ight
red, dip it in cyanide of potassium (or cover it by sprinkling
the cyanide over it), keep it hot until the melted cyanide covers
it thoroughly, and quench in water. Carbon and nitrogen enter the
outer skin of the steel and harden this skin but leave the center
soft. The hard surface or "case" varies in thickness according to
the size of the piece, the materials used and the length of time
which the piece remains at the carburizing temperature. Cyanide
case-hardening is used only where a light or thin skin is sufficient.
It gives a thickness of about 0.002 in.

In some cases of cyanide carburizing, the piece is heated in cyanide
to the desired temperature and then quenched. For a thicker case
the steel is packed in carbon materials of various kinds such as
burnt leather scraps, charcoal, granulated bone or some of the
many carbonizing compounds.

Machined or forged steel parts are packed with case-hardening material
in metal boxes and subjected to a red heat. Under such conditions,
carbon is absorbed by the steel surfaces, and a carburized case is
produced capable of responding to ordinary hardening and tempering
operations, the core meanwhile retaining its original softness and
toughness.

Such case-hardened parts are stronger, cheaper, and more serviceable
than similar parts made of tool steel. The tough core resists breakage
by shock. The hardened case resists wear from friction. The low cost
of material, the ease of manufacture, and the lessened breakage
in quenching all serve to promote cheap production.

For successful carburizing, the following points should be carefully
observed:

The utmost care should be used in the selection of pots for carburizing;
they should be as free as possible from both scaling and warping.
These two requirements eliminate the cast iron pot, although many
are used, thus leaving us to select from malleable castings, wrought
iron, cast steel, and special alloys, such as nichrome or silchrome.
If first cost is not important, it will prove cheaper in the end
to use pots of some special alloy.

[Illustrations: FIGS. 27 to 30.--Case-hardening or carburizing boxes.]

[Illustration: FIG. 31.--A lid that is easily luted.]

The pots should be standardized to suit the product. Pots should be
made as small as possible in width, and space gained by increasing
the height; for it takes about 1-1/2 hr. to heat the average small
pot of 4 in. in width, between 3 and 4 hr. to heat to the c