(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 chlo- ride and sodium fluoride to the
(4)Piping materials for brazing and used braz- ing 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 |
|
|
|
2-5-3. 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
•Attach a reducing valve and a
‚Use a copper pipe to direct the piping material, and attach a
ƒApply a seal onto the clearance between the piping material and inserted copper pipe for Nitrogen in order to prevent backflow of the Nitrogen gas.
„When the Nitrogen gas is flowing, be sure to keep the piping end open.
…Adjust the flow rate of Nitrogen gas so that it is lower than 0,05 m3/Hr or 0,02 MPa (0,2kgf/ cm2) by means of the reducing valve.
†After 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).
‡Remove the flux completely after brazing.
MFlow meter
Stop valve
•Do not enter flux into the refrigeration cycle.
‚When chlorine contained in the flux remains within the pipe, the lubricating oil deterio- rates. Therefore, use a flux which does not contain chlorine.
ƒWhen adding water to the flux, use water which does not contain chlorine (e.g. distilled water or
Nitrogen gas
cylinder
Pipe
From Nitrogen cylinder
Nitrogen gas
Rubber plug
„Remove the flux after brazing.
Fig. 2-5-1 Prevention of oxidation during brazing
– 13 –