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 the 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.
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 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.
Do you need flux to braze copper?
To braze copper, you need flux just so that solder can run freely and is heated by the torch. The rod is then pushed into place without being forced, which would mix up the heat bath and create an unreliable connection between two pieces of copper.
The simplest method for brazing copper is using a flux brush on one surface and then molten metal being brushed onto it – this will also involve water-soluble paste.
The two pieces of metal should have a sanded surface for this process to work efficiently because otherwise, it won’t form an advantageous bond between them. This technique is often how plumbing pipe joints are done because no one wants toxic fumes from soldering around inside their home.
How to braze copper
Brazing is done by heating the copper, then running a layer of constantan (a type of metal with high thermal conductivity and low electrical resistivity) on top. If you line up the two pieces correctly, they will fuse when heated to 250°C.
The flux ensures that any small air bubbles don’t get in there and ruin it all – these will bubble to the surface as it’s heating and can be pierced with a brazing needle before heating starts.
Once they’re heated, apply some braze solder paste – this will melt when heated again to complete the joint! If constantan doesn’t provide enough thermal conductivity, or you want to make sure both pieces are at roughly the same temperature before applying heat for the first time, you can also use a flux-covered brazing rod.
What exactly happens when the filler metal melts?
When the filler metal melts, it will usually form a bright orange glob. This is how you can know that you’ve reached the correct temperature to proceed with the brazing process. You should apply the flux and water-soluble paste, then you should heat the rod and copper pipe for about five minutes before adding solder paste onto the rod.
If you’re wondering how you could do this in construction, it’s typically how plumbing pipes are connected. With plumbing pipes, there are two methods commonly used: brazing copper and soldering copper.
The flux will help the solder flow freely and should be applied to both surfaces for a successful brazing process. The rod is then pushed into place without being forced, which would mix up the heat bath and create an unreliable connection between two pieces of copper.
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.