed simply by placing two pieces of metal end-to-end and then welding along the join.

A butt joint is the most universally used method of joining pipe to itself, fittings, flanges, Valves, and other equipment. This welding technique is widely applied in situations where a quality weld desired, and the weld by X-ray technically should be investigated.

Importantly, in a butt joint, the surfaces of the work pieces being joined are on the same plane and weld metal remains within the planes of the surfaces. Thus, work pieces are nearly parallel and do not overlap, unlike lap joints .

Close attention must be paid to detail in a butt weld to ensure that the maximum strength of the weld is developed. Failure to properly prepare the edges may lead to the production of faulty welds, as correct manipulation of the electrode is impeded. 

Two terms relating to the preparation of butt welds require explanation at this stage. They are:
Root Face: The proportion of the prepared edge that has not been bevelled (Land).
Root Gap: the separation between root faces of the parts to be joined.

Various types of butt welds are in common use and their suitability for different thickness of steel are described as follows:

Square Butt Weld :-

The edges are not prepared, but are separated slightly to allow fusion through the full thickness of the steel. Suitable for plate up to 6 mm in thickness.

Single ‘V’ Butt Weld :-

This is commonly used for plate up to 16 mm in thickness and on metal of greater thickness where access is available from only one side.

Double ‘V’ Butt Weld :-

Used on plate of 12 mm and over in thickness when welding can be applied from both sides. It allows faster welding and greater economy of electrodes than a single ‘V’ preparation on the same thickness of steel and also has less tendency to distortion as weld contraction can be equalised.

Butt Weld with Backing Material :-

When square butt welds or single ‘V’ welds cannot be welded from both sides, it is desirable to use a backing bar to ensure complete fusion.

Single ‘U’ Butt Weld :-

Used on thick plates as an alternative to a single ‘V’ preparation. It has advantages in speed of welding. It takes less weld metal than a single ‘V’, there is less contraction and there is, therefore, a lessened tendency to distortion. Preparation is more expensive than in the case of a ‘V’, as machining is required. This type of joint is most suitable for material over 40 mm in thickness.

Double ‘U’ Butt Weld :-

For use on thick plate that is accessible for welding from both sides. For a given thickness it is faster, needs less weld metal and causes less distortion than a single ‘U’ preparation.

Horizontal Butt Weld :-

The lower member in this case is bevelled to approximately 15° and the upper member 45°, making an included angle of 60°. This preparation provides a ledge on the lower member, which tends to retain the molten metal.

General Notes on Butt Welds :- 

• The first run in a prepared butt weld should be deposited with an electrode not larger than 4.0 mm. The angle of the electrode for the various runs in a butt weld is shown below.
• It is necessary to maintain the root gap by tacking at intervals or by other means, as it will tend to close during welding.
• All single ‘V’, single ‘U’ and square butt welds should have a backing run deposited on the underside of the joint, otherwise 50% may be deducted from the permissible working stress of the joint.
• Before proceeding with a run on the underside of a weld, it is necessary to back-gouge or grind that side of the joint.
• Butt welds should be overfilled to a certain extent by building up the weld until it is above the surface of the plate. Excessive reinforcement, however, should be avoided.
• In multi-run butt welds, it is necessary to remove all slag and surplus weld metal before a start is made on additional runs. This is particularly important with the first run, which tends to form sharp corners that cannot be penetrated with subsequent runs. Electrodes larger than 4.0 mm are not generally used for vertical or overhead butt welds.

Butt welds provide several advantages, including:-

• High strength with complete fusion
• Easy to machine
• Distortion control
• Easy to inspect

Disadvantages :-

• There are some disadvantages such as:
• Welding geometry can limit applications
• Fixturing or backing may be required
• Sensitive to faying surface conditions