it is locked. The preplaced insert method produces a
strong leaktight joint in both sheet metal and pipe work.
SILVER BRAZING TECHNIQUES
You will need some knowledge of the principles of
heat flow through metals to understand silver brazing
The following points are particularly important:
1. Heat always flows from a hotter area to a colder
2. The process by which heat flows from molecule
to molecule through a metal (fig. 9-3) is called
conduction. Conduction takes place quite rapidly in
most metals, but air is a very poor conductor of heat.
Therefore, if two pieces of metal that are to be joined
by silver brazing are not in contact with each other, each
piece must be heated separately. If the two pieces are in
contact with each other, you can heat both by applying
Figure 9-4.--Flow of heat.
heat to one of them. The second piece will be heated by
conduction from the first piece. These principles are
shown in figure 9-4.
3. In the molten state, filler metal flows from the
colder toward the hotter areas on a heated surface.
Therefore, you might say that the filler metal flows in a
direction OPPOSITE to the direction of flow of the heat.
This principle is shown in figure 9-5.
4. The filler metal and the flux used in silver
brazing cannot occupy the same space at the same time.
Therefore, a clearance must be provided in the setup of
the joint so that the filler metal can flow in and the flux
can flow out. Heat should be applied in the manner
shown in figure 9-6 so that the flux will flow out when
Figure 9-5.--Flow of molten filler metal.
the filler metal reaches the bonding temperature.
5. Heat travels faster through some metals than
through others (fig. 9-7). All metals and alloys have
substances, but there is a good deal of variation in the
speed with which various metals and alloys will conduct
heat. Copper, for example, is a very rapid conductor of
heat. When two pieces of different metals are to be
joined by silver brazing, the difference in heat
in heating. For example, if you are trying to join a steel
part and a copper part by silver brazing, you will find
that the steel part reaches the joining temperature more
Figure 9-3.--Conduction of heat.
rapidly and has a tendency to overheat because the heat