US3233994A - Method of forming refractory casting - Google Patents

Description

Feb. 8, 1966 H. 1.. PENBERTHY METHOD OF FORMING REFRACTORY CASTING Filed Sept. 21, 1961 INVENTOR Harvey Larry Pezzbertlzy ATTORNEYS United States Patent 3, 4 METHOD 0F FORMING REFRACTQRY CASTING Harvey Larry Penberthy, 5624 SW. Admiral Way,

Seattle, Wash. Filed Sept. 21, 1961, Ser. No. 139,766 3 Claims. (Cl. 65-66) This invention relates to a method of casting and more particularly to a method of limiting the formation of internal voids in castings.

In the fabrication of relatively large refractory blocks by presently known casting methods, the cast blocks are internally divided into a plurality of a well-defined sections of varying structure. For example, a zirconiaalumina-silica refractory block or" the type used in glass furnaces and being 36 inches square and 12 inches high will, due to rapid chilling, have a lower section extending upwardly approximately 4 inches from the bottom of the block which is extremely dense and non-porous and which is fine-crystalline in nature. The side walls of the block have an outer shell of the same dense and nonporous material extending inwardlyapproximately 2 to 3 inches. The top of the'rnold has a similar dense outer shellextending downwardly approximately 1 inch.

The central space bounded by the outer dense shell consists of two parts: the lower portion, immediately adjacent and attached to the lower 4 inch layer, is formed of large, widely spaced honey-comb type crystals, ranging up to 2 inches in length and the spaces being up to of an inch in depth. The large crystalsin this region indicate that the cooling of this portion of the block was extended over many hours.

In the central block portion immediately above the honey-comb section, there are usually a succession of cavities having a shape of the type which would be enclosed between two saucers placed together in face-toface relationship. These cavities are surrounded by thin septums of relatively dense material. Their inner surfaces are usually glazed, indicating that they were formed during the early stages of the solidification of the blocks, after casting.

The reasons for the formation of the internal cavities and honey-comb structure in refractory blocks such as those described above are best understood by reference to the casting method by which they normally formed. The zirconia-alumina-silica mixture which is made into the refractory blocks described above requires cooling to prevent cracking. Since there is a small amount of interstitial glass in the final glass cast block, the cooling process employed is similar to that used in annealing large glass castings. In the formation of large castings, there is an excess of heat and, due to the size of the casting, its outer portion will solidify While the central portion is still molten. Due to the refractory nature or the material, this outer shell will serve as an insulating barrier around the central molten portion and, due to the inability of the central portion to lose its heat at the same rate as the outer shell, the central molten material will remain molten for a considerable number of hours after the outer shell has solidified. Since the volume shrinkage of a given weight of a zirconia-alumina-silica refractory mixture is approximately 30% from the molten state to the solid state, the formation of cavities and a honeycomb structure as above-described are to be expected.

The internal void problem exists, as well, in the formation of smaller refractory blocks. Thus, under conventional commercial practice, the molds in which the smaller blocks are formed are usually filled at a comparatively fast rate relative to their size, the entire pouring operation taking but a few seconds. The outside skin of the block tends to chill very quickly and cavities are formed ice when the liquid at the center of the block solidifies. No honey-comb structure is formed, however, since there is not enough time for the large crystals to grow.

D With intermediate size blocks, the problem is diminished somewhat since the total heat brought in by the molten material is sufficient to heat the mold and still leave the block at a high enough temperature so that it will remain in the annealing range, but significant internal voids will nevertheless be formed.

Since these cavities and honey-comb structures constitute Weaknesses in the block and present potential passages through which molten glass might leak out of a glass furnace constructed from such refractory material, it is, of course, extremely desirable that they be elimi nated.

There are several methods available to substantially reduce the formation of voids at least in smaller and medium sized refractory blocks. For example, a large riser (i.e., see riser 8 in the drawing) can be provided above the mold, which serves to feed molten material down into the block cavity as solidification takes place. If the riser is removed soon after casing, as is the customary practice, the surface of the block at the point at which the riser has been removed chills quickly and any molten material still in the center of the block shrinks, thus forming a cavity. In any event, the riser technique is wasteful due to the necessity for having an additional volume of material in the riser and to the requirement for cutting off the riser.

Another solution, at least with respect to small and medium sized blocks, is to pour the molten material into the mold cavities at a much slower rate than is customarily done. solidification will take place progressively because heat can fiow from the molten material to the mold wall at a sufficiently fast rate relative to the pouring rate. In' practice, this process is not feasible due to the added cost resulting from the increased pouring time.

It is accordingly 2. primary purpose of the present invention to present a novel method of casting refractory blocks which greatly reduces and minimizes the formation of voids in the central portion of the cast block.

It is another object of the present invention to present a novel method of casting refractory blocks in which large blocks can be cast without the significant formation of undesirable central cavities and which essentially comprises the addition to the molten material during the casting operation of solid particles of such material after the mold has been partially filled with a sufficient quantity of the molten material to form the relatively dense and non-porous bottom portion of the casting which results from rapid chilling of this portion.

It is still a further object of the present invention to accelerate progressive solidification of the central portion of a molten mass of refractory material in a casting, so that solidification of the mass is substantially complete before the end of the pouring operation, by means of the addition to said molten mass, during the pouring operation and after the mold has been partially filled with a sufiicient quantity of the molten material to form the relatively dense and non-porous bottom portion of the casting which results from rapid chilling of this portion, of sufiicient solid material of the same consistency to remove the excess sensible heat and most of the excess heat of fusion of said molten material.

Another object of the present invention is to accelerate progressive solidification of the central portion of a molten mass of refractory material through the addition of solid particles of said material simultaneously with the pouring of the molten material into the mold, said solid particles being heated sufficiently by the molten material to cause them to lose their discrete shape and to mix in- .3 timately with the molten liquid, said solid particles not being added to said molten material untiliafter thesrnold has been partially filled with a sufiicient quantity of the molten material to form the relatively dense and 11011.

porous portion of the casting which results from rapid chilling of this portion and until the mold has. been heated to at least the annealing temperature of said casting, the addition of said solid particles resulting in substantial equalization of the temperature of the casting at at least the annealing temperatureof therefractory material, the cast material being subsequently cooled with sufiicient uniformity to prevent major cracking;

These and further objects and advantages of. the pres-. ent invention will become more apparent upon reference to the following specification and claims, andappended drawing, wherein:

The sole figure illustrated is a vertical section through a mold containing a solidified casting made according to the techniques of the prior art;

A mold, which may be used in the practice of the present invention may be identical to those used in -prior art processes, such as that shown in the drawing. This mold consists essentially of an oversize steel box lined with 2-inch thick slabs 12 of a'silica sand-organic binder composition in a manner well known in the art.. Slabs 12 are held temporarily together in the center of steel box 10 by means of wire (not shown) and then refractory insulating powder 14 is poured into the. cavity between blocks 12 and steel box 10. The empty moldv is then moved to the melting furnace-and themolten;

refractory material is poured into the cavity.

The first step in filling the mold with molten material in accordance with the present invention involves pouring only molten material into the bottom of the mold to form the relatively dense and non-pourous bottom portion of the .casting corersponding" to portion 16 of the casting made .by priorart techniques as shown in the drawing. The quantity of molten material added should be sufiicient to transfer enough heat to the mold to heat it to the annealing temperature while the materialgitself is transformed from the liquid to the solid state by giving up its excess heat over its melting point, its latent heat of fusion and some of its sensible heat, whilecooling from the melting point to a temperature slightly above the annealing temperature. The quantity of molten material to be added to accomplish this result will necessarily vary according tothe size of the block to be cast, the initial temperature of the molten material, the material of construction of the mold, the temperature of the:

mold and the like. In thecase of a zirconiaraluminasilica refractory block of thedimensions described above, in which the pouring temperature of the molten material is approximately 3275 F., theimold cavity should be filled to a level approximately 4 inches above its bottom;

After the initial molten material has been added to the mold cavity, solid granules of the same consistency.

as the molten material and approximately $4 to .%-inch in size are metered into the flowing stream of moltenmaterial until such time as the mold cavity is. nearly. filled. As the granules are added tothe flowing stream;

of molten material, the granules will reach their fusion temperature and will lose theirgranular structure and discrete shape, resulting in an intimate mixture of semiliquid material from the granules. and similar material from the molten material.

The zirconia-aluminia-silica material described above passes through a stage in which the glass matrix therein softens significantly at a point between the temperature at which the material is fully liquid andfully solid. In

dition to the molten material (so-that they lose their discrete shape and mix intimately with the molten ma- I terial) but that the individual crystals within the granules" theprocess of the instant invention, it is desirablethat the granules of material reach this stage upon their addo not melt.

As the glassy matrix. cools, it also partially devitrifies to form interlocked crystals with the material' below'it in the, mold which has previously solidified, This pro-. gressive solidi-ficationcauses allofithe crystals to hem-- terlocked in :one large network, as distinguished from an undesirable system; in. which individual centers of crystallization are present. 1.

The solid granular material should be added in a quark tity such that a sutficientzamount'of heat willzbe absorbed I from. the molten material to result in the; progressive solidification of the central portion of the mass so that solidification of: the mass is substantially complete at the end' of the pouring operation, followed by at leastpartial a equalization of =the temperature after solidification at:a

pointabove the annealing temperature .of the refractory material. sufficient uniformity to prevent major cracking.

In practice, sufiicient solidmaterial will be added to withdraw most of the excess sensible, heat above the fusion point 0f, the moltenzmaterial as well as most of the excess latent heat-of fusion 'of this material. In addition to the foregoing, the upper limit onthe quantity of solid materialwhich may ;be added is limited by the fact that substantially all of'the granular material added'should lose its. granular .structuregand Idiscrete shape. :so as to blend intimately with material introduced in the molten state as previously described. In -.a zirconia-altnnina-silica block of the dimensions described previously, and. at the pouring temperature mentioned, a quantity of solid granular material equal, to approximately 10% of theweight .of molten material used may bemetered' into the stream'of molten material to give satisfactory results, though this figure may vary between approximately S-l5%;

If the casting to be formed is sufficiently small, there may not be enough total heat in'the mold cavityto keep the refractory block above its annealing temperature after solidification if a cold=mold .is used. In such case,

the mold shouldbe preheated before casting.

As a result of the above-described process, the honey! comb region present in prior art castings is almost'entire- 1y eliminated .and the saucer-shaped cavities are reduced to small size, few in number. The reason forrthe reduction in honey-comb structure and cavity formation is that,

since the molten material loses heat to. the cold solid, material during the mold-filling step,-:solidification ofthe molten material, with its accompanyingshrinkage, takes place .whilethe filling operation is still being carried out,

giving the subsequently poured molten material an op-- .portunity to fill voids and cavities as they are formed-3 Since solidification and shrinkage take. place before fill-.

ing ;of the mold has been completed,a resulting advantage of the-instant processis that approximately 15-20% fication; Littleton adds solid unmelted particles to a.

molten casting batchtoserve as nuclei aroundwhich the molten material can solidify so as to r'esultjin a heterogeneous crystalline arrangement. Littleton expressly -specifies,-and it is obviously essential to the success of his process, that the granular particles remain unmelted. even after final solidification .ofthe molten material. Thus,.by,virtue .of the Littleton-method,1each granular particle will be surrounded by a layer of cooled molten material to which itfmay or; may not be bonded and which, in turn, is connectedto .its neighbors by flat contact points such as might be formedwhena number V of balls are placed together. Asa result, tiny cavities or:

Thereafter; the block shouldbe cooled with filled only with porosity will form, since the when it freezes on the granular particles;

Similarly, the instantprooess is significantly different from that disclosed in Linder Patent No. 1,878,870 whichteaches the addition of molten material around solid lumps of the same material. Due to'therelativequantities of solid. and molten material used in the Under process, and sincethe individual lumps'of materiatcannot be significantly -compressedfurther than their initial size, the newly formed skin of refractory material-Which solidifies around the dumps will be put under significant tension as it shrinks.- The-resultant--block will'xbeextremely-weak and fragile as: a result of these internal stresses. Annealing .serves to relieve-these stresses only A patent. in .point is. Patent'No: 2,294,170,; issued-rto' Francis et al. This patent discloses a metal casting process in which solid particles are-"addedto'a casting so as to eliminate defects in the casting such as center zone porosity, pipe, etc. The primary embodiments taught in Francis et al. involve the addition of such solid material to the casting after the mold is substantially filled with molten metal. Because of the high thermal conductivity of molten metal as compared to refractories, the Francis et al. technique is satisfactory, since the skin of solidified material in the metal casting does not form immediately. This is illustrated by the fact that a crucible of molten metal may be exposed to room temperature until the body of molten metal cools to a temperature quite close to the melting point of the metal before any surface crust or skin will form.

In contrast to this, refractory materials, such as are involved in the instant case, will form a surface skin almost immediately, and the center of the body of refractory material will remain liquid for many hours. The refractory crust which forms is sufficiently high in thermal insulation that it prevents the interior from losing heat. This property creates difficulties in introducing cold material after the mold cavity has been completely filled. More specifically, the addition of cold solid particles to the casting after the mold filling operation would result only in the formation of a pocket of uncohered granular particles which would not be able to pick up enough heat to fuse and which would not remove heat from the lower portion of the mold progressively so as to cause progressive crystallization of the material from the bottom upwards.

The present invention solves this problem through the addition of the solid material concurrently with the pouring of the molten material after the mold has been brought up to temperature with an initial fill of molten material only, as has been previously described.

The Francis et al. patent also describes an embodiment (see FIGURE 9 and pages 6 and 7 of the Francis et al. specification) in which solid particles are not added to the molten material of the casting until after the casting has received an initial fill of molten material. Reference to page 7, lines 18, of the Francis et al. specification, however, makes clear that this embodiment involves the addition of the solid material immediately adjacent the walls of the mold, the sole purpose of this addition being to quickly solidify the peripheral portions of the mold. This procedure is, of course, diametrically opposed to that folmolten material-will shrink"- mold-with molten material only so as to bringthe mold body up to temperature-and-toformthe relatively denseand-non-porous portion ofthe casting which iresults from rapid-chilling-ofthis portion: This-initialstep isa critical one as far asthepresent'invention is concerned, since if the continuous mixing method' of .Frornson were em ployed with refractory materials, theinit-ial material in the mold would not reach the intimate-mixing stage and the resulting casting would be, at-least in part, crumbly and porous.

In actuality, the: castingof metals is "quite different from --the casting of refractory materials, to which the instant invention i-s-d-irected.= Rferact-ory materials are not similar to metals in the senseof having discrete melting points, nor are they similar to some alloyswhich have a slushy range between the liquidusand solidus temperatures at-whichcrystals are in equilibrium with liquid metal of a somewhatdifferent-composition. On the contrary, the refractories-of the" presentinvention have a glassy matrix which *fillsthe spaces between crystals of refractory oxides. This glassy phase remains as a glass to some degree, and has the long extended softening range characteristic of glass.

When used in the specification and claims, the term excess sensible heat and excess heat of fusion of the molten material is to be construed to refer to that quantity of heat which would cause the casting to remain molten, in whole or in part, after the pouring process has been completed.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. A method of forming refractory castings whose outer shell cools sufficiently faster than its inner portion and which shrinks sufficiently in solidifying to result in the formation of internal voids, comprising: filling the lower portion of the mold in which the casting is being formed with an initial quantity of molten refractory material until said mold is approximately one third full to form the relatively dense and non-porous bottom portion of the casting which results from rapid chilling of this portion; mixing with the subsequently added molten material during the mold-filling operation a quantity of solid particles of said refractory material such that said particles withdraw from at least the central portion of said molten material the excess sensible heat above its fusion point and at least a portion of its excess latent heat of fusion; said solid particles being heated sufficiently by said molten material to cause them to reach a temperature between that at which they are fully solid and that at which they are fully liquid so that they lose their discrete shape and mix intimately with said molten liquid.

2. A method of forming refractory castings whose outer shell cools sufiiciently faster than its inner portion and which shrinks sufliciently in solidifying to result in the formation of internal voids, comprising: filling the lower portion of the mold in which the casting is being formed with an initial quantity of molten refractory material until'said molten refractory material is approximately 4 inches above the bottom of said mold to form the relatively dense and non-porous bottom portion ofthe casting which results from rapid chilling of this portion; mixing with the subsequently added molten material during the mold-filling operation a quantity of solid particles, of

said refractory material such thatsaid particles Withdraw from at least the central portion of said molten material the excess sensible heat above its fusion point and at least a portion of its excess'latent heat of fusion; said solid particles being heated sufficiently by said molten material to cause the glass matrix in said solid refractory particles to soften sufliciently so that they lose their discrete shape: and mix intimately with said molten liquid but not sufficiently so that complete fusion of the crystals in said particlestakes place.

3. A method of forming refractory castings whose added so that said particles withdraw from at least the central portion of said molten material the excess sensible heat above its fusion point and-at least a portion ofv its excess latent heat of:fusion; suflicient heat being removed from said-molten "material'thr'ough. the ,addition'of said solid particles to resultin substantialequalization of the temperature of'the-casting' atatleast the annealing tem-. peraturegof the refractory'material; said solid particles beingheated sufiiciently by said molten material .to cause;

them to reach a temperature between that at which they are fully solid and that atrwhich they are :fully liquid so that they lose their discrete shape and mix intimately with outer shell cools sufficiently faster than its inner portion and which shrinks sufiiciently in solidifying to result in the formation of internal voids, comprising: filling the" lower portion of the mold in which the casting is being formed with an initial quantity of molten refractory material until said mold is approximately one third full to. form-the relatively dense and non-porous bottom portion of the casting which results from rapid chilling of this portion; pouring additional-refractory material into said mold while metering solid particles of said refractory material into said molten material as it is poured into. said mold; a sufiicient quantityof said solid particles being said molten liquid; and cooling said/casting with suflicient uniformity to prevent major cracking; J

Referenees Cited bythe Examiner DONALL H. SYLVESTER,ZPrimary Examiner.

WILLIAMVB. :KNIGHT, Examiner;

Claims (1)

1. A METHOD OF FORMING REFRACTORY CASTINGS WHOSE OUTER SHELL COOLS SUFFICIENTLY FASTER THAN ITS INNER PORTION AND WHICH SHRINKS SUFFICIENTLY IN SOLIDIFYING TO RESULT IN THE FORMATION OF INTERNAL VOIDS, COMPRISING: FILLING THE LOWER PORTION OF THE MOLD IN WHICH THE CASTING IS BEING FORMED WITH AN INITIAL QUANTITY OF MOLTEN REFRACTORY MATERIAL UNTIL SAID MOLD IS APPROXIMATELY ONE THIRD FULL TO FORM THE RELATIVELY DENSE AND NON-POROUS BOTTOM PORTION OF THE CASTING WHICH RESULTS FROM RAPID CHILLING OF THIS PORTION; MIXING WITH THE SUBSEQUENTLY ADDED MOLTEN MATERIAL DURING THE MOLD-FILLING OPERATION A QUANTITY OF SOLID PARTICLES OF SAID REFRACTORY MATERIAL SUCH THAT SAID PARTICLES WITHDRAW FROM AT LEAST THE CENTRAL PORTION OF SAID MOLTEN MATERIAL THE EXCESS SENSIBLE HEAT ABOVE ITS FUSION POINT AND AT LEAST A PORTION OF ITS EXCESS LATENT HEAT OF FUSION; SAID SOLID PARTICLES BEING HEATED SUFFICIENTLY BY SAID MOLTEN MATERIAL TO CAUSE THEM TO REACH A TEMPERATURE BETWEEN THAT AT WHICH THEY ARE FULLY SOLID AND THAT AT WHICH THEY ARE FULLY LIQUID SO THAT THEY LOSE THEIR DISCRETE SHAPE AND MIX INTIMATELY WITH SAID MOLTEN LIQUID.

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