Presentation - After Preparation, Two Approaches May be Adopted
One approach is to have the root pass welded in the short arc mode on the inside of the pipe with multiple torches. Production rates can be high, but the process requires great care in joint preparation and fit-up. If this is not perfect, then there is a greater chance that many poor-quality welds will be made in a very short period of time.
A second option is to weld the root pass from the outside utilising an internal line-up spacer clamp. The sequence of operation is somewhat different to conventional internal clamps, whereas the internal line-up/spacer clamp is first located in the pipe and the first set of internal chucks are expanded into a hard chuck. This aligns the clamp with the inside diameter of the pipe and removes any ovality.
The second step is to bring into close proximity the next section of pipe. The second set of chucks is then engaged, which grabs the piece of pipe in a soft chuck. Once in a soft chuck, the clamp is spaced-in, which forces the two pipes together.
Figure 2
With one set of the chucks being in a soft state, they allow the pipe to move, so that the two faces of the pipe are butted up together tightly. The second set of chucks is then actuated into a hard chuck, which again takes out any ovality. The final stage is to space the clamp out, giving the journeyman/fitter a predetermined uniform gap around the pipe.
Gas Metal Arc Semi-automatic/Automatic Welding Processes
Gas metal arc semi-automatic/automatic welding processes provide an opportunity for increased productivity and deposition rates by virtue of fewer stop/starts. However, traditionally the shortcircuit mode as been identified as having a risk of lack of fusion.
Normal short-arc welding is considered to be low heat input. However, the current, which is proportional to wire feed speed, can be increased to give higher input and increased penetration. This can result in a fine balance between too little and toomuch and requires considerable operator skill in order to avoid defects such as internal ‘whiskers’. There needs to be enough heat to fuse the inside edges but not so much as to ‘blow through’. The operator has to ride the puddle with the arc to achieve the correct penetration and has little room for deviation.
These defects can easily be detected by industry standard non-destructive evaluation, such as radiography and ultrasonic inspection.
On the other hand, GMAW does have its advantages. Since hydrogen levels with GMAW are typically low, pre-heat requirements are minimal and cracking is rarely an issue.
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