Hard soldering requires
temperatures of 1200 to 1400 degrees F. and produces shearing strength
in the range of 30,000 to 60,000 pounds per square inch.
The hard
soldering alloy for use on steel or brass contains silver, copper, zinc,
and cadmium.
A flux is required and is usually applied to the joint in
the form of a paste typically made from borax, boracic acid and zinc
chloride.
An inexpensive propane torch may be used for most small jobs
in model aircraft work. However, one of the newer high temperature
designs incorporating a pressure regulator will prove more useful. These
usually include a large and a small burner assembly. These torches use
either propane or MAPP gas in 14 ounce bottles. The MAPP gas flame is
about 400 degrees F. hotter than the propane flame and may be necessary
on jobs involving more massive parts to be joined.
Hard soldering must be done in a well ventilated space.
All equipment and supplier's rules and warnings
should be followed.
Several combinations of dissimilar metals may also be hard soldered
such as steel or brass. All joints must be closely fitted since the
molten hard solder filler metal is distributed in the joint by capillary
action. In this case, it means joint clearance in the range of 0.002 to
0.006 inch.
It is very important that joints be clean. A rotary
wire wheel, steel wool and emery cloth are very useful in getting a good
clean finish on the joint after all dirt, oil and etc. are removed.
Any
contaminant left in the joint may result in an imperfect joint and
difficult removal of the flux residue. Flux removal from a clean joint
simply requires a good scrubbing under hot water. Flux removal from a
dirty joint is just like chipping glass since the flux captures the dirt
and makes a glass hard coating on the joint. All flux must be removed to
prevent corrosion.
