How to Braze Pipes: A Guide to Brazing in Plumbing
Nov 17, 2021
When it comes to joining two pieces of metal pipe, plumbers often choose between brazing and soldering. Both methods require a torch and a molten filler metal, and some other specialized tools. Though the techniques are similar, soldering is much more popular. Brazing is a useful skill for plumbers to know so they can work with high-pressure lines, and is also a good skill to know for HVAC and refrigeration specialists working with gas pipes.
In this article
What is brazing in plumbing?
Brazing, also known as silver brazing or silver soldering, is the act of joining two pieces of metal with a molten filler material known as a brazing filler material.
First, the plumber cuts, cleans, and prepares a pipe and a fitting for brazing. Then the plumber uses a torch to heat the pipe and fitting to a very high temperature. Once sufficiently hot, the plumber touches the copper joint with the tip of a brazing filler material—a stiff rod made of different metals (typically made from copper, nickel, and silver). The heat of the metal surfaces melts the braze alloy, which then flows into the joint. If brazed properly, the brazed joint will be gas and liquid-proof once the brazing filler material hardens.
Like soldering, brazing does require flux, though it is often part of the brazing filler material. After brazing a joint, the plumber purges the system with nitrogen to remove the flaky oxidation left behind from the joining process.
The brazing process is typical in refrigeration or HVAC scenarios, but it can also join plumbing pipes, such as copper to copper joints or brass for water transportation.
How does brazing work?
To understand how brazing works, there are a few principles with which to become familiar. Note that brazing might seem similar to TIG welding, however in welding both the base metal and the filler material melt. When brazing, only the filler material melts, leaving the base materials intact—this molten filler metal creates the seal.
The phenomenon that allows molten filler metal to flow into the joint rather than just spill to the floor is capillary action. Capillary action is the result of molecular surface tension, where the liquid is drawn up through a narrow space. This is what occurs with the molten filler metal and the hot pipes being brazed—the surface tension and molecular adhesion between the liquid filer and the pipes draws the liquid metal into the joint.
Brazing Temperature Control
Compared to soldering, brazing requires much higher temperatures. In fact, the melting temperature of the brazing rod is an excess of 842 degrees Fahrenheit (as opposed to 450 to 500 degrees Fahrenheit for a solder). An acetylene torch can easily heat metals beyond this temperature, allowing the brazing filler metal to melt and the base metals to remain intact.
However, an acetylene torch can also heat the base metals past their melting points. If the brazer isn’t careful, they can warp the pipes and fittings, or cut through them all together. For this reason, it’s important to heat the metals enough for free-flowing brazing, but not constantly for extended periods of time. Doing so can lead to warps and leaks.
4 Ways to Braze Pipes
1. Torch Brazing
In torch brazing, the plumber uses a handheld acetylene torch to heat the base material (typically copper or brass). Once the joint is hot enough, they’ll introduce the brazing rod to the joint, allowing it to melt between the metals. Once the molten filler metal cools and hardens, it creates a solid joint.
Torch brazing is the most common method of brazing used in plumbing. It’s highly portable, and the brazed joints that result from the process are incredibly strong.
2. Induction Brazing
Induction brazing is similar to torch brazing except that instead of a torch, a machine produces electric induction heat. The machine never touches the workpieces, which is a distinction from other types of brazing. This process is popular in controlled atmospheres like manufacturing settings. Though the machines are large, they heat metals very quickly.
3. Furnace Brazing
In furnace brazing, technicians place small pieces of brazing filler material across the material at each desired joint, and the entire workpiece enters a furnace. Once the material reaches the correct temperature, the brazing material automatically melts and flows between the joints. Furnace brazing requires specialized furnaces, a controlled atmosphere, and typically, a conveyor belt, hence this type of brazing is typically performed in industrial manufacturing plants.
4. Resistance Brazing
In resistance brazing, a technician applies an electrical current to the base material. Due to the base material’s resistance to the current, it becomes hot. Once hot enough, the brazer fills the joint with the brazing filler material. This method requires large equipment, so it’s best for manufacturing settings.
What tools do you need for brazing plumbing pipes?
Brazing plumbing pipes requires a special set of tools. Fortunately, most of the following tools work for brazing and soldering, allowing for a bit of flexibility:
- Emery cloth is a piece of metal mesh used to clean the end of the pipe before soldering. This helps clean any accumulated dirt and oil, and clean any corrosion from the pipe that may hinder the brazing process and cause pipes to leak.
- Fitting brushes clean the inside of a fitting, and they come in sizes specific to pipe diameters. Similar to the emery cloth, this helps clean the inside of the fittings to better facilitate brazing the pipes together. Be sure you have fitting brushes that are the correct diameter for the pipe you are joining together.
- A pipe cutter features a small cutting wheel and a set of jaws that tighten to cut pipes to length without bending the pipe or leaving burrs behind.
- Acid brushes are soft-bristled brushes with metal handles that the user can dip into the brazing flux to then apply the flux inside the fitting and on the outside of the pipe. These are sometimes called flux brushes or solder brushes. They’re only necessary if the brazing filler material doesn’t contain phosphorus.
- Pipe reamers can remove any small burrs that do remain, as they can hinder a good seal.
- An acetylene torch is a tool you use to heat the pipe and joint beyond the melting point of the solder.
- Lighting a torch requires an ignitor of some type, such as a lighter or striker.
- Having a fire extinguisher on hand is important when working with a propane torch, as the possibility of unintentionally igniting a fire is elevated when working with intense flames and high temperatures.
- Position a piece of fire cloth behind the pipe if you’re soldering in tight quarters.
- A spray bottle filled with water is also good to keep around, as you can dampen any wood studs or materials before soldering to prevent burning.
- Heat-resistant gloves to protect your hands and goggles to protect your eyes.
How to braze plumbing pipes
1. Cut the pipe to size and ream the burrs
Use the pipe cutter to cut the pipe to length. Ensure that it’s long enough to fit inside the fittings. Use the pipe reamer to remove any burrs from the fresh cut.
2. Clean the pipe and fitting
Corrosion, oxidation, and carbon can prevent a good seal. To avoid this, clean the ends of the pipe well. Wrap the emery cloth around the end of the pipe and hold it in your hand. Twist the end of the pipe back and forth in the cloth until it’s perfectly clean. Clean the fitting by twisting the appropriately sized fitting brush inside the fitting until it’s clean.
3. Apply flux to the fitting and pipe
If your brazing rod doesn’t contain phosphorus, you need to apply flux to the joint. Using the acid brush, apply flux to the inside of the fitting and around the outside of the pipe. Slide the pipe into the fitting and twist it one-half turn to ensure the flux spreads around the joint. This is also a good time to position the fire cloth behind the joint and dampen anything that can burn with the spray bottle.
4. Prepare the brazing filler material
Prepare the brazing filler material by bending it downward approximately 4 or 5 inches from the end of the solder wire.
5. Heat the joint
With a fire extinguisher nearby, use an ignitor or striker to light the torch. Adjust the gas valve so the flame is relatively short (just an inch or two long). Be careful not to burn anything behind or around the pipe, move the torch back and forth across the fitting to heat it. A brazed joint must be considerably hotter than a typical soldered joint, and it will change color and glow while it heats up.
6. Braze the joint
Every few seconds, tap the tip of the brazing filler materials against the joint to see if it’s hot enough for brazing. The joint is ready when the material melts. Continue to apply heat while working the tip of the rod around the joint.
7. “Cap” the joint
With the torch still heating the joint, use the end of the rod to push additional brazing material into the joint. This is called capping. Continue all the way around the joint.
8. Clean up
Remove the torch from the joint and shut it off. Carefully place it somewhere that it cannot burn or melt anything nearby.
You’ll need to clean up the joint after brazing. If you applied flux, start by wiping the excess off the joint. Also, you’ll want to remove the flaking oxidation on the outside and inside of the joint. On the outside, a quick wipe with a damp cloth will remove the flaking. On the inside, you can use a fitting brush, but professionals with compressed nitrogen on hand will typically purge the system.
Brazing vs. soldering: How to choose between the methods
For most plumbing scenarios, soldering is more than sufficient. It’s quick, there’s very little risk of damaging the pipe, and it’s strong enough to hold up to the pressure of most water systems. And soldering can be less likely to burn framing components and other materials in the area due to the lower heat requirements, though it requires just as much safety awareness as brazing.
But, for high-pressure systems and systems containing gases (such as air conditioning), brazing is the preferred choice. In fact, a brazed joint is typically up to five times stronger than a soldered joint, as long as the base metals are still intact. Also, for joining dissimilar metals, such as ferrous and non-ferrous base materials, brazing is typically the method for the job.
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