Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CASTING OF MOLTEN IRON AND
FILTERS FOR USE THEREIN
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This invention relates to the casting o~
molten iron in a mould and to filters or use therein.
When molten iron is treated with an inoculant
prior to casting there is a tendency for the effect of
the inoculant to b2 diminished, (known as "fading"),
before the metal is cast into moulds. Various methods
have therefore been proposed Eor inoculatin~ molten
iron as late as possible :in the casting process, either
by treating the iron just be~o~e it entars ~he moul~ or
by treating the iron in the mould itsel~.
~ n inoculant for iron is a substance which
when added to mol~en iron will ~orm nuclei for
crystallisation when the iron solidifies on casting.
By creating favourable condit.ions for solidi~ication
the inoculant controls the graphite structure . or
morphology, eliminates or reducas the formatiGn of iron
carbides known as chill, increases the eutectic cell or
nodule count, reduces castin~ section sensitivity and
prevents undercooling.
.
Inoculation in the mould involves placing the
inocul~nt 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.
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~ ttempts have been ma~e to utili32 an
inoculant in tha form of fine particles, for e~ample
fine particles of ferrosilicon for inoculating grey
cast iron or spheroidal graphite iron, but they have
not been succassEul because the particles of inoculant
tend to get washed into the mould cavity where they can
form inclusions in thP casiing produced when the molten
iron solidifies r and because there is a t~n~ency ror
castings having variations in their microstructur2 to
be produced.
In ordar to overcome the problems associated
with the use of fine particles methods have b2en
proposed whirh utilise inserts mad~ of ~onded,
compressed or s~ntered particulata :Lnoculants, o~Jer
which or thxough which the mol~en iron ~lows and in ona
such method the insert rests on a strainer core.
However, none of ~hese ~ethods has been wholly
successful and none has achie~f~ed wide commercial us~.
~ast inserts have also been used but becf~us~ they tend
to shattar under the influence o~ thermal shock they
can gi~Je rise to inclusions 1n the castings.
When casting molteI1 iron into moulds it is
o,ten desirable to include in t~e mould some means for
pr2~J~nting inclusions ~rom b~ing incorporated in
cas~ings produced in t'~e moulds.
~ ith grey and m~llea~lé irons inclusions can
~s Eorned ~ e to reEraci_ory particles and/or slag being
c~rried ov~r r^rom ~ ~urnace or a ladle into the mould
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cavity or due to particles of sand from the runner
system of a sand mould ~eing washed into the moulà
cavity.
Inclusions are most prevalent in ductile or
nodular irons because in addition sticky magnesium
silicate slags, often associated w.ith particles of
magnesium ox~de and magnesium sulphide, are formed
~uring the nodularising process and these are difficult
to remove prior to pouring the molten metal into the
mould, even though special precautions such as a
~luxing treatment, the use of a teapot la~le vr th~ usa
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
func~ion is as a means for controlling the flow of
molten iron into the mould and they have only a limi~ed
filtering effect.
In recent years it has become common practice
to incorporate cellular ceramic ~ilters in moulds for
casting ferrous metals. ~uropean Patent Application
Publicatio~ 0234825 describes a procPss for casting
molten ferrous metal in a mould in which mol~en 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 seaied plastics container
containing particles of a treatmen~ agent for the
mo1ten ferrous meta' located in a chamber in the runner
system on tnat side of .he filter which is further from
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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 thare is
provid~d 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 lnto
the mou~d so tha~ ~he iron passes through the filter
and into the mould cavity.
According to a further faature of the
invention there is provided a filter for filtering
molten iron comprising a body having a plurality of
cells, at least 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.
Cera~ic honeycomb structured ~odies 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
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plurality of fee~ opaninys exter.din~ partially throuqh
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 United States Patent 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 aæ recticulated polyurethane foam, with an aqueou~
slurry of ceramic material containing a binder, dryiny
the impregnated foam to remo~Je water and then firing
the dried impregnatad foam to burn of e the organic foam
~o produe a ceramic ~oam replica. The production o~
ceramic foams by such a method is described in United
States Patent 3090094, in British Patents 932862,
916784, 1004352, 1054421, 13776~1, 1388911, 138891~ and
1388913 and in European Patent Application Publication
0074978.
The material used for -the ceramic filter must
withstand the temperature of and be resistan't to molten
iron and suitable materials include alumlna, 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
~0 -85~ by weiqht si icon and small ~uantities of
calcium and/or aluminium. Special types of
ferrosilicon containing other elements such as
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titanium, chromium, zirconium, manganeser copper,
bismuth, alkaline earths, e~g. barium or s~rotltium, or
rare earths e.g. cerium may ~lso be used.
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The cells of the filter may be coated with the
inoculant by a variety of techniques such as plasma
spraying or coating using a dispersion of particulat2
inoculant in a suita~le medium.
When a dispersion of inoculant is used
particles of the inoculant may be disperse~ in water or
in an organic carrier liquid, containing a binder, and
the dispersion can be applied as a coating to the! cell
walls o~ the cellular hody by, ~or example, sprayin~ or
dipping the body in the dispersion. Aeter the coating
has been applied it is clried to remove the water or
organic car-rier liquid.
Alternatively -the particles of inoculant may
be dispersed in a medium of wax or a substance ha-Jing
the ph~Isical characteristics of wax. The use of such
dispersions in the treatment of ~olten ferrous matals
is clescribed in British Patents 1105028 and 1257168 and
suita~le media include natural waxes such as ~eeswax,
carnau~a wax or montan wax, paraffln ~Ala~, fatty acids
such as stearic acid and fatty acid esters such as
stearates. The particles of inoculant are added to the
medium which has ~een heated so that it is liqu-cl and
are cllspersed, and the dispersion is thsn applied to
the cell r~alls of the cellular body bv for example,
spraying, pouring or ~y dip~ g tlie cellular ~od~ in
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the dispers.ion. After application the dispersion is
allowed to cool and an adherent coating o~ the
inoculant is obtained.
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, ars used. Relatively large particles
-tend to produce slower fading because they dissolve
relatively slowly but they may produce insuffiaient
nucleation cen~res. Relatively small particles produce
sufficient nucleation centres and therefore improv~ the
mechanical properties of tha cast metal, but beause
they dissolve ~asker they ~end to produce more rapid
fading.
If desired the particles of inoculant may be
encapsulated in a material which will retard the
dissolution rate of the inoculant in the molten iron.
The inoculant - coated filters of the
invention may take a number of forms. For example the
whole wall sur~ace of all the cells may be coated, part
only of some of the cell walls may ~e 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 areas of the cellul~r ~ody may be masked when
the inaculant is applied or the cellular body may be
only par~ially immersed in the inoculant dispersion.
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The thickness of the coating of inoculant
agent may be controlled for example, by controlling the
time t he cellular body ls immersed in the inoculant
dispersion or by removing excess dispersion after
application.
In use the inoculant - coated filter is
located in the runner system of a mould, prefera~ly as
near to the mould cavity as possible and molten iron is
poured into the mould so that it Elows through~ the
filter in which the iron is inoculated and inclusions
are removed from the metal before flowing into the
mould cavity.
~ he ~ilter of the invention offers the
following advantages:-
1) It enables the use of a single methcd ofapplying both a filter and an inoculant in a mould
cavity.
2) It provides a substrate with a hi~Jh surfaca
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 inoculani
and the need to place such inoculants in the mould
o,avity.
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4) Incorporation of an inoculant with a f .ilter
reduce 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 example will serve to illustrate
the invention:-
Two test moulds in phenol-formaldehyde resin
bonded silica sand were produced as shown in the
accompanying drawings in which
Figure 1 i8 a schematic vertical ~ection Qf
the mould.
Figure 2 is a section along a - a of Figure 1
Figure 3 is a section along b - b of Figure 2
Figure 4 is a section along c - c of Figure 1
and
Figure 5 is a section along d - d of Figure 1.
Referring to the drawings the mould consis~s
of a sprue 1, a sprue well 2, a runner 3, having a
print 4 capable of accepting a 55 m~ x 55 mm squa_e
filter 5 of 12 mm thickness, and 10 vertical mould
cavities 6A - 5J to produce test bar castings 1 - 10
interconnected so that when mol~en iron is poured into
the mould and passes through the filter the vertical
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mould cavities 6A - 6J fill sequentially. Each of the
test bar mould cavities 6A - 6J is connected to three
small caviLias 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 piace cavities
7A - 7J solidiLies instantan~ously.
A cordierike/mullIte extruded ceramic filter
having 40 cells per cm2 was inserted into the print of
one of the moulds, and an inoculant - coated filter
ac~ording to the invention was insert~d into the print
of the other mould.
The ~:ilter used in the second mould was the
same co~position as the filter used in the first mould
and its cell walls were coated ~y dipping the filter
in~o a dispersion consisting of 75% by waight
ferrosilicon in 25% by weight paraffin wax. Ths
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 wa~ carried by tlle coated
filter was 20.7 g.
A charge o refined pig iron and steel scrap
was melted in a medium frequency induction furnace and
heated to 150~ C. The r~olken iron was tapped into a
clean p~e-heated ladie containing a ~.9% by ~eight
addition of magnesiu~-ferrosilicon (5% by weighk
~agnesium) based on the weight ol iron to prcduce
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spheroidal graphite iron~ The iron was than inoculated
by the addition of 0.4% by weight based on the waight
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 ths
two moulds at a temperature of 1410 - 1~30 C. The
ca~tings produced ea~h 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 tsn test bars.
The central chill pieces were sectioned at
right angle~ 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 ncdules per mm2).
The results obtained for the nodule count of
chill pieces taken from different test bars are
recorded in the table below.
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TEST BAR CASTING FROM CASTING FROM
No. MOULD WITH MOU~ WITH
UNCOATED FILTER - INOCULANT COATED
N~DUL~ COUNT FILTER - NODULE
PER MM2 COUNT PER MM2
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1 113 131
3 131 163
16~ ~84
7 137 170
9 122 160
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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 decreases so the nodule count decreases and
fewer of the bars contain acceptable nodule numbers.
~ence it is Fossible to assess the effectiveness of in-
mould inoculation by esti-,nating 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 nodu'e COUIIt of the test bars of the
casting produced without inoculation ~n the mould.
