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Design Part 4


Part 1
Part 2
Part 3
Part 5

The previous article looked briefly at butt weld design where mention was made of the increased risk of producing defects as the bevel angle or the root gap is reduced. Bevelling the plate edges allows access to all parts of the joint, enabling good fusion throughout the weld to be achieved. The bevel can be on one or both edges of the items to be joined. What is important is the included angle which is dictated by the need both to achieve the correct torch/electrode angle and to maintain the required arc length and wire stick-out. as shown in Fig.1. The angle on a single bevel joint, as in Fig.1(c) obviously needs to be greater than that on a double bevel V-joint if access problems are not to be encountered. Experience has shown that a weld preparation angle of 45° on a single bevel joint is usually sufficient to allow adequate access.

Fig.1. Effect of a narrow weld preparation angle
Fig.1. Effect of a narrow weld preparation angle

A similar effect is produced by too narrow a root gap where, as above, there is insufficient access to permit a correct arc length to be achieved and the arc cannot be placed in the correct position. Conversely, too wide a root gap on an unbacked weld will require a large, wide weld pool to bridge the gap, resulting in melt through, a loss of control of the pool and the formation of localised excess weld metal - known colloquially as 'noddies' or 'dangleberries'.

As may be guessed from the above, the most problematic region in a weld is that of the root pass. Single sided joints require dimensionally accurate weld preparations and fit-up and skilled welders to ensure full penetration welds with an acceptable root contour. The best root pass appearance using conventional arc welding processes will be achieved using the TIG process but acceptable root conditions can also be achieved with MMA, MIG/MAG and FCAW welding.

When welding, it is obviously easier for the welder when there is a sound base on which to deposit the weld metal; hence the need for a very skilled welder when making full penetration single sided welds.

When permitted by design, it is possible to use partial penetration welds as illustrated in Fig.3 of Connect article no. 92. However, note that this type of joint is not recommended when fatigue is an issue.

Where access to the reverse side of a partial penetration weld is available, then the fabricator has the option of depositing a sealing pass. Remember, however, that most welding processes have only limited penetration and there is a real risk that not all of the unfused land will be melted away. To be certain of removing the unfused land, 'back gouging' the root and filling the groove with sound weld metal is generally carried out. Backgouging, or removal of the unfused land, can be done by any of the conventional metal removal techniques; machining, arc air gouging, chipping, grinding etc (Fig.2). Of these methods, arc air gouging is probably the most cost effective and can produce a smooth contoured U-shaped groove with an included angle of 50 to 60 degrees, allowing adequate access.

The back gouging must be sufficiently deep that any lack of penetration is removed. To confirm that this has been done it is good practice to perform magnetic particle or liquid penetrant inspection of the gouged groove.

Fig.2. Backgouging to achieve full penetration
Fig.2. Backgouging to achieve full penetration

An alternative to backgouging, or when access to the reverse side is not available, is to use a backing strip which will provide support for a fully penetrated root pass. The backing strip may be permanent or temporary, (see Fig.3 below).

Fig.3. Various forms of backing
Fig.3. Various forms of backing

The permanent backing strip weld does not have as good a performance in fatigue loading as a single sided TIG root butt weld and the crevice is a site for preferential corrosion. Whether a permanent backing strip weld is acceptable for service is therefore a design decision.

In addition to providing support for the root pass, a further major advantage of the backing strip weld is that fit-up tolerances may be relaxed as the strip acts as a locating feature. This is particularly so when pipe butt welding where the strip forms a spigot on which to centre the joining pipe. In addition root gap may be varied, the only real limitations being those of cost; the wider the root gap the greater the volume of weld metal and distortion.

The strip must be compatible with the filler metal and the parent base metal. It must be correctly fitted, in close contact with both edges of the weld preparation and welded into position using intermittent tack welds. Any gap between the backing strip and the plate edges is a site for slag entrapment and results in a poor root profile. To ensure full fusion in the root of the weld it is advisable to use a feather edge and to direct the welding arc at the plate/pipe edges.

When a permanent backing strip cannot be used, then a temporary backing bar may be used (conventionally a permanent backing is known as a 'strip', a temporary backing as a 'bar'). As the name suggests this is a backing that is easily removed at the end of the welding operation; it has not become fused to the root pass.

It may be made of a ceramic or of copper, chromium plated for use on stainless steel and nickel based alloys to prevent contamination. Austenitic stainless steel has also been used. The metal backing bars may be water cooled to aid heat loss and may be grooved to provide a mould for the molten weld metal. Welding conditions and fit-up must be carefully controlled to prevent the welding arc from impinging directly on the bar, otherwise there maybe melting of the bar and contamination of the weld pool.

Ceramic backing bars can be obtained in a variety of sizes with shaped grooves to form a weld pool mould. They may be rigid bars of ceramic or articulated such that they can be wrapped around the inside diameter of a pipe or tube. Ceramic tapes are also available, as illustrated above.

These tapes have wide strips of adhesive either side of the ceramic tile to enable the tape to be held in place during welding and peeled off on completion. As with the permanent backing strip, care needs to be taken to ensure that the ceramic tile is in close contact with the metal surfaces otherwise slag and/or weld metal will run into the gap, giving an irregular weld root.

This article was written by Gene Mathers.

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