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Up Topic Welding Industry / Technical Discussions / Nitrogen as backing gas
- - By debj0124 Date 10-23-2001 13:50

May nitrogen instead of argon be used as backing gas for the TIG process? Will porosity be a problem?

Parent - By - Date 10-23-2001 18:11
If I can remember correctly, this has been discussed before on this forum, so if you search for this, you will probably get some usefull info.

Just to get you on your way, I will give you my opinion:

In general, N should not be used as backing gas, because you can still form nitrides in the back of the weld. Nitrides are rather brittle and can have a negative result on both the mechanical and corrosion properties of the material being shielded.

Where there is a bit of a deviation from this is the use of Ar with some N mixed in, for the backing when welding Duplex S/Steels. (Modern Duplex S/Steels contain N as a strengthening element as well as an austenite stabilizer.)

At any rate, I have never used N in the backing gas, even for the welding of duplex S/Steels.

Hope this helps

Niekie Jooste
Parent - By Lawrence (*****) Date 10-23-2001 20:17
Johannes, your question is worded perfectly!

"May" being the active word. If your building a stainless birdbath or Bass fishing lure the answer *may* be yes.

If you are working to a specification you must look to it for the authority to substitute nitrogen for argon in purge.

Story Break: Once upon a time, long ago, during my long and poorly compensated, non-subsidized, non-union apprenticeship, sometime after the Wright Brothers made their historic flight at KittyHawk. As a TIG welder I made many field welds in 3 inch Schedule 5 308 Stainless pipe at various wineries in Northern and Central CA. The Vats were Gigantic and the piping system was measured in Miles, Argon was expensive, Nitrogen and the mechanical contractor who employed me was cheap. As mentioned in the post above, the Nitride layer was very present on the back side of the weld and manifested itself as a kinda dull gray rather than the shiny silver *wedding band*. Furthermore extra care in fitting was required since the smallest of leaks of nitrogen into the arc-zone will wreck arc transfer and foul the weld. Being a kid and happy to have a job I asked few questions about this procedure and really haven't given it much thought until recently. I now wonder how those joints are performing today.

The moral of the story is this...... employment of Nitrogen as a purge gas is not a standard practice in any industry or qualification that I'm aware of. The only stories I have heard extolling the merits of Nitrogen are like mine, "just stories", and not to be trusted or emulated in production conditions without some trustworthy authority to back you up.

Parent - - By sparx (**) Date 12-17-2001 20:55
I realize that this won't answer your specific question, but I thought I would share something with you about tig gases.....normally, argon is used as a purge gas for stainless pipe welding...this is an effective inert atmosphere and does a fine job...however, I have done some research on tig gases for 300 series stainless and have had some great success with argon/hydrogen blends... the hydrogen is a reducing gas, and therefore acts as a de-oxidizer during welding....for a tig gas, travel speeds are increased, bead appearance is excellent, puddle fluidity is increased, heat affected zones are greatly reduced due to the increase in travel speed..... blends of 2% hydrogen are excellent on materials under 16g. and 5% hydrogen for materials thicker. If you get a chance to try this out, I highly recommend it.....
not sure who your gas supplier is, but Praxair has these gases as Hydrostar H2 and Hydrostar H5.
Parent - By George-kh (**) Date 12-21-2001 16:57
Just to complete sparx answer. Argon and H2 mixture can be used just for austenitic S.S.
Best Regards
Parent - By Jorge Giraldo (**) Date 12-28-2001 23:15
Hi Johannes:
Take a look on the previous answers gived to Daon´s question in order to complemment your information about purging gases in GTAW, wich is the discussion that Niekie 2 says. I posted it again in the page 1 of the forum.

Jorge Giraldo
Medellín, Colombia
Up Topic Welding Industry / Technical Discussions / Nitrogen as backing gas

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