Stainless Purging?

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Is it mandatory to purge the back side of Stainless no matter what type of weld it is...pipe, flat, thin, thick..etc.????For instance:  I restore old Ford Stainless radiator shells.  Sand, buff, polish and weld any cracks that have formed.  I took one in to welding class and the instructor just welded the cracks with no purging, it looked like crap but I wil be grinding it flat and buffing.Is this a case where purging is not needed?
Reply:I repair alot of Delorean skins. I at least put a block behind to catch what little gas can get thru the crack. Simple purge hoses are not that hard to rig up either. The hose ferrule can be taped to the backside of the skin also. There is some metalized welder's tape also, and some solar flux that can do the job also.So the instructor did this? Hmmm.Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.
Reply:yeah the back side on SS will oxidize and sugar up, ruining the integrity of the stainless in this area and weakening the weld. in something ornamental like this probably not going to be a big problem as theres no stress or load but to maintain the integrity of the weld you need to either back purge or at least get some solar flux & coat the back of the seem before welding would be the right way to do it.miller 330a bp TIGmiller dynasty 200DX TIGmillermatic 185 MIGthermal dynamics cutmaster 101 plasma cuttersnap-on YA5550 plasma cutterhypertherm powermax 30 plasma cutterbaileigh CS225 cold sawetc....
Reply:That weld may rust faster than the surrounding metal as well.  If you polish and apply a clearcoat, probably not a problem.  But if that welded stainless is left bare, it may very well rust before the unwelded areas show any corrosion.Too much heat and no purge will deteriorate the corrosion resistance of the steel.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Originally Posted by A_DAB_will_doThat weld may rust faster than the surrounding metal as well.  If you polish and apply a clearcoat, probably not a problem.  But if that welded stainless is left bare, it may very well rust before the unwelded areas show any corrosion.Too much heat and no purge will deteriorate the corrosion resistance of the steel.