Gas tungsten arc welding has been used to great advantage in all kinds of pipe welding applications. Besides affording the welder excellent visibility due to the lack of smoke and spatter, there is no porosity due to slag entrapment, and the deposits are essentially hydro-

gen free. The GTAW process is particularly useful, due to more pre­cise heat control, in welding thin wall pipe and in many cases heavy wall pipe may have it’s first root pass done with GTAW while subse­quent fill and cap passes are completed with conventional stick weld­ing. In such cases where root penetration must be as smooth as possi­ble, a consumable insert is placed between the mating pipe edges and fused into the bottom of the root opening.

However as pipe welding is most often done in various “out of posi­tion” situations, the GTAW process needed even more precise heat control to avoid weld pool sagging and bum through. The 1960’s saw the introduction of “pulsed current “ welding control in which both high and low values of current could be preset on the power supply. During the “high” portion of the pulse cycle, actual fusion would take place. Then as the torch moves away, the current automatically switches to a lower value allowing the puddle to partially solidify before the next high current pulse begins. The length of time for each pulse is also adjustable making possible optimum welding conditions over a wide variety of pipe welding applications.

The pulse current mode is shown in Fig 6-15.

Pulse control offers a number of advantages over the inconsistencies that are experienced during the time the root bead is deposited. These advantages of using pulse current-gas tungsten arc welding include:

(a) control over puddle size and fluidity

(b) ability to increase and maintain adequate penetration

(c) ability to maintain uniformity of weld deposit

(d) ability to preset the pulse mode to any condition, as needed by

the welder.

(e) ability to position the torch in its rightful position before the

high pulse is activated

Comparing shielded metal arc welding with pulse arc welding for depositing the root bead on thinwall pipe, the latter shows absolute

control of the puddle size, prevention of excessive penetration, com­plete protection from oxidation, and a clean inner surface of the root bead. Therefore, pulse arc welding is often preferable for welding pipe of both medium alloy and high alloy steel Pulse arc welding is a use­ful innovation for thinwall pipe, including when thinwall pipe needs two passes to be completed.

Before using pulse arc welding for heavywall pipe, the welder must be fully acquainted with the appropriate variables. Heavywall pipe offers an excellent heatsink. The size of the tungsten electrode must be considered. If a slight oscillation will be used while depositing the root bead, it is important to know where the tungsten tip will be located and when the current mode changes to low. The tip should be placed so that welding occurs under a blanket of inert gas; the hot weld will then be protected from oxidation. The position of the tungsten tip should be one sixteenth of an inch on the weld, where there is a chance of only a slight fissure or tiny void. It is from that position that the welder should begin when the high pulse is enacted.

The filler metal should be placed between the root opening where the tip of the rod is in contact with the apex of the groove. The rod must not be allowed too close to the starting point. When the high pulse is initiated, the rod should be inserted, making contact where the rod touches the root bead.

Because the thickness of the pipe makes an excellent heatsink, the high pulse mode duration should be four times that of the low mode. When the low mode is inactive, the tungsten-tip should be raised one sixteenth of an inch. It should then be advanced forward when the high mode is enacted, with a slight oscillation to remove cumulative grains that may have form during the cooling pattern, and to assure that the tie-in to the root is properly fused. The slight backstep becomes nec­essary because the heat-sink, which is so effective, can cause a shrink­age crack. This depends on the frequency of changes between the modes. The high mode should be enacted while the cooling weld is still in a semi-solid state.

The power source design for pulse current welding calls for adjust­ing both the high mode current and low current pulse based on time duration. Adjusting both depends on wall thickness, preheating, fix or roll position, and the rootface thickness.

The internal surface of the pipe edges to be welded should be clean of oxide (rust) and other impurities.

Prequaiified Base Meta!—*Filler Metal Combinations for Matching Strength8 (see 3.3)

Notes:

1. In joints involving base metals of different groups, either of the following filler metals may be used: {1) that which matches the higher strength base meial, or (2) that which matches the lower strength base metal and produces a low-hydrogen deposit. Preheating shail be in conformance with the requirements applicable to the higher sircngth group.

2. Match API standard 28 (fabricated lubes) according to Stee! used.

3. When welds are to be stress-relieved, the deposited weld metal shall not exceed 0.05 percent vanadium.

4. Only low-hydrogen electrodes shail be used when welding A36 or A709 Grade 36 steel more than і in. (25.4 mm) thick for cyclically loaded structures.

5. Spccial welding materials and WPS (e. g., E8GXX-X iow-alloy electrodes) may be required to match the notch toughness of base metal (for applications involving impact loading or low temperature), or for atmospheric corrosion and weathering chaiacleristies (see 3.7 3)

6 The designation of ER70S-IB has been reclassified as ER80S-D2 ш A5.28-79. PrctjuaSified WPSs prepared prior to 1981 and specifying AWS A5.18, EK70S-1B, may now use AWS A5.2B-7911R80S-D2

when welding steels in Croups 1 and U.

7. Filler metais of alloy group B3, B3L, Ё4, B4L, Б5, B5L. B6, B6L, 87, B7U B8. B8L. or B9 in ANSI/AWS A5.5, A5.23, A5.28, or A5.29 are not prtqoaHfied for use if) the as-wcidcd condition.

8. See Tables 2.3 and 2.5 for allowable stress requirements for matching filler metal.

Structural Steel Plates

Structural Steel Pipe and Tabular Shapes

Table C2.6 Structural Steel Shapes (see C2.42.2)

Earlier the importance of maintaining the proper tungsten electrode tip to work distance was stated. Welders will often employ the tech­nique of “walking” the cup along the sides of the groove in order to help maintain that critical electrode tip to work distance. There is how­ever a danger of losing effective shielding gas coverage because of the tendency to lay the gas cup over so that it is more parallel to the joint rather than pointing down into the groove. This occurs due to the curve of the basic torch path as it circumnavigates the pipe and as the welder tries to extend his vision before finally stopping and repositioning the torch. The welder must realize just how critical inert gas shielding is to the soundness of the weld.

The inert gas or gasses which are conveyed through the gas cup, must be so directed that both areas, the molten pool and the area which is in the solidification stage be exposed to inert blanket of protective gases which is supplied through the gas cup. Therefore, the angle in which it is held should be able to provide ample back wash of inert gas to prevent oxidation and other reactions within the grain boundries.

From a welder’s standpoint,, the indication of these drawbacks will be the appearance of the bead surfaces, which may be is dark and dull. A weld which was fully protected from oxidation at high temperature is a weld with a sheen and show no unusual amount of discolloration.

The seriousness of oxidation as described here is based on the fact that temperature of the weld metal one hundred degrees below the crit­ical range is weak, and with gains boundaries further apart as corn - paired to much lower temperatures. Without proper protection from the atmosphere, oxygen may be absorbed within the grainboundries and combining with other residual elements form compounds that will weaken the grain structure as temperature decreases, with construction taking place simultaneously, which leads to separation along grain boundaries.