Question

I want to torch braze copper, but I’ve heard there can be problems. Can you help explain what they might be?

Answer

Torch Brazing
Torch brazing of copper and the establishment of settings that produce a "reducing" flame is common throughout the heat-treating industry. However, certain types of copper, including electrolytic tough pitch (ETP) copper (C11000), are sensitive to embrittlement when heated to 700°F (370°C) or above in a reducing atmosphere. Embrittlement can be rapid if hydrogen or carbon monoxide (CO) is present in this reducing atmosphere. For this reason, oxyfuel gas is not recommended for brazing.[1]

Properties of Acetylene
Acetylene (C₂H₂) and other fuel gases are hydrocarbon-based products and, as such, have a high percentage of carbon in their chemistry. Acetylene has two carbon and two hydrogen atoms in every molecule.

Flame Characteristics
When the torch flame is adjusted so there is a larger amount of fuel-gas flowing through the torch tip than oxygen, there will be an excess of carbon fuel in the torch flame. CO is produced, creating the reducing flame. A reducing flame is also known as a carburizing flame.

When metal surfaces are being heated using the torch, they will have a greater and greater tendency to react with oxygen in the air around the components being brazed. This applies to all metals. However, when the torch flame is adjusted to be "fuel-rich" (i.e. a carburizing flame), all the extra carbon being thrown at the copper surface will quickly react with the oxygen in the mixture and the oxides on the hot metal surface to form CO and carbon dioxide (CO₂).

The formation of these gases actually then "reduces" the surface oxides on the copper, partially eliminating them, and the surface of the copper becomes brighter where the "reducing" flame impinges on the hot copper surface.

Thus, by adjusting the gases flowing through the torch so that a reducing flame is being used, you can even think of the flame as being a "self-fluxing" atmosphere since the flame can actually get rid of surface oxides. A so-called "neutral" flame will not do this.

Additionally, it is important to realize that a neutral flame setting is very difficult to hold since "neutral" is actually just the demarcation point between oxidizing settings and reducing settings. There really is no neutral "zone" as some people believe. They like to treat oxidizing, neutral and reducing as three equally wide zones of potential torch settings. Not true. There are actually only two zones – oxidizing or reducing. Neutral is merely the line that divides them.

We will discuss more on flame characteristics and alternative heat-treatment methods in Part 2.