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.)