(2)Characteristics required for flux
•Activated temperature of flux coincides with the brazing temperature.
•Due to a wide effective temperature range, flux is hard to carbonize.
•It is easy to remove slag after brazing.
•The corrosive action to the treated metal and brazing filler is minimum.
•It excels in coating performance and is harmless to the human body.
As the flux works in a complicated manner as described above, it is necessary to select an adequate type of flux according to the type and shape of treated metal, type of brazing filler and brazing method, etc.
(3)Types of flux
•Noncorrosive flux
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 flux
Most of fluxes generally used for silver brazing are this type.
It features an increased oxide film removing capability due to the addition of compounds such as potassium fluoride, potassium chloride and sodium fluoride to the
(4)Piping materials for brazing and used brazing filler/flux
Piping | Used brazing | Used |
material | filler | flux |
|
|
|
Copper - Copper | Phosphor copper | Do not use |
|
|
|
Copper - Iron | Silver | Paste flux |
|
|
|
Iron - Iron | Silver | Vapor flux |
|
|
|
1Do not enter flux into the refrigeration cycle.
2 When chlorine contained in the flux remains within the pipe, the lubricating oil deteriorates. Therefore, use a flux which does not contain chloring.
3 When adding water to the flux, use water which does not contain chlorine (e.g. distilled water or
4 Remove the flux after brazing.
6-5-3. Brazing
As brazing work requires sophisticated techniques, experiences based upon a theoretical knowledge, it must be performed by a person qualified.
In order to prevent the oxide film from occurring in the pipe interior during brazing, it is effective to proceed with brazing while letting dry Nitrogen gas (N2) flow.
Never use gas other than Nitrogen gas.
(1)Brazing method to prevent oxidation
1 Attach a reducing valve and a
2Use a copper pipe to direct the piping material, and attach a
3Apply a seal into the clearance between the piping material and inserted copper pipe for
Nitrogen in order to prevent backflow of the Nitrogen gas.
4When the Nitrogen gas is flowing, be sure to keep the piping end open.
5Adjust the flow rate of Nitrogen gas so that it is
lower than 0.05 m3/Hr or 0.02 Mpa (0.2 kgf/ cm2) by means of the reducing valve.
6After performing the steps above, keep the Nitrogen gas flowing until the pipe cools down to a certain extent (temperature at which pipes
are touchable with hands).
7Remove the flux completely after brazing.
MFlow meter
Stop valve
Nitrogen gas
cylinder
From Nitrogen cylinder
Pipe
Nitrogen gas
Rubber plug
Fig. 6-5-1 Prevention of oxidation during brazing
– 20 –