US3515201A - Method of casting - Google Patents

Description

United States Patent 3,515,201 METHOD OF CASTING Philip D. Zimmerman, Crystal Lake, lll., assignor to Amsted Industries Incorporated, Chicago, Ill., a

corporation of New Jersey No Drawing. Original application Sept. 15, 1965, Ser. No. 487,575. Divided and this application Nov. 14, 1967, Ser. No. 682,969

Int. Cl. B22d 23/00; B22c 3/00 US. Cl. 164-66 8 Claims ABSTRACT OF THE DISCLOSURE This application is a division of application Ser. No. 487,575, filed Sept. 15, 1965', now abandoned.

No drawing accompanies this specification.

The invention in its broadest aspects is related to providing a smooth surface on the casting and protection of the mold material from chemical attack or erosion by the molten metal.

Castings often have irregular or roughened surfaces that result from several causes, including defects in the surface of the cavity. The defects in the surfaces of the cavity may result from improperly applying a coating thereto or using improper materials in the coating, such that in the actual pouring operation, portions or particles of the coating are dislodged therefrom and a defective surface thereby results.

Castings which have surfaced efects are required to be surface conditioned to provide the desired and necessary smooth surface, but this operation is quite extensive and correspondingly expensive, and an additional objection exists in that the surface of the casting even after being so conditioned is not always in perfect condition.

A broad object of the present invention, therefore, is to provide a novel method of casting, including a method of applying the coating, in the use of which the objections noted above will be overcome.

A more specific object is to provide a method of casting which effectively eliminates such imperfections as laps, wrinkles, and folds in the surface of the cast article.

Still another object is to provide a method of pouring and casting in conjunction with a novel mold coating, which aids in overcoming objections referred to above in connection with previous casting operations.

Attention is now directed to the following detailed description of the composition of the mold coating and the method of applying the coating to the mold, as well as the method of pouring, constituting one example of the invention.

The invention has particular adaptability to graphite molds. Graphite possesses high thermal conductivity, and as the molten metal flows into the mold cavity, the rapid conduction of heat from the molten metal often causes wrinkles and other surface defects in the metal being cast which, at that time, may be freezing on the surfaces of the cavity. Those wrinkles and other defects are extremely objectionable, and when they occur the castingmust be surface conditioned, as noted above. The coating of this invention forms an effective heat barrier between the meta1 being cast and the graphite, and the method of apply- "ice ing the coating renders the coating more effective, and additionally the method of pouring of the invention assists in prevention of wrinkles and other defects in the cast material. In the pouring operation, the molten metal must reach a quiescent state before it freezes in order to form a perfect casting, since if any portion of it should freeze before it reaches a quiescent state, the casting will not be shaped perfectly according to the mold cavity. The invention has particular adaptation to pressure pouring, i.e., in which the molten metal is poured into the mold from the bottom as contrasted with gravity pouring in which the molten metal is poured into the mold from the top.

Additionally, in the pouring operation, the molten metal must not attack the coating in such manner as to impair it by causing portions or particles of it to be dislodged, and thereby forming defects or irregularities in the casting corresponding to the defects in the surface defining the cavity.

A mold coating has two main functions; (a). to impart a smooth surface to the casting and (b) to afford protection to the mold material from chemical attack or erosion by the molten metal. The ability of a coating to accomplish (a) is a function of its thermal conductivity, the inherent properties of the material with respect to wetting by the metal and the effectiveness with which the coating is applied. On the other hand, with respect to (b), the ability of the coating to protect the mold material is controlled by the refractoriness of the coating and its ability to adhere to the surface to be protected. These two functions of the coating tend to be mutually exclusive whereby, in attempting to provide a coating of optimum characteristics and effectiveness, there must be a compromise with respect to these main features.

The features of the invention are adaptable to use in molds of different sizes and shapes, as for example molds having rectangular cavities and those having cavities of other shapes such as cylindrical for casting rounds. These different shapes will have significance in connection with the invention from the standpoint of heat transfer from the molten metal through the coating to the walls of the cavity which is affected by the mass of molten metal as well as dimensions in linear directions, as well as other factors, together with rate of filling, temperature of the mold during filling, etc. The manner in which these different kinds of molds are affected. or have significance will be pointed out hereinbelow.

The coating of the present invention includes a refractory oxide such as alumina (Al O or silica (SiO as the most important constituent. Alumina is preferred in most instances, particularly where large shapes of steel are being cast and high temperatures and much heat are encountered, although other materials have been found satisfactory such, for example, as zirconia (ZrO and combinations of these various oxides. This oxide is then mixed with other materials, as pointed out specifically herein below, in accordance with the preferred and specific features of the invention.

The particles of refractory oxide are milled to a very small size, e.g., as small as 2 microns. Although a completely uniform size of particles is ideal, from a practical standpoint it is not possible to attain that perfect condition, and a considerable range of sizes is permissible and results in successful practice of the invention. For example, the particles may range from 2 microns to 30 microns, while a range of between 5 and 15 microns is practically feasible and produces better results. It has been found that the greater the uniformity of size in the particles, the lesser will be the heat conductivity of the coating.

While the more nearly uniform the particle size of the 3 refractory is, the more effective the coating will be, all deviation from this condition is not necessarily critical. For example, the greater number of the particles that are within the permissible range, the greater will be the effectiveness of the coating, and any particles that are outside of this range (e.g., of larger size), while they are not as effective as the smaller particles, will detract from the effectiveness of the coating only as a matter of degree and will not by their mere presence render the coating ineffective. More specifically, it has been found that if the refractory be ground to a condition in which 90 percent of the particles lie within the desired small range of, for example, 2 to 20 microns, or in the more practical range of 5 and microns, a coating made of particles of this size will be very effective.

It is desired that the coating be of low conductivity and a correspondingly good heat insulator, so that in the operation of pouring the metal into the mold, the heat of the molten metal is not conducted away so rapidly as to enable the molten metal to freeze in the pouring operation. In other words, as much as possible of the heat of the molten metal should remain therein until the pouring step is completed and the cavity is filled. The coating of this invention is particularly effective in this respect.

The following table shows comparison of heat conductivity relative to the mean particle size in microns.

Mean particle size Heat transfer in microns: value in seconds Denser coatings are more effective for protecting the mold as contrasted with providing a smooth surface on the casting, and where protection of the mold is of primary importance a dense coating is preferred. The density of the coating is very determinative of the thermal properties of the coating. Density is dependent upon particle size distribution in the coating material, the technique of application, as well as the binder used. It has been found that the greater the dispersion of the particles in the coating material, the less is the density.

Density is also dependent upon the temperature of the mold surface, as will be brought out herein below.

Referring again to particle size distribution, this phenomenon influences the packing of a sprayed coating. Narrow size distribution results in less packing and lower rate of heat transfer. On the other hand, wide distribution results in the opposite phenomenon or greater heat transfer. Particle size distribution, as used herein, refers to the range of sizes of the particles, e.g., a narrow size distribution may be, for example, between 5 and 10 microns while a wider size distribution may be between, for example, 2 and 20 mircons.

The binders used in mold coatings have considerable effect on heat transfer of the coating. In one case, VeeGum, which is a micaceous material, was utilized and this resulted in a loose coating, while in contrast thereto, colloidal alumina produced a tightly packed coating. Under the same conditions, the former transmitted a given quantum of heat in 48 seconds, as compared with 29 seconds for the latter, indicating lesser heat conductivity in the former.

An addition of ball clay in the amount of between about .25% and 4.0% was found to give sufficient adherence for many applications, and still resulted in a low density coating, but an amount of about 1.0% was most satisfactory in most applications.

With reference to the temperature of the mold surface being coated, since a water suspension of the coating is sprayed onto the surface, there must be enough heat available to vaporize the water, but not enough to cause such a violent vaporization that the coating material is blown off. The preferred upper temperature limit for graphite is in the neighborhood of 325 F. to 400 F., depending upon the binder used and the spraying technique. The other extreme in coating density occurs when the surface temperature is between 180 F. and 225 F. At these lower temperatures the water does not boil immediately upon contact with the surface, but instead, the vaporization takes place from the outside surface of the coating, allowing the solid material to form a compact and extremely dense layer. When this type of drying is occurring, water vapors can be seen coming from the surface for 10 to 15 seconds after spraying.

In the case of a mold for casting rounds, higher mold preheat temperatures may be utilized and may be preferred as compared with molds having rectangular cavities. For example, a preheat temperature of 400 F. in the case of casting rounds is very common and temperatures within a range of about 375 F. to 500 F, normally give satisfactory results. Also in the case of casting rounds mold preheat temperatures may exceed 500 without having deleterious effects on the casting, but in the case of the higher temperatures there is more likelihood of rapid oxidation of the graphite after casting. In all cases, higher preheat temperatures of the mold reduce the rate of heat extraction from the casting, and this facilitates maintenance of uniform cooling, and reduces the rate of contraction of the skin of the casting and consequently stresses are reduced and made more uniform, but as indicated above, upper limits must be given proper regard from the standpoint of deleterious effects on the graphite of the mold. Insofar as the specific practice of preheating the mold is concerned, the same practice may be utilized in the case of molds for rounds and other items.

Spraying a graphite mold at too low a temperature may have another adverse effect in that graphite will pick up and hold water, as steam, at temperatures above the boiling point of water. Tests have shown graphite will hold steam for some time at temperatures up to 400 F. Above 325 F. the time is of the order of fifteen minutes or less, while at temperatures less than 250 F., steam is still retained after one hour.

An interesting phenomenon is that the thermal conductivity characteristics of the refractory portion of the coating may not be as determinative as other factors such as binder, or as application techniques. However, it must not be overlooked that in any event it is desired that the thermal conductivity of the coating be reduced to an absolute minimum, and the refractory material should be selected to that end.

According to the invention, the coating is preferably applied in the form of a water slurry in which the proportion of solids to water is such as to maintain a viscosity thereof near that of water. The slurry is supplied by means of a spray gun which may be either pneumatic or hy draulic.

The mechanism of coating adherence to the mold surface is that of vaporizing the water from the spray shortly after contact, leaving a layer of solids held to the surface by the binder in the formulation. If the mold is too hot, water droplets containing the solids are kept from intimate contact with the mold surface by steam and the coating will not adhere. Too low temperature allows the spray to wet the surface and run, and vaporization in this case is too slow and occurs only from the outside surface of the coating.

By applying the coating in a plurality of layers, it is possible to have the water vaporize slowly at the mold surface as the first layer is sprayed on, and then to pro gressively vaporize outwardly as additional layers are sprayed on. The steam leaving through the undried coating in this manner results in a coating of lower density and uniform depth. The rate of application of a coating is limited by the amount of water which can be vaporized by heat from the mold surface. As the depth increases, the time for vaporization increases. Most formulations can be applied at a rate of between approximately 0.0015" and .005" per pass with a spray gun, while layers of .003 have been found to produce excellent results in most instances. The time required for vaporization of water from a single layer is about one second. The final coating depth, as in the case of other features, will depend somewhat on the kind of mold involved. For example, in the case of molds having rectangular shaped cavities the depth may be between about 0.005 and 0.045", although it will be understood that variations from these dimensions are permissible. Generally the greater the depth of the mold coating, the less will be the heat conductivity and the less rapid freezing of the surface of the casting.

In the case of molds for rounds, a silicon oxide (SiO type of mold coating is preferable, (see below) particularly for small diameters such as in the neighborhood of 6" to 8 and in this case the thickness of the mold coating should be in the neighborhood of 0.015 to 0.020". For molds having cavities of greater diameter, an alumina (A1 0 is preferred (see below) and in this case preferably the thickness of the mold coating should be in the neighborhood of 0.030 to 0.040".

In the case of all molds, it is important that the mold wash coating be applied uniformly in thickness over the entire cavity surface so as to produce even heat extraction from the casting, since uneven heat extraction is a major cause of splits in the surface of the casting.

The refractory material utilized in the coating must withstand the usual high temperatures of, and chemical attack from, molten steel.

The binder must be such that, when used in small percentages, it will cause the refractory material to adhere to the mold surface. The binder, like the base material, must also be hydration stable and should not detract from the refractoriness of the coating. The most successful binders used in this investigation are various ball clays. Bentonite produces a very dense coating, but does contain some water, thereby eliminating it as one of the more successful binders.

Suspending agents and dispersing agents are of secondary importance since they have very little effect on the properties of the coating. Suspending agents generally include a gel, used to retard settling of the refractory material in the spraying system. The quantity used is generally of the order of less than 0.5%. The dispersing agent or electrolyte is used to deflocculate particles of the refractory powder. This has some influence on the apparent particle size of the oxide, thus making suspending easier. Also, by complete dispersion of the particles, the density of the coating is reduced.

To illustrate the relative proportions of these various ingredients, several typical compositions are listed below:

SiO -2400 g. VeeGum-24 g. (binder) Carboxy methyl cellulose2.4 g. (suspending agent) Waterl000 to 2000 cc.

Al O -2l00 g. Ball clay21.0 g. Carboxy methyl cellulose2.1 g. Acetic acid2.5 (dispersing agent) Water-4000 to 2000 cc.

g. Ball clay42 g. C.M.C.2.1 g. Acetic acid2.0 g. (dispersing agent) Waterl000 to 2000 cc.

The rate of pouring the molten metal into the mold is of importance in predetermining the desired results in the casting operation and on the cast article Experiments conducted have shown variations in surface conditions of the cast article with different rates of pouring. It has been found that slower rates of pouring tend to produce rough surfaces on the casting while faster rates accelerate mold wear. The mold wear is due to mechanical erosion of the mold coating, followed by both chemical reaction, solution, and mechanical erosion of the graphite. The desired pouring rates will differ according to various factors involved, e.g., the grade of steel being cast, the thickness of slab, etc. The following is believed to be the explanation of the variation in results produced by different rates of pouring but, however, the invention is not limited to this explanation as an essential theory: there is required to be a certain amount of superheat, i.e., heat above the freezing temperature, in the metal as it fills the mold. The heat from the molten metal is conducted therefrom into the mold, and as the heat of the metal drops below a predetermined quantity the metal may freeze too soon and before it properly fills the mold. The greater the mass of molten metal in the mold the greater the reserve quantity of superheat will be, and the heat will be conducted into the mold at a slower proportional rate than in the case of lesser masses. Therefore, generally speaking, and other things being equal, the greater the mass of molten metal the slower will be the permissible rate of pouring, the mass in this case being generally proportional to the dimension between the closest spaced opposed walls. Considering another factor affecting the rate of pouring, it is pointed out that carbon steels when cast normally have less superheat than stainless steels and must be poured more rapidly than stainless steels to produce the same result in accordance with the foregoing postulation.

Pouring rates also will vary according to the size and character of the mold cavity. For example in the case of molds having rectangular cavities pouring rates may be utilized in the range of between about 0.5" and 5.0" per second vertical rise in the mold. The pouring rates will vary in inverse relation to the width of the cavity i.e., the thickness of the cavity in direction between the most closely spaced opposed surfaces, in a rectangular cavity. The lesser this distance is, the faster should be the pouring rate. Generally the pouring rate will be greater for rounds than for rectangular castings, for any diameter of round similar to the thickness of rectangular cavity. In the case of a rectangular cavity, and in casting stainless steel of the 200, 300 and 400 series, a pouring rate of 1.0:25 is most satisfactory, while in the case of low carbon and low alloy steels, a rate of 1.58":25%, has been found highly satisfactory.

In the case of rounds, pouring rates are preferably between about 1.5" and 2.5 per second rise, inversely in accordance with the diameter of the cavity, i.e., the higher rate should be employed for rounds having a diameter in the neighborhood of 6" while the slower rate is utilized for those of the diameter in the neighborhood of 26", with interpolated rates for other diameters.

It is also of importance in connection with the above specified rates of pouring that the mold be maintained at a certain predetermined temperature in the pouring operation. The temperature range of the mold mentioned above has been found satisfactory.

The temperature of the molten metal in the pouring operation is another factor affecting the condition of the finished casting. The effect produced by the temperatures of the molten metal is closely related to the temperature of the mold, in that the higher the: temperature of the molten metal the greater length of time will be required for the temperature thereof to be lowered suificiently to freeze, or partly freeze, and thereby cause defects in the surface of the casting. Generally, the smaller the mold cavity, the greater should be the temperature of the 7 molten metal; similarly, in the case of rounds the pouring temperature should be higher than in the case of rectangular cavities; also low carbon steels should be poured at a higher temperature than high carbon steels and stainless steels.

The following are representative examples of pouring temperatures; in the case of Type 304 stainless steel a temperature of 2900 F. is found satisfactory in pouring large diameter rounds of in the neighborhood of 20 to 26" in diameter, while in the case of rounds in the neighborhood of to 6", a temperature of 2925 F. should be utilized; in pouring 26" diameter rounds of low alloy steels in the neighborhood of .15 carbon steel, a temperature of in the neighborhood of 2930 F. could be utilized, while on the other hand in pouring .50 carbon steel the temperature of the molten metal might be reduced to 2850 F. In general the purpose of the higher temperatures for casting rounds is to increase the initial heat input into the mold prior to the contractionof the skin of the casting so that there will be a reduced amount of initial heat extraction from the casting followed by a more uniform heat extraction.

An additional factor of the invention is that the atmosphere of the mold cavity should be purged so as to remove the oxygen therein which would react with the molten steel as it enters the mold. Such reaction when permitted to occur forms a film on the top of the rising metal surface which may freeze to the mold and form a fold or lap on the surface of the casting. It has been found that the atmosphere can be purged effectively by filling the cavity with such inert gases as argon or nitrogen, neither of which combines with the molten metal, from a practical standpoint, leaving the surface of the metal perfectly smooth and pure. It is also practicable to introduce a material into the mold cavity which combines with the oxygen present and prevents reaction between the oxygen and the molten steel. An example of such material is magnesium in fine or small form, such as shavings or shreds. It is also practicable to use a combination of these two steps for purging the mold cavity.

While I have herewith disclosed a certain preferred embodiment of the invention, it will be understood that changes may be made therein within the scope of the appended claims.

I claim:

1. A method of reducing surface defects in a bottom pour casting mold having graphite portions with surfaces defining a cavity, comprising the steps applying a coating including refractory oxide to said surfaces of between approximately .005" and .045" in thickness, maintaining the temperature of the mold between approximately 250 F. and 400 F. in the step of applying the coating, and pouring molten steel into the cavity, 'while maintaining the mold at above 212 F. after the application of the coating and until the pouring step.

2. A method of casting according to claim 1 in which the temperature of the mold is maintained at approximately 300 F. after the application of the coating and until the pouring step.

3. A method of casting according to claim 1, in a plurality of molds having cavities of different opposed wallto-wall dimensions, and varying the rate of pouring inversely to such dimensions.

4. A method of casting according to claim 3 in which the rate of pouring is within a range of about 0.5 to 5.0" per second vertical rise in the mold.

5. A method of casting according to claim 4 in which stainless steels of the 200, 300, and 400 series are poured at a rate in the range of about 0.75" to 1.25" per second vertical rise in the mold.

6. A method of casting according to claim 4 in which low carbon and low alloy steels are poured at a rate in the range of about 1.18 to 1.98" per second vertical rise in the mold.

7. A method of casting according to claim 1 in which the cavity of the mold is purged of oxygen before the pouring step.

8. A method of casting according to claim 7 in which the purging step is accomplished by introducing an inert gas of a group including argon and nitrogen into the cavity.

References Cited UNITED STATES PATENTS 1,583,248 5/1926 Durville l64133 3,023,119 2/1'962 Anderson et a1. 164-14 X 3,184,815 5/1965 Reuter 1-6472 3,230,056 l/l966 Arant et a1. 164-14 3,402,757 9/1968 Halliday l6466 J. SPENCER OVERHOLSER, Primary Examiner J. E. ROETHEL, Assistant Examiner US. Cl. X.R. l6472, 133