Soldering and brazing have been the two most utilized methods of joining copper tubes and fittings for many years. These traditional joining methods are similar in many ways, but differ in several significant ways.
This paper describes the similarities and explains the differences in joining processes to help understand which joining procedure is most desirable.
What is Copper Brazing?
The typical means of joining copper tubes is the use of a socket-type, copper, or copper alloy fitting in which the tube sections are inserted, and the pipe device is then locked together using a filler metal.
The filler metal is melted into the space between the tube and the fitting to create a capillary-equalized joint using high temperature processing known as soldering, or brazing.
These types of joints are referred to as capillary or lap joints because the socket of the fitting comes over the end of the tube and the gap to be filled occurs between the tube and the cap illary fitt ing provides and faying surface.
The joint surfaces between the tube and the fitting are the faying surfaces. The tube and fitting are thereby solidly constructed using a filler metal that melts into the capillary space and bonds to the faying surfaces.
A filler metal is a metal alloy that melts and liquifies below the melting temperature of the tube or fitting. A copper (Cu) alloy UNS C12200 has a melting point of 1981°F/1082°C.
If the molten element of choice to fill the joint is below 840ºF, the process being performed is soldering; above that temperature the process being performed 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
Copper with oxygen and without oxygen can both be brazed to produce a joint with acceptable characteristics. A lap joint will develop the full strength of an annealed copper brazed joint.
The flame employed 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 use of a flux will produce a better looking joint.
Copper Soldering Vs. Brazing
Solder is typically melted between 350 degrees and 600 degrees. Copper brazing is completed, for example, braze joints are typically made with filler metal melted between 1100 degrees and 1500 degrees.
Brazing is a metal-joining process in which two or more metal pieces are joined together by melting and flowing filler material into the joint, the filler having a lower melting point than the adjoining metal.
The difference between brazing and soldering is the temperature required to melt that filler metal. The temperature is established by the American Welding Society (AWS) to be 842ºF 450ºC and rounded to 840ºF most times. If the filler metal melts below 840ºF, that process is soldering.
You tend to find brazed joints with refrigeration, high-pressure airlines, and HVAC where maximum joint strength is required. You’ll find soldered joints in low-pressure applications such as potable water lines and maybe some heating systems.
Do you need flux to braze copper?
To braze copper using an external helping agent, a flux medium can be used to aid the solder to run easier and ensure the torch pushes heat into the solder. The rod is then pushed in place without forcing it which would disrupt the heat bath and create a poor joint joining the two pieces of copper.
Another basic way to braze copper could be to flux brush one side then flow some molten metal onto that – this would include water-soluble paste.
Both surfaces of the two pieces of metal should have some degree of sanded surface, if not there will not be proper bonding. This is often the way plumbing pipe joints are done irreigardless if one wants poisonous fumes from soldering to remain in in the house.
How to braze copper
By first heating the copper, brazing is accomplished by running a layer of constantan (a metal with high thermal conductivity and low electrical resistivity) on top. If the two pieces are lined up correctly the two pieces will fuse when heated to 250°C.
To make sure that some small air bubble doesn’t enter there and wreck everything, these will bubble to the surface as it is heated and can be pierced with a brazing needle before you start heating.
Once they are heated, you can apply some braze solder paste. This will melt again when you heat it to complete the joint! However, if constantan doesn’t provide enough thermal conductivity, or if you want to ensure that both pieces are at roughly the same temperature prior to applying heat for the first time, you can also use a brazing rod covered in flux.
What exactly happens when the filler metal melts?
Typically a bright orange glob will form when the filler metal melts. This means you have achieved the correct temperature to start brazing.
You will apply the flux and also a water-soluble paste, then you will want to heat the rod and the copper pipe for roughly five minutes before you apply some solder paste onto the rod.
If you are wondering how you would do this in construction, this is usually how plumbing pipes are connected. For plumbing pipes, there are two typical methods to an effective union: brazing copper and soldering copper.
The flux will allow for the solder to flow freely, and should be used to coat both surfaces to continue brazing properly. When the rod is pushed into place, do not force it because you will mix up the heat bath, and unify the two pieces of copper to an inaccurate connection point.
Types Of Flames
The flame has three states, one of which is not desirable, which is a carburizing flame. Here’s what a carburizing flame looks like, with a unique 3 color appearance, and that results in excessive carbonization on the pipe through no fault of your own, which could ultimately impact the joint.
The other flame you do not want is an oxidizing flame. As the name suggests, an oxidizing flame oxidizes the workpiece which you don’t want to happen. Lastly, there is a neutral flame.
A neutral flame is what you’re looking for in that it neither oxidizes nor carburizes, it has the perfect mix of oxygen and acetylene and is a beautiful clear blue color like this.
Tools/Materials for The Project
Examples of tools and filler materials varies from a soldered joint to a brazed joint, so we will cover those together.
When soldering potable water lines you use 95/5 solder. You cannot use 95/5 solder when brazing so you will need a filler material. There is an immense selection of filler material out there so you will need to decide which is best to use, relative to the job you are doing.
If you are unsure, Harris products does provide a clear chart on which brazing rod to use for which materials joined, in case of any doubt. For this demonstration, I will be using Stay-silv 15.
Stay-silv 15 is 80% copper, 15% silver and 5% phosphorus. You do want to get a rod that has phosphorus in it. This is because phosphorus is self-fluxing and therefore saves you from needing to apply any flux to the pipe.
Seeing silver solder melts at a higher temperature you will need more heat than an average solder joint.
It is possible to braze smaller pipes with a small propane torch, however larger pipes, such as 1″ and up, require more heat, therefore it’s suggested to use an oxy/acetylene system with a proper torch seeing silver solder melts at a higher temperature than standard solder.
Oxyacetylene burns much hotter at approximately 3500C or 6330F compared to propane burning at approximately 1995C or 3620F.
Lastly the torch. When soldering you can use a torch like this because all we need is one gas. Typically, an oxy/acetylene torch does give off a lot more heat than propane torch.
I am using the Harris oxy/acetylene port-a-torch kit which is a Lincoln electric company and good for light duty brazing like this one. It comes in a carrying tote with 2 tanks, 1 oxygen, 1 acetylene, 2 regulators for each gas and the torch itself.
You will notice the torch tips that come with the kit. You could use a simple air/acetylene setup like this, but it is quite a bit longer and doesn’t give you as clean of a joint as an oxy/acetylene does but is cheaper and lighter to carry in tight spaces, so you can just use what works best for you.
One more thing you may want to think about is making sure your torch has flashback arrestors both on the oxygen and acetylene side. Most new kits come with them built in.
Back in the day these used to be separate and you needed to install them on the hose itself. So just make sure your kit has them because they will stop the flame from going back into your bag, which you don’t really want.
FAQs
What type of brazing is used for copper?
Silverphos / Phoscopper. Copperphos brazing alloys are designed to braze copper-to-copper and copper-to-brass assemblies. The presence of phosphorus acts as a fluxing agent during the braze process and deoxidizes copper.
Is it better to solder or braze copper?
Brazing is commonly employed when high joint strength is necessary, such as in HVAC systems, refrigeration units, and high-pressure pipelines. Meanwhile, soldering is often utilized in applications where lower joint strength is acceptable, such as in water supply lines, drainage systems, and smaller pipe connections.
What temperature is needed to braze copper?
In actual practice for copper systems, most soldering is done at temperatures from about 350°F to 600°F, while most brazing is done at temperatures ranging from 1100°F to 1500°F.
How strong is brazing copper?
When you braze a copper–to–copper joint with any of the commonly used AWS classifications of brazing filler metals, such as BCuP or BAg, the shear strength of the braze metal is about 25,000 PSI.
Do you need flux when brazing copper to copper?
However, in copper to copper applications, the phosphorus in BCuP alloys acts as the fluxing agent. No additional flux is needed, although a small amount of flux may be used for applications with prolonged heating times to improve the wetting action of the filler metal.
What brazing rod to use for copper to copper?
Copper-Phosphorus Brazing Rods (Cup 181) – CuP7 2,0 x 500mm – 500gr. Brazing alloy with excellent fluidity for convenient copper-copper and copper-brass joints. For joining copper to copper no flux is necessary. CuP7 is a copper-phosphorous brazing alloy, with good flow properties.