Refractory and method of making it

April 11, 1939.

G. J. EASTER ET AL

REFRACTORY AND METHOD OF MAKING IT Filed July 21, 1936

2,154,153

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BY

INVENTOR.

GEORGE J. EASTER KARL BRO NELL

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ATTORNEY.

Patented Apr. 11, 1939

2,154,153

UNITED STATES PATENT OFFICE

2,184,153 REFRACTORY AND METHOD OF MAKING IT George J. Easter and Karl Brownell, Niagara Falls, N. Y„ assignors, by mesne assignments, to The Carborundum Company, Niagara Falls, N. Y„ a corporation of Delaware Application July 21, 1936, Serial No. 91,748

Claims. (Cl. 49—79)

This invention relates to an improvement in refractory articles and the method of making them, and particularly to refractory articles which are cast in monolithic form from a molten mass.

In the manufacture of cast refractories which are made by pouring molten material into a mold, serious difficulty is encountered in securing castings which are solid throughout and 10 which are not characterized by a pipe or hollow in the center of the casting. This is particularly true in the case of castings composed of 85 % or more of alumina, because material of that type shows a volume shrinkage of approximately 20% 15 as it cools down from the molten condition to the point at which it solidifies. Certain mixtures, notably beta alumina, do not show this shrinkage and accordingly do not tend to have hollows in the center of the casting. Our invention is, how20 ever, advantageous in certain respects for use with castings of even this latter material. This is because the object of this invention is not merely to produce solid castings, but also to accelerate the solidification of the casting and to 25 provide an interlocking internal structure which increases the strength of the casting and moreover permits profitable utilization of a considerable quantity of material which would otherwise be wasted.

In the previous work on the casting of refractories, the tendency toward piping has been constantly objectionable and efforts have been made to overcome the difficulty in various ways, as, for instance by providing a casting with a very large 35 font or header from which the still molten material could drain down into the casting proper as the latter solidified. This has been successful only in instances where the font has been of inordinately large size and has resulted in the 40 loss of excessive quantity of material when the header was scrapped. Attempts have also been made to secure solid castings by multiple pouring, whereby the casting is allowed to cool and shrink and the font is then refilled with addi45 tional molten material which is supposed to run in and take the place of the material which previously occupied the cavity in the center of the block. This has been only partially successful, inasmuch as great care is necessary to pour the 50 second charge at exactly the right time. If it is poured too late the secondary material fails to knit together with the former supply and results in a physically weak casting. _ Moreover, it is frequently found that the gate, leading from the 55 font to the casting proper freezes up so solidly that the secondary charge cannot penetrate into the cavity in the block.

By our method, these disadvantages are overcome and we secure a cast block which has no central cavity but is instead solid throughout and 5 possesses a structure which greatly increases its strength and decreases its tendency to crack during the process of manufacture. The time required for manipulation of - the casting during the filling of the central cavity is, moreover, de- 10 creased from around 40 minutes to perhaps 10 minutes.

Our method is briefly to cast the molten material in the usual manner and thereafter use an electric arc to keep the font full of molten ma- 15 terial and the gate open while we add to the center of the casting lumps of material such, for in-, stance, as pieces of scrap formed from the broken headers from previous castings, thereby chilling the center of the casting so as to cause it to so- 20 lidify at approximately the same time as the outer portions. In this way, relatively small fonts or headers are sufficient and the material which forms them is, to a considerable extent, ultimately salvaged and fed into the center of 25 succeeding castings. While this reuse of scrap is advantageous, our invention is not limited to the use of such material but contemplates the addition of any relatively cold solid material in the manner indicated. 30

In the accompanying drawing,

Fig. 1 shows a section through the center of a casting in a mold immediately at the conclusion of the casting operation; and

Fig. 2 shows a section through the center of a 35 casting near the conclusion of our subsequent treatment.

We shall now explain our method of procedure by describing the manufacture of a casting made of fused alumina, such for example as the fused 4c alumina secured as excess fines from the abrasive industry. This material is composed of approximately 95% alumina, the remainder being miscellaneous impurities such as titania, silica, iron oxide, etc. This example is chosen at ran- 45 dom as one of numerous materials to which we have found our invention applicable and is to be regarded as an example rather than as a limiting disclosure.

In the manufacture of cast refractories, we go first prepare a nielt in an electric arc furnace in the manner well known to operators in the abrasive industry and as described in several patents relating to cast refractories. When the material has become molten and its chemical and 55

2,164,163 physical characteristics have been adjusted to the desired condition in the usual way, we pour the molten material into a mold, which in Fig. 1 is designated as I. This mold may be made of 5 cast iron, graphite slabs or granular refractory material. It should, however, preferably be made of slabs which are clamped together and can be quickly disassembled when so desired. The mold is surmounted by a cover 2 preferably 10 of graphite slabs, having a center opening or gate which is preferably tapered as illustrated in the drawing. This cover is held in place by a heavy font-retaining ring 3 which surrounds the gate.

The molten material 4 is poured into the mold up to the top of the retaining ring 3, and the casting allowed to stand for a few minutes while solidification occurs next to the mold walls. As soon as these walls form sufficiently to be self20 supporting, the mold sides are removed and a sheet iron ring 5 placed around the casting several inches away from it. This intervening space is then filled with insulating material 6 such as finely divided alumina or “Sil-O-Cel”.' Any crust 25 which has formed over the header is now broken and, if need be, more molten material added to again bring the level of molten material up to a point where it can be contacted by electrodes T, which are connected to a source of electric •cur30 rent (not shown) and are suspended in such a manner that their position is readily adjustable. These electrodes are lowered into the melt in the font and a current passed between them. If necessary this current may be started by using a 35 small carbon rod to make contact between the electrodes.

Alternatively, instead of allowing the casting to stand for a few minutes before the electrodes are inserted, they may be inserted in the font im40 mediately after casting and additional charge melted in the font to supply the requirements as shrinkage draws molten material into the casting below.

As soon as is practical after the casting has first ; been poured, we have found it advantageous to remove the slabs which form the mold walls. In this way, damage to the mold itself is minimized and the tendency of the casting to crack upon cooling is considerably decreased.

The presence of the arc in the header maintains the alumina or other cast material in the header and upper portion of the casting at a high degree of fluidity and prevents premature closing of. the gate between the header and casting. Additional ‘fused material may be added from an outside source or unfused material may be fed into this miniature arc furnace at this stage so that it becomes molten and flows down into the center of the casting, replacing material which । tends to contract as the casting solidifies.

As soon as the molten material has solidified for a short distance inward from the mold faces, as perhaps to the position indicated by a dotted line 8, we drop into the molten font irregular lumps 9 of cold material of any convenient shape. These lumps may be made from the headers from previous castings or from any other source. While in general we prefer that they should be of the same composition as the casting itself, I it is permissible to use lumps of other high refractory materials which do not react chemically with the melt which is being cast. These lumps 9 which are in general cold and hence of greater density than the molten material, tend to sink l promptly through the molten material until they come to rest in contact with the outer solidified shell of the casting. We prefer to add these lumps at regular intervals, a few seconds apart, for example at the rate of one lump approximately every eight (8) seconds in making cast- 5 ings approximately one foot cube. In this way the lumps become sufficiently heated on their outsides by contact with the molten bath to become integrally welded into the structure of the block. The lumps may be conveniently dropped through 10 a funnel mouthed pipe (not shown) terminating six or eight (6 or 8) inches above the bath. This drops them into the bath at the proper point and protects the operator from the heat.

It is usually possible to locate the approximate 15 boundary of such lumps in a broken casting, but in general we prefer that the lumps should be so proportioned and so slowly added that there is no separating crack at the interface between the lump and the surrounding matrix material. 20 The size of lumps which can be added varies somewhat in proportion to the dimensions of the casting, pieces approximately 1 inch across being desirable for light castings such as Six 10 x 12 inches, whereas on 18 inch castings, 2 inch lumps 25 are permissible and on larger pieces even larger lumps may be used.

This gradual addition of lump material is continued until it is ascertained by poking into the melt with an iron or carbon rod that the mate- ₃₀rial in the casting proper is no longer fluid. When this stage is reached one or two more lumps are added and the electrodes are then withdrawn, the mold cover 2 and retaining ring 3 removed, and the block covered with finely divided alumina ₃₅or other insulating material and allowed to cool slowly or it may be placed in a furnace and brought down to room temperature sufficiently slowly to prevent cracking.

Addition of lumps in this way permits them to ₄θ become firmly welded into the structure of the casting as a whole, each being substantially surrounded by a subsequently solidified matrix which in general prevents their direct contact with one another. A considerable volume of material may .. be added in this way, amounting to from 15 to 30% of the weight of the entire casting. The added lumps act as a sort of tie-rod, binding the refractory together so that its strength in service is materially superior to the strength of simi- __ lar pieces to which lumps have not been added.

The porosity of the casing is not materially affected except that the central cavity is eliminated. We have, for instance, made castings with as low as 3% porosity by this method. « Where the material is intentionally made porous, the porosity is not materially reduced except insofar as the added chunks may be of more solid stock.

We have had successful results not only by go adding alpha alumina chunks to alpha and beta alumina castings, but also to mullite castings and vice versa. In general, it is preferable that the lumps added be of the same material as the surrounding matrix except in isolated instances 55 where the solid material is less dense than the molten form so that the lumps float on top instead of sinking into the casting proper. This condition prevails, for instance, with beta alumina where the solid material seems to be less dense 70 than the melt from which it forms.

Due to the fact that the interior of the casting transfers its heat to the cold lumps, the temperature of the interior portion falls off rapidly to the solidification point while the temperature

2,164,163 of the outer portion of the casting remains almost as high. This results in a very material decrease in the strains which are otherwise set up due to shrinkage resulting from the more 5 rapid cooling of the outer shell and delayed solidification of the center of the casting. This result is quite beneficial throughout the entire life of the block and considerably simplifies the annealing practice. The presence of the added 10 lumps does not affect the general character of the crystalline structure of the casting, but rather permits it to be either coarsely crystalline with strongly oriented crystals or finely crystalline without predominant orientation, depending upon 15 the other conditions of casting, such, for instance, as the temperature of pouring.

By utilizing our process as above described, we produce a new and valuable block structure in the case of materials having strong crystal20 lizing proclivities (such as alumina or mullite) when the original casting is poured when the melt is well above its fusion point. Blocks made in this way to which lumps are then added show an outer shell of coarsely crystalline, highly 25 oriented material, which (as described in copending application Serial #51,113 filed November 22, 1935, by Easter and Benner) is very beneficial for use in resisting slag attack. Inside this outer shell, however, the crystals, while rela30 tively coarse, are found to be oriented in clusters in which they radiate out for an inch or more from the inserted lumps. This structure is illustrated diagrammatically in Fig. 2. It produces a block which combines the advantages of high 35 mechanical strength with the superior slag resisting qualities associated with oriented crystals.

This structure is not secured when the melt is allowed to closely approach its solidification point before pouring. In either case the major ._ portion of the block retains its characteristic crystal orientation and the structure of only the innermost 15 to 40% of it is in any way affected by the added lumps. In either case the block is strong, solid and very serviceable as a refractory.