An oxidizing flame is, as the name implies, oxygen-rich, and will thus 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 in the air to form oxides on its surface, and the rate of this oxidation increases as the temperature rises. Unfortunately, BFMs cannot effectively bond to oxides, and thus, oxygen must be kept away from the surfaces to be brazed!
By using a fuel-rich 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. Thus, 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.
Any good vacuum brazing shop will encounter components from time to time that:
- don’t completely braze, and must therefore be repair-brazed, but the part that needs to be repaired is a very small portion of the whole;
- or perhaps a customer is asking you to consider brazing a component which turns out to be too big for your furnace, even though the area to be brazed is not that large;
- or perhaps you need to do some additional brazing (or repair brazing) on a part that was previously brazed, then had some new, delicate components attached to it which aren’t allowed to see high temps. Thus, the entire assembly can no longer be furnace brazed.
These are just a few of the reasons that may cause your vacuum-brazing personnel to consider the viability of adding in some torch-brazing capability to your brazing shop, so that you can do more of a “turn-key” job for your customer, and perhaps be able to more cost-effectively do some needed braze-repairs on some of your vacuum-brazed parts.
So, if the need arose, could your shop do this, or would you be forced to send the parts out to other brazing companies who could perform such torch brazing for your customer?