Brazing

Low-temperature silver alloy brazing makes joins that are strong, permanently leak-tight and maintenance-free. Some of the factors to consider when torch brazing similar and different materials together, and for safety,

  It is important not to get flux inside refrigeration tubing or medical gases piping to prevent contamination.

  Highly polished metal surfaces tend to restrict filler metal flow.

  Molten filler metals tend to flow towards areas of higher temperatures.

  As a general rule, don't skimp on flux which absorb oxides. It's your assurance against oxidation during heating.

  Copper-to-copper joins without a flux require a special brazing filler metal with phosphorous which acts as a fluxing agent on copper.

  To maximize corrosion resistance, keep joint tolerances close and filler metal fillets to a minimum.

  Similarly, to obtain minimum electrical resistivity (or increase electrical conductivity), keep joint tolerances as close as possible.

  Thin films make the highest strength joints; clearances of .001 to .005-in. or .025 to .13 mm are sufficient.

  Good thermal conducting metals, such as copper, require more heating than poor thermal conducting metals such as carbon steel.

  Brazing temperatures generally range from about 1150oF to 1600oF or 620oC to 870oC; copper, for example, melts at 1981oF/1083oC, steel at 2500oF/1370oC.

• o Ensure tubes are cut square and burrs are removed.
• o Sharp corners in joint design impede capillary action.

CLEAN SURFACES VITAL

TO SUCCESSFUL BRAZING

 Capillary action will work properly only when the surfaces of the metals are clean-and in the proper order! Coatings of rust, scale, grease, oil or just plain dirt will form a barrier between the base metal surfaces and the brazing materials.

  An oily base metal will repel the flux, leaving bare spots that oxidize under heat and result in voids.

  Oil and grease will carbonize when heated forming a film over which the filler metal will not flow.

  Brazing filler metal won't bond to a rusty surface.

  An acid pickle solution to remove rust and scale won't work on a greasy surface.

  Using an abrasive to get rid of oil will scrub the oil and fine abrasive powder into the surface.

 1.) Get rid of oil and grease first, by dipping the parts in a degreasing solvent, by vapor degreasing or alkaline cleaning.

 2.) Remove oxide or scale with an acid pickle treatment (if the chemicals are compatible with the base metals-and no acid traces remain in crevices or blind holes).

 Or a mechanical abrasive cleaning, particularly in maintenance repair brazing, using emery cloth, grinding wheel, file or grit blast, followed by a rinsing, will clean dirty or rusted surfaces.

SAFETY REMINDERS

 The following well-tested precautions should be followed to prevent the possibility of dangerous fumes and gases arising from base metal coatings, zinc and cadmium-bearing filler metals, and from fluorides in fluxes:

 o A surface contaminant of unknown composition may add to fume hazard, could cause a too-rapid breakdown of flux and result in over-heating and fuming.

 o Full flux coverage reduces fuming.

 o Similarly, direct flame on filler metal causes over-heating and fuming.

 o Intense localized heating uses up flux and increases the danger of fuming; apply heat only to base metals.

 o A cadmium coating on a base metal will volatize and produce toxic fumes during heating, and should be removed before heating.

 o Zinc (galvanized) coatings will also fume when heated, and should be removed before heating.

 o Look for and understand warnings on labels when using filler metals that contain cadmium.

 o In confined and closed spaces, use ventilating fans and exhaust hoods to carry all fumes and gases away from the work-or use air-supplied respirators where required.

This Article was sent in by Jim Jeffries of National HVAC in Loma Linda. CA email to jin5343@aol.com