Torch Brazing Procedure

Torch brazing joins metal with a filler that melts above 840 F, using a flame you control by hand. It makes strong, leak-tight joints on copper tube, steel fittings, carbide tooling and mixed-metal assemblies. A good brazer reads color, flux and flow to know when the joint is ready.

What You Will Be Able to Do

• Choose a filler alloy for the metals in the joint.
• Set a neutral or slightly reducing flame.
• Purge tubing with dry nitrogen.
• Heat a joint evenly and read when it is ready.
• Feed filler, cool the joint and inspect it.

Choosing a Filler Alloy

Brazing filler names begin with B, then a symbol for the main metal. BCuP means copper-phosphorus, and BAg means silver. The number gives a specific alloy.

Brand names cause confusion. Sil-Fos is a trade name used for a family of copper-phosphorus rods, and the number matters. For example, Sil-Fos 15 is BCuP-5 and Sil-Fos 5 is BCuP-3. Always read the AWS classification on the label.

BCuP alloys are the most common fillers for copper refrigeration tubing, because they flux themselves on copper. Silver in the alloy adds ductility, which is why the higher-silver rods handle vibration better. The rules for BCuP are strict:

• On copper to copper, use no flux.
• On copper to brass or bronze, apply flux.
• On steel, stainless steel or nickel alloys, do not use BCuP. Use a BAg filler with flux.

Warning: BAg-1 and other cadmium-bearing alloys give off cadmium fume, which can kill. Check the label and the Safety Data Sheet before you heat an unknown rod. Use cadmium-free alloys unless your employer has a specific requirement and exposure controls.

Choosing and Setting a Flame

Use a tip big enough to heat the whole joint. A tip that is too small makes you hold the flame in one spot too long. Larger joints and thicker metal need larger tips.

The flame should be neutral or slightly reducing. A neutral flame has a sharp, clear inner cone. A slightly reducing flame, also called carburizing, has a short feather of extra fuel around the cone. It limits oxidation of the metal. An oxidizing flame has too much oxygen and a short, pointed cone. Avoid it, because it creates oxide and burns the flux.

Follow the torch and tip maker's chart for pressures. Your cylinders, hoses and regulators need the same setup and leak checks as any oxy-fuel rig.

Purging Tube With Nitrogen

Copper heated in air forms black oxide scale on the inside of the tube. On refrigeration and medical gas lines, that scale breaks loose, plugs metering devices, and contaminates oil and compressors.

To stop that, flow dry nitrogen through the tube while you braze:

1. Connect a nitrogen cylinder with a regulator, and a flowmeter if you have one.
2. Purge the air out of the line before heating.
3. Keep a low flow during brazing. A common setting is 2 to 5 psi, or a few cubic feet per hour.
4. Leave an open path for the gas to escape, so pressure does not build up.
5. Keep the flow on until the joint cools below about 500 F.

Purge rules vary by equipment maker and code, so follow your procedure.

Warning: Use only dry nitrogen. Never use oxygen, compressed air or fuel gas to purge. Nitrogen also displaces breathing air, so ventilate small rooms and enclosed spaces.

Warning: Never heat a refrigerant line until the refrigerant has been properly recovered and the line is open. Heat can burst a charged line, and refrigerant breaking down in a flame makes toxic and corrosive gases.

Preparing the Joint

1. Cut square and remove burrs.
2. Clean both surfaces to bright metal.
3. Check that the fit gives a gap of about 0.001 to 0.005 inch. A tube end that is swaged or fits a socket should overlap by at least the amount the fitting requires.
4. If the filler needs flux, brush a thin coat on both surfaces right away. Keep flux out of the inside of tube.
5. Assemble and support the parts so that they cannot shift.
6. Protect nearby parts. Wet rags or heat paste can protect valves and seals, and many valves must be opened or have parts removed. Follow the maker's instructions.

Heating and Feeding the Filler

1. Heat the tube first. It is thinner and heats fast. Move the flame steadily, and keep it moving.
2. Move to the fitting. The fitting is heavier, so it needs more heat. Sweep around the whole joint so that it all reaches temperature together.
3. Watch for the signs. If you are using flux, it will dry, then go milky, then turn clear and glassy when the joint is ready. Without flux, on copper, watch for a dull cherry-red color.
4. Touch the rod to the joint. Touch it at the joint line, not in the flame. If it melts on contact and flows, you are ready. If it balls up, keep heating the metal.
5. Feed the rod. Let capillary action pull filler into the gap. Keep the flame moving, and lead the filler around the joint.
6. Stop when a full fillet shows. More filler will only run inside the tube.
7. Take the heat away. Do not keep heating after the joint has filled.

On vertical joints, heat the bottom of the joint last so filler doesn't run out. Filler flows toward heat, so you can draw it around a joint with the flame.

If you braze with a TIG torch instead of a flame, keep the hot rod tip close to the puddle and inside the gas shield, so that it stays clean for the next dip.

Cooling and Cleaning

Let the joint cool in still air. Do not quench it. Thermal shock can crack the filler or the base metal, and moving the joint while it freezes can open a crack. Keep the nitrogen flowing on tube.

Once the joint has set and cooled enough, remove flux residue. Most brazing flux comes off with hot water and a brush. Do not leave it, because it can corrode the joint.

Brazing Steel and Mixed Metals

Steel needs a bit more care than copper.

• Use a BAg filler and a flux rated for the temperature.
• Flux both surfaces, and keep plenty on, because steel oxidizes more readily than copper.
• Heat the heavier part first, and bring the joint up evenly.
• On copper to steel, remember that copper pulls heat away fast and that the two metals expand at different rates. Heat the copper more, and check that the gap stays open at temperature.
• On stainless, use a flux for stainless and avoid overheating, since that can reduce corrosion resistance.

Inspection and Testing

Look at each joint when it is clean and cool. A good joint has a smooth, continuous fillet all the way round, with no pinholes, gaps or unfilled spots. Then test as the job requires. Refrigerant and medical gas lines are pressure tested, usually with dry nitrogen, to the pressure in the code or the equipment maker's instructions.

Do not try to patch a leaking joint by adding a blob of filler. A joint that leaked once usually needs to be taken apart, cleaned and made again.

Common Mistakes

• Using BCuP on steel. Brittle compounds form and the joint can fail.
• Skipping the purge. Scale inside a line can ruin equipment.
• Holding the flame still. You get a burned spot and a cold spot.
• Melting the rod in the flame. It drops onto a cold joint and does not bond.
• Quenching the joint. It can crack.

Check Your Understanding

You are joining a copper tube to a brass valve body. Can you braze it with BCuP and no flux?

No. BCuP fluxes itself only on copper. Use flux on the brass, and protect the valve from overheating.

After brazing a line, you see black flakes inside the tube. What happened, and why does it matter?

Nobody flowed nitrogen, so copper oxide scale formed. In a refrigeration system, it can plug metering devices and contaminate the compressor.

Your flux has turned clear and glassy, and the rod flows when you touch it to the joint. What do you do next?

Keep the flame moving, feed the rod until a full fillet shows, then remove the heat. Let the joint cool on its own.

A label says "45% silver, contains cadmium." What should you do?

Do not use it unless your employer has specifically required it and put cadmium controls in place. Ask for a cadmium-free alloy such as BAg-5.

Key Takeaways

• Choose the filler by the metals in the joint, and read the AWS classification, not the brand name.
• BCuP is for copper. Steel needs a BAg filler with flux.
• Use a neutral or slightly reducing flame, and keep it moving.
• Flow dry nitrogen through tube while brazing.
• Heat the metal, not the rod, and let capillary action do the rest.
• Cool naturally, clean off flux, and inspect or pressure test the joint.