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diameter : 22"
material X52 (12mm) + 3mm inconel 625
filer : Er-NiCrMo-3
Let's clarify something else: I presume you mean after the back purge is considered complete and during the welding process? What is the oxygen level to be at in the vicinity of the purge, while welding from the opposite side?
If the purge is complete there should be no oxygen to speak of. Displacement should be total for all intents and purposes. Is there an acceptable level of residual oxygen? With purging from the opposite side and leakage of the purge gas at the joint and the shielding gas from the welding operation there really should be no significant oxygen readings in the vicinity of the welding operation, from either side.
That is the purpose of the purge, oxygen displacement.
My two tin pennies worth. Hopefully understood the question correctly.
He Is In Control, Have a Great Day, Brent
Not sure if I'm answering the correct question here.
Some alloys are more "finicky" than others so you may need to get the Oxygen to nearly zero for the back purge.
Using the standard O2 meter we were generally satisfied with any reading under 1% preferably 0.5% or lower. Anything above 1% was a little risky. This was for 5%-9% chrome and most SS alloys. On larger diameters like you are dealing with, 22", it might take a long time to remove all oxygen and maintain the argon environment…depending on how far apart and tight your purge dams/blocks are. You might need 2 sources of argon for volume.
I went with the above mentioned method until we were tasked with R&D on a welding procedure for a challenging Nuclear Reactor Head repair that involved welding many modified inconel 4" caps to an existing "super alloy" reactor head. We, (a 4 man team of very experienced welders/supervisors) thought using a standard 02 meter and getting it down to .250% oxygen was surely good enough but we had no success with the quality of the root due to oxidation.
The PM, who was a mad scientist, but not as much as a seasoned welder as we were, bought and insisted that we used an expensive and highly accurate meter that measured PPM-parts per million. He set the benchmark at 500/PPM. If we couldn't get that, don't even attempt to weld. It seemed ridiculous to us but we fought it until we were able to achieve the 500/PPM and below and the root pass ID's looked perfect. Most alloys won't require that low of an O2 atmosphere but this one did and a valuable lesson was learned that day. Don't dismiss any input or advice without full consideration first. We learned that "that's the way we always did it" wasn't good enough this time.
Also, leave your back purge on for the hot pass plus one more layer for best results.
By Superflux 
Date 09-06-2014 17:14
Edited 09-06-2014 17:55
It is something that would have to be developed for the WPS.
Way too many variables for it to be addressed in the codes.
BUT!!!
Are you sure this is an O2/purge issue or perhaps there might be another source of contamination?
Just wondering how or why you have determined it to be purge related.
Improperly designed or applied purge dams will cause problems. Turbulence and the venturi effect "sucking" air into the joint can cause contamination. Flow rate, angle of shielding gas nozzle, and insufficient masking of an open root can inject the atmosphere in to the weld.
Not being there to witness the set-up and the limited info given, I'm just tossing ideas for consideration.
Keep us posted.
Rules of thumb:
1) use enough purge gas to displace the system volume by a factor of 6. Good enough for most applications, but not for Titanium or other reactive metals.
2) use a U-groove with a 1/32 to/16 inch root face, 1/4 inch bevel radius, and NO root opening.
The volume of gas (6 x the system volume) should reduce the Oxygen to about 0.3 to 0.4% and Nitrogen down to about 1.25%.
The system must be dry, i.e., free of moisture. If the system was used, it has to be dried. Drain and purging the system initially with dry Argon (Nitrogen is a poor second choice) may be necessary to dry the system.
If a U-groove with a zero root opening is not used, the joints must be taped with a suitable tape to seal the open joints.
Clean the inside of the pipe as well as the groove face and outside. Swab the pipe ID for a minimum distance of 4 inches from the groove to be welded. Acetone or Isopropyl alcohol should work. The preference is Acetone followed by alcohol. Swab until the ID is clean.
Seal all openings to stop the free flow of purge gas.
Vent all high points. Air is lighter than Argon, so it (air) will rise to the high points. Typically a 1/16 inch hole is sufficient to allow the air to escape.
Do not use Nitrogen as a purge gas. Nitrogen is not inert and it is an interstitial element and will alloy with certain base metals (certain stainless steels and reactive metals). In the case of other metals, it may cause porosity (carbon and high strength low alloy steel).
Do not over pressurize the system. A Magnehelic (regulator) can be used to prevent over pressurization.
Best regard - Al
