What effect does backweld have on process piping joint.
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The current version of API 1104 (20th Edition) does not require the use of a specific welding
procedure specification (WPS) for backwelding (i.e., bead sequence is not an essential variable
for procedure qualification). However, if backwelding is to be permitted, it is good practice to
weld and test joints with and without backwelds (i.e., it is good practice to consider the addition
of a backweld to be an essential variable for procedure qualification). The 20th Edition requires
that the sequence of beads must somehow be designated and this is normally accomplished by
providing a sketch. If backwelding is to be prohibited for a given WPS, this should be specified.
The proposed 21st Edition of API 1104 will have specific provisions for qualifying welding
procedures with backwelds.
When qualifying a procedure for backwelding, consideration should be given to specifying and
using low-hydrogen electrodes for the backwelds. While backwelds made using cellulosiccoated electrodes are acceptable provided that a WPS has been developed and qualified using
cellulosic-coated electrodes, backwelds made using low-hydrogen electrodes are more desirable
since the risk of hydrogen cracking is significantly reduced. Preheating requirements for welds
made using low-hydrogen electrodes are typically less than for welds made using cellulosiccoated electrodes, which alleviates concerns for achieving adequate preheat temperatures with
the welder inside the pipe.
Hard crack-susceptible weld
microstructures have been known to result from backwelds that are deposited last.
potential problem with backwelds is that their location (often 40 feet or more from the open end
of the pipeline during production welding) makes them difficult to make (confined space,
difficult to see [poor lighting, smoke], potential for arc burns, etc.) and visually inspect (often
inspected by no one other than the welder).
Achieving adequate preheat temperature with the welder inside is another potential problem.
Because of these potential problems, backwelding is
normally thought of as an undesirable practice. It is often carried out as a last-ditch effort to
make the root region of an otherwise completed weld acceptable. When carried out properly
however, backwelding is an attractive and perfectly acceptable solution to misalignment