Purging gas question.

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I recently read of a gas mix used for "back purging" TIG welds. It consists of 95% Nitrogen, and 5% Hydrogen. According to the author of this info, the Hydrogen scavenges any Oxygenout of the purged area, and promotes better root bead penetration, with lower amperage.Have any of you folks heard of, or used this method?Bgbkwndo.
Reply:Never heard of it. We purge with pure argon.
Reply:Hello Bgbkwndo, I believe the backing gas that you are referring to is used for certain applications on steel pipe. It is not designed for use with stainless steel, titanium, or other similar alloys. The main reason for it's use is cost, the secondary reason(s) for it's use are covered by the comments that you included in your post. I also believe that it is more readily used in Europe and has worked it's way to our shores in more recent times. Best regards, Allanaevald
Reply:Never heard of nitrogen/hydrogen mix being used as a purge gas, where did you see it? Argon/ nitrogen mixes, or pure nitrogen are sometimes used over here to purge Duplex/ Super Duplex steels, aparantly the nitrogen is a strong austenite former and an important alloying element in these materials.Also,  would the benifits of using such a gas mix not be outweighed by the increased chance of hydrogen cracking?Last edited by Baila La Pinza; 08-11-2011 at 12:44 PM.
Reply:I was doing some investigation on welding stainless steels recently and came across the same back purge gas.  95/5 N2/H2 or 90/10 N2/H2.  Specifically it's for use with Duplex Stainless steels for just the reason that aevald mentions.Here's a link to a good discussion on backpurging https://boc.com.au/boc_sp/downloads/...rochure_v7.pdfOne thing to keep in mind is that Hydrogen gas mixes are extremely flammable over a wide concentration range.  both of the mixtures mentioned above are flammable.  The brochure in the link I posted mentions that these back purge gases can be burned off downstream of welding to prevent uncontrolled fires or explosions.  Also remember that H2 fires are nearly invisible as the flames are almost colorless.Straight N2 is a common backpurge for stainless welding here in the states where the corrosion resistance properties of the stainless pipe aren't a major concern and/or where the cost to back purge with Argon is prohibitively expensive.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector