CAST IRON INOCULATION - Elkem

6. Control of Inoculation
Although nuclei cannot be observed
directly at solidification temperatures,
they have an effect on some properties
which can be measured by:
• recording cooling curves
• measuring depth of chill in chill
wedges
• counting the number of eutectic cells
• counting the number of graphite
nodules
6.1 Cooling Curves
Cooling curves record the changes
in temperature with time as a consequence
of a change of energy within
the system. A deviation from normal
cooling indicates the occurrence of a
source of heat such as the heat of
crystallization released by a precipitating
phase. The location of the inversion
points on the generally S-shaped
cooling curve in the region of eutectic
crystallization indicates the tendency
of the melt to solidify “grey” or “white”.
A high level of nucleation promotes a
higher arrest temperature which, by
avoiding the white eutectic, will result
in less risk of carbide formation.
Conversely, when the inversion point is
at a low level on the cooling curve, there
will be a tendency for cementite to
precipitate instead of graphite giving a
“white” structure. An increased cooling
rate, as found in thin sections, will increase
the degree of undercooling that
must be balanced by an increased
number of active nuclei to avoid the
formation of white iron. In the iron-carbon
system there is only a 7 ºC interval
between “grey” solidification and sufficient
undercooling to cause “white”
solidification. In Figure 7 the cooling
curve for an uni noculated reference
melt is compared with a curve from a
melt inoculated with 0.25% inoculant
addition.
The uninoculated melt shows inversion
at 1145 ºC whereas inversion occurs
at 1162 ºC for the inoculated melt. This
means that the uninoculated melt is
undercooled by 20 ºC and the inoculated
melt by 3 ºC, which gives “white” and
“grey” solidification, respectively.
Figure 7: Solidfication curves for
uninoculated ductile iron (a), and
inoculated ductile cast iron (b)
(30 mm section size).
6.2 Chill Testing
The traditional method to determine the
tendency of a melt to solidify “grey” or
“white” is by examining chill wedges.
The larger the zone of white iron, the
fewer the number of nuclei that were
active in initiating a “grey” solidification.
Figure 8 shows chill wedges from a
foundry which had an average 11.2 mm
of chill for a period of two week on uninoculated
cupola iron. By adding 0.2%
FeSi (85% Si), the average chill depth
was reduced and with 0.125% Superseed
® inoculant addition, the chill depth
was reduced even further.
6.3 Eutectic Cell Count
The number of eutectic cells in grey iron
can be determined on etched microspecimens.
If an effective inoculant has
been added to the melt, there will be a
large number of active nuclei to promote
graphite precipitation at low undercooling
during solidification. This will
be represented on the micro-specimen
by a high cell count for grey iron and
a high nodule count for ductile iron.
Table 5 shows the result of cell counts
after inoculation. The eutectic cell
number increases as the inoculant
addition to the base melt is increased.
Other factors, such as over-inoculation
leading to shrinkage proprensity, will influence
the optimum inoculant addition.