Brazing filler metals suitable for joining copper tube are of two classes:
- the BCuP series alloys containing phosphorus
- the BAg series alloys containing a high silver content
The two classes differ in their melting, fluxing and flowing characteristics and this should be considered in selection of a filler metal (See Table 14.12.) While any of the listed filler metals may be used, those most commonly used in plumbing, HVAC refrigeration and fire sprinkler systems are BCuP-2 (for closer tolerances), BCuP-3, 4 or 5 (where close tolerances cannot be held) and BAg-1, BAg-5 and BAg-7. The BCuP series filler metals are more economical than the BAg series, and are better suited for general piping applications. BAg series filler metals should be used when joining dissimilar metals, or the specific characteristics of the BAg series filler metals are required. For joining copper tube, any of these filler metals will provide the necessary strength when used with standard solder-type fittings or commercially available short-cup brazing fittings.
According to the American Welding Society (AWS), the strength of the brazed joint will meet or exceed that of the tube and fitting being joined when the joint overlap and the depth of the filler metal penetration is a minimum of three times the thickness of the thinner base metal (tube or fitting), and a well-developed fillet is present.
The strength of a brazed copper tube joint does not vary much with the different filler metals but depends mainly on maintaining the proper clearance between the outside of the tube and the cup of the fitting. Copper tube and solder-type fittings are accurately made for each other, and the tolerances permitted for each assure the capillary space will be within the limits necessary for a joint of satisfactory strength.
The rated internal working pressures of brazed copper tube systems at service temperatures up to 350° F (the temperature of saturated steam at 120 psi) are shown in Table 14.4a. These pressure ratings should be used only when the correct capillary space has been maintained.