Essential Variable Breach in a Short-Circuit GMAW Requalification
A fabrication shop holds a qualified GMAW procedure for 6 mm carbon steel using short-circuit transfer, ER70S-6 1.2 mm wire, 75/25 Ar/CO2 shielding gas at 18 L/min, and a single-pass-per-side butt joint with a 60-degree included bevel. Production needs to move to 8 mm plate on the same joint design. The welding coordinator proposes reusing the existing procedure and simply adding a second pass, arguing that the filler metal, gas, and transfer mode are unchanged. A recent production weld made this way passed visual inspection but showed incomplete fusion on the root side during a spot ultrasonic check. The client specification requires procedure qualification before production and permits only one retest after a failed mechanical test.
1. Determine whether the proposed change requires a new procedure qualification test, and identify the controlling essential variables.
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The change from 6 mm to 8 mm base metal thickness, combined with the addition of a second pass, is not a trivial adjustment. Under most construction codes, base metal thickness and the number of passes per side are essential variables for GMAW procedure qualification. A procedure qualified on 6 mm does not automatically extend to 8 mm unless the qualification thickness range covers it, and the addition of a pass changes the deposition and heat-input pattern the original qualification was based on.
The controlling variables are: base metal thickness and its qualified range, transfer mode (unchanged here, which helps), filler classification and diameter, shielding gas composition and flow, joint design and bevel geometry, and the number of passes. Because the joint design and thickness both changed, the coordinator cannot rely on the original qualification. A new procedure qualification test is required before production. The incomplete fusion symptom is consistent with a short-circuit procedure that was tuned for a thinner section and a single pass; the second pass may have been deposited with insufficient penetration into the root.
2. The first procedure qualification test fails the bend test. Outline the correct retest path and the documentation the coordinator must produce.
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The client allows only one retest after a failed mechanical test, so the coordinator must not simply rerun the same procedure. First, determine the cause of the bend failure: check whether the failure is in the weld metal or the heat-affected zone, inspect the fracture surface, and review the welding parameters actually used against the proposed procedure. Common causes include insufficient root penetration, excessive heat input, or contamination.
Correct the procedure, then perform one retest that includes the full required set of tests, not just the failed test. If the retest passes, the procedure is qualified for the tested range. If it fails, the procedure is not qualified and a new qualification must be developed.
The documentation must include the procedure qualification record showing actual parameters, the test results, the essential variable ranges qualified, the welder or operator identification, and the reference to the client specification. The coordinator should also record the root cause analysis and the corrective action taken, so the qualification history is traceable.