Level 1, Unit 5, Section 3 – 3.5 Brazing Operation

3.5.1 — Torch Setup and Lighting for Brazing

Brazing requires significantly more heat than soldering — joint temperatures of 600–870°C (1,100–1,600°F) compared to 230–260°C (450–500°F) for soldering. Torch setup and lighting must produce a stable flame suited to brazing heat levels so the fitting and tube can be heated evenly without burning flux or overheating the base metal. For copper tube up to approximately 28 mm (1-1/8 in), an air-fuel (propane or MAPP) torch with a large tip is adequate; for 34.9 mm (1-3/8 in) and above, an oxy-fuel or oxy-MAPP torch with higher heat output is recommended.

Torch and Tip Selection

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Air-Fuel (Propane / MAPP)

Suitable for brazing copper tube up to about 28 mm (1-1/8 in) diameter with a large rosebud or high-output tip. MAPP burns hotter than propane and reduces the time to reach brazing temperature, which reduces the risk of flux burning before filler flows. Torch setup follows the same assembly and leak-check procedure as for soldering (see Section 1.5.5), with a larger tip and higher working pressure.

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Oxy-Fuel (Oxy-Acetylene / Oxy-MAPP)

Required for larger tube sizes (34.9 mm / 1-3/8 in and up) and dissimilar-metal joints where higher heat input is needed to bring two different metals to brazing temperature simultaneously. Setup follows the full oxy-fuel assembly procedure (see Section 1.4.1). Use a welding or heating tip sized for the joint; a neutral to slightly reducing flame is preferred for copper brazing.

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Tip Size Selection

Select the tip based on tube diameter. A tip that is too small requires excessive dwell time to reach brazing temperature — the flux burns before the joint is hot enough and the copper oxidizes internally. A tip that is too large overheats the fitting face and tube wall before the fitting body reaches temperature, causing the filler to flow onto the surface rather than into the joint.

Flame Setting for Brazing

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Neutral to Slightly Reducing Flame

For copper brazing with both air-fuel and oxy-fuel torches, set a neutral to slightly reducing flame. A neutral flame minimizes oxidation of the copper surface and keeps the flux active longer. A slightly reducing flame (small carbonizing feather — 2–3 mm beyond the inner cone) provides a mildly oxygen-depleted atmosphere at the joint, which further reduces copper oxide formation. Avoid an oxidizing flame — it produces heavy copper oxide that the flux cannot fully dissolve at typical brazing temperatures, leading to a contaminated joint.

For oxy-fuel: adjust to neutral first (blunt, well-defined inner cone, no feather), then add a very small amount of fuel to create a slight feather — 2–3 mm maximum
For air-fuel: open the valve to a full working flame; most air-fuel torches naturally produce a slightly reducing environment at brazing temperatures
A stable, quiet flame with a well-defined inner cone indicates correct gas pressure and mixture; a hissing or roaring flame indicates excess oxygen or fuel — adjust before beginning heating

3.5.2 — Joint Cleaning, Preparation, and Fit-Up for Brazing

As with soldering, brazing requires clean, properly fitted joints. Contamination and gaps outside acceptable clearances reduce capillary flow and joint integrity. The preparation steps for brazing are identical in principle to those for soldering (covered in Section 1.5.1 and 1.5.2) but two aspects require additional attention at brazing temperatures: flux selection and joint gap tolerance.

Cleaning and Measuring

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Surface Preparation

Clean tube ends with fine emery cloth (120–150 grit) using a circular motion to bright copper; clean fitting cups with the correct-size internal fitting brush
Wipe both surfaces with a clean, dry cloth immediately after abrasive cleaning; apply flux without delay
Do not touch cleaned surfaces with bare hands — skin oils contaminate copper instantly
Measure tube runs using face-to-face or face-to-centre dimensioning (refer to Section 1.5.2); cut square ends using a tubing cutter and deburr fully before assembly
Confirm the tube seats fully at the bottom of the fitting cup; mark insertion depth on the tube before final assembly to verify engagement

Joint Gap and Flux for Brazing

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Joint Clearance for Brazing

The recommended joint clearance for brazing copper fittings is 0.05–0.13 mm (0.002–0.005 in) — similar to soldering but at the tighter end of the range. At brazing temperatures, copper expands; if the gap is too large when cold, capillary flow is unreliable. Standard wrought copper solder-end fittings (ASME B16.22) are designed to produce this clearance on ACR tube.

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Flux for Silver-Alloy Brazing

Silver-alloy (BAg) brazing requires a flux rated for brazing temperatures (typically 600–870°C / 1,100–1,600°F). AWS Type FB3-A or equivalent (white or off-white paste). Apply a thin, even coat to the tube end and fitting cup immediately after cleaning. The flux must remain active (liquid, not charred) throughout the heating cycle or the joint will oxidize before the filler flows.

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No Flux for BCuP on Copper

When using BCuP filler on copper-to-copper joints, no external flux is needed or recommended. The phosphorous in the filler performs the fluxing action at brazing temperature. Adding a separate flux with BCuP on copper can interfere with filler flow and produces additional residue inside the joint that can be difficult to remove.

Tubing Design and Routing Considerations

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Design for Brazability and Serviceability

Plan joint locations where there is clear access for torch, filler rod, and both hands — joints in tight corners or against walls are difficult to heat evenly and dangerous to perform
Provide adequate support for copper tube runs; unsupported spans that allow the pipe to flex during heating can create stress at brazed joints while the filler is still molten, causing voids or cold joints
Avoid placing joints in locations subject to high vibration without appropriate vibration isolation — the joint itself may be sound, but long-term vibration fatigue can crack the heat-affected zone adjacent to the fitting
Allow sufficient clearance between adjacent joints so the heat from one joint does not re-flow a previously completed joint; a minimum of 150 mm (6 in) between joint faces is a general guideline

3.5.3 — Brazing Technique and Positions

Brazing technique follows the same fundamental principle as soldering: heat the fitting, not the filler. The filler must melt by contact with the hot base metal — not by contact with the flame — so that capillary action draws it through the joint. The higher temperature and longer heating time required for brazing mean that position and heat distribution are even more critical than in soldering; gravity acts on a larger volume of liquid filler for a longer period.

Heat Application Sequence

Start on the fitting body: apply heat to the larger mass first, sweeping the flame continuously around the full circumference. The fitting must reach brazing temperature before the tube is brought up, or the filler will freeze in the joint as it tries to flow toward the cooler fitting.
Move heat to the tube just behind the fitting cup — about 25–50 mm (1–2 in) back from the joint face. This establishes a temperature gradient that draws the molten filler into the joint toward the heat.
Watch the flux: the flux transitions from white paste (cold) to clear liquid (active — joint approaching brazing temperature) to dark and charring (overheated). Feed filler only when the flux is clear and liquid.
Test the temperature by touching the filler rod to the joint face away from the direct flame. If the filler melts on contact with the fitting and flows smoothly into the joint, the temperature is correct.
Feed the filler steadily at the joint face while keeping the flame moving. Feed until a continuous, uniform fillet appears around the full circumference. Do not over-feed — excess filler forms a large external bead that does not contribute to joint strength.
Remove heat and filler simultaneously when the fillet is complete. Do not disturb the joint until the filler has solidified and the fitting has visibly cooled from orange to dark.

Positions

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Horizontal

Heat both the top and bottom of the fitting evenly before feeding filler. Begin feeding at the bottom of the joint face — gravity assists solder flow around the lower half. Move the feed point around the circumference. Watch for the filler sagging downward at the bottom; if sagging occurs, reduce heat briefly, allow the filler to stiffen slightly, then continue. A slow, controlled feed at a consistent flame distance produces the most uniform fillet in horizontal work.

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Vertical

Heat the fitting sides evenly. Feed filler at the joint face and allow capillary action to distribute it around the circumference. With the fitting above the tube (tube pointing down): heat the fitting sides and lower rim; filler fed at the bottom face will be drawn upward into the joint. With the fitting below (tube pointing up): apply heat higher on the fitting to prevent the filler from running downward before it fills the upper portion of the joint gap.

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Inverted (Overhead)

The most demanding position. Use the minimum heat necessary to reach brazing temperature. Feed filler in small, controlled amounts — capillary force must exceed gravity to hold the molten filler in the overhead joint. Over-feeding causes drips. PPE is critical: face shield, leather gloves, and a leather welding jacket protect against molten filler falling from the joint face. Place a welding blanket below to catch any drips and protect adjacent equipment or finished surfaces.

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Do Not Re-Heat a Completed Joint to “Touch Up”

Re-heating a brazed joint to add filler does not repair voids — it re-melts the filler, redistributes it unpredictably, and can cause the flux to burn out, leaving an oxidized root. If the fillet is incomplete or a void is suspected, the joint must be disassembled while hot (or cut out), fully cleaned, re-fluxed, and re-brazed.