THIS PREVENTS SLAG FROM MIXING WITH UNSOLIDIFIED WELD METAL (SLAG INCLUSION)
• REMOVE SLAG FROM CRATER AND TWO INCHES OF BEAD.
the electrode downward, away from the keyhole, as shown in Fig. 8-19. This will prevent slag from mixing with unsolidified weld metal to form slag inclusions.
Before restarting to weld the bead, the slag must be removed from the bead and also from about 2 inches of the weld bead away from the keyhole.
To restart the weld, the arc is struck at a distance of about % inch back on the bead, and moved toward the keyhole, with a long arc length. After the arc has stabilized and the gaseous shield has formed, it is buried in the weld joint at the end of the previous bead. When a pool of molten metal reappears, the weld is continued as before.
Tie-ins. When the edges of the tack welds are feathered there is no difficulty in making the tie-in, The electrode is dragged along in the joint in the normal manner. As it approaches the end of the bead to which the tie-in is to be made, the electrode is moved up the sloping sides of the feathered edge. The welder must watch the molten pool of metal, and when it blends smoothly between the beads, he must reverse the direction of travel for a very short distance and then break the arc by quickly withdrawing the electrode away from the work.
Preparation for the Hot Pass. A finished root bead is shown in Fig. 8-20. The edges of the outside surface are undercut and particles of slag are buried along these edges. A high crown is usually produced on the top of the root bead when it is welded by the downhill method. This is caused by the temperature difference in the liquid metal, which is cooler at the edge of the bead than it is in the center. Filler metal that has not fused at the edge of the weld will tend to
Fig. 8-20. A finished root bead deposited by downhill welding with the electrode buried in the weld joint. A. Outside of weld joint; B. Inside surface at the bottom
of the joint.
flow toward the hotter region of the pool, which is at the center. Thus, the metal tends to gather in the center of the weld to form a crown.
The slag coating must first be removed from the root bead. The weld joint should then be ground to partially reduce the high crown and clean out the worst areas of undercut and slag inclusion at the edge of the weld to prepare these areas so that they can readily be filled by the next pass, as shown in Fig. 8-21. The root bead should not be ground excessively, and too much time should not be spent on this operation.
After the root bead has been ground, it should be thoroughly cleaned with a rotary wire brush to remove as much of the entrapped slag as possible. Any defects that remain will be corrected by the hot pass.
The Hot Pass. The primary objective of the second pass, usually called the hot pass in downhill pipe welding, is to burn out the remaining slag and to complete the edge fusion between the root and the base metal. Only a small amount of metal is added by this pass. As mentioned earlier in this chapter, the weld joint should be warm when this and all subsequent passes are deposited. The remaining passes should be made as soon as possible after the previous passes have been completed.
The current setting used to deposit the hot pass should be slightly
Fig. 8-2 J, A ground root ready for welding the hot pass-
higher than for welding the root bead. While an exact recommendation cannot be given, the current setting should be in the range of 110 to 150 amps DC, The same type of electrodes used to weld the root are used here, namely, V8 to %2 in., E6010, E6010IP, or E7010-A. The electrode angle at which the electrode is held should be 10 to 15 degrees in all positions around the pipe,
In order to make a sound weld joint, each bead should start and end at a different position from the bead over which it is deposited. This means that the hot pass should not be started exactly at the 12 o’clock position, but at about 1 inch away, on either side of this position.
To start the weld, the arc is struck in the usual manner by holding a long arc until it has stabilized and the gaseous shield has formed. It should be struck ahead of the weld and then brought back to the starting point in order to preheat the metal over which it is to be deposited. At the starting point the arc is carefully shortened to the normal arc length, which, for the hot pass, is %2 to % inch. At this point the electrode should be held at the correct angle, or 10 to 15 degrees.
The normal arc is held at the starting point for a few moments to allow the molten pool of metal to form. The weld is then started by immediately starting to “whip” the electrode as shown in Fig. 8-22.
The purpose of the up-and-down whipping motion is to control the weld puddle and to force some of the liquid metal to flow into the corner of the weld, thereby filling the undercut. To reduce the fluidity of the puddle and to prevent overflowing, the whipping motion should be about 1 % electrode diameters long and made in
Courtesy of the Hobart Brothers С о.
Fig. 8-22. Whipping procedure used to weld the hot pass.
the direction of welding without a change in the arc length. While whipping, the arc should be made to pause in order to deposit filler metal in the crater.
The entire hot pass is deposited using the whipping procedure. As the bead is being deposited, the welder must maintain the correct electrode angle in all positions around the pipe. He must watch the formation of the bead to see that the edges are well filled with deposited metal. In this step he has the assistance of the higher current flow which burns out the slag and promotes edge fusion.
When necessary, the arc is quenched by quickly flicking the electrode downward, away from the weld. Before restarting the weld, all of the slag must be removed from the crater and from the second pass for a distance of about 1 inch behind the crater. The arc should be struck on the root bead about xh inch ahead of the crater, as shown in Fig. 8-23. After the arc has stabilized and the gaseous shield has formed, the arc is brought into the crater and shortened to the normal arc length. Here it is held until the crater is well filled with molten metal. The weld then can be continued by whipping the electrode as before.
There should be no difficulty in making the tie-in at the bottom of the joint. The first half of the deposited layer should be deslagged and cleaned before the weld on the second half is begun. As the weld deposit approaches the first layer at the bottom of the joint, the welder will not slow down the speed of welding. He must watch to
of the Hobart Brothers Co.
Fig, 8-23. Restarting procedure for the second, or hot, pass.
see that the molten pool of metal fills up to blend together neatly with the first layer and he can then break the arc by quickly flicking the electrode away from the pipe.
The Intermediate Passes. After the hot pass has been deposited, one or more intermediate layers are deposited to fill the weld joint with sound weld metal. The last intermediate layer should be flush, or У32 inch below the top of the weld joint.
Since the objective of making intermediate layers is to fill the weld joint, slightly larger electrodes should be used, the actual size depending on the thickness of the pipe wall. For the practice pipe, a 5/32-mch electrode is recommended as more current can be used with the larger electrode. The recommended setting is approximately 130 to 150 amps DC. Again, the electrode angle is 10 to 15 degrees, which should be maintained at all times. The arc length should be maintained at about one electrode diameter.
Before each new layer is deposited, the slag coating must be removed from the previous layer which should also be cleaned with a wire brush. When starting each intermediate layer, care must be taken to begin in a different place from the start of the layer below.
While the same welding procedures are used to weld all of the intermediate layers, the procedure does change in different positions around the pipe. These positions are: flat, II to 1:30 o’clock; vertical, 1:30 to 5 o’clock; and overhead, 5 to 7 o’clock. An especially difficult area to weld on the pipe is that between the 2:30 and 4 o’clock positions. On this portion of pipe, a different technique must be used to deposit the bead.
Welding in the flat position at the top of the pipe presents no great difficulty for an experienced welder. As shown in Fig. 8-24, the bead is deposited by manipulating the electrode in a loop-shaped weave.
Courtesy of the Hobart Brothers Co.
Fig. 8-24. Loop-shaped weave used to weld the intermediate layer at the top of
The “diameter” of the loop should be approximately I % to 2 times the diameter of the electrode so that a gaseous shield is maintained over the molten metal at all times. When the arc is in the puddle, the electrode should pause to allow time for the addition of filler metal.
Manipulating the electrode smoothly, this procedure is continued until the bead has reached the position where the weld starts to assume the characteristic of a vertical (3G) weld. At this time the weaving procedure must be changed to a slant weave, as shown in Fig. 8 -25. The weave should advance downward about one electrode diameter per stroke, and the electrode should pause at the end of each stroke in order to obtain good fusion at the edges of the weld.
When the weld is at approximately the 2:30 position, the molten metal has a tendency to roll downward at a faster rate than before. To counteract this tendency, the speed of travel must be increased
Courtesy of the Hobart Brothers Co. Fig. 8-25. Slant weave used to weld the intermediate layer at the side of the pipe.
Fig. 8-26. Electrode angle used to counteract the tendency of the molten metal to roll at the side of the pipe,
slightly while weaving is continued as before, and the electrode angle must be increased to 20 to 25 degrees, as shown in Fig. 8-26. Changing the electrode angle is very important in order to control the puddle. By holding the electrode at the steeper angle and keeping the arc at the edge of the crater, the arc force helps to support the puddle by forcing molten metal near the arc to flow upward. It is also an important practice to maintain a smooth and even motion while weaving, keeping the arc ahead of the downward-moving puddle.
When the tendency of the puddle to roll downward lessens noticeably, the welding speed can be slowed down and the electrode angle changed back to the normal 10 to 15 degrees. Usually, this occurs near the 4 o’clock position. The slant weave is continued until overhead welding starts near the 5 o’clock position.
On reaching the overhead welding position the molten metal will naturally have a tendency to drip downward out of the weld. It is held in the weld joint by surface tension, assisted by the arc force. However, if the puddle is permitted to get too large and the molten metal too fluid, it can drip. To overcome this tendency and to control the puddle, a horseshoe weave, Fig. 8-27, is employed.
The objective of this weave is to reduce the size of the puddle and to decrease the fluidity of the molten metal, while at the same time promoting good edge fusion. The electrode is moved in the semicircular pattern shown in Fig. 8-27, with a pause at the end of each
Courtesy af the Hobart Brothers Co.
Fig. 8-27. Horseshoe weave used to weld the bottom of the intermediate layer.
weave to melt the edges of the joint and to deposit filler metal there in order to obtain good edge fusion. While pausing at one edge of the weld, the molten metal at the other edge solidifies, and at the center of the weld it either solidifies or becomes very mushy, thereby losing its fluidity. A smooth wrist motion should be used and the electrode advanced about one electrode diameter for each weave.
In the regions of the joint roughly bounded by the 2 to 4 o’clock and the 8 to 10 o’clock positions, the speed of travel must usually be faster than at the top and bottom of the weid, in order to control the puddle. Usually, for this reason, less metal will be deposited in this region. Thus, after depositing the next to the last layer, the weld joint will not be as thick in these regions as it is on the top and bottom of the weld.
The joint must be evenly filled with weld metal before the cover pass is made. Therefore, additional beads, called “stripper” beads, will frequently have to be deposited on the sides of the joint, as shown in Fig. 8-28. The same welding procedure is used to deposit the stripper beads as that described previously for welding in the 2 to 4 o’clock positions.
Cover Pass. The cover pass must be a bead with a neat appearance and having a slight crown (about Vie in.) to reinforce the weld joint. The electrode, arc length, and electrode angle will be the same as those used to weld the intermediate passes.
Two slightly different weaves, shown in Fig. 8-29, are used to
weld the cover pass. In each case, the weave must be wider than the previous beads in order to cover the weld joint. At the end of each weave the electrode should pause when it is centered over the edge of the previous layer which will provide the correct bead width as well as fuse the bead with the edge of the joint without undercut.
As shown in Fig. 8-29A, from the 12 to the 5 o’clock positions, a slant weave is used. For overhead welding from the 5 to 6 o’clock positions a semicircular weave (Fig. 8-29B) is used. In each case the speed of travel should be such that the weld metal will build up to form the Vie-inch crown. To avoid undercutting and gas pockets the correct arc length, electrode angle, and speed of travel should be maintained while the electrode is manipulated with a smooth movement.
ELECTRODE MOTION ) FOR 5-TO 6-0’CLOCK POSITIONS