S/s Purge. Argon Vs. Nitrogen

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So I Am Wondering What The Difference Is Between Purging With Nitrogen Rather Than Argon.I Will Be Welding A 4 Inch S/s Line That Is About 500 Ft Long And There Is 3 Position Welds To Do. The Stainless Line Is A Nitrogen Line And Is Tied Into An Existing Nitrogen Line. So Why Couldnt I Crack The Valve And Purge With Nitrogen Rather Than Using Argon? What Differences Will There Be To The Weld In The Pipe?
Reply:I'm no metallurgist nor a welder, but I do know that Nitrogen is NOT a noble gas and can react with other elements including Iron. Argon is completely inert noble gas and can't react.There is a possible that it would work, but it could go either way.
Reply:This is directly out of the Modern Welding Technology book, Nitrogen is often used for purging stainless steel pipe and tubing systems. It is much less expensive than argon and it keeps the oxygen away from the root of the weld.
Reply:There has been discussion on Nitrogen purging before, try a search.Basically, Nitrogen is a diatomic (2 atoms) molecule under "normal" conditions, and as this N2 molecule it is basically inert.  But, N2 breaks down to single atoms in the welding arc and becomes very reactive with molten and hot steel, so Nitrogen cannot be used as an arc shielding gas.  But, inside pipe or tubing with closed root weld joints, N2 can be used as a purge gas since it is not exposed to the welding arc.
Reply:Originally Posted by Go1lumThis is directly out of the Modern Welding Technology book, Nitrogen is often used for purging stainless steel pipe and tubing systems. It is much less expensive than argon and it keeps the oxygen away from the root of the weld.
Reply:Originally Posted by pulserThere has been discussion on Nitrogen purging before, try a search.Basically, Nitrogen is a diatomic (2 atoms) molecule under "normal" conditions, and as this N2 molecule it is basically inert.  But, N2 breaks down to single atoms in the welding arc and becomes very reactive with molten and hot steel, so Nitrogen cannot be used as an arc shielding gas.  But, inside pipe or tubing with closed root weld joints, N2 can be used as a purge gas since it is not exposed to the welding arc.