Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
be increased to levels similar to those of GMAW. Many commercial applications
of narrow-gap GTAW employ hot wire addition and an interesting example
is the Roboweld system which was designed for welding transmission
pipelines both on land and on pipe lay barges. Seam tracking and accurate
positioning of the electrode may be achieved by one of the methods described
in Chapter 10, but some systems also include facilities for video monitoring
of the arc and torch position with the ability for the operator to override the
automatic control to correct the torch position. To improve the sidewall
fusion the electrode may be oscillated or alternatively the arc may be deflected
magnetically.
9.3.2 GMAW
The potential for using narrower weld preparations and smaller included
angles for GMAW has often been claimed as an advantage of the process
when compared with SMAW. However, although some concessions are allowed
in the construction standards, this advantage has not been fully exploited in
manual welding probably due to the difficulty of maintaining consistent
fusion with reduced access. Reduced preparation widths are, however, used
in automatic welding where improved control is possible. The developments
in narrow-gap GMAW fall into two categories:
∑ the use of conventional automatic systems and narrower gaps;
∑ the use of special narrow-gap GMAW systems and narrow gaps.
Reduced gap/angle GMAW
A major application in the first category are the systems developed for
transmission pipeline welding. The CRC Evans system, [184] for example,
uses a compound bevel of the type shown in Fig. 9.2. In comparison with a
standard American Petroleum Institute (API) bevel, this preparation reduces
the weld metal volume by more than 20%. The pipe position is fixed with its
longitudinal axis in the horizontal plane and welding must therefore be carried
out in the 5G position (i.e. with the welding system rotating around the
pipe). This is achieved by using a tractor equipped with an oscillator and
typical operating conditions are shown in Table 9.1.
Systems such as this have been used successfully for welding pipe diameters
of 600–1500 mm with wall thicknesses of 8–22 mm.
Narrow-gap GMAW developments
The use of narrow-gap GMAW for the welding of submarine hulls was
reported as early as 1966 [185] and, by the mid 1970s, a range of developments
of the process were being used on production applications in Japan.