1. Pure tungsten

2. 1 or 2 percent thoriated tungsten

3. Zirconiated tungsten.

Recommendations for the applications of the types of electrodes are provided in Table 6-1. Thoriated tungsten electrodes are used for most pipe welding applications, including mild steel pipe.

The shape of the electrode tip has a marked influence on the contour, penetration, and width of the face of the weld deposit. It is

especially important to shape the electrode tip correctly, similar to a sharpened pencil, when it is to be used to weld a root head on a pipe joint.

The working end, or tip, of the electrode is shaped by grinding it with a very fine grinding wheel that should be used only for this purpose. A fine-grained wheel should be used in order to obtain a very smooth surface finish on the tip of the electrode, which is helpful in maintaining a more stabilized arc. For this purpose, a 60 grain, О to M grade silicon carbide grinding wheel should be used, such as a C-60-M-V wheel. If the electrode becomes contaminated with metal, it should be reground immediately.

Exact specifications for the shape of the electrode tip are given in Fig. 6-5. The included angle of the point should be about 22 to 23 degrees, or the length should be ground back to a distance equal to about 2V2 electrode diameters. It is important to blunt the tip slightly by grinding it flat at the end for a distance slightly less than %4 inch from the point.

Shielding the Weld Metal. In all welding processes, the very hot metal in the region of the puddle is protected by some kind of shielding. A gas formed by heating the coating of the electrode protects the top of the weld in the shielded metal~arc process. In addition, the molten metal is protected by a coating of liquid slag that rises to the surface. On open butt joints, this coating protects the top of the weld and some of the slag flows through the liquid metal to the bottom of the joint where it also protects the exposed surface.

TUNGSTEN ELECTRODE

Fig. 6-5. Grinding the thoriated tungsten eiectrode tip for root bead welding Point angle specification is shown as 22° to 23°.

Actually, there is some uncertainty about the amount of protection provided to the exposed surface at the bottom of the joint. However, when welding mild steel pipe with the Consumable Electrode Method, this presents no real problem.

Mild steel pipe can also be welded by the GTAW process without special precautions. A sufficient quantity of the inert gas reaches the bottom of the joint to provide adequate protection and the top of the weld is covered with a heavy blanket of inert gas. Highly alloyed steel pipe, however, when welded with the GTAW process, will require extra protection at the bottom of the joint. This is done by filling the inside of the pipes, in the region of the pipe joint, with an inert gas.

Several methods are used for containing the inert gas in the pipes; two of these are shown in Fig. 6-6. Two “pistons” having rubber seals are inserted on each side of the pipe joint (see Fig. 6-6A), The pipe joint itself is taped shut to prevent the inert gas, which is blown into the pipe at a very low pressure, from escaping. A small portion of the joint is left open to allow the air to escape. When welding, the sealing tape around the pipe is removed in sections just ahead of the weld, and additional inert gas is blown inside of the pipe to make up for the gas lost through this opening. This method can be used when welding short lengths of pipe; however, it is somewhat awkward to use on longer pipe lengths and on larger-diameter pipes.

Fig. 6-6. Methods of containing the inert gas shield insidea pipe. A. Two pistons used for short, smaller-diameter pipes. B. Plastic gas bags used in conjunction with longer pipe lengths.

in field conditions, where the pipe diameters and lengths are frequently large, the joint can be sealed by inflating two plastic balloons in the pipe, as shown in Fig. 6-6B. They must be positioned far enough from the weld joint so that the heat from the weld will not burst them. The location of the balloons must also be marked on

the outside of the pipe, for when the root bead is finished, just sufficient heat is applied by an oxyacetylene torch at these markings to burst the balloons. The joint itself is sealed with tape, as before, and the inert gas is blown into the pipe through a rubber hose or a length of small-diameter pipe or tubing. Another method of sealing the inside of the pipe is to tape a wall of polystyrene to the inside of the pipe. The advantage of these two sealing methods is that the materials used to make the seal can be blown out by compressed air or a stream of water after the pipe joint is finished.

Preparation of the Weld Joint. The procedure for preparing the weld joint is the same as for preparing the joint when welding by the Shielded Metal-Arc process. The principal difference is in the dimensions of the joint. The width of the root face is reduced to Уі6 inch and the root opening is narrowed to Vis to 3/32 inch, as shown in

Fig. 6-7.

The pipe joint should be carefully fitted together to obtain an accurate alignment and the correct width of root opening. If alloy steel pipes are being welded, an inert gas should be blown into the inside of the pipe as described in the previous section. Highly alloyed

Fig. 6-7. Weld joint specification for GTAW welding of the root bead.

steel pipes should be preheated at the pipe joint from 200 to 300F prior to welding the tack welds and the remainder of the root bead.

When all of the preparations have been made, four evenly spaced tack welds are made around the pipe joints. Since these welds will become a part of the root bead, they must be made with the same care and craftsmanship as the remainder of the root bead.

Procedure for Welding the Root Bead. Even before the tack welds are made, the GTAW welding torch must be adjusted and the welding machine must be set to provide the correct current amper­age and gas flow.

To weld 8 in., Schedule 60, mild steel pipe nipples, a %2-inch diameter thoriated tungsten electrode and a '/8-inch diameter mild steel filler rod are used. The electrode should be ground to a point with a fine grinding wheel. In order to hold the electrode, а ;1/:ї2-іпсЬ collet is placed in the welding torch, after which the electrode can be inserted.

For welding a root bead, the electrode must be positioned in the torch so that it will extend the correct distance beyond the end of the nozzle. This is done by placing the torch in an upright position with the nozzle resting on the bevel of the weld joint, as shown in Fig.

6- 8, While this is being done, the welding current should, of course, be shut off. The electrode is then adjusted so that it is positioned

Fig. 6-8. Position of GTAW torch and tungsten electrode when adjusting the electrode for root bead welding.

with the end almost flush with the bottom of the joint or the inside surfaces of the pipe.

Reference to Table 6-1 shows that the welding current character­istic should be direct current, straight polarity (DCSP), when weld­ing mild carbon steel pipe. Table 6-2 provides the range of the welding current amperage to be used. Because of the many variables encountered, it is not possible to give a more precise recommenda­tion. In this case, using a %2-inch electrode and a direct current, straight polarity current characteristic, the welding current flow should be 150 to 225 amps. The gas supply should be adjusted to provide a flow rate of about 20 cubic feet per hour.

When the pipe is in the 5G position, the tack welds should be made in the 8:30, 4:30, 1:30, and 11:30 positions. The regular root bead is started in the 6 o’clock position and the weld is made by uphill welding around one side of the pipe to the 12 o’clock position.

Then the other half of the pipe is welded, starting again at the 6 o’clock position and welding uphilL І There is no difference in the welding procedure used, whether

* welding in the 5G or the 2G positions. For this reason, the proce­

dure that is recommended in the following paragraphs can be used in either position. In both cases (5G or 2G positions), the root bead should not be started against a tack weld. It is somewhat difficult to start a weld in the 6 o’clock position because of the stance that the welder must take; however, the procedure that is used to start the weld will be the same, regardless of the position of the weld.

The arc should not be struck in a manner similar to the procedure used for Shielded Metal-Arc Welding with consumable electrodes as this procedure would seriously damage the tungsten electrode.

To start the weld, the current and the inert gas supply to the welding torch must be shut off. The GTAW welding torch is held in the right hand (if left-handed, in the left hand) and the filler rod is

Table 6-2, Current Range Chart for GTAW Wefding Current (amperes) Electrode Diameter (Inches) A. C.H. F. Alternating Current with High Frequency D. C.S. P. Direct Current, Straight Polarity (electrode neg.) D. C.R. P. Direct Current, Reverse Polarity (electrode pos.) Pure Tungsten * tThoriated or Zirconium* * tThoriated tThoriated or Zirconium*!l; .020 5 to 35 .040 10 to 40 15 to 60 30 to 100 Vie 30 to 70 60 to 100 70 to 150 10 to 20 70 to 100 100 to 160 150 to 225 15 to 30 % 100 to 150 140 to 220 200 to 275 25 to 40 150 to 225 200 to 275 250 to 350 40 to 55 200 to 300 250 to 400 300 to 500 55 to 90 '/« 275 to 400 300 to 500 400 to 650 80 to 125 *Pure Sungsten — green tip **Zirconium — brown lip tThoriated tungsten — 2r'A red lip. і c/< yd)ow tip

held in the other hand. The torch is brought into position for welding by carefully placing the nozzle against the two beveled surfaces of the weld joint, which acts like a V-block to locate the electrode in the weld groove. In order to position the electrode in the center of the joint opening, the side angle of the electrode and the welding torch must be at zero degrees; i. e., the electrode must be perpendicular to the pipes in this direction.

However, the electrode angle must be steep, as shown in Fig.

6- 9A and B. As shown, the welding torch to start with, should be positioned to hold the electrode at an angle of approximately 55

Fig. 6-9. A. Electrode angle and filler rod angle in preparation for root bead welding. The welding current is off when the electrode is in this position. B. Welder ready to start a bead.

Fig. 6-10. Correct angles of the tungsten electrode and the electrode when welding.

degrees. At the same time, the filler rod should be positioned close to the weld in preparation for insertion in the weld after the arc is established and the molten pool of metal has formed. It should not be positioned in the - arc area when the arc is first established; however, the welder should position the filler rod so that he can dip it into the molten puddle without any disturbing body movements that might affect the smooth manipulation of the electrode. When welding, the filler rod should be held at an angle that is between 45 and 70 degrees, as shown in Fig. 6-9A and Fig. 6-Ю.

With the welding torch and the filler rod held in the starting positions, the welder must then close his face mask and switch on the welding current using the foot switch or the switch on the welding torch. Slowly and carefully he will straighten the welding torch thereby reducing the electrode angle and bringing the tip of the electrode into the groove. In doing this the welding torch is pivoted around the end of the nozzle, which is held in position on the bevels of the weld joint.

While the welding torch is pivoted, the arc will be formed and become established. As shown in Fig. 6-10, the welding torch is pivoted until the electrode angle has reached 20 degrees, the correct angle for welding. The filler rod is held at an angle of 45 to 70 degrees. As the welding current is started, it also causes the flow of the inert gas to start to form a protective shield over the weld area.

After the arc has been established, the welding torch is held in place until a puddle of molten metal has appeared between the two edges of the pipe. After the puddle is established, the filler rod is dipped into the molten metal. The welder then begins to oscillate the welding torch from side to side, as shown in Fig. 6-11, allowing the arc to travel slightly beyond the edges of the pipe.

Fig. 6-М. Electrode oscillating procedure for welding the root bead with a GTAW torch.

Fig. 6-і2. Welding a root bead. (Left) Welder in welding position: (Right) Welder's view.

Welding a root bead is shown in Fig. 6-12. When the puddle has been built up to the required size by the addition of filler metal, the welding torch is then moved on by slowly and steadily gliding it over the bevel while at the same time the torch is oscillated from side to side. Filler is added continuously to the molten puddle from the end of the rod which is held in the puddle.

While welding, the welder must watch the puddle at all times to see that it is built up to a uniform height and forms a smooth bead with only traces of a ripple. As with most welding processes when welding pipe in the 5G position, there is some tendency for the puddle to drip, causing excessive penetration. When this occurs, corrective action should be taken by slightly increasing the speed of travel and the speed of the oscillating movement. If possible, more filler metal than usual should be added to the puddle which tends to chill the puddle and to reduce its size.

The welder must be careful at all times to prevent the electrode from making contact with the sides of the joint and the molten puddle. This would not only contaminate the end of the electrode but cause the arc to become unsteady. Should this occur, the weld should be stopped and the tip of the electrode reground before restarting the weld.

One problem that is occasionally encountered is that the nozzle will tend to stick or grab as it is lightly dragged over the bevel during welding. The end of the nozzle gets very hot, a factor which promotes its sticking~to the heated bevel. As shown in Fig. 6-1 ЗА, the edge of the nozzle is square, and sticking can be avoided by grinding a radius on the edge of the nozzle as shown in Fig. 6-13B.

Stop and Restart. The weld should never be stopped unnecessarily. However, there are occasions when it must be stopped, such as when
welding a tack weld or when the 12 o’clock position is reached. The weld must also be stopped and restarted when making a tie-in with a tack weld which wiil be discussed in the next section.

To stop the weld, the welding current is simply switched off. If the filler rod is to be replaced or if the weld bead is finished, the filler rod should be withdrawn beforehand. However, if the bead is to be continued, the filler rod is left in the puddle to solidify against the end of the bead.

Fig. 6-13. A. Standard nozzle with square edge; B. Nozzle edge rounded to facilitate dragging the GTAW torch when welding a root bead.

J O'

Fig. 6-14, A perfect tie-in on a root bead, deposited by the GTAW welding process.

The procedure for restarting the weld against a layer of weld metal is similar to the starting procedure previously described. With the welding current off. the welding torch is positioned on the bevel
adjacent to the weld bead. This time, when positioning the welding torch, it should be held at the angle used for welding; i. e., the electrode angle should be about 20 degrees. The position of the electrode is carefully adjusted so that it does not quite touch the weld bead. The welding torch is then rotated about the nozzle until the electrode angle is about 55 degrees, as was shown in Fig. 6-9. After the filler rod is in position, the welding current is switched on and the welding torch is slowly rotated to the 20-degree position in prepara­tion for welding, with care being taken not to allow the electrode to touch either the bead or the sides of the joint. When the arc has been stabilized and a sufficient quantity of molten metal has appeared, the filler rod is placed in the puddle and the electrode is oscillated, as before, and the weld is continued.

Making a Tie-in. When the weld approaches another bead, the normal GTAW welding procedure is continued until the weld is approximately Vi6 inch from the bead. At this point the filler rod is withdrawn, but the welding torch is slowly moved on; it is tilted as necessary to prevent the electrode from coming in contact with the weld metal. The welder must watch the filling up of the gap between the beads. When a smooth blend between the beads has been obtained the current is switched off. A perfect tie-in, such as that shown in Fig. 6-14, will result if this procedure is followed with care and attention.

Second Pass. When a second pass with the GTAW torch must be made, the procedure is the same as that used in welding the root bead. However, it is necessary to readjust the position of the elec­trode in the GTAW welding torch. The length that the electrode protrudes from the end of the nozzle must be shortened so that it will not come in contact with the root bead or the molten pool of metal. The end of the electrode is ground to a blunted point, as before.

While welding, the electrode and the filler rod are held at the same angles as before. The welding torch is advanced by slowly and uniformly gliding and oscillating the welding torch with the end of the nozzle resting in the V-shaped groove of the weld joint. By welding slowly and carefully, the second layer will be deposited uniformly and with only a trace of a ripple.