METHOD OF MAKING AND USING A CERAMIC SHELL MOLD

Description

Technical field

[0001] This invention relates to the preparation of a ceramic shell mold useful for investment casting purposes, and particularly to a method of making a shell mold that will effectively reduce the amount of surface decarburization of a ferrous article formed in the shell mold.

Background art

[0002] Investment casting, also referred to as the "lost wax" process, typically involves alternate applications of a ceramic coating composition and a stucco composition to an expendable pattern in order to provide a multi-layered shell mold. The pattern is usually made of wax, plastic or similar material which is melted out to leave a correspondingly shaped internal cavity into which molten metal is poured.

[0003] Unfortunately, there have been many attempts to control the surface finish and the amount of decarburization of steel investment castings. The problem of a metal-mold-atmosphere reaction at the time of pouring and initial stages of solidification of the molten metal has continued to cause an undesirable carbon-free zone adjacent the surface of the article as well as surface blemishes. The methods of minimizing this phenomenon have included casting in a vacuum, use of inert gas shrouding, the addition of reducing agents into the mold cavity prior to pouring, preheating the mold in a carbonaceous atmosphere prior to casting, etc. All of these production steps are costly, time- consuming or raise issues of safety to foundry personnel such as by producing noxious vapors.

[0004] U.S. Patent No. 3,184,813 issued to P. J. O'Shea on May 25, 1965 and U.S. Patent No. 3,296,666 issued to N. G. Lirones on January 10, 1967 are representative of the large number of ceramic dip coat compositions used in building up multi-layered shell molds. Frequently, the compositions of the shell mold layers are tailored for the specific metal,

[0005] U.S. Patent 3,153,826 discloses a method of making a ceramic shell mold including Step (a) alternately applying a coating composition including a ceramic powder and a binder, and then a stucco composition including granular refractory material to an expendable pattern a preselected number of times, drying the coating between applications, and forming a resultant multi-layered mold, Step (b) heating the multi-layered mold, removing the pattern, and forming a resultant hardened mold, and Step (c) applying a barrier coating.

[0006] In the past, for example, graphite has been added to the usual coating composition of a ceramic powder and a binder in order to improve surface finish and to minimize the amount of decarburization of steel articles. But while the use of a relatively uniform amount of graphite throughout the full cross section of the shell mold wall has resulted in some improvement in the quality of the castings, surface irregularities and localized carburization have been observed because of the undesirable contact of the molten metal directly with the graphite particles. Moreover, the strength of the individually applied layers is reduced by graphite addition and the shell mold is more costly than desired.

[0007] The present invention is directed to overcoming one or more of the problems as set forth above.

Disclosure of invention

[0008] In accordance with one aspect of the present invention, a ceramic shell mold is made according to claim 1.

[0009] In another aspect of the invention a method of investment casting of a ferrous article in a shell mold is disclosed, the features of which method are claimed in claim 10.

Brief description of drawing

[0010] The sole figure is a diagrammatic and enlarged, fragmentary cross sectional view through a multi-layered shell mold having a barrier coating thereon in accordance with the present invention.

Best mode for carrying out the invention

[0011] A preferred method of making a ceramic shell mold 6 comprises the steps of alternately applying a ceramic coating composition 8 and a stucco composition 10 to an expendable or thermally meltable pattern a pre-selected number of times, firing such multi-layered mold to remove the pattern and provide a hardened mold 12 having an internal casting cavity 14, and applying a barrier coating 16 including a ceramic powder, a binder and a preselected amount of graphite as is generally illustrated in the drawing. The presence of any significant amount of graphite is preferably avoided in the multi-layered mold, particularly adjacent the casting cavity 14, and is preferably controlled to a range of about 13 to 17 Wt.% graphite of the total amount of the solid portion of the barrier coating 16.

[0012] The aforementioned ceramic coating composition 8 basically includes a ceramic powder and a binder. Typically, the ceramic powder is selected from the group consisting essentially of fused silica, vitreous silica, crystalline silica, alumina silicate, alumina, magnesium silicate, zircon, zirconium silicate, and clay treated to remove impurities, and can be mixtures thereof. The binder is selected from the group consisting essentially of colloidal silica sol, ethyl silicate, aluminum phosphate, and aqueous alkali metal silicate.

[0013] The stucco composition 10 basically includes conventional granular refractory materials such as zircon.

[0014] The multi-layered mold made by alternately applying the ceramic coating composition 8 and the stucco composition 10 a preselected number of times to the pattern is substantially free of graphite. By this term it is meant that less than 0.5 Wt.% graphite is present in the multi-layered mold before the barrier coating 16 is applied.

[0015] More particularly, a preferred method of making the ceramic shell mold 6 includes the following steps:

Step (a) Forming an expendable or meltable pattern of wax, plastic or similar material of a construction having the desired shape;

Step (b) Applying a prime or first ceramic coating composition 8 including fused silica flour, finely divided zircon, a limited amount of nitrile polymer latex for low temperature strength, for example 2 Wt.%, and colloidal silica sol including water in the form of a slurry to the pattern by dipping the pattern into an agitated thixotropic slurry thereof, removing the coated pattern therefrom and allowing a preselected amount of draining and initial stages of setting thereof;

Step (c) Applying a coarser or stucco coating composition 10 including granular refractory material such as zircon to the still wet first coating composition 8 by sprinkling same thereon from a conventional rainfall sander, or alternately by immersing it in a conventional fluidized bed, and with the AFS grain size of the stucco coating composition being generally limited to a range of from about 35 mesh to 20 mesh (about 0.5 mm to 0.8 mm);

Step (d) Drying the coated and stuccoed pattern for a preselected time period, for example 30 minutes to 6 hours, to a waterproof or gelled shape and providing a first layer 18;

Step (e) Alternately repeating Steps (b), (c), and (d) a preselected number of times while preferably increasing the relative coarseness of the solid particles therein, for example for nine cycles, and providing a multi-layered "green" mold having a plurality of the layers 18, each layer being about 1 mm (.040") thick and intimately associated with each other as is representatively indicated in the drawing;

Step (f) Heating the multi-layered "green" mold in an autoclave at a preselected first temperature of about 180 to 200°C (350 to 400°F) for about 5 to 25 minutes, melting out and removing the pattern, and providing some strength to the mold;

Step (g) Firing the multilayered mold in a furnace at a preselected second temperature of about 800 to 1400°C (1500 to 2500°F), and preferably about 1000°C (1800°F) for about one hour to provide a hardened mold 12 having an exterior surface 20, and an interior surface 22 facing the casting cavity 14 as shown in the drawing;

Step (h) Applying a barrier coating layer 24 to the exterior surface 20 of the hardened mold 12 while it is at a preselected third temperature of about 200°C (400°F), the barrier coating layer including a mixture of zircon, fused silica, finely divided graphite, and colloidal silica sol, the AFS grain size of the graphite particles being preferably limited in size to passing through a 200 mesh sieve (less than about 0.075 mm or 0.003"), and being most desirably limited to a range of about 600 mesh to 325 mesh (about .01 mm to .05 mm), and limiting the amount of graphite to a range of about 4 to 20 Wt.% of the solid or dry portion of the mixture;

Step (i) Drying the barrier coating layer for a preselected period of time;

Step (j) Repeating Steps (h) and (i) a plurality of times, for example three times, to provide a plurality of the graphite containing barrier coating layers 24 to define the multi-layered barrier coating 16 as shown in the drawing; and

Step (k) Heating the hardened mold 12 and the barrier coating 16 in a furnace of the like to a pre-selected third temperature of about 900 to 1400°C (1650 to 2550°F), and preferably about 1050°C (1920°F) to make the ceramic shell mold 6.

[0016] Subsequently, a ferrous molten metal such as steel is poured into the casting cavity 14 of the ceramic shell mold 6. Most desirably, the mold is maintained at a temperature of about 1000°C (1830°F), or slightly below, since the molten metal poured therein is about 1350 to 1700°C (2460 to 3100°F) and this minimizes the temperature differential therebetween.

[0017] Various modifications of Steps (a) through (k) set forth above can be visualized without departing from the scope of the present invention as defined by the appendant claims. For example, drying Step (d) can be achieved under ambient air conditions for a period of about one-half to one hour, or alternatively the drying can be achieved in an oven or furnace at a temperature slightly above ambient temperature to reduce the holding time. Of course, the temperature cannot be elevated too much because the pattern either can melt or can expand to the point of unduly stressing the relatively weak walls of the partially complete mold.

[0018] One of the advantages of this method of investment casting is that it is easier to melt out and remove the pattern from the multi-layered mold because it has a thinner section during intermediate Step (f) than the equivalent strength prior art shell mold has at the time of pattern removal. I have also noted a consistently higher quality of the hardened molds 12 when compared with the thicker prior art molds. Furthermore, Step (g) can be achieved without the need for a reducing atmosphere because the multi-layered mold is substantially free of graphite at that stage.

[0019] Moreover, in Step (h) zircon can be replaced by an equivalent amount of alumina silicate. The barrier coating is preferably about 78 Wt% of dry materials including the aforementioned zircon or alumina silicate, fused silica, and graphite, and the remaining 22 Wt.% is substantially liquid binder including the colloidal silica sol. Specifically, the preferred proportions of the dry materials in the barrier coating 16 are about 75 parts zircon, 25 parts fused silica, and 11 to 25 parts graphite by weight.

[0020] In actuality, Steps (h), (i), and (j) were achieved by repetitively dipping the hardened mold 12 while hot into an agitated thixotropic solution of the aforementioned ceramic and graphite materials for about four or five seconds and removing the mold to permit substantial gelling of the ceramic materials during periods of about 30 seconds therebetween in ambient air. The fact that the mold is hot accelerates the gelling and tends to bridge the ceramic materials over any minor imperfections. Such dipping was automatically accomplished by a known mechanical dipping apparatus provided with a suitable timing and counting control system, not shown.

Industrial applicability

[0021] In order to determine the optimum range of graphite in the barrier coating 16, various weight percentages of graphite were added to the zircon and fused silica portions thereof. Steel articles were made by pouring steel of about 0.3 Wt.% carbon into the heated ceramic shell molds 10 as mentioned above, and the carbon free depth (CFD) and maximum affected depth (MAD) from the surface of the article measured after sectioning of the article. The carbon free depth (CFD) is a measure of the thickness of the surface zone that has experienced substantially total decarburization. The maximum affected depth (MAD) is a measure of the thickness of a thicker surface zone that has experienced at least partial decarburization or a substantive deviation from the carbon level of the central body portion of the article. The test results were as follows:

[0022] Thus, the test data indicates that the prior art ceramic shell mold with substantially no graphite therein exhibited an undesirably high level of decarburization, and the articles prepared in accordance with one aspect of the present invention exhibited a decreasing degree of decarburization as the proportion of graphite in the barrier coating 16 increased up to about 17 Wt.96.

[0023] In addition to such decarburization measurements, which typically reflect the amount of surface material that must be removed so that any subsequent heat treatment effect of the carbon will be uniform throughout the steel article, the surface smoothness of the test articles was noted. For example, the relatively frequent valleys of about 1.5 mm (0.060") maximum depth in the prior art articles were proportionately reduced to minimal blemishes of less than about 0.4 mm (0.015") with the addition of graphite toward 15 Wt.% in the barrier coating 16. I found out also that at about 3.4 Wt% graphite the effect on decarburization was minimal, whereas at the other end of the range at about 20 Wt.% graphite, the graphite was difficult to keep in suspension, tended to agglomerate and thereby weaken the layers, and did not appear to result in any significant change in the results from that of about 15 Wt.% graphite proportion.

[0024] In view of such beneficial results, the broad range of graphite in the barrier coating 16 is about 4 to 20 Wt.%, the preferred range is about 13 to 17 Wt.%, and the most desirable amount is about 15 Wt.%.

[0025] It is of note to appreciate that the problems of decarburization and surface blemishes of investment cast articles is more severe when the amount of carbon in the ferrous molten metal is reduced toward 0.1 Wt.96 carbon. Thus, the method of the present invention is particularly useful for minimizing decarburization of steel articles with less than 1.5 Wt.% carbon. Graphite is reactive to oxygen, and the reaction is accelerated as the temperature increases. In a crystalline material such as the shell mold, graphite will travel in the porous interstices thereof during heating. I theorize that during pouring of molten metal into the shell mold a portion of the graphite in the barrier coating 16 diffuses inwardly toward the casting cavity 14 while at the same time a portion of the carbon in the molten metal tends to diffuse into the shell mold where oxygen is available. Under any theory, however, carbon depletion is greatly minimized by the method of present invention.

Claims

1. A method of making a ceramic shell mold including:

Step (a) alternately applying a coating composition (8) including a ceramic powder and a binder, and then a stucco composition (10) including granular refractory material to an expendable pattern a preselected number of times, drying the coating between applications, and forming a resultant multilayered mold,

Step (b) heating the multi-layered mold, removing the pattern, and forming a resultant hardened mold (12), and step (c) applying a barrier coating (16) characterized in that:

the multilayered mold of Step (a) has less than 0.5 Wt.% graphite; and

Step (c) includes applying the barrier coating (16) to the exterior surface of the multi- layered mold (12) while the hardened mold (12) is at a preselected temperature above ambient, said barrier coating (16) including a mixture of a ceramic powder, a binder, and a preselected amount of particulate graphite within a range of 4 to 20 Wt.% of the solid portion of the barrier coating (16).

2. The method of claim 1 wherein Step (c) includes applying the barrier coating (16) to the hardened mold (12) while the hardened mold has a temperature of 200°C.

3. The method of claim 1 wherein Step (c) includes selecting the barrier coating mixture as a thixotropic solution of zircon, fused silica, particulate graphite and colloidal silica sol.

4. The method of claim 1 wherein Step (c) includes selecting the barrier coating mixture as a thixotropic solution of alumina silicate, fused silica, particulate graphite and colloidal silica.

5. The method of claim 1 including maintaining said preselected amount of particulate graphite in a range of 13 to 17 Wt.% of the solid portion of the barrier coating (16).

6. The method of claim 1 including selecting said preselected amount of particulate graphite at 15 Wt.% of the solid portion of the barrier coating (16).

7. The method of claim 1 including maintaining said preselected amount of particulate graphite in a size range less than 0.075 mm.

8. The method of claim 1 including maintaining said preselected amount of particulate graphite within an AFS grain size range of 0.01 mm to 0.05 mm.

9. The method of claim 1 wherein Step (c) includes repetitively dipping the hardened mold (12) into and removing the hardened mold (12) from an agitated thixotropic solution of the barrier coating (16).

10. A method of investment casting of a ferrous article in a shell mold (6) comprising:

Step (a) applying a coating composition (8) including a ceramic powder and binder to an expendable pattern, said ceramic powder being selected from the group consisting essentially of fused silica, vitreous silica, crystalline silica, alumina silicate, alumina silicate, alumina, magnesium silicate, zircon, zirconium silicate, and clay, and binder being selected from the group consisting essentially of colloidal silica sol, ethyl silicate, aluminum phosphate, and aqueous alkali metal silicate;

Step (b) applying a stucco composition (10) including a granular refractory material;

Step (c) alternately repeating Steps (a) and (b) a preselected number of times and forming a multi-layered mold, said multi-layered mold having less than 0.5 Wt.% graphite;

Step (d) heating the multi-layered mold and forming a hardened mold (12) having an internal cavity (14);

Step (e) applying a barrier coating (16) to the exterior surface (20) of the hardened mold (12) at a location spaced from the internal cavity (14) and while the hardened mold (12) is hot, said barrier coating (16) having a solid portion and being a mixture of a ceramic powder, a binder, and a preselected amount of finely divided graphite, said preselected amount of graphite being within a range of 4 to 20 Wt.% of the solid portion of the barrier coating (16);

Step (f) heating the hardened mold (12) and barrier coating (16) and forming a hot shell mold (6); and

Step (g) pouring a ferrous molten metal into the internal cavity (14) of the hot shell mold (6).

11. The method of claim 10 wherein Step (e) includes maintaining said preselected amount of graphite in a range of 13 to 17 Wt.%.

12. The method of claim 10 wherein Step (e) includes dipping the hot hardened mold (12) into a thixotropic solution a preselected number of times to apply the barrier coating (16).

13. The method of claim 12 wherein Step (e) includes maintaining the finely divided graphite in the thixotropic solution within an AFS grain size range of 0.01 mm to 0.05 mm.

Ansprüche

1. Verfahren zur Herstellung einer keramischen Schalenform einschließlich der folgenden Schritte:

Schritt a: Alternatives Aufbringen einer Beschichtungszusammensetzung (8) einschließlich eines Keramikpulvers und eines' Bindemittels, und sodann einer Gipszusammensetzung (10) einschließlich eines granularen feuerfesten Materials auf ein verbrauchbares Muster und eine vorgewählte Anzahl von Malen, Trocknen der Schicht zwischen den Aufbringungen und Ausbildung einer sich dadurch ergebenden mehrerer Lagen aufweisenden Form,

Schritt b: Erhitzung der mehrere Lagen aufweisenden Form, Entfernung des Musters und Bildung einer sich ergebenden gehärteten Form (12), und

Schritt c: Aufbringen einer Sperrschicht (16), dadurch gekennzeichnet, daß die mehrere Lagen aufweisende Form des Schritts a weniger als ungefähr 0,5 Gewichts% Graphit besitzt und daß im Schritt c das Aufbringen der Sperrschicht (16) auf die Außenoberfläche der mehrere Lagen besitzenden Form (12) vorgersehen ist, während sich die gehärtete Form auf einer vorgewählten Temperatur oberhalb der Umgebungstemperatur befindet, und wobei die Sperrschicht (16) eine Mischung aus einem Keramikpulver, einem Bindemittel und einer vorgewählten Menge teilchenförmigen Graphits, und zwar innerhalb eines Bereichs von 4-20 Gewichts-% des festen Anteils der Sperrschicht (16), einschließt.

2. Verfahren nach Anspruch 1, wobei der Schritt c das Aufbringen der Sperrschicht (16) auf die gehärtete Form (12) umfaßt, und zwar während die gehärtete Form eine Temperatur von 200°C besitzt.

3. Verfahren nach Anspruch 1, wobei der Schritt c die Auswahl der Sperrschichtmischung als eine thixotrope Lösung von Zirkon, geschmolzenem Silizium-Dioxyd, teilchenförmigem Graphit und kolloidalem Kieselsäuresol umfasst.

4. Verfahren nach Anspruch 1, wobei der Schritt c die Auswahl der Sperrschichtmischung als eine thixotrope Lösung aus Aluminiumsilikat, geschmolzenem Siliziumdioxyd, teilchenförmigem Graphit und kolloidalem Siliziumdioxyd umfaßt.

5. Verfahren nach Anspruch 1, wobei die vorgewählte Menge an teilchenförmigem Graphit in einem Bereich von 13-17 Gewichts% des festen Anteils der Sperrschicht (16) gehalten ist.

6. Verfahren nach Anspruch 1, wobei die vorgewählte Menge an teilchenförmigem Graphit mit 15 Gewichts% des festen Anteils der Sperrschicht (16) ausgewählt ist.

7. Verfahren nach Anspruch 1, wobei die vorgewählte Menge an teilchenförmigem Graphit in einem Größenbereich von weniger als 0,075 mm erhalten ist.

8. Verfahren nach Anspruch 1, wobei die vorgewählte Menge an teilchenförmigem Graphit innerhalb eines AFS Korngrößenbereichs von 0,01 mm bis 0,05 mm gehalten ist.

9. Verfahren nach Anspruch 1, wobei der Schritt c das weiderholte Eintauchen der gehärteten Form (12) in eine gerührte thixotrope Lösung der Sperrschicht (16) und das Herausnehmen der gehärteten Form (12) umfaßt.

10. Präzisionsformgußverfahren für einen Eisen enthaltenden Gegenstand in einer Schalenform (6) unter Verwendung folgender Schritte:

Schritt a: Aufbringen einer Beschichtungszusammensetzung (8) einschließlich eines keramischen Pulvers und eines Bindemittels auf einem verbrauchbaren Muster, wobei das keramische Pulver aus der im wesentlichen aus folgendem bestehenden Gruppe ausgewählt ist: Geschmolzenes Siliziumdioxyd, glasiges Siliziumdioxyd, kristallines Silizium- dioxyd, Aluminiumsilikat, Aluminiumoxyd, Magnesiumsilikat, Zirkon, Zirkonsilikat und Ton, wobei ferner das Bindemittel aus der im wesentlichen aus folgendem bestehenden Gruppe ausgewählt ist: Kolloidales Kieselsäuresol, Äthylsilikat. Aluminiumphosphat, wässriges Alkalimetallsilikat,

Schritt b: Aufbringen einer Gipszusammensetzung (10) einschließlich eines granularen feuerfesten Materials;

Schritt c: Abwechselnde Wiederholung der Schritte a und b für eine vorgewählte Anzahl von Malen und bildung einer mehrere Lagen aufweisenden Form, die mindestens 0,5 Gewichts% Graphit besitzt,

Schritt d: Erhitzen der mehrere Lagen aufweisenden Form und Ausbildung einer gehärteten Form (12) mit einem Innenhohlraum (14);

Schritt e: Aufbringen eines Sperrüberzugs (16) auf die Außenoberfläche (20) der gehärteten Form (12) an einer gegenüber dem Innenhohlraum (14) in Abstand angeordneten Stelle, und zwar während die gehärtete Form (12) heiß ist, wobei die Sperrschicht (16) einen Festanteil besitzt und eine Mischung aus einem Keramikpulver, einem Bindemittel und einer vorgewählten Menge an fein verteilten Graphit ist, und wobei ferner die vorgewählte Menge an Graphit innerhalb eines Bereichs von 4--20% des Festanteils der Sperrschicht (16) liegt:

Schritt f: Erhitzung der gehärteten Form (12) und der Sperrschicht (16) und Ausbildung einer heißen Schalenform (6); und

Schritt g: Eingießen eines eisenhaltigen geschmolzenen Metalls in den Innenhohlraum (14) der heißen Schalenform (6).

11. Verfahren nach Anspruch 10, wobei der Schritt e die Aufrechterhaltung der vorgewählten Graphitmenge im Bereich von 13-17 Gewichts% umfaßt.

12. Verfahren nach Anspruch 10, wobei der Schritt e das Eintauchen der heißen gehärteten Form (12) in eine thixotrope Lösung umfaßt, und zwar für eine vorgewählte Anzahl von Malen, um die Sperrschicht (16) aufzubringen.

13. Verfahren nach Anspruch 12, wobei der Schritt e die Aufrechterhaltung des fein geteilten Graphits in der thixotropen Lösung innerhalb eines AFS Korngrößenbereichs von 0,01 mm bis 0,05 mm umfaßt.

Revendications

1. Procédé de fabrication d'une coquille de moulage en céramique comportant:

Etape (a) l'application alternée d'une composition de revêtement (8) comportant une poudre céramique et un liant, puis d'une composition de stuc (10) comportant une matière réfractaire granuleuse, sur un modèle non réutilisable, un nombre de fois prédéterminé, le séchage du revêtement entre les applications, et la formation d'un moule à couches multiples résultant,

Etape (b) le chauffage du moule à couches multiples, l'enlèvement du modèle, et la formation d'un moule durci résultant (12), et Etape (c) l'application d'un revêtement protecteur (16), caractérisé en ce que:
le moule à couches multiples de l'Etape (a) contient moins de 0,5% en poids de graphite; et

l'Etape (c) comporte l'application du revêtement protecteur (16) sur la surface extérieure du moule à couches multiples (12) tandis que le moule durci (12) se trouve à une température prédéterminée au-dessus de la température ambiante, ledit revêtement protecteur (16) comportant un mélange d'une poudre céramique, d'un liant, et d'une quantité prédéterminée de graphite en particules dans une proportion de 4 à 20% en poids de la partie solide du revêtement protecteur (16).

2. Procédé selon la revendication 1, dans lequel l'Etape (c) comporte l'application du revêtement protecteur (16) sur le moule durci (12) tandis que le moule durci est à une température de 200°C environ.

3. Procédé selon la revendication 1, dans lequel l'Etape (c) comporte le choix du mélange de revêtement protecteur sous la forme d'une solution thixotropique de zircon, de silice fondue, de graphite en particules et d'un sol de silice colloïdale.

4. Procédé selon la revendication 1, dans lequel l'Etape (c) comporte le choix du mélange de revêtement protecteur sous la forme d'une solution thixotropique de silicate d'alumine, de silice fondue, de graphite en particules et de silice colloïdale.

5. Procédé selon la revendication 1, comportant le maintien de ladite quantité prédéterminée de graphite en particules dans l'intervalle de 13 à 17% en poids de la partie solide du revêtement protecteur (16).

6. Procédé selon la revendication 1, comportant le choix de ladite quantité prédéterminée de graphite en particules à 15% en poids de la partie solide du revêtement protecteur (16).

7. Procédé selon la revendication 1, comportant le maintien de ladite quantité prédéterminée de graphite en particules à une granulométrie inférieure à 0,075 mm.

8. Procédé selon la revendication 1, comportant le maintien de ladite quantité prédéterminée de graphite en particules dans un intervalle de granulométrie AFS de 0,01 mm à 0,05 mm.

9. Procédé selon la revendication 1, dans lequel l'Etape (c) comporte une répétition des opérations de trempage du moule durci (12) dans une solution thixotropique agitée du revêtement protecteur (16), puis de retrait du moule de cette solution.

10. Procédé de moulage à cire perdue d'un objet ferreux dans une coquille de moulage (6) comprenant:

Etape (a): l'application d'une composition de revêtement (8) comportant une poudre céramique et un liant sur un modèle non réutilisable, ladite poudre céramique étant choisie dans le groupe qui se compose essentiellement de la silice fondue, de la silice vitreuse, de la silice cristalline, du silicate d'alumine, de l'alumine, du silicate de magnésium, du zircon, du silicate de zirconium, et de l'argile, et le liant étant choisi dans le groupe qui se compose essentiellement des sols de silice colloïdale, du silicate d'éthyle, du phosphate d'alumium, et des solutions aqueuses de silicates de métaux alcalins;

Etape (b): l'application d'une composition de stuc (10) comportant une matière réfractaire granuleuse;

Etape (c): la répétition alternée des étapes (a) et (b) un nombre de fois prédéterminé, et la formation d'un moule à couches multiples, ledit moule à couches multiples contenant moins de 0,5% en poids de graphite;

Etape (d): le chauffage du moule à couches multiples et la formation d'un moule durci (12) comportant une cavité interne (14);

Etape (e): l'application d'un revêtement protecteur (16) sur la surface extérieure (20) du moule durci (12) à un endroit situé à une certaine distance de la cavité interne (14) et tandis que le moule durci (12) est encore chaud, ledit revêtement protecteur (16) comportant une partie solide et étant formé d'un mélange d'une poudre céramique, d'un liant, et d'une quantité prédéterminée de graphite finement divisé, ladite quantité prédéterminée de graphite étant comprise dans l'intervalle de 4 à 20% en poids de la partie solide du revêtement protecteur (16);

Etape (f): le chauffage du moule durci (12) et du revêtement protecteur (16) et la formation d'une coquille de moulage chaude (6); et

Etape (g): la coulée d'un métal ferreux en fusion dans la cavité interne (14) de la coquille de moulage chaude (6).

11. Procédé selon la revendication 10, dans lequel l'étape (e) comporte le maintien de ladite quantité prédéterminée de graphite dans un intervalle de 13 à 17% en poids.

12. Procédé selon la revendication 10, dans lequel l'Etape (e) comporte le trempage, un nombre de fois prédéterminé, du moule durci chaud (12) dans une solution thixotropique pour appliquer le revêtement protecteur (16).

13. Procédé selon la revendication 12, dans lequel l'Etape (e) comporte le maintien du graphite finement divisé contenu dans la solution thixotropique dans un intervalle de granulométrie AFS de 0,01 mm à 0,05 mm.

Drawing