Question: We are setting up to do some oxy-acetylene torch brazing of copper components and have been told that we should use a "reducing" flame. What is "reducing" about such a flame, and why shouldn't we just use a "neutral" flame?

Answer: Torch brazing of copper is commonly done by many companies, and the persons who suggested the "reducing" flame to you are correct. A reducing flame is also known as a carburizing flame. Acetylene and other fuel gases are hydrocarbon products and thus have a lot of carbon in their chemistry. 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, and that's the primary reason why it's called 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 since all metals want to react with oxygen to form surface oxides as they get hotter and hotter. It's a standard metallurgical reaction. These surface oxides are very dark in color.

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 oxides on the hot metal surface to form CO and CO2. The formation of these gases actually then "reduces" the surface oxides on the copper, eliminates them, and the surface of the copper becomes bright and shiny 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, folks need 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!

So now you have the rest of the story.