stainless steel without backing gas 2

[nde3rich]

Please refresh me someone...is there any code prohibition on welding open root stainless steel when the joint is to be backgouged and the welded from the opposite side?

 

[metengr]

If you don't use backing gas for welding, the heat from welding will result in surface oxidation. This would be unacceptable for an open root butt weld that will be left as-is. If backgouging will be used on this weld, this approach will be acceptable because you would be removing the surface oxidized material and welding with backing. The Code does not address these specific process details only details for weld procedure qualification. Make sure your WPS supports this approach.

 

[unclesyd]

There is another option and that is to use a backup welding flux, like the Solar or the Superior material linked below. These materials require more than wipe it on and go with any welder. As stated in the available information test coupons should be made. Another thing to do to insure a good job is to close the end of the pipe to prevent the wind tunnel effect.
In our case there are certain parts of our process where we will use a N2 purge or N2 purge + flux. The real critical areas we still use Argon with 02 monitoring.

http://www.solarflux.com/index.html

http://www.superiorflux.com/welding.html

 

[Duwe6]

What metengr said: Stay in the limits of your PQR. Backing gas is not an Essential variable for SMAW. It is for GTAW. You have to have a PQR coupon made without backing gas to not use it, even with a backgouge & weld on second side. Counter-intuitive & probably a Code oversight.

 

[stanweld]

It is common practice when backgouging and reverse side welding stainless steels to not use a backing gas. There are times when you may wish to do so to minimize or eliminate the need to backgouge/backgrind.

 

[pradipgoswami]

Hi nde3rich,

See the relevant ASME Sec-IX clause on joint design.It is a non-essential variable,i.e a change of which would not affect properties of weld joints.

Use of backing gas is not a must for carbon and low alloy steels(up to 2.25% Cr),but highly recomemnded for stainless and other reactive metals(unless flux cored fillers of Kobelco TGX series) are used.

If the joint is to be back-gouged(with good accessibilty) then there's no point in specifying GTAW,this would be a waste.

Sometimes if may happen, say for welding smaller dia nozzles to vessel/heat exchanger shells(for S.S and other reactive metals), if the welds are difficult to access for back gouging, one may have to specify GTAW root pass to ensure proper weld joint quality.Under those circumstances backing shall be must.

Thanks.

Pradip Goswami
Welding Specialist
Ontario Power Generation Inc.

QW-401.4 Nonessential Variable (Procedure). A change in a welding condition which will not affect the mechanical properties of a weldment (such as joint design, method of back gouging or cleaning, etc.)