backpurging with nitrogen

[Bruce Simpson]

I was told today that nitrogen wa a satisfactory alternative to
expensive argon when backpurging a stainless steel weld.

Has anyone had experience with this and are there any caveats?

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[John Dyson]

It is used as an economical alternative to inert gasses, but it not as good.
You get a slight grey tint which I think will reduce corrosion resistance
and it may harbour contamination making it unsuitable for food products.
Not many companies I know use it, most people stick to inert gasses. You
may get some further information from material suppliers / manufacturers.

For some good information on purging :-
http://gowelding.com/met/austenitic.html

Regards

John Dyson www.gowelding.com

"Bruce Simpson" <see.si...@ct.info> wrote in message
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[Tom Doody]

Info from an AWS document:

AWS D10.4 Recommended Practices for Welding Austenitic
Chromium-Nickel Stainless Steel Piping and Tubing

From section 7.4 Inert Gas Purging

"The gases used for weld root purging are generally argon and helium.
It has been established that nitrogen may be used satisfactorily for
purging purposes when welding stainless steel pipe. Where weld
discoloration due to slight surface oxidation is not objectionable,
use of commercial or standard dry nitrogen is acceptable. It should
be recognized that nitrogen absorption can reduce the ferrite content
of the root pass."

The discoloration mentioned both by this reference and by John Dyson
is probably due to the purity of the nitrogen rather than an effect
attributable to the nitrogen itself. Higher purity nitrogen will cost
more than commercial grade nitrogen. The cost is normally why
nitrogen was being considered in place of argon in the first place.

AWS also has a publication (D18.2) that shows the effect of back purge
oxygen levels (ppm range) on the internal oxide tint (weld
discoloration) for stainless steel TIG welding.

Having said all this, the company standards for my previous company
(an oil company) prohibited the use of nitrogen as a backing purge gas
for TIG welding of stainless steel. This was because of the many
critical corrosion-resistant applications and questions about purity
for the nitrogen.

Tom Doody

[Brian]

would it be suitable for backpurging when building automotive exhaust
headers? the critical factor is eliminating the crusty mung on the inside
of the beads, not discolouration or corrosion resistance in particular.

Brian

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