Why are ground welds "magnetic"?

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I have noticed over the years that after I grind a MIG weld down (I am an auto body tech) the metal shavings will stick to the ground weld. Even pretty large drill shavings, it is like the darn weld is a magnet. Anyone know the reason for this?I have a feeling it is going to be very simple and I will be a doof, oh well.  Brian
Reply:It's the effect of the DC current flow through the material. Your are polarizing the zone or piece in question.. Not significantly, but to an irritating amount.
Reply:If you heat it with a torch or shock it with a hammer, you can knock the molecules back out of their polarized orientation and demagnetize it.  Not completely, but some.  A good reason not to run your permanent magnet DC motors too hot or drop them on the floor.
Reply:Any magnetic metal that is molten is above its currie point, the point at which it looses its magnetism.  When cooled through the currie temperature in a magnetic field, it retains this magnetism just like the tape in a tape recorder.  Welding with AC should generally minimize this residual magnetism.
Reply:wanna de magnetize it?put a de-gauser next to it  ...zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Yuh want a case of magnetic misery ? Once i was assigned to weld on this big "thing" . it was thick metal, about 1 1/8th inch thick. where i had to start was an open butt, with about a 1/4" inch GAP. i was told to use 7018 . ARC blow supreme . it was fierce . even going backwards did not help. finally , the boss let me root it with 6011.( A C )  then a pass with 7024 (a c) . after that i went to dc and 7018 , no more problems .Oh yeh, we tried wrapping the weld leads around it, NO luck. Even more grounds to it , we had to go to the ac & 6011 to get it going .Last edited by Roy Hodges; 06-07-2006 at 10:50 PM.
Reply:Roy, if you made that whole post up, I wouldn't know.  You are talking chinese to this bodyman.  Ok guys, this is interesting. One more, I have never checked it later, is the darn thing magnetic for ever if not heated or hit?Brian
Reply:All steel has residual magnetism.  A steel plate would lose the strong magnetism over time but will always have some magnetic characteristics.   A steel boat if berthed for months in one position will develop a magnetic memory that can have an effect on the ship's compasses.  From what I understand the boat will be influenced by the earth's weak magnetic field.
Reply:MartinSR & lotechman  brought up interesting points I never thought  of. Like a car body being magnetised ? and a ship getting also magnetised . I don't think i ever heard that mentioned at Mare Island , except that we were NOT supposed to let welding cables dangle in the water , from shore to ship, that THAT could cause more electrolisis. or maybe i forget somethings? couldn't be because i am almost 67 yr. old ? Heaven forbid , how'd father time sneak up on me ?
Reply:If you looked closely at a ships compass you would note two small adjustment screws, knobs, whatever.  These were connected to two bar magnets stationed at 90 degrees to the compass centerline. below these were two stationary bar magnets.  To correct the compass reading to true north/south and due east/west you would sight known landmarks to establish a true reading and adjust the n/s line.  Then the ship was turned 90 degrees and the e/w reading was trued.  You then went through this process several times to get a reasonably accurate set of readings throughout the compass range.  when properly calibrated, the bar magnets would compensate for the ships magnetic field.  Even the nonmagnetic minesweepers compasses had to be adjusted because of the cast iron engine and other small magnetic objects.  In Vancouver Canada it was easy because the streets going up the mountain in North Van were true north south and could be seen almost anywhere from English Bay.  Lots of room to manouver and sight.