I assume you are referring to a pressure vessel, not an aircraft engine. Further, assuming you are welding carbon steel and you are considering using the GMAW with short circuiting transfer, the problem is one of incomplete fusion type discontinuities associated with the low heat input of the transfer mode, i.e., short circuiting.

The process can be used if you qualify the procedure using the short circuiting transfer mode. ASME places thickness restrictions of the range of thickness qualified. The range for the voltage, wire feed speed (amperage), electrode extension, and travel speed must be closely controlled to assure consistent results in production. This is not reflected in the essential variables listed in ASME Section IX. I recommend that the variables listed above be monitored and recorded while the test coupon is being welded. Again, the variable listed above are usually considered to be non-essential, and are not required to be recorded on the PQR document per ASME Section IX.

The welders should be trained as to the importance of following the qualified WPS and working within the listed ranges for voltage, wire feed speed, electrode extension, etc. Monitoring during production is very important as well.

Due to the low heat input of GMAW(SC), cleaning is very important as well. Mill scale, surface oxides, paint, etc. will lead to fusion type defects. The weld deposit can look perfectly acceptable, yet not be fused to the base metal if the surface wasn't properly cleaned before welding.

In contrast to GMAW(SC), the spray transfer mode is much more forgiving due to the higher arc voltage, wire feed speed, and high heat input. The fact that you are limited to the flat and horizontal positions limits it's usefullness if you have to weld in all positions.

Good luck.

Best regards -Al