Nitriding involves the diffusion of nitrogen into the surface
layers of a low carbon steel at elevated temperature. The
formation of nitrides in the nitrided layer provides the increased
hardness. Nitriding is typically carried out in the temperature
range of 500 - 575°C, this is in the ferritic state rather
than the austenitic used for
carburising.
This is possible since
ferrite
has a much higher solubility for nitrogen than it does
for carbon.
The advantage of nitriding in the ferritic state is that any
previous heat treatment of the steel component is not disrupted
and there is little / no distortion of the final component
shape. This means that higher carbon steels that have been
previously heat treated, such as through hardened steels which
have been
quenched
and tempered, can be surface hardened by nitriding. Nitriding
can be carried out using solid, liquid or gaseous media but
the most common is gas nitriding using
ammonia
(NH
3) gas as the nitrogen carrying species. To
get a hardening effect on nitriding the steel must contain
strong nitride forming elements such as Al, Cr and / or V.