We braze steel components together using a copper brazing paste. After furnace brazing, we notice a carbon residue around the edge of the brazed joint. Where is this coming from, and what can be done to prevent it?

Carbon residue is not an uncommon thing in a number of brazing shops that do copper brazing using paste. The copper acts as a catalyst to break down some of the organic binders into carbon, hydrogen, etc. Thus, you will see carbon residues on some of the braze joints. It is a hit-or-miss thing, does not happen all the time and sometimes is rare in some brazing shops. But the cause is fairly well understood, namely, the catalytic reaction of copper with the organic binders. Sometimes a carbon residue shows up even though the brazing shop is only using solid copper pre-formed brazing filler metal (BFM) inserts. That can happen when the preforms or base metals have not been cleaned well and some forming/drawing lubricant is left on the surface of the metals. Then, at brazing temperatures, the copper can react with the residue of drawing lubricant still on the braze rings, base metals, etc., and some carbon residue can be seen as a result.

Carbon residue usually starts to show up when a furnace atmosphere is TOO good. That is, the oxygen content (dew point) may be too low/dry. If this happens only once in a while, it would tend to indicate that the "dryness" of the atmosphere may be varying from time to time because the dewpoint control is not adequate.

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. Thus, it may be necessary to introduce a tiny amount of oxygen into the system to be sure that the carbon residue does not form. This can be accomplished in a number of different ways, but a couple of methods used may be to introduce a tiny amount of airflow from a separate air line into the atmosphere (after experimenting to find out how much needs to be used) or slow down the flow-rate of the current dry-atmosphere being introduced into the furnace.