Cover Pass

When the pipe joint is filled almost to the surface, a pipe weld is ready to be capped, using the downhill technique and stringer beads. The bevel edge should be visible on both sides to serve as a guide for depositing straight stringer beads side by side, as the weld is complet­ed. When the first stringer bead has been completed using the bevel edge as a guide, the second stringer should use the inner edge of the first as it’s guide. The electrode should be placed directly over the edge, allowing the puddle to wash up on the first. All other passes should follow the same pattern.

Welding large diameter and heavy-wall pipe requires two welders, one on either side of the pipe. They need to coordinate their starting of the weld at the top of the pipe. The tie-in at that point should be smooth and staggered two inches from the previous tie-in, as shown Fig 8-37.

An allowance should be made for the same procedure, staggering the stringer beads on all filler passes or when depositing stringers for cap­ping. In addition, tie-in on the overhead can be difficult under certain conditions. For example, when the filler pass lacks the proper level in preparation for capping the weld. As the prceeding fill pass should be slightly below the surface. Therefore, the welder is able to maintain a constant speed of travel without having to worry about filling and cap­ping at the same time.

As for the tie-in on the overhead when capping, the same practices are applied at the top as on the bottom. Regarding the overhead posi­tion, (a) welders can slightly lower their current setting, but maintain a smooth advancing arc at a continuous speed of travel, (b) one welder should reach the stop point, four inches ahead of the second welder, and (c) welders’ view of the overhead position should be at 45 degrees; as the arc moves away from them, and they should maintain the oscillation process or movement. This allows for the slight wash-up at the edges of the puddle. The focus from that angle of viewing allows a welder to keep track of the build-up of the stringer surface, and to monitor the edges of the deposit. It is now easier for the welder to keep a uniform weld. Getting directly under the pipe with a clear view of the arc and the puddle ahead actually can add to confusion. However, by looking from the recommended angle, the welder can control the speed of travel, while maintaining a clear view of the puddle edges. The build-up and solidification pattern determines the speed of travel or any changes made during the welding process.

This chapter has covered several important steps, among them filling the pipe almost to the surface, and maintaining the straight bevel edges of the pipe. The straight edges can effectively be used as a guide line for making or depositing the first stringer bead, keeping the cap as straight as possible and uniform in width. The inner surface (edge) of the first stringer can be used as a guide for additional stringers and filler. The welder in training must continually strive for perfection, which is not easily achieved. Important considerations include:

(a) The instructor should continually help welders improve their efforts.

(b) The welders on both sides of the pipe must concentrate on making proper tie-in from the start of their training.

(c) A complete root bead should have little or no undercut; if undercut does occur, it is extremely important to maintain the preheating temperature.

(d) In some instances, porosity can be avoided by proper manipulative practices.

(e) Wire brushing is the first stage of preparing the root bead surface. Next, grinding removes the humps and other regularities on the surface of the root bead. Overheating due to friction of the grind ing disc on the weld surface should be avoided; it is indicated by discoloration on the surface of the weld (black, blue, or brown) caused by oxidation, which can lead to porosity when the hot pass is deposited.

Composition-based unweldability influences toughness. The steel has a tendency to develop higher levels of hardness when the cooling rate is faster then recommended. The heat-affected zone may not develop an entire martensite structure that would have the highest susceptibility to hydrogen embrittlement cracking. Nevertheless, this mix structure can develop or display cracking.

Attention should also be given to welding at ambient temperature. Circumstances often arise when the work is to be joined at a low tem­perature. Naturally, the question should be raised whether satisfactory results can be obtained by applying regular procedures. In addition to the metallurgical effects of welding at ambient temperature, there are other aspects of the joining operation that can affect weld quality under these conditions.

Suppose a weld is to be made applying the downhill technique with ambient temperature, or very low preheating temperature, on pipe material that has.25 percent carbon, .90 percent manganese, and small quantities of other alloying elements. The welder can make a valuable contribution to a weld that is defect free, and adequate both in ductili­ty and toughness. The procedure in this case is not to ignore preheat­ing, but to use another format that will have a preheating effect. When the first three passes are deposited, all of which are thin layers, the tem­perature built-up will be higher or equivalent to the required level.

This technique starts with two or three welders who expertly deposit the root bead with minimum time lost and w'ith no defects. On com­pletion, the welder helpers immediately grind the restarts areas. These two steps should be completed within 3-to-4 minutes. The hot pass then begins with two welders working on either side, starting at the 3 o’clock position, downwards to the overhead position. The welder at the top extends the hot pass from 12 o’clock to 2, and from 12 o’clock to 10.

After the welders have completed their respective sides to the over­head, where the tie-in has been made, the welder helpers brush that area. Time is important in completing those three passes, raising the temperature above the preheating temperature within ten minutes. Care must be taken that the root bead have no undercut. The hot pass should be properly deposited, fusing both sides of the bevel; likewise the third pass.

Porosity can develop from inadequate heat input on the bevel surface. Fortunately, this is not the case at this point. What is involved from here on is that when the stringer bead is deposited on the bevel face, the welder must supply ample heat input by angling the electrode towards the bevel surface, and then oscillating the electrode in order to

have the fluid puddle wash-up on the third pass with adequate fusion, as shown in the figure on page 130. Angling the electrode also supplies sufficient heat to the bevel surface so that there is no cold surface in the close vicinity. Because of this, the slag that enters the puddle will quickly dissolve to form the weld surface coating. A fluid puddle indi­cates that there is fusion. The gases that enter the molten puddle will be able to escape, avoiding porosity.

The success of this approach requires (a), taking advantage of time, (b) using highly skilled welders, and (c) applying techniques that induce heat input to maintain the interpass temperature effectively. In short, angling the electrode by turning the wrist on the bevel is effective in eliminating defects, even when the pipe is preheated.

When preheating is stipulated for a procedure, the temperature must be maintained at the start of depositing the root bead, and continues even when the root bead is completed, as well as the time when the root bead is being brushed and grinded in preparation for depositing the hot pass. Effective preheating is based on understanding variables such as lapse of time between heating and starting the weld, the drop due to conduction, known as heat sink must also be taken into consideration. Furthermore, the wall thickness of the pipe also affects the drop in temperature. Thus, the actual preheating temperature specified in the procedure is likely to be higher when taking these variable into account

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