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kishbud (Materials) (OP)
21 Dec 06 22:12
     I need to demagnetize various grades of SS seamless tubing in lengths less than 12". The tubing is first hand selected and tested for low to non-magnetic properties using 1/8oz neodymium magnets. All cutting, filing and finishing are all done by hand to prevent frequency build up from high-speed tools. The only process I cannot avoid is welding, and of all the modalities of welding TIG (GTAW) w/316L SS rod leaves the least residual magnetization and the cleanest weld. The finished SS product ends up slightly magnetized, indicated by the neodymium magnet sticking under its own weight or only momentarily attracted then falling away. This is unacceptable around the sensitive measuring equipment being used.

Thanks Mike
Helpful Member!  EdStainless (Materials)
22 Dec 06 8:42
you need to change your selection method.  You should check at lower field levels, it will be more sensitive.
You are confusing two effects.  When the magnet sitcks to the parts it is because they are ferromangetic.  They may or may not be mangentised, but they are able to be magnetized.  The only way to fix this is to re-anneal the material.  Or come to me for some good welded tubing that has been properly anneled in the first place.
After welding you have two things happening.  If the material is ferromagnetic it will have become magentized from the welding operation.  You would detect this this a Gauss meter or a very fine iron needle, not a magnet.
The second thing (and what is really happening) is that you have residual delta ferrite in teh welds making them slightly ferromagnetic.  You can either re-anneal the entire assembly and suffer some distortion. Or you can look at using a  filler metal that is higher in Ni and less prone to ferrite formation.  The risk is that you may get some weld cracking.  I would look at fillers like 310 or 904L for this.
If you just need demagnetization then take your parts to a local tool and die shop. they will have an AC demag unit.  It uses an AC field that decays.  Witha little playing around you can get very low residual field levels.

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