ss exhaust system, slip joints

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i have a full stainless steel exhaust system on my 03' bmw 330, its kinda hard to describe but where one pipe goes into the other one the end if flared out about 3in down the pipe with a 1in notch with a bolt and nut on ether side, so that the other pipe can slip inside. when i go to tighten it down it will not seal all the way, so i have a exhaust leak because of this. there is a small gap around the pipe about 1/16- 1/8in, i would like to tig it all up. is there anything special i should do?
Reply:if you dont want to tig it you could use copper rtv
Reply:Originally Posted by neoslimSnip..... is there anything special i should do?
Reply:Originally Posted by David RClean it!It should be back purged.Take pictures!Daivd
Reply:Originally Posted by TSORI know they do it and why for food and pharmaceutical stuff.Why back purge an exhaust system??
Reply:i did not even think about purging it. I can tig ok, so what size cups and tig setup should i use, the pipes are 2 3/4in and are about 2in apart. The back purge is to get all of the oxygen and impurities out of the system which equal a better weld.
Reply:use an exhaust band clamp, we used them on a custom exhaust for a jeep last week and they worked like a charm.  Local napa has them, 7 bucks a piece.No reason to back purge SS for an exhaust.  If you burn through then you gonna suck the soot through from the other side and a purge wont prevent that anyways.  Weld it cold so as to not burn through if you must weld it but I would use a clamp.  You can also remove it easier in the future.Have we all gone mad?
Reply:New fuel injected cars burn relatively clean (12.7:1 A/F ratio) . The old carb and spark plug trick works because of an excess amount of unburned fuel.John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:i not worried about the unburnt fuel, its going to be pulled off the car so i can get a complete weld all the way around the pipe.
Reply:Back purge may help prevent the SS from suffering premature corrosion failure.  It will keep the chromium and other alloying additives in the ss from reacting with oxygen in the air and 'sugaring' the back side of the weld bead.  If those alloying ingredients are not dissolved in the steel, they can't do their job of protecting the iron in the SS from corrosion.Also, if you've run this exhaust already, I'd make sure to thoroughly clean the soot and exhaust residue from the inside of the pipes before welding.  Wire brush at least 1" back from the location of the weld joint.The carbon deposits on the surface can react with the chromium and other alloying elements in the SS, when the steel is heated to temps ranging from 800-1600°F.  All welding causes the steel to spend some time at these temps.  If the chromium reacts with carbon, it can no longer provide it's intended function, to protect the iron in the steel from corrosion.  The technical term for this is chromium carbide precipitation.  It's a real problem with 300 series austenitic stainless steels, and I've seen it occur in 400 series ferritic stainless as well.Even if your stainless is one of the ultra-low carbon grades, carbon pickup from surface contamination or the surrounding atmosphere is a potential problem.  Clean you filler metal to ensure that no oils or greases from the wire drawing operation are introduced to the weld puddle.This is why MIG welding of stainless must be done with shielding gas that does not contain any CO2.  Pure Ar, He, or Tri-Mix are OK.  I'd use pure Ar for your TIG welds in this case.Keep your heat input as small as possible.  This will minimize the size of the HAZ and the chances of losing any of the steel's corrosion resistance.Chromium carbide precipitation appears as a line of corrosion or cracking running parallel to the weld bead.  The width of the band is affected by heat input to the weld and the cooling rate of the weld.  The amount of time that passes before failure occurs is dependant on the amount of carbon contamination, the operating environment, and the amount of Chromium, Niobium, and Titanium in the base metal. (perhaps Molybdenum, and Tantalum as well, if the ss alloy contains these elements)I've seen faulty stainless exhaust welds on 409 stainless fail in as little as 10,000 miles of 'time in service', simulated on an engine dyno.Last edited by A_DAB_will_do; 04-18-2008 at 12:30 PM.Reason: add elements and fix small typosBenson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector