What is Copper Brazing and How To Do It?

For many years the two most common methods of joining copper tubes and fittings have been soldering and brazing. These tried-and-true methods are in a number of ways similar, yet there are also several distinct differences that set them apart. This paper explains the similarities and highlights the differences between the two joining processes to help determine which joining method is most desirable.

What is Copper Brazing?

The most common method of joining copper tubes is with the use of a socket-type, copper, or copper alloy fitting into which the tube sections are inserted and fastened by means of filler metal, using either a soldering or brazing process. This type of joint is known as a capillary or lap joint because the socket of the fitting overlaps the tube end and a space is formed between the tube and the fitting.

This space is called the capillary space. The surfaces of fitting and tube that overlap to form the joint are known as the faying surfaces. Tube and fitting are then solidly joined using a filler metal that is melted into the capillary space and adheres to these surfaces.

The filler metal is a metal alloy that has a melting temperature below that of either the tube or fitting. The melting point of copper (Cu) alloy UNS C12200 is 1,981°F/1082°C. If the filler metal melts below 840ºF the process being performed is soldering. Above that temperature, the process is brazing.

Why Copper Brazing Use?

The use of copper brazing is for when greater joint strength is needed or for systems that operate at 350 degrees or higher.

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Typical uses include:

  • Fire protection
  • Air-conditioning and refrigeration
  • Fuel gas distribution
  • Water supplies

Both oxygen-bearing and oxygen-free copper can be brazed to produce a joint with satisfactory properties. The full strength of an annealed copper brazed joint will be developed with a lap joint.

The flame used should be slightly carburizing. All of the silver brazing alloys can be used with the proper fluxes. With the copper-phosphorous or copper-phosphorous-silver alloys, a brazed joint can be made without a flux, although the use of flux will result in a joint of better appearance.

Copper Brazing

Copper Soldering Vs. Brazing

Most soldering is done at temperatures from 350 to 600 degrees. Copper brazing is done, such as for brazing joints at 1100 to 1500 degrees.

Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing material into the joint, the filler material having a lower melting point than the adjoining metal.

The difference between brazing and soldering is the temperature necessary to melt that filler metal. That temperature is defined to be 842ºF/450ºC by the American Welding Society (AWS) but is often rounded to 840ºF. If the filler metal melts below 840ºF the process being performed is soldering.

Brazed joints are used for refrigeration, high-pressure airlines, and HVAC where maximum joint strength is a must, and soldered joints are used in low-pressure applications such as potable water lines and some heating systems.

Copper Brazed Joints

Copper brazed joints are used when great joint strength is required or the system where the joints are used operates at greater than 350 degrees.

Butt, lap, and scarf joints are used in brazing operations, whether the joint members are flat, round, tubular, or of irregular cross-sections. Clearances to permit the penetration of the filler metal, except in large diameter pipe joints, should not be more than 0.002 to 0.003 in. (0.051 to 0.076 mm).

The clearances of large diameter pipe joining maybe 0.008 to 0.100 in. (0.203 to 2.540 mm). The joint may be made with inserts of the filler metal or the filler metal may be fed in from the outside after the joint has been brought up to the proper temperature.

The scarf joint is used in joining bandsaws and for joints where the double thickness of the lap is not desired.

Instructions For Brazing Copper Tube Process

Complete all of these steps the same day.

  • Mark the tube for the proper length.
  • Cut the tube using a hacksaw, tube cutter or other personally preferred methods.
  • Ream the ends of the cut tube to remove any metal spurs. Tools that can ream the tube include a reaming blade (found on tube cutters), a half-round file or a deburring tool
  • Prepare the joints by cleaning areas to be soldered to remove any oxides and oil. Use a sand cloth or abrasive pad. The space between the tube and fitting should be .004 in.
  • Insert the tube onto the fitting to ensure a snug fit, but that also leaves enough room for the capillary action of the solder. About .0004 inches. Turn the tube if possible. Firmly support the tube.
  • Hold the flame perpendicular to the tube and preheat both the tube and the fitting cup. Do not overheat since this could cause the flux to burn. Preferably use an oxyfuel torch with a neutral flame. Keep the flame in motion and do not linger on any one part of the tube. If using a flux to remove and dissolve any oxide use gloves, do not apply with bare hands. Keep away from cuts, the mouth, and eyes. Note that when brazing copper tube to wrote copper fittings using a BCuP brazing alloy, brazing flux can be used but is not necessary.
  • When using a flux heat it uniformly until the flux becomes transparent (follow the manufacturer’s directions). Note that is harder to keep the required uniform temperature on tubes with a larger circumference.  Preheating of the entire fitting is needed. A second torch can also help to maintain the heat needed.
  • Touch the filler metal to the joint which should start to melt. Apply at the point where the tube enters the socket of the fitting. When the filler metal melts, apply the heat source to the base of the cup. If a joint is in a horizontal position apply solder slightly off center at the bottom. Push solder straight into the joint, while keeping the torch at the base of the fitting and just ahead of the point of solder application. Keep the flame away from the filler metal. The temperature of the joint itself should melt the filler metal. The flame should be slight ahead of the filler metal application. Follow the same procedure for joints that are vertical.
  • Stop heating as soon as you see a complete fillet. Allow the joint to cool without the use of water. After cooling, clean off any flux with a damp rag.

Types Of Flames

There are 3 types of flames. One you don’t want is a carburizing flame. Here’s what a “carburizing flame” looks like, it’s easily distinguished by its 3 colors, and leads to excessive carbonization on the pipe which isn’t necessary and could compromise the joint.

Secondly, is an oxidizing flame which you don’t want either. This type of flame, as the name states, will oxidize your workpiece which is unwanted. And lastly, a neutral flame. A neutral flame is what you are looking for as it doesn’t carburize nor oxidize, it has a perfect mix of oxygen and acetylene and is typically a clear blue color like this.

Tools/Materials for The Project

The tools and type of filler materials vary from a soldered joint to a brazed joint, so let’s go thru both of them together.

When soldering potable water lines, you’ll be using this type of solder which is called 95/5. This type of solder can’t be used when brazing so, you’re going to be needing a filler material. There are many types of filler materials on the market and you’ll need to choose the right one depending on the type of job you are doing.

In case of any doubts, Harris products does supply a very clear chart on what type of brazing rod to use for various types of materials being joined. For this demonstration, I’ll be using a Stay-silv 15 which contains 80% copper, 15% silver, and 5% phosphorus. You’ll want to get a rod that contains phosphorus as they’re self-fluxing, removing the need to apply any flux on the pipe.

Seeing silver solder melts at a much higher temperature, you’ll be needing more heat than your typical soldered joint.

Brazing smaller pipes are possible with a small propane torch, but larger pipes such as 1” and up require more heat, therefore, it’s recommended to use an oxy/acetylene system with an appropriate torch seeing silver solder melts at a higher temperature than normal solder. Oxyacetylene burns much hotter, at 3500*C or 6330*F compared to propane which burns at around 1995*C or 3620*F.

And finally, the torch. When soldering, a torch like this is used seeing only one gas is needed. Plus, propane torches don’t give out near as much heat as an oxy/acetylene torch does.

I’m using this oxy/acetylene port-a-torch kit from Harris, which is a Lincoln electric company and is great for light-duty brazing tasks such as this one. It comes in a carrying tote and there are 2 tanks, 1 for oxygen and 1 for acetylene, there are also 2 regulators for each gas and the actual torch.

You’ll also notice the torch tips that come with the kit. You could use a simple air/acetylene setup like this, but it’s considerably longer and doesn’t give you a clean joint like an oxy/acetylene kit does. However, they are more economical and lighter to carry around in tight spaces, so use what’s best for you.

Something else you might want to consider is making sure your torch has flashback arrestors on the oxygen and acetylene side. Most new torches come with them built-in. In the past, these were separate and needed to be installed on the hose itself. So just make sure your kit is equipped with them as they’ll stop a flame from going into your tank, which you don’t really want.

Copper Brazing video