C25 as Back Gas for 304SS TIG Weld

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Hello all:  I am wondering if C25 is "inert" enough to function as the back gas when Tig welding 304SS using 100% Argon as main shield.  Application is for automobile exhaust headers.  Any experiences or comments for a newbie?  Thanks.  Burt
Reply:The awnswer is NO.The whole purpose of the gas is to keep oxygen away from the weld zone.Just put a "Y" fitting on your argon low pressure regulator  with a valve to turn it on or off.Why use another tank?MIG is a different process.75/25 or C25 is used for other reasons with that process.Last edited by Donald Branscom; 07-28-2008 at 06:14 PM.AWS certified welding inspectorAWS certified welder
Reply:I have not actually tried C25 as a backing gas, but in theory the backside CO2 should remain non-reactive if you are welding a closed root and it is not exposed to the welding arc.  I think it is the welding arc which breaks down the CO2 molecule allowing the carbon and oxygen to react with the molten weld pool.  This is the reason nitrogen can be used as a backing gas, because N2 is not reactive. so I think the same applies to CO2.  Try it and let us know.
Reply:If, and a pretty big if, the joint is so close-fitting and tight that it is closed root, then theoretically it can work if the arc never interacts with the purge/backing gas.But why not avoid a possible problem source and just use your argon as the purge and backshield gas?  One tank, a T-fitting to send some gas into the tubing as purge, then weld away.Besides, if your arc or heat starts to get away from you and you start to burn-through, you would have totally 'sugared' that section if you have something other than plain argon.  If you started to burn-through and have argon front-n-back, you could probably TIG the hole closed and then keep going.  The best laid schemes ... Gang oft agley ...
Reply:I have seen major problems with chromium carbide precipitation when SS409L was MIG welded with Ar/CO2 shielding gas mixtures.  Austenitic stainless steels are equally susceptible to this problem, although SS304 may be more tolerant of corrosion problems if only because of it's higher chromium content. If you feel you must do this, keep the CO2 below 5%.  Anything above this progressively compromises the corrosion resistance of the steel.  The problem is less severe with Higher Chromium, Titanium, and Columbium(Niobium) content.  The Chromium is tied up with carbon and eventually the grain boundary concentrations of Chromium drop below levels that provide corrosion resistance(passivation).  The Titanium and Columbium added to some alloys are there to scavenge or trap carbon if it is introduced during welding.  In short, can you weld with Ar/CO2 mixtures?  Yes.  Should you, in my opinion, No.  Use 100% Argon, or tri-mixtures of Argon/Helium/Oxygen.  Note:  I've never worked with the latter shielding gas mixture, I've just read that it's used with MIG welding for improved weld penetration and bead appearance.  I believe the source for this was stainless steel used in food preparation equipment, where a smooth, pore free surface minimizes the opportunity for bacteria to grow.Last edited by A_DAB_will_do; 07-28-2008 at 07:07 PM.Reason: add more detail.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Ummmm, Dab, what you wrote about MIG and corrosion and stainless is all true and such, ...but the OP was asking about TIG and C-25 as a backpurge for SS tube.  The best laid schemes ... Gang oft agley ...
Reply:Thanks to all for their responses.  Using a T-fitting sounds like the best bet.  Have a ways to go with technique before ready to try it but was trying to get a leg up on the information.  Thanks again guys.  Take care. Burt
Reply:Thanks Moonrise,I realized that the post is about back purging with C25..  Seems to me that sufficient heat is present on the back side of the TIG weld to create a real risk of the same type of failure.Maybe I went off the deep end with details....happens to me sometimes... Originally Posted by MoonRiseUmmmm, Dab, what you wrote about MIG and corrosion and stainless is all true and such, ...but the OP was asking about TIG and C-25 as a backpurge for SS tube.
Reply:fluvannabear,I would not recommend using C25 to backgas a stainless weld.Backgas for stainless needs to an "inert" gas.  In other words, it needs to be non-reactive.C25, which contains 25% CO2, is not an inert gas.  That is why the term MIG (Metal Inert Gas) welding is really an incorrect term (as we use it today).  The proper term, GMAW (Gas Metal Arc Welding) provides for the use of reactive gases such as C25 or CO2.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:ive heard it reffered to as metal active gas (MAG) as well, food for thought...