Treatment - Case hardening
Case hardening or Surface
hardening is the process of hardening the
surface of steel while leaving the interior
unchanged. The idea behind case hardening
is to have two different types of steel
in the same item. This allows a relatively
soft, tough core of a component to be combined
with a hard (but potentially brittle) surface.
Case hardening improves the wear resistance
of machine parts without affecting the tough
interior of the parts. Many processes are
available for surface hardening.
Both carbon and alloy steels
are suitable for case-hardening providing
their carbon content is low, usually less
than 0.2%. Case hardened steel is usually
formed by diffusing
carbon and/or nitrogen into the outer layer
of the steel at high temperature.
The term case hardening
is derived from the practicalities of the
process itself. The steel work piece (e.g.
a firing pin, the head of a rifle bolt,
or an engine cam shaft) is placed inside
a case packed tight with a carbon-based
case hardening compound. This is also known
as a carburizing pack. The pack is put inside
a hot furnace for a variable length of time.
Time and temperature determines how deep
into the surface the hardening extends.
However, the depth of hardening is ultimately
limited by the inability of carbon to diffuse
deeply into solid steel and a typical depth
of surface hardening with this method is
up to 1.5mm.
Another common application
of Case hardening is on screws, particularly
Self-Drilling Screws. In order for the screws
to be able to drill, cut and tap into other
materials like steel, the drill point and
the forming threads must be harder than
the material(s) that it is drilling into.
However if the whole screw is uniformly
hard, it will become very brittle and it
will break easily. This is overcomed by
ensuring that only the case is hardened
and the core remains relatively soft. For
screws and fasteners, case hardening is
less complicated as it is achieved by heating
and quenching in the form of heat treatment.