Stainless spool production under deadline, qualification and purge constraints
A shop has to fabricate 40 m of 8 inch, schedule 10, 304L stainless steel pipe spools in five working days. The scope is 12 spools, GTAW root with argon purge and GMAW fill, 100% radiography, and pickling/passivation. Two welders are available: Welder A is qualified for GMAW on 304L, but his GTAW qualification expired three months ago; Welder B is qualified for GTAW only. Purging gas is in short supply, and the customer will not accept lower root quality. The planner wants to use a soluble purge dam and increase the GMAW fill current to save time. The QA manager is on leave, and the customer's inspector visits only on day 4. The budget allows one subcontractor for two days, or overtime, but not both.
1. Set out a production and qualification plan that meets the deadline without compromising the root and the 100% RT requirement.
Read the worked answer
The first constraint is qualification. Welder A's GTAW qualification has expired, so A cannot weld GTAW roots until requalified. Welder B is qualified for GTAW only. The initial plan should therefore use B for all GTAW roots and A for GMAW fill, provided A's GMAW qualification covers the thickness and position. If A's GMAW qualification is not valid for the joint, the plan changes.
Sequence the work so root welding is not the bottleneck. Prefabricate and fit up as many spools as possible on day 1, purge and root them with B, and follow with A on fill. Do not let A weld roots. If the volume is too high for B alone, use the subcontractor for GTAW roots for two days rather than for fill.
Schedule critical welds before day 4 so the customer's inspector can witness or review them. Since the QA manager is absent, nominate a deputy in writing for document review and hold points. Keep 100% RT as planned; do not substitute sampling. Build in time for repair and re-radiography.
2. Evaluate the planner's proposal to use a soluble purge dam and higher GMAW current. What risks and controls apply?
Read the worked answer
A soluble purge dam can be acceptable, but it introduces residue and removal risk. If the dam is not completely dissolved and flushed, residue can contaminate the root or later process fluid. For 8 inch schedule 10, access for removal and verification is possible but must be planned. The dam must be qualified for the procedure, and the purge volume and oxygen level must still be controlled. It does not replace a proper argon purge; it only reduces the volume to be purged.
Higher GMAW fill current can improve deposition rate, but it raises heat input and may increase distortion, spatter and the risk of burn-through if applied near the root. It can also affect ferrite balance in 304L, although 304L is less sensitive than duplex. The main risks are root damage, excessive heat input and a weld that fails RT.
Controls include a written parameter change approved under the WPS, a test coupon at the higher current, purge oxygen monitoring, and inspection of the first production weld. Do not increase current to compensate for poor fit-up.
3. Explain the cost and schedule trade-offs between subcontracting, overtime, and a partial re-sequence, and state your recommendation.
Read the worked answer
Subcontracting for two days gives additional qualified GTAW capacity without committing to overtime for the whole shop. It is best used on the root operation, which is the constraint. The cost is limited and predictable, but the subcontractor must be qualified to the same WPS and inducted into the shop's quality system.
Overtime adds capacity across all operations, including fit-up, purging, welding and inspection. It may be more flexible than subcontracting, but it raises fatigue and supervision risk, and the QA presence is thinner. It also costs more if the bottleneck is only root welding.
A partial re-sequence is the lowest-cost option if it works: pre-fabricate more spools early, let B concentrate on roots, and let A follow with fill. This avoids both subcontracting and overtime if the volume is achievable. The risk is that one welder becomes the bottleneck and the schedule slips.
My recommendation is to re-sequence first, confirm B's realistic root output on day 1, and use the two-day subcontractor only for GTAW roots if the output is short. Keep overtime as a contingency, not the primary plan. This protects the root quality and the 100% RT requirement within the budget constraint.