WILD QUALITY INSPECTION A GOOD WELD
EVEN RIPPLE TOP
SLIGHT CROWN BOTTOM
Fig. 544. Weld quality inspection showing good weld and common welding
COMMON WELDING MISTAKES
1. Wefding current too high
2. faulty joint preparation
3. Lack of arc control
t. Weidmg current too low
2. Travel speed too fast
3. Fk>Ot opening too narrow
1. Root opening too wide
2. Welding current too (ugh 3- Travel spend too slow
1. Current loo tow
2. Wrong electrode angle
3. Wrong starting technique
1. Welding current too high
2. Travel speed too slow
3. Root opening too wide
1, 'Welding current vt^y 1ш;И
2, Travel speed too slow/
3, Root opemriif too wide
Courtesy of the Hobart Brothers Co.
Fig. 5-13. (Left) A perfect tie-in to the keyhole end: A. Tie-in; B. Restart. (Right) A perfect tie-in to the heavy end.
When the tie-in is made at the heavy edge of the weld, the speed of welding should be decreased somewhat, a short distance (approximately % inch) away from the edge of the previous weld. This is done to allow time for the thick edge to heat up. If the arc is too short and the welding speed is too fast when approaching the tie-in, insufficient penetration will result. The welding speed should be slow and the arc length normal, until the puddle joins the thick edge of the previous weld. When the puddle is tied-in smoothly with the previous weld, the electrode is moved slightly in the reverse direction while at the same time lengthening the arc slightly; the arc is then broken with a quick movement.
inspect the Weld
After the entire root bead has been completed, it should be thoroughly inspected for visible welding defects. Figure 5-14 illustrates such defects, which are discussed in detail in Chapter 11. Any major defect should be removed before the second bead is deposited. A portable grinder equipped with a thin edge composition grinding wheel can be used to remove the defect.
If it is necessary to remove a large section or a deeply penetrating section of the root bead to correct a defect, this part of the bead must
be rewelded before the second bead is deposited. However, if the correct welding technique described in these pages has been used with skill and care, very few or no defects should appear on the weld. The welding techniques described so far have been proved to result in a successful pipe weld by countless applications in the field.
Poor Fit-up, Wide Root Opening. With careful workmanship it should be possible to obtain a good fit-up at all times in preparation for welding pipes together. A good fit-up is not uncommon in pipe welding. However, poor fit-up cannot be ignored because, on occasion, welding must be done under these circumstances. Frequent causes of poor fit-up are manual flame cutting of the bevel by inexperienced operators, or mistakes in measurements.
One condition encountered as the result of poor edge preparation is a wide root opening. Wide root openings are difficult to weld and in such cases a decision must be made whether to attempt the weld or to replace the entire length of pipe. Replacing the entire pipe can be very expensive and sometimes cannot be afforded. Usually this decision depends on the type of job. For example, a poor fit-up job would be unacceptable in a chemical or an atomic plant.
If the root opening is too wide, the first step is to join the pipes together with tack welds. If the tack welds must be made with the pipes in the horizontal, or 5G, position, they should be welded in the 12, 3, 6, and 9 o’clock positions around the pipe. If the root openings are normal, the tack welds are made in the 2, 5, 8, and 11 o’clock positions. When tack welds are made in the 6 o’clock position, the root bead is started against a tack weld, contrary to the usual procedure.
In starting the tack weld it is first necessary to build a bridge of weld metal across the wide gap at the root. This is done by welding a number of small nuggets of weld metal on the root faces until finally
one of the nuggets joins the two faces together. This procedure is shown in Fig. 5-15.
The welding current should be reduced slightly to weld the nuggets. After the arc is struck it is held just long enough to become stabilized and then it is shortened. A small nugget of weld metal is deposited on the root face after which the arc is quenched. It need not be fused perfectly, the principal objective being to deposit a small lump of metal. The same procedure is then used to deposit a small nugget of metal on the opposite root face. After cleaning each nugget, the same procedure is used to deposit one nugget on top of another until the bridge is built.
The purpose of the bridge is to form a metallic path across the wide root opening that can be used to maintain the arc when starting to weld the bead. These nuggets are imperfectly fused. Because the gaseous shield may not have formed completely, they are likely to be porous and lumpy. For this reason they must be removed after a strong bead of perfectly welded metal has been deposited. Often this is done by grinding after the first half of the weld has been made around the pipe joint. The entire bridge and about inch of additional metal along the bead should be removed with a grinding wheel or with a hammer and a chisel.
After the bridge has been built across the root opening, the remainder of the tack weld can be deposited. A slightly reduced current setting should be used in this case. As usual, the bridge should be cleaned before striking the arc. The arc is struck in the joint ahead of the bridge and a long arc is held until the arc is stabilized and the gaseous shield has formed. It is then brought over the bridge and shortened. The arc length should be slightly shorter than normal. It is moved slowly across the bridge once or twice until some liquid metal appears; the weld bead can then be deposited.
The weld bead is deposited by using a U-weave, as shown in Fig.
5- 16. It is necessary to use this weave in order to bridge the wide root opening. Because the root opening is already wide, a full keyhole may not form; however, the edges of the root face must be melted at the edges of the weld zone.
One danger encountered in welding across a wide root opening is excessive penetration resulting from overheating. For this reason the arc length should be somewhat shorter than normal and the current setting should be somewhat lower than normal.
In making the U-weave, Fig. 5-16, the electrode should be manipulated to bring the arc all the way out of the molten puddle. The arc should be moved along the face of the bevel and kept away from the edges of the bevel. The molten pool of metal should be allowed to
Courtesy of the Hobari Brothers Co.
Fig. 5-16. Welder’s view of the correct procedure for making a U-weave.
solidify completely before the arc is returned to the weld zone. This requires the use of a long U-weave. Then, where the arc is returned to the weld zone, remelting of the solidified metal takes place to form a somewhat smaller and less fluid pool of liquid metal. When the filler metal from the electrode is deposited in this puddle, the molten metal will not overflow the root opening.
If a slight amount of overheating does occur, the electrode angle should be increased. If this does not prevent overheating, it may be necessary to discontinue the arc at short intervals to allow the weld to cool before welding is again resumed. This is indeed a slow process which calls for patience and smooth electrode movement. When the tack weld is about 3/4 inch long, the arc is quenched in the usual manner.
When welding a wide root opening, the root bead is started at the 6 o’clock position, against the end of the tack weld, The arc is struck on the tack weld and a long arc is maintained over the end of the tack weld in order to preheat this area. As soon as some molten metal appears at the edges of the tack, the arc is shortened and welding is commenced.
The root bead is welded as described above, using a short arc and a long U-weave. Care must be exercised to avoid overheating and excessive penetration. The U-weave is used to weld entirely around the joint. Before welding the second half of the pipe joint, the bridge used to start the weld and about У2 inch of the tack weld must be removed.
Poor Fit-up; Narrow Root Opening. The pipe welder will encounter situations where the root opening is smaller than the recommended size; sometimes the root may be closed entirely. This situation may be corrected by recutting the edge of the pipe with an oxyacetyiene cutting torch or by grinding the edge with a hand grinder.
Often, however, the joint is simply welded together without making any alteration to the root opening. To do this, the same techniques for welding a normal size root opening are used with only slight modifications.
When the root opening is too small or nonexistent, the welding current flow must be increased, usually by as much as 10 to 20 amps. The heavier current flow increases the penetration of the weld. Also, it is usually best to decrease the arc length when welding. Sometimes a near “drag” arc length can be used.
The welder must give close attention to the amount of penetration obtained while welding the narrow root opening. If the penetration is too deep or overpenetration occurs, a slight whipping motion may avoid this condition. Increasing the speed of welding may also be helpful. If these measures do not succeed in correcting the overpenetration, the current setting must be reduced. Underpenetration can be corrected by increasing the welding current and by using a somewhat slower welding speed.
When confronted with a narrow root opening, it is always good practice to make some test welds on pieces of scrap metal first. The plates should be beveled and set apart a distance equal to the narrow root opening. In this way the current setting and the welding procedure can be adjusted before welding the pipe; and defective welds, which must later be removed, can be avoided on the pipe.
Poor Fit-up; Wide and Narrow Root Openings. Sometimes a combination of narrow and wide root openings is encountered in a pipe joint. This can be the result of the ends of the pipe being cut incorrectly or of the pipes being misaligned — that is, oriented at a slight angle to each other.
Welding a combination narrow and wide root opening requires the application of both of the methods previously described. The method used obviously depends upon the root opening encountered.
It is usually necessary to adjust the welding current setting several times when welding around the pipe.
First of all, of course, the four tack welds should be welded in place. To prevent restraint cracking, the first tack welds should be made in the region of the narrow root openings. For the same reason and because it is less difficult to obtain a start, it is desirable to weld the narrow root opening first, when welding the root bead. This is not always possible, however, because the basic method of welding the pipe from the bottom toward the top should not be abandoned. For this reason it may be necessary to start the root bead in a region having a wide root opening.
Poor Fit-up; Root Face Too Wide. When the root face is too wide, it may be possible to correct this condition by recutting the end of the pipe. Normally, however, the pipes are welded together in the usual manner, using a higher current setting to obtain complete penetration.
Poor Fit-up; Root Face Too Thin. In this case the edge, at the root, will melt very rapidly while the rest of the weld is relatively cold. The method of overcoming this difficulty is to reduce the current flow and to use a U-weave for depositing the bead. By reducing the current setting less heat is generated by the arc and the possibility of burning through the thin edges is diminished. The U-weave preheats the bevel ahead of the edge, thereby insuring that the weld deposit will fuse properly with the parent metal. As before, when using the U-weave, it may be necessary to allow the puddle to become mushy before the arc is returned each time.
Welding the Root Bead with how-Hydrogen Electrodes. Root beads are seldom welded with low-hydrogen electrodes because very highly skilled welders are required to make welds that are free from defects. High-pressure pipe joints are usually welded by welding the root bead with a deeply penetrating type electrode, such as E6010, or by the Gas Tungsten Arc-Weld process (GTAW, described in Chapter 6), and the remainder of the joint with a low-hydrogen electrode. However, with skill and care it is possible to weld root beads with low-hydrogen electrodes. The following instructions describe how this may be done.
As shown in Fig. 5-17, the diameter across the coating of the Vg-inch E7018 low-hydrogen electrode is larger than that of the V8-inch E6010 electrode. The heavier coating of the E7018 electrode does not allow the arc to be taken close enough to the root face, thereby making it difficult to establish the correct arc length. Welding with an arc that is too long can cause suck-in (see Fig.
Є 6010 (АРРЯОХ.)
E 7018 (LOW HYDROGEN)
Fig. 5-17. A. Diameters across the coatings of.125 in. E6G10 and Е70І& low-hydrogen electrodes; B. Diameter across the coating of a,093 in. E70!8 low-hydrogen electrode.
—4' —’l25(CCAT, NG)
E 7018 (LOW HYDROGEN)
(COATING),186 - Hf I;
(WIRE) .125 -
5- 14). Furthermore, the heavier coating interferes with the manipulation of the electrode when making a weave. For this reason a smaller size (.093 inch) hydrogen electrode should be used to weld the root bead. It has a diameter across the coating that is approximately equal to that of the Vg-inch E6010 electrode.
When welding with low-hydrogen electrodes, pinholes can be caused by incorrect arc striking, chipped flux coating, moisture in the weld joints, or wet electrodes. To avoid pinholes while striking the arc, strike just ahead of the starting point and shorten the arc as quickly as possible to the proper length, Then back-up the arc to the starting point and proceed to weld as soon as the molten pool of metal has formed. Chipped spots on the electrode coating will cause the arc to be erratic at that point, resulting in pinholes and a hard zone in the weld. For this reason electrode containers should be handled with care and damaged electrodes discarded. Wet joints should be heated with an oxyacetylene torch to drive off all moisture.
The electrode flux coating is sensitive to moisture and must be kept dry. Open containers should be stored in a “dry box” or controlled-humidity storage oven, where the electrodes are kept at a temperature of 300 to 400F. Electrodes that have been exposed to moisture can be reconditioned by drying for one hour at a temperature of 600 to 800F, the exact temperature depending on the make of the electrode.
A higher current setting is almost always used for welding with low-hydrogen electrodes and, therefore, more heat is liberated. The arc characteristic is also different. The low-hydrogen electrode produces an arc that is relatively smooth but lacks the penetrating power of the more lightly coated electrodes.
The heavy electrode coating will form a heavy blanket of slag over the liquid puddle of molten metal, which causes the cooling rate to be slower and the metal to remain liquid for a longer period. The viscosity of the molten slag and weld metal is lower; i. e., it will flow
more readily than a liquid having a higher viscosity. The combined effect of the slower cooling rate and the lower viscosity of the liquid will cause the molten puddle to drip readily. For this reason, difficulty is experienced when welding in the overhead and vertical positions with low-hydrogen electrodes.
The general procedure for welding the root bead with low - hydrogen electrodes is the same as before. Tack welds are made around the pipe, after which both sides of the joint are welded from the bottom to the top of the pipe. The welding technique, however, is different.
A very short arc should be used at all times when welding with low-hydrogen electrodes. The electrode must be kept very close to the root face in order to obtain adequate penetration. When the arc is struck, it should be shortened immediately, as explained before. The whipping procedure should never be used when welding with low-hydrogen electrodes.
A V-shaped weave, Fig. 5-18, must be used for welding the root bead entirely around the pipe when using the low-hydrogen electrode. The objective of this weave is to allow the molten slag and metal in the puddle to cool and to lose some of its fluidity (or to increase its viscosity) in order to prevent dripping. Using this weave also preheats the metal ahead of the weld.
The weave should be made by smooth and precise movements of the wrist. The arc should be brought out of the puddle and up along the bevel with a quick movement. Allowing just enough time for the puddle to lose some of its fluidity by slowing down the return movement, the arc is returned to the puddle and held there for a short pause. This movement is then repeated up and along the other bevel.
The liquid metal must never be allowed to solidify during the weave. Some of the slag may also solidify and, if this occurs along
Fig. 5-І 8. Welding the root bead with a low-hydrogen electrode using a V-weave.
the edges of the weld, it may be difficult to dissolve the heavy slag entirely — resulting in a lack of fusion. If the puddle is always kept liquid, the slag will seldom be trapped because it has a lower melting point than the weld metal. Since the arc is smooth and lacks deep penetrating power, the movements must be smooth and precise to avoid solidification of the puddle.