1. Preparation of the pipe ends is considered a very important step in this welding operation. A diagram of a typical weld preparation detail is shown in Figure 3. It is recommended that a weld angle of about 35 degrees be machined onto the pipe ends.
Further, it is recommended that both the ID and OD be cleaned to bright, shiny metal for a distance of about 1" from the weld preparation edge and that a land be machined or ground per the dimensions shown in Figure 3. After all machining and grinding operations are completed, thorough cleaning and degreasing of the entire area is required.
It should be recognized that the pipe will have some ovality which can make for a mismatch in the ID during fit-up and welding (see Figure 3), it may be necessary to counter-bore the ID of the pipe sections in order to obtain an acceptable match around the inside diameter of the pipe.
2. Any of the purging systems discussed above, except thermally disposable dams, are considered acceptable for HASTELLOY® alloy pipe systems. As a way to insure that the purge gas enters the enclosed weld zone with a minimum of gas flow turbulence, a gas diffusing device is recommended. Figure 4 shows one possible design, for use with large diameter pipes, which can be fabricated in the field. Modifications to such a general design will certainly be required to meet other specific job requirements. The purge dams must be installed deep enough in the pipe section so that the heat of welding will not cause damage to the dams.
3. Fit-up of the pipe sections requires care so as to establish proper root-gap tolerance between the two pipe sections. It is generally recommended that the root gap be at least 1/32" larger than the diameter of welding filler wire to be used. This will allow for manipulation of the filler wire even when some shrinkage occurs during tack welding and closure welding.
4. Purging of the weld zone is done at about 40 CFH. The weld joint should be sealed around the circumference of the pipe except for a small opening at the top position of the pipe. The intention is to have the shielding gas enter at the bottom of the weld zone and exit at the top. If pipe welding is being performed in the vertical (2G) position, the gas should enter at the bottom purge dam and exit through a small hole in the top purge dam. Normally, flow rates and times are selected so that a minimum of 5 to 6 shielding gas volume changes take place prior to welding.(1)
5. Tack welding is necessary to ensure that the two pipe sections do not move during subsequent closure welding. The tack welds must be large enough and placed often enough around the joint circumference so that the root gap remains open and wider than the welding filler wire diameter. Generally, the weld joint is kept sealed except in the area were welding is being conducted.
It is recommended that the tack welds be ground to a feather edge. This will help ensure that the closure welds will tie into the tack welds and not leave small defects which might be detected during radiographic examination. The shielding gas flow rates used during tack welding, and during subsequent closure welding, should be about 40 CFH for the back purge and about 10 CFH for the welding torch. Other welding parameters are documented in Table 1.
Torch position and filler wire position are critical to the success of root-pass welding. A diagram showing the positions of the GTAW torch body and welding filler wire relative to the pipe are shown in Figure 5.(1) Torch and filler wire manipulation consist of torch movement from sidewall to sidewall which forms a "keyhole". Two drops of filler material are melted into each sidewall with each back and forth motion of the welding torch. The filler metal is generally positioned in the root-gap opening. If the root gap "closes-up", not allowing the filler wire to be placed in the root-gap opening, the amount of weld reinforcement at the root will be limited.
6. The closure welding is performed much like the tack welding. Again, the joint is kept sealed except in the area were welding is being performed. During the last portion of the closure weld, the gas purge flow rate may have to be lowered so that the dams will not be damaged due to purge gas pressure increase. It is generally recommended that back purge gas be maintained for the first two fill pass welds. This will help ensure that the root pass is not heavily oxidized during these subsequent weld passes.
The selection of welding process for the fill passes is left up to the discretion of the field welding engineer.