3.3 Heating, Filling & Inspecting Brazed Joints
Key Takeaways
Where flux is used, heat the joint slowly until the flux liquefies, then heat quickly to brazing temperature; without flux, heat quickly to brazing temperature without overheating.
After brazing, wash the outside of every joint with water and a wire brush (hot water if flux was used) and visually inspect it.
Unacceptable joints include flux residue, base metal melting or erosion, unmelted filler, filler not visible all the way around, cracks in the tube or filler, and joints that fail installer pressure tests.
Joints with base metal melting or erosion, or with cracks in the tube or component, must be replaced; other defective joints may be repaired.
No brazed joint may be reheated more than once before it is replaced.
Heating the Joint
NFPA 99 gives two heating rules and points to the CDA (Copper Development Association) Copper Tube Handbook, chapter on brazed joints, for technique. That chapter covers applying heat, applying filler metal, and making horizontal, vertical, and large-diameter joints.
| Joint type | NFPA 99 heating rule |
|---|---|
| Flux used (BAg on a dissimilar-metal joint) | Heat slowly until the flux has liquefied, then heat quickly to brazing temperature |
| No flux (BCuP on copper) | Heat quickly to brazing temperature, taking care not to overheat the joint |
Technique the brazing course teaches
These points follow the CDA handbook approach. Your qualified procedure governs.
- Heat the tube first, sweeping the flame around it near the fitting, then move the heat to the fitting cup so the whole joint reaches temperature evenly.
- Feed filler at the joint once it is at temperature. Capillary action draws molten filler into the clearance between the tube and the socket. Do not melt filler in the flame and drip it on, because that leaves unmelted or poorly bonded filler.
- Horizontal joints: begin feeding slightly off-center at the bottom of the joint, work across the bottom and up one side to the top, then return and complete the other side so the fill overlaps.
- Vertical up-flow joints: the filler must travel upward against gravity. Even heating draws it up, while overheating the tube can make it run back out. NITC's BTS15 practical includes two vertical up-flow joints.
- Large diameters need more heat input and often a multi-tip torch, so the whole circumference reaches temperature together.
- Keep the nitrogen purge flowing through heating and cooling until the joint is cool to the touch.
Cleaning and Inspection After Brazing
- The outside of every joint must be cleaned by washing with water and a wire brush. This removes residue so the joint can be inspected clearly.
- Where flux was used, the wash water must be hot.
- Each brazed joint must be visually inspected after its outside surfaces are cleaned.
Conditions That Make a Joint Unacceptable
NFPA 99 lists joint conditions that are not permitted:
| # | Prohibited condition | What usually causes it |
|---|---|---|
| 1 | Flux or flux residue (where flux or flux-coated BAg rods were used on dissimilar metals) | Too much flux, or poor post-braze washing |
| 2 | Base metal melting or erosion | Overheating, or holding the flame in one spot |
| 3 | Unmelted filler metal | Joint not at temperature; filler melted by the flame instead of the joint |
| 4 | Filler metal not clearly visible all the way around the joint at the socket-to-tube interface | Uneven heating, poor cleaning, or wrong clearance |
| 5 | Cracks in the tube or component | Thermal or mechanical damage |
| 6 | Cracks in the braze filler metal | Movement during cooling, or a problem with the alloy or procedure |
| 7 | Failure to hold test pressure in the installer's initial pressure test or standing pressure test | Voids, incomplete penetration, or pinholes |
Repair or Replace?
| Defect category | NFPA 99 disposition |
|---|---|
| Base metal melting or erosion (2) | Replace the joint |
| Cracks in the tube or component (5) | Replace the joint |
| Flux residue, unmelted filler, filler not visible all around, cracks in filler, failed pressure test (1, 3, 4, 6, 7) | May be repaired |
There is one more limit on repairs: no joint may be reheated more than once before being replaced. In practice, a joint gets one repair attempt. If it fails again, cut it out, use new fittings and clean tube, and braze it under purge.
Tip
Exams often pair a defect with the wrong fix. Erosion and tube cracks call for replacement. A gap where filler is not visible all the way around calls for repair, but only once. If a question says the joint has already been reheated once, the only acceptable answer is replacement.
What Visual Inspection Cannot Show
From the outside you can confirm a continuous filler ring, the absence of erosion and cracks, and a clean surface. You cannot see how far the filler penetrated or whether there is internal oxidation. Those are checked in brazer qualification, where coupons are sectioned (Section 3.4), and in pressure, particulate, and purity testing (Chapter 7). This is why NFPA 99 relies on qualified procedures and qualified brazers rather than on visual inspection of field joints alone.
Worked Scenario
A brazer finishes a 2 in. vertical up-flow joint on an oxygen riser. After washing, a short arc of the socket interface shows no visible filler, but the copper is not eroded or cracked.
- Classify: filler not visible all the way around, which is condition 4 and repairable.
- Repair once: re-establish the purge, then reheat the joint evenly and feed filler at the gap.
- Re-inspect: wash with water and a wire brush, then inspect visually.
- If it still fails, or if erosion appears from the second heating, replace it. The joint has already used its one reheat.
During post-braze inspection, a brazer finds that the copper tube next to a fitting has been eroded by overheating. What does NFPA 99 require?
Replace the joint
Repair it by reheating and adding filler
Accept it if it holds the initial pressure test
Accept it if the erosion is less than 1/8 in. long
A dissimilar-metal joint was brazed with BAg filler and flux. How must the outside of the joint be cleaned before visual inspection?
Wiped with a dry cloth only
Sanded with sand cloth to bright metal
Washed with hot water and a wire brush
Sprayed with an ammonia-based cleaner
A joint failed visual inspection because filler was not visible all the way around. It was reheated once and repaired, and it still shows a gap. What is the correct next step?
Reheat it a second time and add more filler
Coat the gap with an oxygen-safe sealant
Leave it for the verifier to evaluate
Replace the joint
Sections you finish are checked off in the contents.