Casting of molten iron and filters for use therein

Molten iron is cast into a mould through a filter located in the runner system of the mould using a filter comprising a body having a plurality of cells, at least some of the cells having their walls at least partially coated with a first layer of wax and a second layer of an inoculant, such as graphite, calcium silicide or ferrosilicon, for the iron.

This invention relates to the casting of molten iron in a mould and to 
filters for use therein. 
When molten iron is treated with an inoculant prior to casting there is a 
tendency for the effect of the inoculant to be diminished, (known as 
"fading"), before the metal is cast into moulds. Various methods have 
therefore been proposed for inoculating molten iron as late as possible in 
the casting process, either by treating the iron just before it enters the 
mould or by treating the iron in the mould itself. 
An inoculant for iron is a substance which when added to molten iron will 
form nuclei for crystallisation when the iron solidifies on casting. By 
creating favourable conditions for solidification the inoculant controls 
the graphite structure or morphology, eliminates or reduces the formation 
of iron carbides known as chill, increases the eutectic cell or nodule 
count, reduces casting section sensitivity and prevents undercooling. 
Inoculation in the mould involves placing the inoculant at a point in the 
runner system, preferably as near to the mould cavity as possible, so that 
the molten iron is treated as it flows through the runner system. 
Attempts have been made to utilise an inoculant in the form of fine 
particles, for example fine particles of ferrosilicon for inoculating grey 
cast iron or spheroidal graphite iron, but they have not been successful 
because the particles of inoculant tend to get washed into the mould 
cavity where they can form inclusions in the casting produced when the 
molten iron solidifies, and because there is a tendency for castings 
having variations in their microstructure to be produced. 
In order to overcome the problems associated with the use of fine particles 
methods have been proposed which utilise inserts made of bonded, 
compressed or sintered particulate inoculants, over which or through which 
the molten iron flows and in one such method the insert rests on a 
strainer core. However, none of these methods has been wholly successful 
and none has achieved wide commercial use. Cast inserts have also been 
used but because they tend to shatter under the influence of thermal shock 
they can give rise to inclusions in the castings. 
When casting molten iron into moulds it is often desirable to include in 
the mould some means for preventing inclusions from being incorporated in 
castings produced in the moulds. 
With grey and malleable irons inclusions can be formed due to refractory 
particles and/or slag being carried over from a furnace or a ladle into 
the mould cavity or due to particles of sand from the runner system of a 
sand mould being washed into the mould cavity. 
Inclusions are most prevalent in ductile or nodular irons because in 
addition sticky magnesium silicate slags, often associated with particles 
of magnesium oxide and magnesium sulphide, are formed during the 
nodularising process and these are difficult to remove prior to pouring 
the molten metal into the mould, even through special precautions such as 
a fluxing treatment, the use of a teapot ladle or the use of a specially 
designed runner system incorporating slag traps are adopted. 
Strainer cores are often used in moulds in malleable and grey iron 
foundries, but their principal function is as a means for controlling the 
flow of molten iron into the mould and they have only a limited filtering 
effect. 
In recent years it has become common practice to incorporate cellular 
ceramic filters in moulds for casting ferrous metals. European Patent 
Application Publication 0234825 describes a process for casting molten 
ferrous metal in a mould in which molten ferrous metal is poured into a 
mould having a ceramic filter having an open-cell foam structure located 
in the runner of the mould, and a sealed plastics container containing 
particles of a treatment agent for the molten ferrous metal located in a 
chamber in the runner system on that side of the filter which is further 
from the mould cavity, such that part of the container is in the sprue 
well, so that molten ferrous metal is treated by the treatment agent 
before flowing through the filter and into the mould cavity. 
According to the present invention, there is provided a process for casting 
molten iron in a mould comprising providing a mould having a mould cavity 
and a runner system, locating in the runner system a filter having a 
plurality of cells, at least some of the cells having their walls at least 
partially coated with an inoculant for the iron, and pouring molten iron 
into the mould so that the iron passes through the filter and into the 
mould cavity. 
According to a further feature of the invention there is provided a filter 
for filtering molten iron comprising a body having a plurality of cells, 
at least at some of the cells having their walls at least partially coated 
with an inoculant for the molten iron. 
The body forming the filter may be for example a ceramic body having a 
honeycomb type of structure having cells extending between opposite faces 
of the body, a porous pressed ceramic body, or an open-cell ceramic foam. 
An open-cell ceramic foam is preferred. 
Ceramic honeycomb structured bodies can be made by extruding material 
through a die having an outlet face provided with a gridwork of 
interconnected discharge slots and an inlet face provided with a plurality 
of feed openings extending partially through the die in communication with 
the discharge slots and drying and firing the honeycomb structure 
so-formed. The production of ceramic honeycomb structures by such a method 
is described in U.S. Pat. No. 3790654. 
Open-cell ceramic foams which are suitable for use as filters for molten 
ferrous metals may conveniently be made by impregnating an organic foam, 
such as recticulated polyurethane foam, with an aqueous slurry of ceramic 
material containing a binder, drying the impregnated foam to remove water 
and then firing the dried impregnated foam to burn off the organic foam to 
produce a ceramic foam replica. The production of ceramic foams by such a 
method is described in U.S. Pat. No. 3,090,094, in British Patents 923862, 
916784, 1004352, 1054421, 1377691, 1388911, 1388912 and 1388913 and in 
European Patent Application Publication 0074978. 
The material used for the ceramic filter must withstand the temperature of 
and be resistant to molten iron and suitable materials include alumina, 
high alumina content silicates such as sillimanite, mullite and burned 
fireclay, silicon carbide and mixtures thereof. 
Examples of suitable inoculants are graphite, calcium silicide and 
ferrosilicon, usually containing 50-85% by weight silicon and small 
quantities of calcium and/or aluminium. Special types of ferrosilicon 
containing other elements such as titanium, chromium, zirconium, 
manganese, copper, bismuth, alkaline earths such as barium or strontium, 
or rare earths such as cerium, may also be used. If desired one or more of 
the elements listed above may be used in conjunction with an inoculant 
such as ferrosilicon and either mixed with the ferrosilicon and applied to 
the filter so as to constitute a single inoculant layer or applied to the 
filter on top of the ferrosilicon so as to constitute a second inoculant 
layer. 
The size of the particles of inoculant may be up to about 10 mm but 
preferably particles having a narrow size range of less than 6 mm, more 
preferably 0.05 mm-2 mm, are used. Relatively large particles tend to 
produce slower fading of the inoculation effect because they dissolve in 
the molten iron relatively slowly but they may produce insufficient 
nucleation sites. Relatively small particles produce sufficient nucleation 
sites but because they dissolve faster they tend to produce more rapid 
fading. 
The cells of the filter may be coated with the inoculant by a variety of 
techniques such as plasma spraying, coating using a dispersion of 
particulate inoculant in a suitable medium or preferably by coating with a 
first layer of an adhesive and a second layer of particulate inoculant. 
When a dispersion of inoculant is used particles of the inoculant may be 
dispersed in water or in an organic carrier liquid, containing a binder, 
and the dispersion can be applied as a coating to the cell walls of the 
cellular body by, for example, spraying or dipping the body in the 
dispersion. After the coating has been applied it is dried to remove the 
water or organic carrier liquid. 
Alternatively the particles of treatment agent may be dispersed in a medium 
of wax or a substance having a physical characteristics of wax. The use of 
such dispersions in the treatment of molten ferrous metals is described in 
British Patents 1105028 and 1257168 and suitable media include natural 
waxes such as beeswax, carnauba wax or montan wax, paraffin wax, fatty 
acids such as stearic acid and fatty acid esters such as stearates. The 
particles of treatment agent are added to the medium which has been heated 
so that it is liquid and are dispersed, and the dispersion is then applied 
to the cell walls of the cellular body by for example, spraying, pouring 
or by dipping the cellular body in the dispersion. After application the 
dispersion is allowed to cool and an adherent coating of the inoculant is 
obtained. 
In the preferred embodiment in which the cell walls are first coated with 
an adhesive, the adhesive may be any type of adhesive which will remain 
tacky after application to the cell walls of the filter. The adhesive may 
be for example a wax or a substance having the physical characteristics of 
a wax such as the materials listed above. Such adhesives may be applied to 
the filter by heating the adhesive until it is liquid and then spraying it 
into the filter or dipping the filter into the liquid adhesive and 
draining off excess adhesive. The adhesive may also be a resin such as an 
acrylic resin which can be applied to the filter in the form of a 
dispersion or a solution in a liquid medium such as water or an organic 
solvent by spraying or dipping and then drying to remove the liquid 
medium. 
The inoculant particles may be applied to the adhesive-coated cell walls of 
the filter for example by dropping the particles through the filter under 
gravity or by blowing the particles into the filter using compressed air, 
and allowing excess inoculant to pass through the filter. The inoculant 
particles may also be applied to the filter by immersing an 
adhesive-coated filter in a fluidised bed of the inoculant particles. 
If desired the particles of inoculant may be encapsulated in a material 
which will retard the dissolution rate of the inoculant in the molten 
ferrous metal. 
The inoculant-coated filters of the invention may take a number of forms. 
For example the whole wall surface of all the cells may be coated, part 
only of some of the cell walls may be coated or some of the cells may be 
filled with inoculant throughout the whole or only part of the thickness 
of the filter. Depending on the form which it is desired to achieve, 
certain area of the cellular body may be masked when the inoculant is 
applied or the cellular body may be only partially immersed in the 
inoculant dispersion or precoating adhesive. 
The thickness of the coating of inoculant may be controlled for example, by 
controlling the time the cellular body is immersed in the inoculant 
dispersion or by removing excess dispersion after application. 
The pick-up of inoculant by the filter will be dependent on the surface 
area of the filter cell walls and on the particle size of the inoculant 
used. For example for a rectangular ceramic foam filter 75 mm long, 50 mm 
wide and 22 mm thick having 4 pores per linear cm and weighing 38-40 g the 
inoculant coating using an inoculant of particle size 0.2 mm-0.5 mm is 
32-35 g. For a similar filter of 8 pores per linear cm the amount of 
inoculant coating using the same inoculant is 20-25 g. 
In use the inoculant-coated filter is located in the runner system of a 
mould, preferably as near to the mould cavity as possible and molten iron 
metal is poured into the mould so that it flows through the filter in 
which the iron is inoculated and inclusions are removed from the iron 
before flowing into the mould cavity. 
The filter of the invention offers the following advantages: 
1) It enables the use of a single method of applying both a filter and an 
inoculant in a mould cavity. 
2) It provides a substrate with a high surface area which permits rapid and 
uniform distribution of an inoculant in a metal stream and a reduction in 
the amount of inoculant required for effective treatment. 
3) It eliminates the separate manufacturing operation needed to produce 
bonded or cast inoculants and the need to place such inoculants in the 
mould cavity. 
4) Incorporation of an inoculant with a filter reduces casting inclusions 
caused by undissolved inoculant, oxidised inoculant or alloy slags. 
5) The filter is adaptable to automatic placement in a mould thus reducing 
manpower requirements. 
The following examples will serve to illustrate the invention:

Referring to the drawings the mould consists of a sprue 1, a sprue well 2, 
a runner 3, having a print 4 capable of accepting a 75 mm.times.50 mm 
rectangular filter 5 of 22 mm thickness, and 10 vertical mould cavities 
6A-6J to produce castings 1-10 interconnected so that when molten iron is 
poured into the mould and passes through the filter the vertical mould 
cavities 6A-6J fill sequentially. Each of the test bar mould cavities 6A-J 
is connected to three small cavities 7A-7J for producing chill pieces of 
cast iron. As each of the test bar cavities 6A-6J fill with molten iron so 
do the chill piece cavities 7A-7J and the iron in the chill piece cavities 
7A-7J solidifies instantaneously. 
A rectangular ceramic foam filter of silicon carbide, alumina and silica, 
and bonded by aluminium orthophosphate, having a size of 75 mm.times.50 
mm.times.22 mm and 4 pores per linear cm was inserted into the print 4 of 
one of the moulds, and an inoculant-coated filter according to the 
invention was inserted into the print 4 of the other mould. 
The filter used in the second mould was the same composition and size as 
the filter used in the first mould and its cell walls were coated with 
montan wax by dipping the filter in molten montan wax and then with 
inoculant by allowing particles of the inoculant to fall through the 
filter under gravity. The inoculant used had a nominal composition by 
weight of 65% silicon, 1.4% aluminium 1.4% calcium, 4.0% manganese, 3.75% 
zirconium and balance iron, and a particle size of 0.2 mm to 0.5 mm. The 
uncoated filter weighed 39.7 g and the amount of inoculant material 
carried by the filter after coating was 36.2 g. 
A charge of refined pig iron and steel scrap was melted in a medium 
frequency induction furnace and heated at 1500.degree. C. The molten iron 
was tapped into a clean pre-heated ladle containing a 2.9% by weight 
addition of magnesium-ferrosilicon (5% by weight magnesium) based on the 
weight of iron to produce spheroidal graphite iron. The iron was then 
inoculated by the addition of 0.4% by weight based on the weight of iron 
of foundry grade ferrosilicon. 
The analysis of the treated iron was: 
carbon--3.60% 
silicon--2.30% 
sulphur--0.005% 
magnesium--0.054% 
manganese--0.062% 
phosphorus --0.023%. 
The iron was poured from the ladle into the two moulds at a temperature of 
1410.degree.-1430.degree. C. The castings produced each of which weighed 
40 kg were allowed to solidify and cool, and after the sand had been 
removed from them the chill pieces were removed from each of the ten test 
bars. 
The central chill pieces were sectioned at right angles to the fractured 
face along their length, and the cut face of one of the sections was 
prepared and examined microscopically in order to measure the nodule count 
(number of graphite nodules per mm.sup.2). 
The results obtained for the nodule count of chill pieces taken from 
different test bars are recorded in Table 1 below. 
TABLE 1 
______________________________________ 
CASTING FROM CASTING FROM 
MOULD WITH MOULD WITH 
TEST UNCOATED FILTER - 
INOCULANT COATED 
BAR NODULE COUNT FILTER - NODULE 
No. PER MM.sup.2 COUNT PER MM.sup.2 
______________________________________ 
1 151 1048 
2 -- 551 
3 192 443 
4 -- 320 
5 185 310 
7 180 324 
9 177 291 
______________________________________ 
Using the test mould shown in the drawings and described above highly 
effective inoculation will produce a high nodule count in the chill pieces 
from all ten of the test bars. As the effectiveness of inoculation 
decrease so the nodule count decreases and fewer of the test bars. As the 
effectiveness of inoculation decrease so the nodule count decreases and 
fewer of the bars contain acceptable nodule numbers. Hence it is possible 
to assess the effectiveness of in-mould inoculation by estimating in terms 
of test bar number the point at which effective inoculation ends. In the 
present tests the filter coated with inoculant gave a higher nodule count 
for all the test bars compared to the nodule count of the test bars of the 
casting produced without inoculation in the mould. 
EXAMPLE 2 
Two moulds as shown in the drawings and the procedure described in Example 
1 were used to determine the effectiveness of a filter coated with a 
mixture of ferrosilicon and copper as an in - mould inoculant. 
One mould contained a ceramic foam filter of the type used in Example 1 and 
the other contained a similar ceramic foam filter which had been coated 
with montan wax and then with a mixture of 80% by weight of the inoculant 
used in Example 1 and 20% by weight copper powder of 99% purity and 0.5-1 
mm particle size. The uncoated filter weighed 39.5 g and the amount of 
inoculant material carried by the filter after coating was 32.7 g. 
Molten spheroidal graphite iron which had not been inoculated was poured 
from a ladle into the moulds at a temperature of 1410-1430.degree. C. The 
analysis of the iron was: 
carbon--3.50% 
silicon--2.26% 
sulphur--0.008% 
magnesium--0.032% 
manganese--0.089% 
phosphorus--0.022%. 
Chill pieces from the resultant casting were prepared as described in 
Example 1 and their nodule count determined. The results obtained for the 
central chill pieces from different test bars are tabulated in Table 2 
below. 
TABLE 2 
______________________________________ 
CASTING FROM CASTING FROM 
MOULD WITH MOULD WITH 
TEST UNCOATED FILTER - 
INOCULANT COATED 
BAR NODULE COUNT FILTER - NODULE 
No. PER MM.sup.2 COUNT PER MM.sup.2 
______________________________________ 
1 146 841 
2 -- 718 
3 181 400 
4 -- 335 
5 176 223 
7 222 323 
9 208 248 
______________________________________ 
As the results show the filter coated with inoculant gave a higher nodule 
counter for all the test bars compared to the nodule count of the test 
bars of the casting produced without inoculation in the mould. 
EXAMPLE 3 
Two test moulds in phenol-formaldehyde resin bonded silica sand were 
produced as shown in the accompanying drawings except that the print 4 was 
dimensioned so as to accept a 55 mm.times.55 mm square filter of 12 mm 
thickness. 
A cordierite/mullite extruded ceramic filter having 40 cells per cm.sup.2 
was inserted into the print of one of the moulds, and an inoculant coated 
filter according to the invention was inserted into the print of the other 
mould. 
The filter used in the second mould was the same composition as the filter 
used in the first mould and its cell walls were coated by dipping the 
filter into a dispersion consisting of 75% by weight ferrosilicon in 25% 
by weight paraffin wax. The ferrosilicon used had a nominal composition of 
75% silicon, 0.3-1.0% calcium, 1.5-2.0% aluminium and balance iron, and a 
particle size of less then 75 microns. The uncoated filter weighed 23.1 g 
and the amount of inoculant and wax carried by the coated filter was 20.7 
g. 
A charge of refined pig iron and steel scrap was melted in a medium 
frequency induction furnace and heated to 1500.degree. C. The molten iron 
was tapped into a clean pre-heated ladle containing a 2.9% by weight 
addition of magnesium-ferrosilicon (5% by weight magnesium) based on the 
weight of iron to produce spheroidal graphite iron. The iron was then 
inoculated by the addition of 0.4% by weight based on the weight of iron 
of foundry grade ferrosilicon. 
The analysis of the iron was: 
carbon--3.61% 
silicon--2.45% 
sulphur--0.005% 
magnesium--0.041% 
manganese--0.062% 
phosphorus--0.021%. 
The iron was poured from the ladle into the two moulds at a temperature of 
1410-1430.degree. C. Chill pieces from the resultant castings were 
prepared as described in Example 1 and their nodule count determined. The 
results for the central chill pieces from different test bars are 
tabulated in Table 3 below. 
TABLE 3 
______________________________________ 
CASTING FROM CASTING FROM 
MOULD WITH MOULD WITH 
TEST UNCOATED FILTER - 
INOCULANT COATED 
BAR NODULE COUNT FILTER - NODULE 
No. PER MM.sup.2 COUNT PER MM.sup.2 
______________________________________ 
1 113 131 
3 131 163 
5 164 184 
7 137 170 
9 122 160 
______________________________________ 
The filter coated with inoculant gave a higher nodule count for all the 
test bars compared to the nodule count of the test bars of the casting 
produced without inoculation in the mould.