CA2021505A1 - Casting of molten iron and filters for use therein - Google Patents

Abstract

ABSTRACT

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 an inoculant, such as graphite, calcium silicide or ferrosilicon, for the iron.

Description

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CASTING OF MOLTEN IRON AND
FILTERS FOR USE THEREIN
__ _ _ 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.
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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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- ~ - FS L~lf3 ~ 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 - 3 - _S 1418 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 ~,7~

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 ~ ~ ~J ~

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 s,~

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 2~2~

- 7 ~ FS 1418 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 2~

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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 ~2i5~i ~ FS 14i8 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

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16~ ~84 _ __ _ . ~

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.

Claims (14)

1. 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.

2. 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.

3. A filter according to Claim 2 wherein the body is a ceramic body having cells extending between opposite faces of the body.

4. A filter according to Claim 2 wherein the body is a porous pressed ceramic body.

5. A filter according to Claim 2 wherein the body is an open-cell ceramic foam.

6. A filter according to Claim 2 wherein the inoculant is graphite, calcium silicide or ferrosilicon.

7. A filter according to claim 6 wherein the ferrosilicon contains aluminium, titanium, chromium, zirconium, manganese, copper, bismuth, an alkaline earth and/or a rare earth.

8. A filter according to Claim 2 wherein the inoculant has a particle size of up to 10 mm.

9. A filter according to Claim 8 wherein the inoculant has a particle size of 0.05 - 2 mm.

10. A filter according to Claim 2 wherein the cell walls are coated with a dispersion of the inoculant in a wax or in a substance having the physical characteristics of wax.

11. A filter according to Claim 10 wherein the wax is beeswax, carnauba wax, montan wax or paraffin wax.

12. A filter according to Claim 11 wherein the substance having the physical characteristics of wax is a fatty acid or a fatty acid ester.

13. A filter according to Claim 2 wherein the whole wall surface of all the cells is with the inoculant.

14. A filter according to Claim 2 wherein only the walls of some of the cells are partially coated with the inoculant.