Welder Performance vs. Procedure Qualification
A certified GTAW welder with 12 years of experience is assigned to a D17.1 Class A joint in 0.063 in (1.6 mm) titanium. The welder's performance qualification was earned on 0.125 in stainless steel in the 6G position two years ago. The shop supervisor argues the welder is 'more than qualified' because of experience and because titanium is 'just another alloy.' The joint is a tube-to-fitting butt weld that will be radiographed. The welder has never welded titanium before.
1. Assess whether the welder may perform this joint and identify the qualification gaps.
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The welder may not perform this joint on the strength of the existing qualification. Two independent gaps exist: (1) base-metal group - titanium is a different P-number/group than stainless steel, and welder performance qualification is base-metal-group specific; (2) thickness - 0.063 in is thinner than the 0.125 in coupon and may fall outside the qualified thickness range. Experience is not a substitute for qualification under D17.1. The supervisor's 'just another alloy' reasoning is exactly the error the exam targets: titanium's reactivity and shielding requirements make it a distinct qualification case, not a cosmetic variation. The welder needs a performance qualification on the applicable titanium group and thickness before production welding, plus demonstrated shielding-gas practice.
2. What additional process controls matter for titanium that a stainless-steel-qualified welder might overlook?
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Titanium's high reactivity with oxygen and nitrogen at welding temperature means shielding must extend beyond the arc: trailing shields, back-purge, and post-flow until the weld cools below the oxidation threshold. Color inspection of the finished weld (silver/straw acceptable; blue/purple/white indicates contamination) is a practical control. The welder must also avoid iron contamination from tooling and brushes. These are not cosmetic details; contamination can embrittle the joint and produce discontinuities that radiography may not reveal. A welder qualified only on stainless steel may not know to back-purge a tube joint or to allow extended post-flow, so the WPS should specify these parameters and the welder should be briefed before the first production weld.