A Spray-Transfer WPS Does Not Cover Short-Circuit Vertical Welding
A shop has a qualified WPS and PQR for GMAW spray-transfer welding of 1/2 in. A36 plate to 1/2 in. A36 plate, single-V groove, using 0.045 in. ER70S-6 with 90% Ar/10% CO2 at 28 V, 260 A, and 12 in./min. The PQR was made in the 1G position. A new job requires 3/8 in. plate, single-V groove, in the 3G vertical-up position. The shop wants to use GMAW short-circuit transfer at 20 V and 130 A to control the puddle. The engineer says, 'Same process, same filler, so the existing WPS covers it.'
1. Is the existing WPS adequate for the new job? Identify the essential variables that are affected.
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No. At least three essential variables change:
- Transfer mode: Changing from spray transfer to short-circuit transfer is a significant change for GMAW. The two modes have different metal-transfer behavior, heat input, and fusion characteristics, and the change generally requires requalification.
- Position: The PQR was made in 1G. The new job is 3G vertical-up, which is a different position and is not covered by a 1G qualification.
- Thickness: 1/2 in. to 3/8 in. may or may not fall within the qualified thickness range, depending on the code and the PQR details. This must be checked rather than assumed.
The engineer's statement that 'same process, same filler' is sufficient is incorrect. Process and filler are only part of the essential-variable list.
2. What requalification or alternative would you recommend, and what are the trade-offs?
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Options:
- Write a new PQR for GMAW short-circuit in 3G on 3/8 in. plate. This is the direct route if short-circuit is the desired production method. It requires a new test weld, mechanical testing, and a revised WPS.
- Use a process better suited to vertical-up. FCAW or SMAW are commonly used for 3G work and may already be qualified in the shop. This avoids developing a new short-circuit procedure.
- Use pulsed spray or a controlled spray transfer in 3G if the equipment and procedure support it. This still requires qualification for the new position and mode.
Short-circuit GMAW is attractive for out-of-position work, but on 3/8 in. plate it needs careful procedure control to avoid lack of fusion.
3. If the shop proceeds with short-circuit GMAW in 3G, what quality risks should the procedure and welder controls address?
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- Lack of fusion at the sidewalls and root is the main risk. Short-circuit transfer produces a fast-freezing puddle and relatively low heat input, so the arc must be directed at the joint faces and the puddle must wet into both walls.
- Cold lap can occur if travel is too fast or the arc is held too far from the leading edge of the puddle.
- Porosity can result from poor gas coverage, drafts, or a wet surface; the gas flow and joint cleanliness must be controlled.
- Procedure controls: specify current, voltage, wire-feed speed, travel speed, gas flow, work angle, and any weave limits. Do not let the welder improvise settings outside the WPS.
- Verification: the qualification test should include bend or macro examination that reveals fusion at the root and sidewalls, not just a visual pass.