Soldering and Brazing
Soldering and brazing are two methods used to join copper tubing with non-ferrous filler metal. Soldering uses filler metals that melt below 800° F., usually in the 375° F. to 500° F. range. Filler metals used for brazing melt above 650° F.
Soldering, also called sweat soldering or soft soldering is commonly used to join copper tubing and fittings. It is used on piping that carries liquids at 250° F. or lower.
The soldering together of metals requires heat and a non-ferrous filler metal. The melting point of the filler metal must be below the melting point of the two metals being joined.
Soldered joints depend on capillary action to draw the melted solder into the gap between the surfaces to be joined. Capillary action is most effective when the space between the tubing and fitting is between .002″ and .005″.
Solder is an alloy consisting of two or more metals with a melting point below 800° F. The most common solder alloy used in HVAC is composed of 95% tin and 5% antimony. Lead was once common in solder, but its use is now prohibited by law.
Different fluxes are available; selection depends on the solder alloy being used. Flux cleans the metal surface, prevents oxidation and allows solder to easily flow into the joint. It also helps float out oxides ahead of the filler metal and promotes wetting of the metals that allows the solder to better flow into the joint.
Prepare tubing and fittings for soldering by measuring, cutting, and reaming the pipe and cleaning the tubing ends with No. 00 steel wool or emery cloth. Read the procedure in the text for preparing tubing and fittings for soldering.
Air/acetylene torches are used for soldering. The nitrogen in air does not support combustion, resulting in a lower flame temperature than that possible with an oxygen/acetylene torch.
Soldering involves pressurized gases and an open flame and can be hazardous. Read paragraph 2.3.0 in its entirety with special emphasis on the NOTE, WARNING and CAUTION statements for a review of procedures for soldering joints.
Since hotter temperatures (between 1200° F and 1550° F) are required for brazing, different equipment using oxygen along with a fuel gas such as acetylene is required.
Oxygen is supplied in a cylinder with pressure near 2000PSI. Acetylene cylinders are pressurized at about 250PSI. To prevent a dangerous release of pressure, both cylinders are supplied with protective valve caps that must be in place if the cylinders are moved.
Since acetylene is unstable above 15PSI, never set an acetylene regulator above that pressure.
Oxygen supports combustion, so it must be stored away from flammable materials. Never use oil or grease around an oxygen cylinder or allow anyone to stand in front of an oxygen cylinder when the valve is opened.
Both oxygen and acetylene cylinders are equipped with shut-off valves and pressure regulators that must be properly adjusted for safe and correct operation.
Once the brazing equipment has been set up, the torch can be lit and adjusted for brazing. Make sure the correct tip is installed.
There are three types of flames. The neutral flame is commonly used for brazing. The carburizing flame is cooler and is used if melting of the base metal might occur. The oxidizing flame is the hottest and is sometimes used for brazing cast iron.
You can tell when the flame is properly adjusted because it will return to the tip, the inner cone will appear soft and luminous, and the torch will make a soft and even, blowing sound.
When heat is applied to copper tubing during brazing, air inside can cause the tubing to oxidize. Purging the tubing during brazing with an inert gas such as nitrogen or carbon dioxide will displace the air and prevent oxidation.
When nitrogen is used, the high cylinder pressure of the gas must be regulated down to a safe working pressure.
A typical hookup consists of a pressure regulator that contains cylinder and regulated pressure gauges, a pressure relief valve, and a shutoff valve.
To prepare for brazing, clean the surfaces to be joined, apply flux, and select the proper brazing alloy.
If required, purge the tubing with regulated nitrogen. Wear tinted goggles for eye protection. Move the torch flame back and forth over the tubing and fitting.
Allow the fitting to receive more heat than the tubing by pausing at the fitting while continuing to move the flame back and forth.
When the brazed joint is complete, wash it with a cloth and warm water to remove flux residue.