The Intermediate and Cover Passes
Multilayer welds are used to weld all thick-wall and most thinwall pipes. The multilayer weld is more ductile and free from defects than a large single-layer weld would be. A multilayer weld is also easier to make because it is more difficult to control the large puddle of molten metal of a single-layer weld.
Since a large weld bead cools more slowly than a small bead, excessive grain growth occurs, which causes a decrease in the ductility of the weld. In the case of a weld consisting of normal-size beads, the ductility is improved because of the heat treating effect that each succeeding bead has upon the others. When a large bead cools slowly, the hot metai is exposed to the atmosphere longer causing oxides to form and this can lead to harmful effects.
This chapter is concerned with welding the intermediate and the cover passes on heavy-wall pipe. As before, each bead is started at the lowest part of the pipe and, welding uphill, the bead is welded to the top of the pipe. Since the joint is now closed by the root bead, a different welding technique is used. This is a slant weave, which is used at all positions around the pipe.
The Multiple Layers. The first layer, or bead, in a multiple-layer butt joint is the root bead. This is followed by one or more intermediate layers (also called filler layers). The final layer is called the cover pass. Each intermediate layer or bead is identified by the order in which it was welded, starting with the second pass.
As shown in Fig. 7-1, the number of beads or passes that are welded around the pipe joint depends upon the thickness of the pipe wall. Heavy beads should be avoided for reasons given earlier in this chapter. For this reason it is obvious that thick-wall pipes will require more beads than will thin-wall pipes.
The recommended placement of the various layers or beads across the section of the pipe wall is shown in Fig. 7-1. This illustration can be used as a guide when deciding how to place the beads.
Each bead must be firmly fused to the metal with which it is in contact, whether it is another bead or the base metal at the walls of the joint. In effect, the entire weld should be a solid body of metal, free from voids and other defects. The joint must possess strength
and ductility sufficient to withstand any load to which it might be subjected.
The intermediate beads should fill the joint and be flush with the outside surfaces of the pipes. This is a very important condition prior to welding the cover pass. It is very difficult to weld a good cover pass unless the metal over which it is welded is flush with the outer surfaces of the pipes. The cover pass should be slightly crowned and the sides should blend smoothly into each pipe without undercut.
Preparation of the Root Bead. A root bead that is perfectly formed internally can be very rough on the outside, as shown in Fig. 7-2 A and B. Depending on the skill and care used by the welder, it may
have humps and undercuts, especially where restarts occur. To clean and to prepare a surface such as that shown in Fig. 7-2 requires more than simple chipping and wire brushing.
The root bead should be prepared by grinding out all of the undercuts, rough spots, and humps to avoid the risk of having an invisible defect after the second pass is finished. Visible defects can be corrected, but this is costly.
Under ordinary circumstances, root bead grinding can be somewhat overemphasized and removing an unnecessary amount of weld
Fig. 7-2. Root bead that is perfect internally, but rough externally. A. Outside surface; B. Underside of root bead.
metal should also be avoided. Deep penetrating type electrodes, such as E6010, E6011, and E7010, will easily melt the metal in any defective areas and fill the voids at undercuts. When these electrodes are used, the smallest humps can be removed by melting; only the most serious defects will require grinding to correct them.
When shallow penetration type electrodes, such as low-hydrogen electrodes, are used to weld the joint, much greater care must be exercised in preparing the joint. Since these electrodes are used to weld high-pressure pipe joints, extra work and care are required in the preparation of these joints.
Because of their lack of penetrating power, it is much more difficult to melt humps and to fill-in undercuts with weid metal when using low-hydrogen electrodes. To maintain the short arc used with these electrodes, the surface over which the weld is made should be smooth.
For these reasons, the surface of the root bead must be ground to a flat contour, as shown in Fig. 7-3. The root bead surface is widened by grinding to remove undercuts at the fusion line between
Fig. 7-3. Root bead ground io flat contour In preparation for welding second pass with a low-hydrogen electrode.
the weld metal and the base metal. Since the low-hydrogen electrodes have a heavier coating, widening the root bead also provides more space for electrode manipulation and the fiat contour will enable the welder to maintain a constant arc length.
In summary, the degree of preparation of the root bead for welding the second pass depends upon the type of electrode being used. When high-pressure joints are to be welded there can be no compromise in preparing the joint. In other cases, the welder must exercise a certain amount of judgment; he must decide which defects are serious enough to affect the quality of the second pass, and remove them leaving only those minor defects that can be corrected on the second pass.
The same conditions prevail when preparing the joint for welding the beads that will be made following the second pass. When welding with low-hydrogen electrodes, the surface over which the weld is to be made must be smooth and free of all defects. When welding with deeply penetrating electrodes, however, only major defects must be removed.
The Current Setting. Because of the varying conditions of the job location, the machine, and other factors detailed in Chapter 3, it is not possible to give an exact current setting that can be used in all cases. In order to provide a relative idea of the current setting to use for the intermediate passes, the current setting for the root bead will be used for comparison.
For the second pass and for subsequent passes, the current setting will depend primarily on the temperature of the pipe joint. If the pipe is still very warm (about 200 to 300F) when starting to weld the second bead, it is possible to use the same current setting that was used to weld the root bead. At this temperature the pipe joint is too hot to touch with the bare hand. If the temperature of the pipe joint has dropped significantly below this temperature, the current setting must be increased to provide more heat to melt the parent metal and to maintain the puddle.
When the welding operation has been suspended for a longer period of time, such as overnight, the weld joint will be at the temperature of the surrounding air. In this event it is advisable to preheat the joint with an oxyacetylene torch. In many instances, such as when welding highly alloyed pipe, preheating is mandatory. This reduces the thermal shock on the joint when welding is resumed, and also reduces the severity of the residual stresses locked in the weld when it has cooled to the temperature of its surroundings.
When several layers of weld beads have been deposited, the heat will transfer out of the weld at a slower rate. In other words, they tend to store heat, at least for a short time. When this occurs, there is no need to change the welding current. However, if these layers have been allowed to cool to the temperature of the surrounding air, they will act as a heat sink by rapidly withdrawing heat from the weld. In some cases, the cooling rate of the weld may be so fast that it will act like a quench. Moreover, more heat is required to raise the temperature of the layers of cold weld metal, often more heat than can be supplied by the arc.
For this reason, preheating with an oxyacetylene torch is highly recommended when several layers have been deposited and which have been allowed to get cold. If several layers have been deposited in pipe having a wall thickness of 3/4 inch or more, preheating is almost mandatory.
In summary, the current setting depends on the temperature of the pipe joint. If it is warm (about 200 to 300F), less current can be used than if it is at the temperature of the surrounding air. If the pipe joint has been allowed to cool, preheating is very advisable and often mandatory in certain cases.
Electrode Angle. The correct electrode angles for welding the intermediate and cover passes around the pipe joint are shown in Fig. 7-4A and B. The side angle should be such that the electrode is perpendicular to the surface of the pipe.
Figure 7-4C shows one error that is sometimes made, even by experienced welders, when welding from the 9 o’clock to the 12 o’clock positions. The electrode angle in this case is too great. By facing upward, the arc will cause the puddle to be less fluid causing slag inclusions and improper fusion at the edges of the weld.
When the molten pool of metal is mushy, it will not fuse properly
Fig. 7-4. Correct and incorrect electrode angles for welding intermediate and cover passes. A, Correct electrode angle for 6 to 9 o’clock positions; 8. Correct electrode angle for 9 to 12 o’clock positions; C. Incorrect electrode angle for 9 to
12 o’clock positions.
nor will it flow readily. The slag, having a lower melting point than the metal, will be very fluid and it will flow swiftly to the edges of the weld where it will become trapped between the base metal and the weld metal. Therefore, it is necessary to prevent the molten pool of metal from becoming mushy while welding. While several factors affect the fluidity of the puddle, one that must not be ignored by the welder is the electrode angle; it should always be as shown in Fig.
7- 4 A and B.
Welding the Intermediate Passes. Before each pass is welded, the joint must be cleaned and prepared as described previously in this chapter. Each layer should start in a different position on the joint;
i. e., no two layers should be started in the same position. For example, the root bead was started in the 6:30 position; thus the second layer should then not start in this position but in the 6 o’clock position.
The arc should be struck on the bead over which the weld is to be made. It should be struck ahead of the weld to preheat the metal over which the first part of the weld is to be made. As usual, a long arc should be held in this position until it has stabilized and the gaseous shield has formed. It is then held at the 6 o’clock position and shortened. After the puddle of molten metal has formed, the electrode is moved on to form the bead.
Courtesy of the Hobart Brothers Co. Fig. 7-5. Electrode movement for making slant weave used to weld all intermediate and cover beads.
All of the intermediate beads are made by using a slant weave, Fig. 7-5. This weave is used in ail positions around the circumference of the pipe joint. The arc should be moved along at a smooth and steady pace by operating the electrode with a wrist movement. At the end of each weave there should be a slight pause, after which the direction of the arc is reversed. Because the surface of the pipe is curved, care should be exercised to maintain a uniform arc length and electrode angle. Varying the arc length can have a significant effect on the depth of fusion and the size of the puddle. By maintaining a steady movement and a uniform arc length the electrode is seldom away from the edge of the puddle which results in a uniform depth of fusion and a bead with a neat appearance.
It is important to pause momentarily at the end of each weave. This allows the filler and base metals to mix properly, and it also allows the bead to fuse properly to the side of the joint. Any slag that is trapped in the corner of the weld will be remelted and will flow toward the rest of the metal where it will not interfere with the fusion of the puddle and the side of the joint. The molten metal m the puddle will also fill any undercuts in the corners, and will solidify when the arc is moved on.
The puddle must be maintained at all times and should not be allowed to become mushy, or small particles of slag may become
entrapped in undercuts at the corners or in cavities. The welding speed should be adjusted to maintain a fluid puddle, yet care must be exercised not to allow the puddle to sag when welding from the 6 o’clock to the 3 o’clock positions. As the weld advances beyond the 3 o’clock position, the tendency for the puddle to sag decreases and, for this reason, the speed of welding can be gradually decreased to maintain a uniform build-up of weld metal.
For light coated electrodes (E6010, E6011, E7011), the arc length should be about %2 to Va inch. This will provide enough heat so that the puddle of molten metal will be large enough to accept the globules of filler metal without excessive build-up. If a short arc (Vie inch) is used, the size of the puddle of molten metal is decreased considerably. In this case the size of the pool of molten metal in which the filler metal can be deposited is limited. When the globule enters the smaller body of liquid metal, it will rise and, at the same time, cool more rapidly causing the bead to have a high crown (Fig.
7- 6) and, perhaps, lack good edge fusion.
Fig. 7-6. Bead wim nigh crown and lack of edge fusion caused by welding with
a short arc.
The use of an excessive amount of current must also be avoided. When this occurs the welder will increase the welding speed and the rate of electrode manipulation to keep the molten metal from sagging or from overflowing. He will tend to become erratic in the manipulation of the rod* varying both the arc length and the weave. To control the puddle he will often resort to a U-weave which periodically will remove the gaseous shield from the molten metal and produce a harmful effect on the quality and appearance of the weld.
The third pass should be made wider than the second pass and it should fill the groove from side to side, as shown in Fig. 7-7. This pass is made by using the same slant weave as before, pausing at the end of each weave. The width of subsequent weaves will depend on the thickness of the pipe and upon the judgment of the welder. The manner of placing these beads in relation to each other was shown in Fig. 7-1. A bead may be deposited by making the center of the electrode pause over the edge of the bead below; or the weave may be widened by making the electrode pause one electrode diameter beyond the edge of the bead below. The appearance of each bead should be smooth and without undercut.
Stop and Restart. When the end of a bead is reached or when the electrode is consumed, the weld must be stopped. This is done by simply reversing the direction of the electrode travel for a short distance and then breaking the arc with a quick movement. The molten metal in the puddle should solidify in the form of a crater, as shown in Fig. 7-8.
Before restarting, the end of the weld should be chipped and wire brushed for a distance of at least V2 inch beyond the crater. All traces of the slag coating must be removed from this area.
To resume welding, the arc should be struck ahead of the crater and a long arc maintained, as shown in Fig. 7-9. When the arc has been stabilized and the gaseous shield has formed, it is brought into the crater and shortened to the normal length for welding. The arc should be moved slowly, from side to side in the crater, until a pool of molten metal has formed; only then can the bead be continued by resuming the slant weave.
Fig. 7-8. View of bead that has been stopped to change electrodes, showing the
The arc should not be struck in the crater. This practice would cause the first few globules of filler metal from the electrode to build up like humps because the pool of molten metal had not formed when they were deposited. It has also been found that porosity results in the restart zone when the arc is struck directly in the crater. This is probably caused by the absence of an adequate gaseous shield to protect the hot metal, and the absence of enough slag from
Fig. 7-Ю. View of a good tie-in.
the electrode coating, in time to thoroughly deoxidize the molten metal.
Tie-in, It is not difficult to make a good tie-in (Fig. 7*10) when welding intermediate and cover beads if care and attention are given to this matter during welding. When approaching a tie-in, the weave may be slowed down slightly to allow the molten metal to build up somewhat. Then, when the built-up pool of molten metal reaches the other bead, the welder must watch to see that the molten metal fills the crater and blends smoothly with the other bead; if necessary, he may have to run the arc a very short distance (yie inch) over the other bead in order to achieve a smooth tie-in. When the smooth blend of the two beads occurs, the arc is broken with a sudden movement.
The Cover Pass. The cover pass is the final pass or bead to complete the weld. It must have a smooth and uniform appearance. As shown
Fig. 7-1 1. Perfect cover passes. (Top) Horizontal (2G) weld made with E6010 electrode, (Bottom) 5G position cover passes, with joint in center made with E7018 low-hydrogea electrode and the two end welds made with Є6010.
in Fig. 7-11, it should have a slight crown. When the pipes are in the 5G position, as shown by the lower pipe in Fig, 7-И, a single bead is used to make the final cover pass. Since this is difficult to do, however, when the pipe is in the 2G position, as shown by the upper pipe in Fig. 7-11, the cover pass usually will require three separate beads to be made. Welding in the 2G position will be treated later, in
Chapter 8; this chapter will treat welding of the cover pass in the 5G position only.
Welding the last intermediate layer, or layers, is a preparation for welding the cover pass. It is important that they be welded flush with the surfaces of the pipes. Undercut and poor edge fusion should be avoided; if they occur, they should be removed and the damaged areas rewelded before welding the cover pass.
In preparation for welding the cover pass, the weld joint must be thoroughly cleaned. If low-hydrogen electrodes are used, any humps on the surfaces of the beads over which the weld is to be made must be removed. Normally, the cover bead should overlap the edges of the bevel for approximately Vi6 inch.
Since the weld must be uniform and have a good appearance, the welder should always be in the most comfortable position that circumstances will allow in order to be able to manipulate the electrode smoothly and accurately. Whenever the welder senses that the manipulation of the electrode is impaired, he should stop welding and change position.
The arc should be struck ahead of the weld and brought back to the 6 o’clock position while holding a long arc in order to stabilize the arc and form the gaseous shield. It is then shortened to a normal arc length. The welding process begins by weaving the electrode two or three times at a slow rate in order to form the molten pool of metal and to obtain good fusion.
As shown in Fig. 7-12, a slant weave should be used to make the entire cover pass. Close attention to the job and careful workman-
Conrit’sy of the Hobart Brothers Co.
Fig. 7-12. Slant weave used to make the cover pass.
ship are required in order to obtain a neat appearing bead* The electrode angle must be correct and a uniform arc length maintained.
When making the weave,, the welder should watch the edges of the underlying bead. They serve as a guide in order to maintain a uniform width of the cover bead around the entire pipe. At the end of each weave, the electrode should pause to prevent undercut and to obtain good edge fusion.
The welder must also watch the puddle and maintain a welding speed so that the crown (also called the bead reinforcement) of the solidified bead remains between У32 and 1Д6 inch above the surface of the pipe. Care must be exercised not to permit the crown to build up too high, especially in the vertical and overhead welding positions.
Difficulties can be encountered in welding the cover pass for the following reasons:
1. The pipe is too hot
2. The joint was not cleaned properly prior to welding
3. The intermediate beads have not filled the pipe joint com - pletely
4. There is undercut and poor edge fusion of the previous bead.
If the pipe is too hot the molten metal will solidify more slowly and it will be very difficult to control the puddle. As a result there will be a rough-looking weld which is unsatisfactory as a cover pass. The remedy would be to allow the pipe to cool until it is warm (about 200 to 300F) before starting to weld. Sometimes it is possible to weld a hot pipe satisfactorily by reducing the welding current.
Cleaning the joint properly prior to welding is the welder’s responsibility. Undercut, poor edge fusion, and improperly filled joints are also the welder’s responsibility. He should not start to weld the cover pass until these matters are corrected. Better yet, he should exercise greater care when welding the intermediate passes so that these conditions do not occur.
Low-Hydrogen Electrodes. As already explained, low-hydrogen electrodes are not a deep penetrating type of electrode and for this reason the surface over which they weld must be smooth. Before each pass, the surfaces of the joint should be cleaned, have all defects removed, and be ground smooth. Proper preparation is an important part in welding successfully with low-hydrogen electrodes.
The general procedure for welding the cover bead with low - hydrogen electrodes is the same as for more deeply penetrating electrodes; however, there are some differences. When the arc is struck it should be shortened immediately. The whipping procedure should never be used to control the puddle. The weld should be made by using the slant weave described in this chapter. At the end of each weave the welder must be sure to pause in order to avoid undercutting. It is very important to maintain a short arc at all times in order to avoid sagging and to prevent pinholes from occurring in the weld.
The electrode coating must be kept dry and electrodes with a chipped coating should not be used. Wet joints must be preheated with an oxyacetylene torch. Welding should never be done in the rain when using low-hydrogen electrodes. If the welding area becomes wet, the electric arc will break down the water, forming hydrogen, which will enter the weld metal with very harmful effect.