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WELDING PROCESSES
Obviously the first step in the welding of pipe is to run the
root pass. This is perhaps the most critical pass on a pipe weld
for several reasons. First, this is the most difficult pass to
make on a pipe weld, requiring good operator skill for manually
applied processes, with good process control combined with good
alignment. Automatically applied processes require operators with
high degrees of technical skill combined with good alignment and
backing systems. The automated process of choice today is gas
metal arc welding and is generally used with either an internal
copper backup ring, or, if the diameter is large enough, an internal
welding system. Both of these approaches add complexity to field
welding and impose certain restrictions the use of traditional
GMAW transfer modes.
With backup rings there is the possibility of unacceptable
copper pick up in the root pass. With internal welding systems
there is a minimum pipe diameter below which the systems are
not practical. The ideal welding process would allow welding
of a root bead without backup rings and internal systems and
would have a root bead with sound weld metal and just enough
buildup to insure a full thickness weld. This weld would also
have no internal undercut, no lack of fusion, no porosity, and
good mechanical properties.
Welding speed must also be considered when looking at the
welding of the root pass. The pace of pipe laying is determined
by how quickly the root pass can be done. While some time can
be gained by putting more operators on this pass, there is a
practical limit to this approach. Therefore, high travel speeds
are essential. Speed is needed to maintain schedules and control
equipment leasing costs.
Much of the pipeline welding done today is in the emerging
economies of the world, often in remote inhospitable climates
and must draw on local labor pools for welders. Tthis means
that the process used must cope with adverse conditions of weather
including wind, temperature extremes, and moisture. The necessary
skills need either to exist in the local labor pool, or be easily
learned. The required welding equipment must also be rugged,
reliable, and durable.
When all of the above factors are considered, two welding
processes emerge as the leading processes, shielded metal arc
welding and self shielded flux cored arc welding. In the case
of shielded metal arc welding, Figure 1, there are advantages
to using cellulosic electrodes run in the vertical down direction
instead of using low hydrogen electrodes, even on higher strength
steels. Because cellulosic electrodes generate a significant
amount of shielding gases in use and have a focused forceful
arc, these electrodes tend to have better root pass properties
and better root pass control. The high arc force helps to maintain
puddle and slag control in vertical down progression, while
also having high travel speeds. Low hydrogen electrodes primarily
use slag to protect the weld pool and this can lead to contamination
of the weld pool from the back side of the bead, reducing weld
properties and increasing the chances for porosity. The relatively
low penetration of low hydrogen electrodes when compared to
cellulosic electrodes also means that wider root gaps must be
used which increase welding time and slow down the welding operation.
Cellulosic electrodes can put in root passes at speeds that
exceed 14 inches per minute (356 mm per minute) and with consistent
inside buildups of under 1/16 inch (1.6 mm).
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