Nitriding Carbonitriding Cyaniding
A very hard, thin case can be produced by exposing an
already quenched and tempered steel to an ammonia atmosphere at
about 510 to 540°C, but unfortunately for periods of 50 to 90 h. The
nitrogen diffuses into the steel and combines with strong nitride formers
such as aluminum and chromium, which are characteristically present in
steels where this process is to be used. The nitrides are small and finely
dispersed; since quenching is not necessary after nitriding, dimensional
control is excellent and cracking is not an issue. The core properties do
not change since tempering at 550°C or higher temperature has already
taken place. A typical steel composition is as follows: C, 0.2 to 0.3
percent; Mn, 0.04 to 0.6 percent; Al, 0.9 to 1.4 percent; Cr, 0.9 to 1.4
percent; and Mo, 0.15 to 0.25 percent.
Carbonitriding (Cyaniding) An interesting intermediate which rapidly
adds both carbon and nitrogen to steels can be obtained by immersing
parts in a cyanide bath just above the critical temperature of the core
followed by direct quenching. A layer of about 0.25 mm (0.010 in) can
be obtained in 1 h. The nitrides add to the wear resistance.
Local Surface Hardening For some parts which do not readily fit in
a furnace, the surface can be hardened preferentially by local heating
using flames, induction coils, electron beams, or lasers. The operation
requires skill and experience, but in proper hands it can result in very
good local control of structure, including the development of favorable
surface compressive stresses to improve fatigue resistance.
Clad steels can be produced by one of several methods, including
simple cladding by rolling a sandwich out of contact with air at a temperature
high enough to bond (1,200°C); by explosive cladding where
the geometry is such that the energy of the explosive causes a narrow
molten zone to traverse along the interface and provide a good fusion
bond; and by various casting and welding processes which can deposit a
wide variety of materials (ranging from economical, tough, corrosion

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