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Up Topic Welding Industry / Technical Discussions / NITROGEN PURGE?
- - By Superflux (****) Date 01-15-2008 23:12
Back in the '70s when I first entered the pipe trades, it was common practice to purge 300 series (18/8) stainless steel lines with nitrogen and GTAW weld with pure argon. Having taken a 15- 20 year break into other ventures, and having reentered, I notice that argon seems to be the purge of choice. Did argon get cheaper, nitrogen more expensive or was there detrimental effects with using nitrogen? What the ????? Yea, this is the kinda stuff that keeps me up late at night.
Parent - By SWP (**) Date 01-16-2008 00:22
I have not personally worked with nitrogen purging of 300 series stainless steel, but have heard about it over the years.  AWS C5.5/C5.5M:2003, Recommended Practices for Gas Tungsten Arc Welding, states that nitrogen is widely used in Europe as a purge gas, and that it is used for austenitic stainless steel, copper, nickel alloys, carbon and low alloy steels, but it should not be used with reactive metals.

I think there could be a couple of concerns with purging austenitic SS.

As I understand it, Nitrogen is a diatomic (2 atoms) molecule under "normal" conditions, and the 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.

Nitrogen is a strong "austenite former", so if nitrogen is "picked-up" by the weld metal, it will reduce the ferrite content.  Somewhere around 3 to 10% ferrite is generally recommended for prevention of hot cracking.
Parent - - By weldgault (**) Date 01-16-2008 01:45
Argon, has been the choice of most Engineers and procedures for the last 20 years, I know of.  John
Parent - - By Kix (****) Date 01-16-2008 13:11
Whats up with the 95/5 (argon/no2) purge gas that they use on duplex stainless.  I noticed that it is a pain in the rear to get a good purge with it.  Very rairly do you get a gold bead or blueish bead i might say on the inside of the pipe when doing an open root.  I don't think i've ever seen a gold bead with that purge gas.  Is this because the arc is slightly exposed and reacts with the NO2 to make for a case of the nasty on the inside of the pipe?
Parent - By SWP (**) Date 01-16-2008 14:27
I think the 5% nitrogen blend is being used to control (reduce) the formation of ferrite in the duplex SS weld.
Parent - - By weldgault (**) Date 01-16-2008 16:36
Argon is still the best and least expensive, in most areas, because having to mix the gases,   John
Parent - - By 803056 (*****) Date 01-16-2008 22:23
Certain stainless alloys utilize nitrogen as an "alloying" constituent. A nitrogen purge with one of those alloys will affect the total nitrogen in the alloy thus it will affect the mechanical properties.

Many people use argon as a means of side stepping the problems associated with using nitrogen with the "wrong" stainless alloy.

Best regards - Al
Parent - - By Shane Feder (****) Date 01-17-2008 16:37
Guys,
I was going to respond but as I am busy making love to a large bottle of rum I couldn"t be bothered.
This subject has been dealt with numerous times on the forum, type "Nitrogen Purge" into Search and I am sure you will find what you are after.
Now I will go back to my rum,
Cheers,
Shane
Parent - By 803056 (*****) Date 01-18-2008 02:36
Good to hear from you Shane!

Best regards - Al
Parent - - By DaveBoyer (*****) Date 01-18-2008 03:59
Drink a half a pint for Me please.
Parent - By Superflux (****) Date 01-18-2008 07:20
Thanks to all who responded to this post. I've been steered in the proper direction, question answered, and now I think I'll follow Shane Feder's lead only with some Crown. "ZZZZZZZZZ"
Up Topic Welding Industry / Technical Discussions / NITROGEN PURGE?

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