What are the effects of backweld?

Asked by Ryan Bidong 2 years, 3 months ago | 2 Answers

What effect does backweld have on process piping joint.

2 Answers

taddpham 1 year, 9 months ago

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[7] 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. 

taddpham 1 year, 9 months ago

Hard crack-susceptible weld microstructures have been known to result from backwelds that are deposited last.

Another 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 problems.