Set screws are made of soft metal, but the points must be hardened
to resist the wear. This is done by heating the set screws red hot,
then sticking the point into the cyanide until it cools off. Re-heat
and repeat the work. During this time the iron is absorbing carbon.
Heat again, plunge into cold water. The carbon forms a case on the end
of the screw. Now you have a screw hard on the outside but of soft
material in the inside. It is able to resist any shock that carbon
steel itself could not stand.
The second method calls for the use of pulverized charcoal and bone.
The principle is the same as in the first method. The iron articles to
receive a case hardening are placed in a cast-iron box with a layer
of charcoal above and a layer of bone dust below, alternating in this
way until the box is filled to the top. The box is then placed in a
heating furnace and heated to a temperature 1200° F. It is kept at
this temperature from three to four hours. The articles all this time
are absorbing charcoal from bone dust. The box is taken out of the
furnace and plunged into a bath of cold water. From this cold bath
it is plunged into a bath of boiling water, then taken out and dried.
The pieces are oiled over slightly to bring out the colours. They have
not only a hard exterior after this process, but they have a beautiful
mottled surface of grays and blues, colours one often sees in parts of
small guns, wrenches, etc.
Surgical instruments, guns, small wrenches, etc., are hardened in this
way. The equipment for the second method is much more expensive than
that for the first; however, the principles involved are the same in
both cases.