6-5-2.Flux; 6-5-3.Brazing - Toshiba HWT-601XWHM3W-ETR Service Manual

6-5-2. Flux
(1) Reason why flux is necessary
• By removing the oxide film and any foreign matter on
the metal surface, it assists the flow of brazing filler.
• In the brazing process, it prevents the metal surface
from being oxidized.
• By reducing the brazing filler's surface tension, the
brazing filler adheres better to the treated metal.
(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 borax-boric acid compound.
(4) Piping materials for brazing and used
brazing filler / flux
Piping material Used brazing filler
Copper - Copper Phosphor copper
Copper - Iron
Iron - Iron
NOTE
(1) Do 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
chlorine.
(3) When adding water to the flux, use water which
does not contain chlorine (e.g. distilled water or ion-
exchange water).
(4) Remove the flux after brazing.
Used flux
Do not use
Silver Paste flux
Silver Vapour flux
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 flow.
Never use gas other than Nitrogen gas.
(1) Brazing method to prevent oxidation
1) Attach a reducing valve and a flow-meter to the
Nitrogen gas cylinder.
2) Use a copper pipe to direct the piping material,
and attach a flow-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 backflow of the
Nitrogen gas.
4) When the Nitrogen gas is flowing, be sure to
keep the piping end open.
5) Adjust 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.
6) 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).
7) Remove the flux completely after brazing.
Nitrogen gas
cylinder
Pipe
Fig. 6-5-1 Prevention of oxidation during brazing
46
M
Flow meter
Stop valve
From Nitrogen cylinder
Rubber plug
Nitrogen
gas