CAST IRON INOCULATION - Elkem

After inoculation with a Ca-containing
ferrosilicon, hexagonal silicate phases of
the CaO•SiO 2
and the CaO•Al 2
O 3
• 2SiO 2
type will form at the surface of the existing
oxide inclusions produced during
nodularisation. These silicates will act
as very favourable nucleation sites for
graphite during solidification, due to
their hexagonal crystal structure, which
matches the graphite crystal lattice
very well (i.e. low energy interface).
Figure 5 shows a typical inclusion in
ductile cast iron which is formed after
nodularisation (left), and a schematic
representation of the inclusion composition
after inoculation (right). The surface
shell contains hexagonal calcium
silicates formed during inoculant addition,
while the bulk particle is a product
of the nodularisation treatment. Hence,
the inoculation does not increase the
total number of nuclei particles in the
melt, but rather modifies the surface of
the already existing products from nodularisation.
This explains why the number
density of particles in uninoculated
and inoculated ductile iron melts are
the same (Table 3), while the resulting
nodule numbers will differ greatly due
to the inclusion surface modification.
When inoculation is carried out with
a strontium or barium containing ferrosilicon
inoculating hexagonal silicates
equivalent to the calcium silicates
(CaO•SiO 2
and CaO•Al 2
O 3
•2SiO 2
) will
be formed (i.e. SrO•SiO 2
, SrO•Al 2
O 3
•2SiO, BaO•SiO 2
and BaO•Al 2
O 3
•2SiO 2
)
5.4 Composition of nuclei in grey iron
Recent research results have identified
a three step nucleation process for
generating graphite flakes in grey iron.
By means of electron microscope investigations,
it has been revealed that
a nucleus for a graphite flake consists
of a particle with a body of manganeseand
calcium-sulphide surrounding a
nucleus core of complex Al 2
O 3
–XO
oxides, see Figure 6. The core oxide
contains elements such as calcium,
barium, strontium, zirconium, and rare
earth elements. Towards the surface
of the manganese/calcium-sulphide
body, even more complex compounds
have been observed on which the
graphite has grown.
The hypothesis is that the oxides form
as stable elements in the iron melt first.
Secondly, manganese and calcium
sulphides grow on these oxides until
a desired size and a more complex
faceted compound appears on the surface.
The third step is that the graphite
starts to grow on this faceted surface
and grows along its base planes of
hexagonal structure.
One interesting observation was that
aluminium seems to play a key role in
the nucleation process in conjunction
with other elements. Testing of iron with
very low levels of aluminium showed
poorer performance than iron with a
certain level of aluminium. It can be
concluded that final content of aluminium
in grey iron should be between
0.005 and 0.010% in order to maximise
eutectic cell count in grey iron. This
aluminium content range is and has to
be less than the 0.015 – 0.25% Al, as
this range for pin-hole susceptibility
influenced by aluminium.
As a result of these observations, Elkem
has invented the Preseed preconditioner
that contains zirconium and
aluminium, to be added to the iron melt
in the furnace or well ahead of inoculation,
in order to increase the potency
of the melt for inoculation
Figure 6: Transmission electron micrographs of complex sulphide/oxide inclusion in grey iron and profile of chemical
composition through the nucleus.