Back to the Basics of Brazing and Soldering

March 6, 2006
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FIG. 1: A heavy oxide can be formed on the pipe preventing a bond from forming between the filler metal and the pipe surface.
The processes of brazing, soldering, and welding are often miscategorized as all being welding.

In the process of welding, both the edge of the base metal and the filler metal are melted. During brazing and soldering, however, only the metal being added to join the parts is melted.

The difference between brazing and soldering is temperature. Soldering takes place at a lower temperature than brazing. Other than that, both processes are similar.

In both brazing and soldering the base metal must be heated to a temperature within a range where the bonding phase between the molten filler and the base metal can occur. This process is called tinning. If the metal being joined is too cool or too hot, tinning cannot occur.

Overheating is a major problem when brazing or soldering copper pipe because the flux will burn (become oxidized), stop working, and become a barrier to tinning. In addition, a heavy oxide can be formed on the pipe itself, preventing a bond from forming between the filler metal and the pipe surface. (See Figure 1.)

MELTING TEMPERATURE

The melting temperature of the filler material determines whether the process is brazing or soldering. The brazing process takes place at temperatures above 840°F and soldering takes place at temperatures below 840°. Both processes are often referred to in conjunction with the filler metal. For example, silver brazing refers to the use of a silver alloy commonly used to join copper tubing for HVAC work.

The term soft solder often refers to an alloy of tin and lead. The term hard solder frequently refers to an alloy of tin and antimony. Currently, tin antimony alloys are the most commonly used alloys for soldering because of the potential health and environmental concerns with lead-based solders.

Because the terms soft and hard soldering are not standardized terms, there may be variations of their meanings depending on local usage. For example, sometimes the term hard soldering is used to refer to alloys containing trace amounts of silver.

FIG. 2: A flux is an active compound that removes light surface oxides and promotes the tinning of the base metal with the liquid filler metal.

FLUXES

In order for the bonding or tinning phase to occur between the filler metal and the base metal, a flux is often required. A flux is an active compound that removes light surface oxides and promotes the tinning of the base metal with the liquid filler metal. (See Figure 2.)

Some fluxes are neutral at room temperature and only become active when heated to a specific temperature. Other fluxes are active, irrespective of the temperature. All fluxes, but particularly active fluxes, must be removed from the completed joint. If flux is allowed to remain on the joint - even though it may be inert at standard temperatures - it can cause problems.

Flux can trap moisture causing pitted corrosion, or it may obscure small pinhole leaks, which could open later causing system leaks. Active fluxes can significantly damage metal parts if not completely cleaned away and neutralized. Not all solder and brazing requires fluxes. For example, some alloys containing silver, which promotes tinning, are often referred to as fluxless alloys. In some cases a flux is needed with these fluxless alloys. For example, sometimes with a heavy surface oxide or when silver braze is used with steel or brass, a flux is needed.

This chart shows common soldering and brazing metal and fluxes showing base metals that can be joined.

BRAZING

Codes require that refrigerant lines be joined with a brazing alloy. The most commonly utilized process is to use a silver brazing alloy and an air acetylene torch. The acetylene gas from a compressed gas cylinder flows through a venturi in the torch drawing air into the gas where they are mixed before being consumed in the flame. An air acetylene flame burns at a temperature of approximately 4220°. Air acetylene torches work very well on copper pipe for soldering pipe from ¼-in. up to about 3 inches and for brazing pipes from ¼-in. up to about 2 inches in diameter.

Although larger diameter copper pipes may be joined using air acetylene torches, the lack of a highly concentrated flame results in a much slower joint completion time. The longer that copper is kept at the higher brazing temperature, the greater the formation of copper oxide. Copper oxide can become a barrier to the successful completion of the joint as well as becoming a contaminant in the refrigerant circuit. For that reason, technicians usually use an oxyacetylene torch for making braze joints in larger diameter copper pipes.

Properly used, the oxyacetylene torch can produce satisfactory brazes in copper tubing ranging in sizes from 1/16-in. through 6 inches. The oxyacetylene torch uses both compressed acetylene and oxygen from cylinders.

SOLDERING

Soldering is used in the HVACR industry to both join electrical wiring and components and for copper pipes used for supply water and condensate drains. Solder is not approved for refrigerant pipe joints.

Electrical soldering is accomplished using an electrical soldering iron. Soldering to join copper pipes may be done with an air acetylene, air Mapp, or air propane torch. Oxyacetylene is not recommended for soldering because its flame temperature is so high that it is easy to overheat the pipe and burn the solder or flux so that no bonding can occur.

Before performing any brazing or soldering, a technician should be familiar with local codes related to the processes and be willing to consult with the manufacturers of equipment and products for guidance.

Larry Jeffus, author of the ARI textbook "Refrigeration and Air Conditioning: An Introduction to HVACR, Fourth Edition," can be reached at larry@jeffus.org.

Publication date: 03/06/2006

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