Torch brazing uses the intensely hot flame from a torch to heat up the metals in the joint area, bringing it up to brazing temperature fairly quickly (from just a few seconds to a few minutes maximum) depending on the size and mass of the part being heated. A typical brazing torch might look like the one in Fig. 2 and generally consists of a metal mixer-body into which two different gases are fed. These two gases are typically oxygen (or perhaps just compressed air) and a combustible fuel gas (e.g., acetylene, propane, etc.). They enter the torch body through separate gas lines/hoses (right side of Fig. 2) and are then blended before exiting out of the curved tip of the torch body, where that blended gas is then ignited and burned (Fig. 1 from part 1). The gas flow rates are controlled by the two knobs on the mixer body.
A torch tip may have a single hole in it (Fig. 2) or perhaps several holes in its tip (Fig. 3). The dual-tipped torch shown in Fig. 3 uses multi-flame (“rosebud”) tips that allow much more even heating of all the joint surfaces of a component since the joint can now be heated from two sides at the same time.
Flux is Needed
When torch brazing out in open air, it is important to use brazing fluxes to prevent oxidation of the surfaces of the part being heated. Fluxes are excellent oxygen absorbers and will keep the brazing surfaces clean and oxide-free until the brazing filler metal (BFM) melts and flows into the braze joint.
Pure copper can be heated in air without flux, by using a specialized phos-copper BFM to compensate for the lack of physically applied paste flux. The phosphorus in the BFM acts as a flux on pure copper and is very effective at doing its job. But it is only effective on pure-copper. Brasses, bronzes, and all other metals will require the generous application of a flux to enable the parts to be heated in open air and locally brazed.