US2023044A - Core for and method of producing hollow refractories - Google Patents

Description

G. J. EASTER Dec. 3, 1935.

CORE FOR AND METHOD OF PRODUCING HOLLOW REFRACTORIES Filed Dec. 18, 1935 INVENTOR. GEORGE J. EASTER ATTORNEY.

Patented Dec. 3, 1935 UNITED STATES CORE FOR AND METHOD OF PRODUCING HOLLOW REFRACTORIES Application December 18, 1933, Serial No. 702,941

9 Claims.

This invention relates to a new method for the manufacture of nonmetallic articles made by casting molten material into refractory molds containing one or more cores for the fabrication of articles with recesses, holes, etc. Such articles are often found to contain cracks when delivered from the molds, this cracking being caused by the thermal inflexibility of the core which sets up internal stresses in the cast article as it cools about the inclosed core.

I have discovered that if the core is made of material of higher thermal expansion than the material being cast, and if the heat capacity of the core be kept low both by keeping the amount of material low and by using material whose inherent specific heat is low, the core rapidly reaches a maximum temperature and size when the molten material is poured around it; and then after the walls of the casting solidify, it shrinks away from the shell of cast material. Due to its higher thermal expansion, the core shrinksfaster than the casting even though air is excluded from the core. In this manner, as the article is being formed within the mold the amount of internal stress is minimized and the article as finally made is free of cracks.

My invention is illustrated in the drawing in which Figure 1 is a vertical cross-section through the mold with the core in place ready for the casting operation; and

Figure 2 shows diagrammatically one method of applying my invention to the manufacture of crucibles of cast alumina or of similar refractory material.

The core in a typical case, such as making a casting of fused alumina, is made of chromite or magnesia both of which have appreciably higher coeflicients of expansion than alumina. In Fig ure 1, the walls 3 of the mold are made of material sufliciently refractory to contain the molten material of the casting without destruction or disintegration of the mold walls. The mold is provided'with a riser 4 of ample size to permit complete filling of the mold without interference by material freezing in the header 5. The riser is preferably made wedge shaped with its minimum section immediately adjacent the mold, so that removal of the excess material constituting the header 5 is facilitated. The walls of the core 6 are in general, as is indicated, relatively thin and exposed to the air on one side. The air space within such a care may be a dead air space, so as not to hasten the cooling of the core. In special cases, I may use as core material refractory alloys PATENT OFFICE of chromium and iron, and chromium, iron an nickel; but when these are used the interior of the core must be air cooled as the core is notso refractory as the casting and .any melting of the core is to be avoided. Such metal cores are in 5 particular useful when materials such as chromite which have in themselves high thermal expansion coefiicients are being cast. 7

In some cases I havefound it advantageous to overlay the core with a layer 1 of blotting paper 10 or other destroyable carbonaceous material against which the molten material will initially come in contact. 1

Should it be desirable, as above referred to, to air cool the core, this may be accomplished as in- 15 dicated diagrammatically in Figure 2. The core 1 may be shaped as required by the form of recess which the desired article is to have. In accordance with Figure 2, a trough shaped refractory piece is to be cast in the mold 36; by the use of a 20 core 6a. Cooling air is supplied to the interior of the core by the inlet 9 and the heated air discharged by way of the outlet In or out at the ends of the trough.

Cores utilizing the method of this invention may 25 be made of ceramically bonded and burned or temporarily bonded granular material. The cores may be either monolithic or may be madeup in sections. For example, I have built upcores from slabs made by bonding chromite grain with sulfite a0 pitch, the slabs being assembled into a hollow shaped core for the manufacturing of crucibles with rectanguluar openings. Furthermore, the core may be precast, either as a single casting, or in sections. When precast, ceramically bonded 35 or metallic cores are used, the cores may be used repeatedly until worn or warped, and even then may be patched or straightened for additional use. The essential requirement is that the refractory material of the core have a higher coefflcient of 4 expansion than the material being cast. The use of chromite cores is particularly advantageous in that the material is highly refractory and yet completely inert, so that no reaction takes place between the core and the molten casting.

The mold proper may, for example, be made of bonded refractory granular material according to regular molding practice as adapted to the casting of refractory non-metallic fusions. The

"core should be interlocked with the mold to prevent displacement by the material being cast using any suitable means not shown other than to indicate the core set in a close fitting groove around its base.

In accordance with my invention, I provide a core which will quickly become heated to its maximum temperature so that it will attain its maximum size before the cast shell next to it completely solidifies. In cooling down, the core will shrink faster than the casting so that strains will not be set up-which will break the casting. The layer of paper or other carbonaceous material has the advantage of imparting a. further degree of flexibility between the core and the casting, and reducing the thermal shock to the core in that it allows additional time for the heating of the core before the cast face solidifies in contact with it. The layer of paper may be omitted successfully in many cases however.

While I have explained in detail the manner of performing my invention as illustrated by certain specific embodiments, it will be understood that my inventionhs not limited to the specific details of these embodiments,'but may be otherwise embodied and practiced within the scope of the following claims.

I claim:

1. In combination with a mold for casting hollow refractories, a core of low thermal capacity and having a higher coefiicient of thermal expansion than the material being cast.

2. In a mold for fabricating cast non-metallic articles, a core whose coefiicient of thermal expansion is greater than that of thematerial being cast.

3. The method of forming refractory castings with openings or recesses comprising casting the molten alumina in a mold including a core having thin walls of chromite.

4. The method of forming refractory castings with openings or recesses comprising casting the molten alumina in a mold including a core having thin walls of fused magnesia.

5. The method of forming chromite castings 5 with openings or recesses comprising casting the molten chromite in a mold provided with a core having thin walls of a ferro-chromium alloy.

6. In combination with a mold for forming cast non-metallic articles, a hollow core having thin walls whose coefiicient of thermal expansion is greater than that of the material being cast.

7. The method of forming refractory castings with openings or recessescomprising casting the molten refractory material in a mold including a hollow core and passing cooling fluid through the hollow core to prevent overheating of the; core during casting.

8. The method of forming refractory castings with openings or recesses comprising casting moiten material in a mold including a hollow core formed of material having a lower freezing point than the temperature of the molten material being cast and passing cooling fluid through the hollow core.

' 9. In the process of casting molten non-metallic refractory materials the steps which comprise shaping the molten material by casting it against a refractory wall, permitting heat to escape through said wall and governing the temperature of said wall by circulation of a cooling fiuid in contact therewith but out of contact with the cast material.

GEORGE J. EASTER.

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