Hydrogen to Vacuum: Which Brazing Changes Force a New BPQR?
A shop's BPS-1, supported by BPQR-1, covers furnace brazing of C12200 copper tube to 304L stainless tube in a hydrogen atmosphere: BAg-1a paste filler, 1500 °F, 10 minutes at temperature, joint clearance 0.002–0.004 in, qualified on 3/4 in OD × 0.065 in wall tube, with visual and sectioned test specimens. The BPS as issued lists brazing temperature "1450 °F–1600 °F" and clearance "as required." A new product line uses the same two base metals but requires: (1) vacuum furnace brazing at 10⁻⁴ torr; (2) 1560 °F with 25 minutes at temperature to soak larger assemblies; (3) BAg-1a paste from a different supplier; (4) joints with 0.005–0.008 in clearance; and (5) a redesigned fixture. The shop intends to run all of it on BPS-1 without any testing.
1. Sort the five changes: which require a new brazing procedure qualification, and which do not? Give your reasoning for each.
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Judge each change against the brazing essential variables, not against how similar the new work looks.
- (1) Hydrogen → vacuum atmosphere: requalify. The furnace atmosphere is an essential variable in brazing because it governs fluxing action and filler flow. Vacuum and hydrogen are different protection mechanisms, not two settings of the same thing.
- (2) 1560 °F / 25 min: requalify. Brazing temperature and time at temperature define the qualified thermal cycle. This is both a higher peak and a hold 2.5 times longer, which changes filler flow, erosion, and the base-metal thermal exposure. It is outside anything BPQR-1 established.
- (3) New supplier of BAg-1a paste: no requalification. The filler metal classification and form are unchanged, so no essential variable moves. Verify the certificate of classification and that the BPS calls out the classification rather than a brand.
- (4) Clearance 0.005–0.008 in: requalify. Joint clearance is an essential variable; capillary brazing depends on it, and the qualified clearance range is 0.002–0.004 in. This is the change most likely to produce silent, intermittent leaks.
- (5) New fixture: no requalification by itself, but the new BPQR must be run with production-type fixturing and heat-sink conditions so the recorded cycle is representative. A fixture that changes heating rate effectively changes the thermal cycle.
2. Is the BPS entry "brazing temperature 1450 °F–1600 °F" defensible, given that BPQR-1 was run at 1500 °F?
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No - this is the brazing version of an unsupported PWHT range, and it has to be corrected whether or not the new product line is accepted.
- The BPS may list only ranges the supporting BPQR established. BPQR-1 documents a single point, 1500 °F for 10 minutes; the BPS cannot manufacture a 150 °F window and a variable hold time out of one qualification.
- Same problem with "clearance as required." Clearance is an essential variable in brazing and the qualified range must appear on the BPS; leaving it open removes the controlling parameter from the document.
- Two acceptable fixes: reissue the BPS with the narrow, supported cycle and leave the other work for a separate BPS, or qualify a BPQR that actually spans the range (for example, brackets of temperature and hold time bracketing the intended production window). Editing the numbers on an existing BPS is not one of the options.
3. The brazer himself has been furnace-brazing these materials for years. Does he need any new qualification for the vacuum-furnace product?
Read the worked answer
Keep the two qualifications separate - procedure and performance answer different questions.
- Brazing performance qualification is tied primarily to the process and position the brazer uses. Moving from hydrogen to vacuum, changing temperature and hold time, or widening clearance are BPS variables; they do not by themselves invalidate the brazer's qualification.
- What would require the brazer to be requalified is a change in the process he personally operates - for example, moving this work to torch or induction brazing rather than the furnace. The brazer must be qualified for the process he actually runs, on a test assembly, with the record certified and held by the employer.
- Both documents must be in place before the first production assembly: a BPS that genuinely covers the vacuum cycle, and a brazer qualification that covers the process and position used. One without the other is not a qualified joint.