(b) Heating can be localized.
(c) Method is applicable to many types of assemblies.
(d) It is an extremely flexible process.
(2) This method of heating also has the following limitations:
(a) Procedure is relatively slow.
(b) Temperature control is difficult.
(c) The flame cannot be easily applied to assemblies having multiple or
inaccessible joints.
(3) Flames resulting from a combination of oxygen and city gas or any of
the bottled gases, such as butane or propane, give a much greater heat and when the
torch is properly designed give excellent results.
(4) The oxyacetylene flame is to be preferred to the others. However, the
flame is intense and must be kept in motion in order to avoid overheating or
melting the work.
(5) A slightly oxidizing flame should be used for capillary brazing. A
highly oxidizing flame retards wetting and flow characteristics of the brazing
alloy.
(6) The size of the torch tip selected should be large enough to minimize
the preheating time, but not so large as to melt the edges of the base metal during
the brazing operations.
(7) The bond strength depends directly upon the proper heating of the base
metal and the proper adding of the filler rod metal.
(8) To avoid overheating, the tip is held at a small angle to the work
surface. The best results are obtained by using a slightly oxidizing flame,
heating to the correct temperature, and applying small quantities of the filler rod
material only as needed. This is done by keeping the filler rod near enough to the
flame so that the rod will maintained at a temperature near its melting point.
(9) The filler rod metal has a tensile strength ranging from two to three
times the tensile strength of the junction zone and, in many cases, a strength
greatly in excess of such metals as cast iron. The tensile strength is
approximately 35,000 PSI.
(10) Parts which are brazed must not be moved in any way that will place a
strain on the joint until the filler metal has solidified completely; otherwise,
the filler metal may form cracks and the brazed joint will fail.
23.
JOINT PREPARATION.
a. The strength of a brazed joint depends directly upon the bond area (fig
18). This area may be increased for additional strength by beveling the edges of
the joint or by depositing a wide face weld.
b. On plates three-sixteenth inch or more in thickness, it is generally
advisable to bevel the plate edges to form a single-vee groove with a 90 included
angle.
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