– 16 –
2. Characteristics required for fl ux
Activated temperature of fl ux coincides with the
brazing temperature.
Due to a wide effective temperature range, fl ux
is hard to carbonize.
It is easy to remove slag after brazing.
The corrosive action to the treated metal and
brazing fi ller is minimum.
It excels in coating performance and is
harmless to the human body.
As the fl ux works in a complicated manner as
described above, it is necessary to select an
adequate type of fl ux according to the type and
shape of treated metal, type of brazing fi ller and
brazing method, etc.
3. Types of fl ux
Noncorrosive fl ux
Generally, it is a compound of borax and boric
acid.
It is effective in case where the brazing
temperature is higher than 800°C.
Activated fl ux
Most of fl uxes generally used for silver brazing
are this type.
It features an increased oxide fi lm removing
capability due to the addition of compounds
such as potassium fl uoride, potassium chloride
and sodium fl uoride to the borax-boric acid
compound.
4. Piping materials for brazing and used
brazing fi ller/fl ux
Piping material Used brazing fi ller Used fl ux
Copper - Copper Phosphor copper Do not use
Copper - Iron Silver Paste fl ux
Iron - Iron Silver Vapor fl ux
1. Do not enter fl ux into the refrigeration cycle.
2. When chlorine contained in the fl ux
remains within the pipe, the lubricating oil
deteriorates.
Therefore, use a fl ux which does not contain
chlorine.
3. When adding water to the fl ux, use water
which does not contain chlorine (e.g. distilled
water or ion-exchange water).
4. Remove the fl ux after brazing.
•
•
•
•
•
•
•
3-5-3. Brazing
As brazing work requires sophisticated techniques,
experiences based upon a theoretical knowledge, it
must be performed by a person qualifi ed.
In order to prevent the oxide fi lm from occurring
in the pipe interior during brazing, it is effective to
proceed with brazing while letting dry Nitrogen gas
(N2) fl ow.
Never use gas other than Nitrogen gas.
1. Brazing method to prevent oxidation
1) Attach a reducing valve and a fl ow-meter to
the Nitrogen gas cylinder.
2) Use a copper pipe to direct the piping
material, and attach a fl ow-meter to the
cylinder.
3) Apply a seal onto the clearance between the
piping material and inserted copper pipe for
Nitrogen in order to prevent back fl ow of the
Nitrogen gas.
4) When the Nitrogen gas is fl owing, be sure to
keep the piping end open.
5) Adjust the fl ow rate of Nitrogen gas so that
it is lower than 0.05 m
3
/Hr or 0.02 MPa
(0.2kgf/cm
2
) by means of the reducing valve.
6) After performing the steps above, keep the
Nitrogen gas fl owing until the pipe cools down
to a certain extent (temperature at which pipes
are touchable with hands).
7) Remove the fl ux completely after brazing.
Nitrogen gas
cylinder
Pipe
Flow meter
M
Stop valve
From Nitrogen cylinder
Nitrogen
gas
Rubber plug
Fig. 3-5-1 Prevention of oxidation during
brazing
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