Once
the austenitizing process has been carried out, a particular thermal treatment
must be made in order to get some metallographic and mechanical characteristics
that are optimum for the wires. This operation is called isothermal
annealing.
It consists in the crossing at a well defined temperature of
the TTT curves for a well determined steel in order to get after the full transformation
of austenite a fine perlitic structure of the sorbitic type with traces of weak
superior bainite reaching a maximum of 2
to 3%. FIB has developed two technologies
in this field.
One of these is based on the quenching in a bath of molten
lead in which cooling systems allow a perfect quenching of wires in a range
of 0,8 mm up to 18
mm diameter.
Aware of the environmental requirements, FIB has also developed
a technology in fluidized bed thanks to which sorbitic structures can be obtained
while freeing oneself from the constraints linked with the use of lead.