CAST IRON INOCULATION - Elkem

7.2 Coarsening of Inclusions
It has previously been discussed that
graphite nucleation occurs from nonmetallic
inclusions in the melt. A significant
coarsening of these inclusions
occurs within the time interval between
inoculation and solidification of the cast
iron. This coarsening of inclusions will
result in a reduction in the inclusion
number density, consequently reducing
the graphite nucleation frequency.
Hence, the fading of inoculation can
be explained by this coarsening of the
inclusion population with time. Due to
the coarsening, the total number of
possible nucleation sites for graphite
during solidification is reduced.
Figure 9 shows a plot of the number
density of inclusions in cast iron as a
function of holding time after inocu lation.
7.3 Effects of Various Inclusions
Inoculants lose their ability to reduce
chill and nucleate graphite if the metal
is held for extended periods before
casting. However, inoculants have
different fading characteristics. The
barium-based Barinoc ® inoculant
produces a high initial number of
nucleation sites throughout the holding
period, thus making it an excellent
inoculant for ladle treatments.
Foundrisil ® inoculant is an effective
chill reducer for both low and high
sulphur grey iron as well as ductile
iron. Another effective inoculant that
maintains the inoculation effect is the
strontium-containing Superseed ®
inoculant. Figure 10 shows the fading
characteristics of some inoculants in
cast iron.
Figure 9: Coarsening behaviour of
inclusions in liquid cast iron during
holding.
Figure 10: Fading characteristics for
various inoculants in cast iron.
8. Inoculation and Cast Iron Properties
8.1 Inoculation and Strength
Inoculation increases the number of
eutectic cells (or nodules) which leads
to a finer structure of the iron, and in
particular, this will cause an increase in
tensile strength in hypoeutectic irons.
Figure 11 shows the increase in tensile
strength by adding an inoculant.
8.2 Inoculation and Machinability
Inoculation increases the number of
potent nuclei that will promote graphite
nucleation at low undercooling. Improved
machinability is achieved by
inoculation suppressing the formation
of hard un-machinable white iron
structures. Inoculation also reduces
section sensitivity. While uninoculated
irons will show a wide variation in
hardness, inoculated grey or nodular
cast irons will show more consistent
hardness values over a wide range of
sections, Figure 12.
Figure 12: The wall thickness sensitivity
of (Brinell) hardness can be
reduced by an inoculant (partly calculated
from Rockwell -B* and -C**).
Figure 11: Increasing inoculant
additions improve tensile strength.
The final analyses of these trial melts
are identical after inoculation.