If the magnetic field is sufficient to divert the welding arc, it is too much. The magnetic field can be the result of a residual field, it can be caused by preparing the groove by grinding, or the cause can be the welding current passing through the pipe.

The field strength is a function of the current flowing through the part being welded or if the welder wraps the welding lead around the pipe, the strength of the magnetic field is directly proportional to the amperage times the number of wraps (of welding lead) around the pipe.

Al

jwleger I know of no standard that addresses magnetism or that says maximum allowed. As Al states, if it affects your welding well it needs to be sorted one way or another!

I don't know of a quantified "maximum" but as others have said - if you have the rod centered over the root and the bead is being deposited on a sidewall - you have a problem.

There are lots of ways to correct this.  You can use a cheap gauss meter to look for "null" points (no magnetism) and start there (even though they will change as you fill in the root), you can CONSIDER a backing bar (subject to engineer approval) which may redirect the magnetic field out of the root) or you can get your NDT guy to degauss the whole weldment if MT is required.

I have only experienced severe magnetism in a few cases.  One was while SMAW of heavy wall pipe in winter in Texas panhandle and again in Maine.  Was able to reduce magnetism by trial & error of wrapping ground and electrode leads in different directions until right combination worked.

Other time we had to also employ a heat ring inside of pipe to reduce magnetism.

Robert

Do you have a gauss meter? There are companies that sell/rent degaussing equipment but you can wrap leads as well. If this is a recurring issue then I would recommend one of the commercial units. They are superior to lead wrapping techniques. 10-40 gauss can cause issues. Storing pipe, pipe construction and/or shallow pipelines near overhead electric transmission lines or substations can be part of an induction field.

Hey jwleger,
You will not see a max. anywhere.

Most gauss meters measure the magnetic lines of flux on the surface of the part.  It cannot measure the lines within the part, nor the direction (which is most important). 

Magnetic lines running perpendicular to your welding direction causes the most arc blow.  Any other direction diminishes the effect.  I've successfully welded on parts with 300 gauss with no arc blow, probably because the lines were parallel to the weld joint.

I've read in literature somewhere that 50 gauss can cause arc blow.  I agree with this statement, but only if the lines of flux is perpendicular.

Tyrone