I don't know if it's applicable here, but I have heard that stainless is
    welded, the back side of the weld needs to be in an O2 purged
    environment.  I wonder if/how that's done when the inside of the pipe is
    being welded to a flange..

    Matt-

    That is true for tube butt joints such as headers. You need a
    separate supply of nitrogen gas behind the weld. Not so for a
    flange welded on the end of a tube as the slag, if any, can be
    mechanically removed.

    Paul Lamar ...No rotor no motor

Paul,
Let's hope that Matt mistyped O2 for N2! Oxygen wouldn't be used in
almost any weld process other than flame cutting.
Bill Jepson

Thanks Bill. I missed that.

Paul Lamar ...No rotor no motor.

Matt and Bill both agree; I think that darned English language is getting in
the way. "Purge" means "to eliminate." In the old Soviet Union, for example,
when an official was "purged" from the party, it meant he was sent on a
permanent vacation somewhere else.

So an O2 purged environment is just one where oxygen is not present and has
been replaced with another gas. Nitrogen was Bill's suggested replacement
gas; argon, available to TIG welders everywhere, would also work well.

Barry Gardner
Wheaton, IL

Darn! One of them there big words. You are right of course.

-- 
Paul Lamar ...No rotor no motor.

If all you are welding are 300-class stainless steels you can use nitrogen
as a purge gas. But nitrogen is soluble in the molten puddle of many alloys
and causes them to be brittle. Argon is the best all around shielding and
purge gas and does not cause brittleness. Inconel alloys if not cleaned
properly WILL crack. The thin oxide layer found on Inconel and stainless
steel alloys must be removed from the weld zone. The reason for this is that
the oxide layer melts at about 1000 degrees F higher than the base metal. If
it is not removed it can become a stratified layer within the weld metal and
a failure point. The oxide layer found on steel (rust) melts at about the
same temperature as the base metal so it is less of a problem. Other sources
of contamination are dirt, oil, pencil marks, even fingerprints can cause
cracks in welds. The oils from a fingerprint can cause cracks in some types
of Inconel. And by the way watch the heat input with high heat input comes a
large heat affected zone. Examine the weld joints that have failed and you
will see that the failure is in the HAZ [heat affected zone] and not the weld itself.

Russ Keith



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