Study the carbonizing and case hardening of steel.
Surface Hardening Treatment
(Case
Hardening)
Many applications require high hardness or strength primarily at the
surface, and complex service stresses frequently require not only a hard,
wear-resistant surface, but also core strength and toughness to withstand
impact stress.
To achieve these different properties, two general processes are used:
1) The chemical composition
of the surface is altered, prior to or after quenching and tempering; the processes used include
carburizing, nitriding, cyaniding, and carbonitriding; and
2) Only the surface layer
is hardened by the heating and quenching process; the most common processes
used for surface hardening are flame hardening and induction hardening.
Carburizing: Carbon is diffused into
the part’s surface to a controlled depth by heating the part in a carbonaceous
medium. The resulting depth of carburization, commonly referred to as case
depth, depends on the carbon potential of the medium used and the time and
temperature of the carburizing treatment. The steels most suitable for carburizing
to enhance toughness are those with sufficiently low carbon contents, usually
below 0.3 percent. Carburizing temperatures range from 1550 to 1750°F (843 to
954°C), with the temperature and time at temperature adjusted to obtain various
case depths. Steel selection, hardenability, and type of quench are determined
by section size, desired core hardness, and service requirements.
Three types of carburizing are most often used: -
Liquid carburizing involves heating the steel in molten barium cyanide or sodium
cyanide. The case absorbs some nitrogen in addition to carbon, thus enhancing
surface hardness.
Gas carburizing involves heating the steel in a gas of controlled carbon content.
When used, the carbon level in the case can be closely controlled.
Pack carburizing, which involves sealing both the steel and solid carbonaceous material
in a gas-tight container, then heating this combination.
