Q: We discovered that some pipe that arrived on the construction site would not take a weld because it was magnetized. What is a good method to demagnetize the pipe so we can weld it?

A: This is a fairly common problem that occurs when lengths of pipe get banged together during the shipping process. Magnetism results in magnetic arc blow, a phenomenon that occurs during direct current (DC) welding, where the arc pulls to one side of the weld joint during the root pass. This deviation of the arc often can be severe and cause weld defects that will need to be repaired.

You have a few options if you suspect your material is magnetized. You could remove the magnetism completely, but that would require expensive equipment. You could also determine the extent of the magnetic pull with a gauss gauge before you place a demagnetizer coil around the pipe, but this type of equipment may not always be available.

One common method that I have used on pipelines in the oil and gas industry is to create a magnetic coil with the work lead. I wrap the lead around the pipe three to six times, about half the pipe diameter away from the weld groove on the opposite side from the direction of the arc blow pull. Then I deposit several ¾- to 1-in. tack welds around the weld joint. Using a current in the lower range of what is suggested, I deposit a root pass with a short arc length based on the size and type of my electrode. If I’m still experiencing arc blow, I then try to wrap the work lead in the opposite direction.

Finally, since arc blow occurs during DC welding, try switching to alternating current (AC) to deposit the initial weld bead. Then switch back to DC for the remainder of the weld joint.

Whatever method you use, be sure to pay close attention to the weld puddle, maintain a short arc length, and remove all slag deposits after each bead. You are the first line of inspection and should be aware of the quality of your work.

Email Jim Mosman, AWS, CWI/CWE

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