QuestionI want to torch braze copper, but I’ve heard there can be problems. Can you help explain what they might be?
AnswerContribution from Fluxes
Copper acts as a catalyst to break down some of the organic binders in the flux into (primarily) carbon and hydrogen. Thus, you may see carbon residues on some of the braze joints. It is often a hit-or-miss thing because it does not happen all the time. But the cause is fairly well understood, namely, the catalytic reaction of copper with the organic binders. A little bit of oxygen is actually needed to react with the carbon in the organics to form CO in order to prevent such residue from showing up.
With the acetylene burning, slowly open the oxygen supply to the torch and allow this gas to join the flame. The flame will turn intensely bright and then blue white. There will be an outer flame from 4-8 inches long and from 1-3 inches thick. Inside of this flame will be two more rather distinct flames. The inner one at the torch tip is very small and the intermediate one is long and pointed. The oxygen should be turned on until the two inner flames unite into one blue-white cone from 0.25 to 0.50 inches long and 1/8 to 0.25 inches in diameter. If this single, clearly defined cone does not appear when the oxygen supply is opened, turn off some of the acetylene until it does appear.
If too much oxygen is added to the flame, there will still be the central blue-white cone, but it will be smaller and more or less ragged around the edges (Fig. 1a). When there is just enough oxygen to make the single cone, and when, by turning on more acetylene or turning off oxygen, two cones are caused to appear, the flame is neutral (Fig. 1b). The other condition is when too much acetylene is present (Fig. 1c). While brazing, test the correctness of the flame adjustment occasionally by turning off some oxygen until the two flames or cones appear. Then regulate as before to secure the single distinct cone. Too much oxygen is harmful when brazing. An excessive amount of sparks coming from the braze denotes that there is too much oxygen in the flame. Should the opening in the tip become partially clogged, it will be difficult to secure a neutral flame. (The tip should be cleaned with a brass or copper wire, never a steel tool or wire of any kind.)