Many brazers and welders talk about adjusting their torch flame so that it is oxidizing, neutral or carburizing. This is important to know about, and it is shown in Fig. 4.
An oxidizing flame is, as the name implies, oxygen-rich and will therefore heavily oxidize the surface of the part being heated. A carburizing flame is one that is fuel-rich, since most fuel-gases are hydrocarbon gases.
When any metal is heated, it tends to react with oxygen to form oxides on its surface, and the rate of this oxidation increases as the temperature rises. Unfortunately, BFMs cannot effectively bond to oxides, andcoxygen must be kept away from the surfaces to be brazed.
By using a fuel-rich (carburizing) gas-flame, the torch brazer will be throwing excess carbon at the surface of the part being heated. This carbon will instantly react with the surface oxidation that is forming as the part is being heated by the flame, and the excess carbon reacts with the surface oxygen to form gaseous CO and CO2, thus keeping the surface of the metal bright and clean. For this reason, a carburizing flame is also known as a “reducing flame,” since it continuously reduces the surface oxidation that is trying to form.
Please be aware that “neutral” is really just a line that separates oxidizing from carburizing. There really is no neutral region to which a torch can be adjusted. Invariably the flame setting will drift, and you don’t want it drifting into the oxidizing region. Therefore, set the torch to slightly carburizing (reducing), and you should then have a good flame that will heat the surface effectively and keep it clean as well.
In summary, use a good, slightly carburizing (reducing) flame; keep the flame far enough away from the part being brazed so that the flame can wrap around the fitting being brazed; and be sure to let the heat from the base metal melt the BFM rather than heating the BFM directly with the torch.