To me what you describe is three seperate issues.

1 possible thermal arc blow
2 magnetic arc blow
3 a ground being located one side or the other and setting up detrimental arc ground to one side or the other.

As for where a seam would be on seamless pipe, there is no seam. There are three prevalent processes for manufacture of Seamless Pipe

Plug Mill - Larger diameters, an ingot is heated to just below melting point, and pierced. The hole is is enlarged on a rotary machine, in the resulting tube (bloom) a plug near the size of the finish diamter is forced in. It's then sent through rolls of a plug mill with each succeeding pass reducing the wall thickness. When it's near size, it's moved to a reeling mill and reducing mill which brings it to it's final finish dimension. After that comes heat treat straighting inspection etc.

Mandrel Mill - for smaller size pipe typically 1 through about 8" an ingot is heated the same as a plug mill and pierced the same. A mandrel is inserted and then it's passed through a rolling mill The key difference between plug and mandrel is the wall thickness is reduced continuously with pairs of curved rollers set at 90 degrees. When it's near size it's reheated and passed through a stretch reducing mill to final diameter and final T. the rest is the same as the plug mill.

Extrusion - This is for small tubes only. A bar stock is cut and heated before being sized and descaled. It's then passed through a steel extruding die. The final size is similar to Mandrel final stage.

Therefore, if you have a seam at all, you don't have seamless pipe. Now for ERW and HFI welded seam pipe you can run into some where the seam is not readily apparent.
That is another story.

As for the remainder, There is nothing more on the magnetism that can be added that I did not already mention to my knowledge. Others feel free to chime in.

Regards,
Gerald

Sourdough, I have seen pipes on pipelines that were magnetized to the point where you could stick electrodes on the bevel and they would stick. We think they became magnetic by being stored under high tension power lines or possibly the FBE Coating process. In any case the pipes seemed to habe a high magnetic permeability.

We would have to wrap AC coils around the pipe and run AC current through it during welding. This seemed to neutralize the magnetic field in the pipe. AC is how you demagnetize parts anyway.

Sourdough,

      Have experienced this lots!!  Thought it was my lack of experience, (broke out a year ago) until I asked a seasoned pipeliner.  He told me that due to the high demand of rods, the part of the machine that holds the rods as they are coated wear out and the result of this is the flux is thicker on one side of the electrode than the other. Hence the fingernailing.  Could be BS, but he said he got the info from an inspector who visited the plant where Lincoln manufactures electrodes.  Recently we got a half pallet of 3/16 hippy that was so bad you could visually see it. The co returned them. Even now with primarily good rod there are 10 to 15 rods per box that have a crack from end to end in the flux. My helper notices them except on bad hangover days.(which occur quite often).

  Also, (a question) what is up with the big bulge in the flux as you are welding about an inch away from the arc? Have any of you noticed this on some hippy rod? I have noticed that my arc is very irradic when this occurs and is usually on the bottom.  Am I just running too hot? 

I have expierienced this while making a pipeline repair.  We cut out the bad section of pipe, then when we installed the new joint of pipe, one end welded up fine then the other end had a little to much gap and the arc was being affected by the magnatism trying to pass across the gap!  Clamp something across the gap, such as apiece of angle or some sort of jumperwire. this allows the magnatism to go where  it wants  to. Once the stringer bead was in it welded like it was suposed to. I don't know if  this is your situation but  it sure helped me make that weld.

Pipe is mangnetized by various industrial causes, intentional and circumstatial, and possilby by local geomagnetic effects.  Magnetic fields in the pipe during welding may be peexisting or generated by the high amperage DC current.  The two fields likely interact in some non linear way during welding, or field effects oscillate, hence noticable changes in arc behaviour during welding.  The best treatment appears to be magnetic hysteresis treatment of the pipe ends before welding. 

Diameter is not an intrisic factor for field strength, but more mass of similarly magnetized material would have a percieved stronger effect.

ERW pipe seam is made by high frequency AC current, which acts to reject resultant common orientation of magnetic domains, ie many randomly distributed local domains cancel each others magnetic fields.  Seamless pipe would be magnetized due to industrial processes applied to the pipe, not due to any phantom seam artefact.

Be carful the accuracy of magnetic field measurements at pipe ends.  The probes have specific orientation relationships that must be matched to the field direction in order to have an accurate reading.  I have found no inspectors that are familiar with this physical detail.