CAST IRON INOCULATION - Elkem

CAST IRON
INOCULAtion
THE TECHNOLOGY OF GRAPHITE SHAPE CONTROL
ISO 9001 ISO 14001 ISO/TS 16949

CAST IRON INOCULATION
THE TECHNOLOGY OF GRAPHITE SHAPE CONTROL
Elkem manufactures and markets a
series of high quality inoculants to treat
cast iron and ensure the production of
an ideal graphite shape, distribution
and freedom from chill (cementite). All
inoculants are available in sizes suitable
for ladle or in-stream additions.
This brochure describes some of the
conditions in the production of cast iron
that call for the addition of an inoculant
to ensure the reliable production of a
sound, strong, tough, machinable casting.
The mechanism of inoculation and
graphite nucleation in cast iron during
solidification is also described.
What is Inoculation of Cast Iron
Inoculation is the means of controlling
structures and properties of cast iron
by minimizing undercooling and
increasing the number of nucleation
sites during solidification. An inoculant
is a material added to the liquid iron
just prior to casting that will provide
suitable sites for nucleation of graphite
during the subsequent cooling. Traditionally,
inoculants have been based
on graphite, ferrosilicon or calcium
silicide. Almost exclusively, inoculants
today are ferrosilicon based containing
small quantities of active elements
such as Al, Ba, Ca, Sr, Zr and RE (Rare
Earth metals).
The purpose of inoculation is to assist
in providing sufficient nucleation sites
for dissolved carbon to precipitate
as graphite rather than iron carbide
(cementite, Fe 3
C). This is done by preventing
undercooling below the metastable
eutectic temperature where
carbidic (white) structures are formed.
The iron solidification mechanism is
prone to form chilled iron structures
when the inoculation is inadequate.
There are several reasons why chilled
structures are normally undesirable.
They are hard and brittle and interfere
with machining, necessitate additional
heat treatment operations, resulting in
nonconformance with specifications
and, in general, increase the total cost
of production.
Inoculation changes the structure of
cast iron by altering the solidification
process. A look at the solidification
process for hypoeutectic grey iron
(iron with a carbon equivalent less
than 4.3) helps in understanding the
effect of inoculation.
The first metal to solidify in hypoeutectic
grey iron is primary austenite. As
cooling continues, the remaining iron
grows richer in dissolved carbon. Eventually,
the liquid reaches the eutectic
composition of 4.3% carbon equivalent,
at which final or eutectic solidification
would start under equilibrium conditions.
However, equilibrium solidification does
not occur under practical foundry conditions.
Due to variations in chemistry,
pouring temperature, solidification rate,
section thickness and other conditions,
the metal will cool below the eutectic
temperature before the start of final
solidification.
If the undercooling is slight, random
graphite flakes form uniformly in the
iron matrix, see Figure 1. This is known
as Type A graphite. As the undercooling
increases, the graphite will branch,
forming abnormal patterns. This is
known as Types B, D and E graphite.
A further increase in undercooling will
suppress the formation of graphite
and results in a hard white iron carbide
structure.
The role of the inoculant is to produce
nuclei in the liquid iron melt which
enhance the graphite nucleation with
a low degree of undercooling. This will
in turn, promote the formation of Type
A graphite structures in grey iron,
and a high number of small graphite
nodules in ductile iron.
Figure 1: Graphite type versus undercooling.
1. Structure and Phases in Cast Iron
The structure of cast iron has a
domi nant influence on strength and
machinability, and in order to obtain a
machinable grey iron structure for thin
sections, the addition of an inoculant
to molten iron is widely practiced
and often absolutely necessary. For
convenience, potential difficulties with
machinability can be determined by
carrying out a hardness test (Brinell
hardness) on iron castings and, in
general, machinability improves with
decreasing hardness. The cast iron
structure can be influenced at two
distinct stages in the production route:
• during solidification
• during heat treatment
However, for economic reasons, the
desired structure should be achieved
during solidification without the necessity
for heat treatment.

The microstructure of an iron casting
consists of several phases, each having
varying levels of carbon, iron and other
elements present. Table 1 shows the
analysis and specific densities of the
solid and liquid phases which take
part in the solidification process. When
solidification is complete, the following
combination of phases may be found:
1) Austenite + Graphite
= GREY structure
2) Austenite + Graphite + Cementite
= MOTTLED structure
3) Austenite + Cementite
= WHITE structure
This review demonstrates that solidifica
tion results in a minimum of two solid
phases; and austenite is present in all
the phase combinations. As the casting
cools, the austenite subsequently transforms
to pearlite and/or ferrite in solid
state (eutectoid transformation).
Of all the solid phases listed above,
cementite has the highest hardness
(~660 HB), whilst graphite is a relatively
soft material of low density, which
can act as a lubricant. Hardness and
machinability of the as-cast structure
are, therefore, influenced by the relative
amounts of cementite and graphite, with
austenite playing only a minor role.
Table 1: Approximate analysis and
specific densities of phases in the
solidification range of cast iron with
2.4% Si.
2. Structure Stability
A metastable white or mottled structure
can be transformed into a stable grey
structure by annealing, but the reverse
transformation is not possible as the
stable structure represents the lowest
possible energy level (at a given
temperature and composition). The
graphite produced by annealing will
have a different structure to that formed
during solidification. Cementite,
austenite and liquid iron have similar
densities and all contain carbon in
solution, see Table 1. No major redistribution
of the atom species is required
for a white structure to be produced
during solidification. However, the
formation of a stable grey structure
containing graphite is quite different.
Graphite precipitated from molten iron
is virtually pure carbon, and since it
has a lower specific density than the
alternative phases; a major redistribution
of atoms is required to develop
a stable structure. A slow rate of solidification
is therefore more likely to
produce a grey iron structure.
The precipitation of cementite, re quir ing
less atom redistribution than graphite,
will be more likely during rapid solidification.
This can be demonstrated by
examining a typical wedge test specimen.
The narrow tip of the wedge solidifies
at a faster rate than the thicker
section at the base of the wedge, and
will show a white structure whilst the
area of slow cooling at the base will
display a grey structure, see Figure 2.
Consequently, a slow rate of solidification
(slow cooling rate) and a small
value of undercooling encourages the
formation of a grey structure with good
machinability and discourages a hard
white structure.
Figure 2: Chill Wedge with fast solidifying
‘white’ tip and slowly cooled ‘grey’ base.
3. Influence of Elements on As-Cast Structure
Within the composition of cast iron,
graphitizing elements will promote the
carbon-carbon bond to produce graphite
in the as-cast structure, whereas
carbide stabilizing elements promote
the carboniron bond and cementite will
appear in the structure. Table 2 lists a
number of such stabilizing elements.
As an example, in malleable cast irons
the need for the as-cast structure to
solidify white determines that the silicon
level is much lower than in grey irons.
Also, since chromium is a carbide
promoting element, it has to be kept at
a low level to allow transformation to a
graphitic structure during subsequent
heat treatment. In normal furnace
charge materials, steel and external
cast iron scrap may be heterogeneous
materials, especially on different deliveries,
with contents of Cr, Cu, Sn, Sb, V,
Mo, Ti, etc., depending on the original
source and ultimately on the ability of
the scrap dealer.
Pig iron produced from steel scrap can
also display a similar variable response
to inoculation due to fluctuating trace
element contents. A more consistent
response to inoculation is attainable by
adopting a charge containing a reasonable
proportion of ore-based pig iron
due to its low level of trace elements of
the carbide stabilising type.
Controlling the concentration of trace
elements allows the foundryman a means
of promoting grey as-cast structures
and, also, helps in avoiding other undesirable
effects of trace elements on
microstructure and properties.
Table 2: Graphitizing and carbide
promoting elements.

4. Influence of Nuclei on Solidfication Structure
When crystallisation of eutectic cast
iron in chilled specimens is studied,
a gradual advance of the solidification
front is revealed. Transformation does not
take place instantaneously or uniformly
over a cross section. Initial solidification
occurs at the surface from distinct
crystallization centres and after some
time a solid/liquid interface forms. Other
isolated crystallisation centres are
active in the remaining melt and initiate
the formation of solid, see Figures 3
and 4. These isolated areas are called
eutectic cells.
Eventually, cells grow at the expense
of the liquid, and a solid cast structure
develops. Each eutectic cell consists of
graphite and austenite with graphite as
the primary phase.
Precipitation is initiated by randomly
distributed crystallisation centres, called
nuclei. These nuclei offer favourable sites
for the deposition of carbon atoms and,
subsequently, precipitation of graphite
and austenite onto existing graphite
continues. The morphology of these cells
for grey iron shows a marked difference
with that for nodular iron, as can be
seen from Figure 3.
• Grey iron: graphite lamellae start
growing from a common centre and
stays in contact with the melt as austenite
fills the spaces between
the lamellae.
• Nodular iron: a graphite nodule forms
first and is surrounded by austenite at
a later stage.
In eutectic nodular iron, the nodule
number is virtually identical with the
number of eutectic cells.
The mechanism described is for eutectic
solidification and is not influenced by
the presence of kish (primary) graphite
or austenite dendrites.
Austenite
Graphite
Austenite
Graphite
Figure 3: Eutetic cells: lamellar (top),
nodular (bottom) graphite .
The nuclei substances can be more easily
observed in nodular graphite iron than
in grey iron, since it is easy to locate the
centre of a graphite spheroid. Measurements
have shown that the nuclei are
between 0.5 to 2.0 microns in diameter,
with a bulk chemical composition of
magnesium sulphide and magnesium
silicate. A similar investigation of nuclei
composition for grey iron has shown that
the nucleus has a core of a complex
aluminium-X-oxide where X can be
Ca, Ba, Sr, Ce, Zr surrounded by the
manganese sulphide.
Figure 4: Solidification of near eutetic
iron 2 .
5. Prerequisites for Successful Inoculation
5.1 Number of Nuclei
About 2.4wt% graphite and 97.6wt%
austenite are formed during the crystallization
of eutectic (nodular) cast iron,
which corresponds to approximately
8 vol% graphite and 92 vol% austenite.
The mean diameter of graphite nodules
is usually between 10 – 80 microns,
although lower and higher values are
possible. This leads to about 3000 to
30,000 nodules per cubic millimetre
depending on the section size (cooling
rate) of the casting. The total number of
possible nuclei for graphite (inclusions)
will be at least one order of magnitude
larger than this graphite nodule density.
This means that the number of inclusions
or possible nuclei for graphite is at least
100,000 per cubic millimetre and that
only a small fraction of these nuclei
actually nucleates graphite during
solidification 1 . Table 3 gives examples
of number densities and mean particle
sizes for nuclei and graphite nodules in
ductile cast iron under various inoculation
conditions. As can be seen from
the table, the number and mean size of
nuclei particles are unaffected by the
inoculant addition, although the nodule
characteristics obtained after solidification
are strongly dependent on the
type of inoculant used. These findings
will be discussed below.
The calculations used to generate these
figures contain certain assumptions,
but one can safely conclude that:
• The number of nuclei per volume of
melt is extremely high, and approximately
one order of magnitude larger
than the number of graphite nodules
actually nucleated;
• The ability of the particles to nucleate
graphite is strongly affected by the
inoculant addition.
In order to obtain a nucleation event, a
certain degree of undercooling during
solidification is required. But since
different nuclei phases initiate graphite
nucleation at different undercooling
levels, it is preferable to have a large
number of nuclei particles which can
initiate nucleation at very small undercooling.
This is achieved by the addition
of an inoculant to the melt just prior to
casting.

5.2 Constituents of an inoculant
Most of the inoculant material is socalled
‘carrier’ material that is doped
with a minor additive (“nucleant”),
which produces nucleating particles
in the iron melt. These particles will, in
turn, initiate the crystallization of graphite.
The carrier (e.g. silicon and iron
combined as ferrosilicon) should have
the following characteristics:
• provide fast and homogeneous distri -
bution of the nucleant in the melt
• have a composition that is compat ible
to the analysis of the melt
• form an alloy between the nucleant
and the carrier
• be cost efficient
Trials using very pure ferrosilicon as an
inoculant have demonstrated that it
does not have any nucleating effect for
graphite 1,3,4 as shown in Table 3.
The nucleant, e.g. Ca, Sr, Ba or Al only
needs a limited presence and it is
beneficial if the nucleant forms an alloy
with the carrier. Also, the nucleant must
have a limited solubility in cast iron,
and form stable compounds with the
other elements forming the nuclei particles
(e.g. sulphur and oxygen). Good
nucleation effect may be achieved if
the ferrosilicon contains small but controlled
amounts of calcium, strontium or
barium in the range of 0.6 to 2.0%.
Table 3: Example of nuclei and nodule
number densities, average
diameters and volume fractions 1 .
5.3 Composition of the Nuclei
in Ductile Iron
Laboratory test results are used in this
section to explain the role of calcium
as an example of a trace element
behaving as the nucleant in ferrosilicon.
Calcium will occur in ferrosilicon as a
silicide (CaSi 2
). Calcium has virtually no
solubility in iron, and reacts with components
in the melt to form sulphides
and oxides.
In magnesium treated cast irons, the
inclusions contain mainly magnesium,
calcium, sulphur, silicon and oxygen.
These are primary reaction products
of the magnesium treatment. The
inclusions are composed of a sulphide
core and a faceted outer silicate shell.
The sulphide core contains both MgS
and CaS, while the outer shell consists
of complex magnesium silicates
(e.g. MgO•SiO 2
, 2MgO•SiO 2
). These
phases will not act as potent nucleation
sites for graphite during solidification
because of a large nucleus/ graphite
interfacial energy barrier. The interfacial
energy barrier is the controlling factor in
heterogeneous nucleation behaviour.
Figure 5: Transmission electron
micrograph of duplex sulphide/oxide
inclusion in ductile iron (left).
Schematic representation of an inclusion
after inoculation by a calcium
containing ferrosilicon. The surface
layer of calcium silicate is the effective
phase for graphite nucleation (right). 1

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.

5.5 Specification of Inoculants
The chemical composition and reliability
of the analysis from lot to lot is important
if a ferroalloy is to be considered as a
good and consistent inoculant. Many
foundrymen insist on silicon and phosphorus
analyses in pig iron, but pay little
attention to the analysis of the inoculant,
or vice versa. The preceding paragraphs
indicate quite clearly that the minor
constituents in ferroalloys, not the major
constituents (usually sili con), are critical
for the performance as inoculants. All the
Elkem inoculants are alloys that have
been smelted and alloyed to the quoted
specifications, and with the exception
of Ultraseed ® inoculant, no further additions
have to be mechanically blended
with the alloy. The analysis guaranteed
by the specification ensures consistent
inoculant properties from lot to lot. The
inoculants listed in Table 4 differ by
analysis, price and application. The
foundry experts of Elkem can give
detailed information on each inoculant
and its individual features, and also
suggestions as to the most suitable alloy
for a specific foundry condition.
5.6 Addition Technique
Chemical considerations alone will not
ensure satisfactory results since equal
attention must be paid to addition technique.
For ladle inoculation this means
a continuous addition of inoculant to the
stream of iron (normally added between
one third and two thirds of ladle filling)
so that the high turbulence encourages
fast and homogeneous distribution of
the alloy. Stream inoculation may be
practised, in conjunction with automatic
pouring furnaces, using finer sized
grades of the above inoculants at lower
addition rates. Similarly, inoculant fade
can be overcome by reducing the time
interval between the inoculant addition
and solidification by placing the inoculant
piece, or insert, into the gating
system. The reaction with liquid iron
occurs within the mould and this is
known as in-mould inoculation.
Fading is the reduction in inoculation
effect with increasing time taken to pour
inoculated iron. Elkem inoculants have
been assessed against untreated reference
melts and even after 10 minutes
the inoculation effect of the treated melt
proved to be good. Provided ladle inoculation
has been carried out in a satisfactory
way and the ladle is not delayed
for an excessive period before pouring,
the need for mould inoculation can be
avoided in most cases.
Table 4: Elkem preconditioner, inoculants and inserts for grey and ductile irons.
Alinoc ® , Barinoc ® , Elcast ® , Foundrisil ® ,Reseed ® , SMZ ® , Superseed ® , Ultraseed ® , Vaxon ® and Zircinoc ®
are registered trademarks owned by Elkem AS. Preseed is a trademark of Elkem AS.

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.

Table 5: Eutectic cell count (30 mm round bars).
Figure 8: Cupola melted grey iron; no
inoculation (left), inoculated with 0.2%
FeSi85% (centre), and inoculated with
0.125% Superseed ® inoculant (right).
7. Fading of Inoculation
7.1 Principle Effects
The effects of inoculation are at a maximum
immediately after the addition of the
inoculant. The rate of inoculant fading,
which depends upon the composition
of the inoculant and the condition of the
iron to which it is added, may be very
rapid and much of the inoculating effect
may be lost in the first few minutes after
the addition. The principal effects of
fading are:
• to cause greater undercooling to take
place during eutectic solidification
and to lead to a greater tendency to
chilling in grey and ductile cast irons,
particularly in thin sections;
• to reduce the number of nodules
formed in ductile iron and to cause a
deterioration in their shape. If suffic ient
ly severe, the deterioration in shape
may affect the mechanical properties
of the casting;
• to reduce the number of eutectic cells
growing in flake graphite irons result
ing in a less uniform size distribution
of graphite in the casting and a
reduction in mechanical properties.
There are some well established facts
concerning fading which are of practical
significance:
• all inoculants fade;
• there is no period after inoculation
during which fading does not occur.
To obtain the maximum effect, metal
should be cast as soon as possible
after the addition of inoculant;
• some inoculants fade more slowly
than others;
• inoculating effects vary according to
inoculant composition. It is desirable
that foundries should carry out tests
to determine which is the most suit
able inoculant for their purpose.

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.

9. Inoculation and Shrinkage
The solidification of grey iron is characterized
by the formation of a skin type
array of eutectic cells at the mould/metal
interface, followed by the development
of eutectic cells ahead of the advancing
solidification front. Newly formed graphite
compensates partly or fully for the liquid
iron contraction, provided it precipitates
within a relatively rigid “skin”, which is
charac teristic of uninoculated grey iron.
How ever, if the mode of solidification is
changed, the good shrinkage characteristics
can be jeopardized, especially if
a rigid skin cannot be developed at the
mould/metal interface leaving the mould
directly exposed to ferrostatic pressure.
Eventually, the mould may yield under the
ferrostatic pressure from the remaining
liquid, and the increased volume of the
mould cavity becomes too high for compensation
by graphite precipitation at the
end of solidification. Some shrinkage may
occur as a result of excessive dilation of
the mould although mould geometry
will have an influence.
Unfortunately, inoculation changes the
mode of solidification in such a way that
the rigidity of the “skin” is decreased.
Inoculant additions should not become
excessive to avoid shrinkage and yet the
addition should be adequate to ensure
“grey” solidification. Test specimens,
Figure 13, show that for an equivalent
chill depth, the eutectic cell count will be
lower when using Superseed ® inoculant
in place of foundry grade ferrosilicon.
The lower cell count reduces the ferrostatic
pressure on the mould and improves
the tendency to avoid shrinkage
defects.
Since the eutectic cell count for nodular
cast iron is much higher than for grey iron,
one would expect a greater shrinkage
tendency, and it is interesting to see
that the solidification pattern is in fact
similar to over-inoculated grey iron.
Ultraseed ® inoculant has proven highly
successful in providing fresh nucleation
sites to ductile irons of long holding time
where the base iron or magnesium
treated iron have been held for prolonged
times before addition of the
post inoculant. Such long hold times
are well known to reduce the overall
capabilities of the iron prior to inoculation
resulting in so-called “dead” iron.
Ultraseed ® inoculant will thus reinstall
good nucleation effectiveness from
reactions with its sulphur and oxygen
content forming new nucleation sites.
Due to the powerful effects of Ultraseed ®
inoculant on raising nodule count and
improving chill protection, it has been
found that the tendency to shrinkage
formation is also reduced with this inoculant.
Especially, the type of shrinkage
that often occurs as small porosities in
hot-spot sections of the complex castings;
appear to be effec tively reduced
or even eliminated by Ultraseed ® inoculant.
Figure 14 shows an example of
microshrinkage porosity that has been
minimized by the use of Ultraseed ®
inoculant.
Figure 13: Comparison of the eutectic
cell count in 5 mm sections at about
equal chill depth (from BCIRA).
Figure 14: Example of micro-shrinkage prorosity in ductile iron part that has been
minimized by Ultraseed ® inoculant (left), compared to manganese-zirconium
containing inoculant (right).

10. Product Development
Based on a comprehensive under standing
of the mechanisms of ino culation
described in this brochure, Elkem has,
over the years, evaluated many alternative
alloy analyses to develop the current
range of inoculants which includes the
well established Superseed ® ,Ultraseed ® ,
Reseed ® and Alinoc ® inoculants.
Development of new improved alloys
goes on continuously.
Recently, Preseed preconditioner
has been added to the portfolio as a
novel preconditioner to enhance inoculation
effect.
For further information on Elkem’s
extensive range of inoculants, please
contact your local representative. The
success of Elkem products worldwide
justifies their elaborate development
and provides a sound base for the
foundryman to select the appropriate
inoculant for his foundry’s particular
requirements.
References:
1) Skaland, T.: Ph.D Thesis, The Norwegian Inst. of Tech., 1992
2) Engler, S.: Giesserei, techn.-wiss.Beih., 17(1965), p 169/202
3) Moore, A.: Brit.Foundrym. 68 (1974) March, p59/69 Patterson, V.H; Foundry 100 (1972) June, p 68/71
4) Riposan et al: Investigation of the Effect of Residual Aluminium on Solidification Characteristics of Un-inoculated Ca/Sr-Inoculated Gray Irons. AFS 2004
Elkem AS
Foundry Products
Hoffsveien 65B
P.O. Box 5211
Majorstuen
N-0303, Oslo, Norway
Telephone : +47 22 45 01 00
Telefax : +47 22 45 01 52
www.foundry.elkem.com
Revised April 2012 © Copyright Elkem AS