Unlike carbon rich steels which can be
hardened by heating, mild steels do not contain enough carbon to perform this operation.
Instead carbon is forced into the skin of the metal to produce a piece of mild steel which
has an outer casing containing more carbon than its core. This process is known as case
hardening. The operation is simple. The mild steel is heated to a bright red and whilst it
is softer it is dipped into a carbon compound which soaks into the outer surface. One
dipping is not enough and re-heating and dipping should be done several times. This will
produce a carbon rich skin or case of about 1mm thick. Once the casing has been produced
the metal is heated to a bright red and this time dipped in water to harden it.
Case hardening can be useful for making pieces which need to be hard on the outside to
resist wear and tear, but softer on the inside to resist shock or sudden blows.