When TIG welding pipe/tubing, is the quality of the weld compromised if...

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you do not do a back purge?  If I understand this well, you do a back purge so that the weld inside the pipe is not oxidized.  Does it make a better weld or just more attractive?Also, can someone explain to me why you leave a space between the 2 ends when welding a pipe/tubing?  Why not just do like a "round butt joint"?  Thanks in advance for any information.
Reply:Yes. Yes, and yes.Sometimes it is good enough without backpurging.  Othertimes it is essential to backpurge, or weld in an inert chamber.Sometimes you need a slight gap between the ends to enable you to get the torch (or the stick, or the wire electrode, etc) down into the place where you want the weld to be.  This way you can get penetration and fusion all the way down into the bottom of the seam, and then you may run multi-passbeads to get 100% fill.  Sort of the same reason why you may need to bevel prep the edges of plate before welding.
Reply:Originally Posted by x1rideryou do not do a back purge?  If I understand this well, you do a back purge so that the weld inside the pipe is not oxidized.  Does it make a better weld or just more attractive?Also, can someone explain to me why you leave a space between the 2 ends when welding a pipe/tubing?  Why not just do like a "round butt joint"?  Thanks in advance for any information.
Reply:On Carbon Pipe( with TIG ), a back purge is not necessary if ER70S-2 is used, because there are enought Deoxidizers in the wire to take care of backside porisitySMAW,GMAW,FCAW,GTAW,SAW,PAC/PAW/OFCand Shielding Gases.  There all here. :
Reply:In some instances, it's just not feasible or even possible to do a backpurge, especially with tubing.  Take 4130 tubing for racecars for instance.  As it is, you will need to drill a small pressure relief hole in each tube that will be capped off at both ends, or the gasses trying to escape from the inside will actually cause an unfixable blowout as you complete the joint.  Although MOST people don't, I actually prefer to use a solar flux on the backside of these joints...
Reply:Thanks for the replies, the information is very much appreciated.  Let's say I want to fabricate an exhaust with SS and intercooler pipes with aluminum.  Would backpurging be necessary in these applications and what about leaving a gap (both are fairly thin walled pieces of tubing).  Thanks again.
Reply:Originally Posted by x1riderThanks for the replies, the information is very much appreciated.  Let's say I want to fabricate an exhaust with SS and intercooler pipes with aluminum.  Would backpurging be necessary in these applications and what about leaving a gap (both are fairly thin walled pieces of tubing).  Thanks again.
Reply:Purging tubing is generally common practice when doing sanitary/food service stuff..Inside of race car header tubes..Sometimes yes..Sometimes no..Exhaust systems..no(I have never seen anyone purge an exhaust system)Something that will be eaten or drank will have the process tubes purged for easy cleaning..By food I mean like peanut butter..anything that gets packed at a rapid pace like that.....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:Originally Posted by zapsterPurging tubing is generally common practice when doing sanitary/food service stuff..Inside of race car header tubes..Sometimes yes..Sometimes no..Exhaust systems..no(I have never seen anyone purge an exhaust system)Something that will be eaten or drank will have the process tubes purged for easy cleaning..By food I mean like peanut butter..anything that gets packed at a rapid pace like that.....zap!
Reply:no need to purge aluminium at all. i'd always purge stainless unless doing a weld with no burn through (fillets for example). solar paste is useful for some set ups as Supe saidfor the thin wall tubing i'd go butt to butt- no gapthe use of a root gaps allows the welder to watch the edge of the pieces melt away to become one with the puddle. without a gap the weld needs to be 'key holed' to guarentee a defect free root. if you rely on the appearence of the puddle to gauge penetration the weld is far more likely to end up with lack of fusion, incomplete penetration and/or uneven penetration- all of which are stress risers that cracks could grow from. remember that many pipe welds are carrying high pressures, flamable contents etc. stress risers reduce service life. with hygenic welds (food stuffs etc) any crevices, rough sufaces etc make good spots for bacteria to gather. no need to worry about any of this for exhausts/car plumbing
Reply:Intercooler pipes...Aluminum I would suspect..I would not bother purging that..Get the fit up real good and just "V" the outside edge of the mating parts and go for it!If your heat is set right and everything fits well you should have no burn thru to worry about anyway..The runners on the 427 Intake I built were 2 piece..No burn thru at all!You can do 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:Originally Posted by hotrodderno need to purge aluminium at all. i'd always purge stainless unless doing a weld with no burn through (fillets for example). solar paste is useful for some set ups as Supe saidfor the thin wall tubing i'd go butt to butt- no gapthe use of a root gaps allows the welder to watch the edge of the pieces melt away to become one with the puddle. without a gap the weld needs to be 'key holed' to guarentee a defect free root. if you rely on the appearence of the puddle to gauge penetration the weld is far more likely to end up with lack of fusion, incomplete penetration and/or uneven penetration- all of which are stress risers that cracks could grow from. remember that many pipe welds are carrying high pressures, flamable contents etc. stress risers reduce service life. with hygenic welds (food stuffs etc) any crevices, rough sufaces etc make good spots for bacteria to gather. no need to worry about any of this for exhausts/car plumbing
Reply:Originally Posted by zapsterThe runners on the 427 Intake I built were 2 piece..No burn thru at all!
Reply:Here ya go..Overall..Top off..Inside runner..(.090 wall)Little practice and you'll do 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:Originally Posted by x1riderJeez, there'a a lot more involved into this than I thought!  I think I'll just keep on pushing beads on flat surfaces for a while!
Reply:Originally Posted by zapsterHere ya go..Overall..Top off..Inside runner..(.090 wall)Little practice and you'll do it!...zap!
Reply:Originally Posted by hotrodderyou were asking why root gaps are used. all the stuff i was talking about is only an issue with critical welds which is why i said no need to worry about this for exhausts and car plumbingback onto purging stainless...if the stainless gets hot enough when in contact with atmosphere (the backside while butt welding) coking/sugaring will result. this is result of the chromium reacting with the atmosphere. the weld WILL have less corrosion resistance as a resultagain, not really a problem with an exhaust as it'll still last a long time.headers are exposed to higher heat levels. a cold weld with no penetration may lead to cracking in service (expansion/contraction forces can cause cracks to grow from the incomplete welds (the unwelded portion is the stress riser).
Reply:Originally Posted by SupeIn some instances, it's just not feasible or even possible to do a backpurge, especially with tubing.  Take 4130 tubing for racecars for instance.  As it is, you will need to drill a small pressure relief hole in each tube that will be capped off at both ends, or the gasses trying to escape from the inside will actually cause an unfixable blowout as you complete the joint.  Although MOST people don't, I actually prefer to use a solar flux on the backside of these joints...
Reply:Originally Posted by zapsterIf your heat is set right and everything fits well you should have no burn thru to worry about anyway.....zap!
Reply:Originally Posted by RojodiabloHA!! You've never seen me weld on a bad day!!!!!
Reply:Some of the high dollar exhaust fabricators do back purge their pieces. On the stainless exhaust I do, Solar flux works for me. It keeps the sugaring down on the backside if I get it a little hot. But it can migrate to the front side. and its a bitch to get off. Make sure it is only on the back side. Wipe it off the face of the joint then it seems to not migrate to the surface.mm135HTP Invertig 201 With water cooler9" Southbend LatheLots of hand tools.
Reply:Nice work zapster.The difference between art and craft is the quality of the workmanship. I am an artist.