In furnace brazing, this tendency for the melted copper filler to flash and spread is generally caused by one of four factors:

• Your brazing zone temperature is set too high. Copper melts at 1984ºF. For carbon steel brazing, the brazing zone should be set at 2050 ± 10ºF.
• The reducing power of your atmosphere is too high, caused by too low a dew point or too high a concentration of hydrogen in the brazing zone. For carbon steels in a muffle type furnace, dew points should range from -10º F to +10ºF, with 5% hydrogen.
• The joint gap is too wide, producing lower capillary forces for the melted copper to flow into the joint; this causes the copper to flow away from the joint.
• The part is in the "hot zone" of your furnace for too long.

For a free copy of our technology guide, "Introduction to Furnace Brazing," please call us at 800-654-4567.