US2698469A - Method of preventing piping in ingot molds - Google Patents

Description

United States Patent METHOD OF PREVENTING PIPING IN INGOT MOLDS Leonard Joseph Mueller, Milwaukee, and John Raymond Bach, Cedarburg, Wis., assignors to The Falk Corporation, Milwaukee, Wis., a corporation of Wisconsin No Drawing. Application March 8, 1950, Serial No. 148,496

4 Claims. (Cl. 22-216) This invention relates to the steel making and foundry art, particularly to the forming of ingots and castings from steel and to a composition of matter and method of preparing the same which may be employed in the granular bulk form as a cover or blanket to assist in maintaining ingot tops, gates, risers and the like in a suitably fluid condition up until solidification of the ingot or casting to improve its quality; and it resides more specifically in a blanketing composition prepared by subjecting a colloidal dispersion of a minor proportion of refractory mineral substance in a major proportion of fusible carbonaceous material to high temperature in an oxidizing atmosphere such as to cause combustion of the carbonaceous ingredient to the point where volatile matter is eliminated and the proportion of residual nonvolatile carbon in the resulting product is reduced to 30% or less.

Heretofore a wide variety of granular bulk materials have been employed to form a floating layer on the upwardly exposed molten metal surfaces of poured ingots or gates, risers, or sink heads of castings for the purpose of impeding loss of heat therefrom and the access of atmospheric oxygen thereto. The intended purpose of so-called risers or sink heads is only served to the extent that the same remain fluid so as to remain effective to supply molten metal to the solidifying and contracting casting proper. Because of the comparatively high solidification temperature of steel the provision of adequate sink heads or risers and the maintenance of the necessary fluid conditions therein has presented serious diificulties.

Some have sought through blanketing compositions to provide thermal insulation suflicient to ensure retention of fluidity in the riser for the requisite interval but the compositions heretofore available have not proven entirely satisfactory for this purpose. In another approach the aim is to supply heat to the riser from an external source either by flames or by causing a strongly exothermic chemical reaction to take place on the surface of the riser or by striking an electric are between an electrode and the exposed surface of the riser. The costs of the latter expedients, however, are excessive and composition changes induced in the riser metal thereby are sometimes disadvantageous.

An effort has also been made to obtain the combined efiect of a heat insulating layer and an exothermic composition by employing a blanketing composition made up chiefly of granules or other small particles of carbon such as powdered charcoal, ground coke, carbonized seed husks, graphite fiakes and the like, sometimes diluted with particles of mineral material such as ashes, ground slag, clay, and mica. When such a blanket is employed, air diffusing into the lower parts of the blanketing layer enters into combustion to some extent with the carbon present causing liberation of some heat. The gain from this action, however, is not realized. The more active convection which is induced thereby adversely aflz'ects the heat insulating properties of the blanket. Inert blanketing materials when properly selected actually are more effective in delaying the dropping of the temperature of the riser. Inert refractory blanketing materials, however, are inclined to induce cavitation, piping and carroting of the riser for other reasons even though its temperature remains high.

The presence of carbon in a blanketing layer on the other hand acts to advantage in that the carbon content of the riser metal is increased and the solidification tem- 2,698,469 Patented Jan. 4, 1955 perature of the riser metal is thus lowered which deters piping, carroting and cavitation. Also the suitability of the riser as scrap for remelting is enhanced where oxidation has been avoided.

It is the discovery of this invention that the advantages of an inert refractory blanketing material in delaying temperature drop, and the advantage of a carbon containing blanketing material in avoiding carroting and decarbonization may both be realized in a single composition without entailing the disadvantages of either. This is accomplished in this invention by providing a granular bulk material, each granule of which is made up of a cellular porous body in which inert refractory material is the predominant constituent and in which there is intimately dispersed, at least to the degree of a colloidal dispersion, a minor proportion of carbon. Such a granular bulk material is distinct from a composition formed of a mere gross mixture of ground coke, charcoal and the like with particles of refractory inert material such as clay, sand, ashes and the like and the results in use are markedly and surprisingly different.

In preparing a composition in accordance with this invention, for example, a major proportion of a fusible carbonaceous substance such as pitch may be brought to a fluid condition and intimately mixed with a minor proportion of an inert mineral refractory substance held in solution, as for example, there may be added 5% by weight of an alkali silicate solution to which there is then added with mixing a substance such as ammonium chloride effective to precipitate colloidal silica gel. The whole is thoroughly mixed to disperse the colloidal mineral constituent while heating and then strongly ignited in an oxidizing atmosphere until volatile matter is substantially eliminated and a residue containing less than 30% of carbon by weight is obtained. The hard porous mass is then ground to form granules of convenient size, say from 0.02 to 0.001 inch, whereupon it is ready for use.

Instead of precipitating silica in situ, as it were, certain naturally occurring inert refractory colloidal substances such as colloidal clay, sometimes called bentom'te, and the like may be dispersed in minor proportion, that is from 1% to 10%, directly in the pitch while in the fluid condition above mentioned to replace all or part of the silica. After thorough mixing the mixture is ignited and ground, as above, to produce a hard granular residue in which carbon is a minor constituent. Other artificial or naturally occurring colloidally subdivided refractory mineral matter including the aluminum silicates may be employed as well.

It has been discovered that the structure of the granules, effective for the purpose of this invention, appears only to be developed by igniting and burning an extremely fine dispersion of inert refractory material in minor proportion in a major proportion of fusible carbonaceous material until the volatile matter is eliminated and carbon is burned away until it is but a minor proportion {30% or less, down to 4%) of the finished composition.

If desired the mixture to be ignited may be first subdivided into small particles or fragments and these fragments ignited to produce the finished granular product. Care should be exercised in so doing to avoid producing a too finely powdered product since best results are obtained where the granular size is kept approximately in the range of from 0.02 inch to 0.001 inch.

A convenient and economical method of preparing a suitable composition in accordance with this invention is to select a naturally occurring colloidal dispersion of refractory mineral material in a fusible carbonaceous matrix such as bituminous coal which is then directly subjected in mass or in granules to oxidizing ignition such that there remains in the finished product from 4% to 30% of carbon and from 96% to 70% of inert refractroy substance or ash, the same being intimately dispersed with the carbon. Most bituminous coals contain sufficient ash resulting from colloidally dispersed mineral matter to be suitable for the purpose and this mineral matter is sufliciently refractory no to be fused at the temperature of the molten steel. For purposes of economy, however, it is preferred that a coal be selected which has a high fusing ash and which at the same time is high in ash content since the yield of useable material is consequently enhanced.

An economical source of material suitable for the purposes of this invention may at times be the solids carried in the flue gas from a furnace burning solid bituminous coal at high rates on grates. In such a case small fragments of coal break away and while suspended in the flue gases are ignited to the point where the carbon content thereof is reduced to 30% or below before being dropped from the flue gases into the furnace breaching. This material when collected is frequently ready for use as a riser blanket without further treatment.

In a typical instance of use of the composition of this invention and as a comparison of the same with a typical chemical exothermic blanket, being the best blanketing material, known to applicants, prior to the improvement of this invention, the mold for a heavy steel casting of a large gear was provided with risers having an approximate cross sectional area of 272 square inches. The mold was poured to fill the risers to an effective depth of 13 inches and the tops of part of the risers were immediately covered with a layer about 4 inches thick formed of the granular bulk composition prepared as above described, and the remaining risers were blanketed with the typical chemical exothermic blanket. Upon breaking out the casting it was found that the tops of the risers blanketed with the composition of this invention had shrunken with uniform paraboloid depressions approximately 4 inches in maximum depth while those blanketed with the exothermic composition were piped or carroted to within a fraction of an inch of the full 13 inch depth of the risers. Upon the basis of this experience the volume of metal contained in risers to ensure a sound casting when blanketed by the composition of this invention could be safely reduced by at least 50%. In actual foundry experience since the above test it has been found that riser bulk may be thus markedly curtailed without unsound castings resulting.

Inasmuch as steel foundry practice has been such that the weight of metal contained in risers has been from 30% to 50% or even higher of the metal poured, any such saving in riser requirements as is made possible by the composition of this invention, has a marked bearing upon the foundry economy as a whole. With consideration given to the numerous compositions heretofore employed as riser blanketing materials the gain or saving made possible by the instant invention is marked and surprising.

In use, the blanketing composition of this invention exhibits the high thermal insulating properties charac teristic of inert blanketing materials since there appears to be little of the rapid convection set up by compositions containing granules of high carbon content. This appears to be due to the reduced accessibility of the carbon due to the nature of its dispersion as well as to the compartively small concentration of the same. At the same time the carbon appears to be present in such a condition and in such amount as to ensure a non-oxidizing atmosphere at the surface of the metal since an actual carbon increase rather than a decrease can be measured near the upper surface of the solidified riser.

The gradual and delayed solidification of the riser due to the heat insulating effect of the blanket composition of this invention, coupled with an absence of composition change which would cause an increased solidification temperature of the metal appear to be the factors which deter piping, carroting and cavitation. These results in turn appear to flow directly from the amount and nature of dispersion of the carbon within the granules making up the composition.

We claim:

1. The improved method of delaying solidification of molten riser metal and the like to avoid piping and carroting thereof during casting, which consists in covering the exposed surface of the molten metal with a bulk material composed of small independent discrete porous cellular granules the substance of said granules consisting substantially, uniformly throughout of inert mineral refractory material in proportion of from to 96% by weight and non-volatile carbonaceous material in proportion of from 4% to 30% by weight, each of said materials being colloidally interspersed with the other.

2. A method in accordance with claim 1 wherein the mineral constituent of the granules consists of bituminous coal ash constituents.

3. A method in accordance with claim 1, wherein the mliineral constituent of the granules consists of colloidal s1 ca.

4. A method in accordance with claim 1, wherein the nfineral constituent of the granules consists of colloidal c ay.

References Cited in the file of this patent UNITED STATES PATENTS 1,920,854 Gathmann Aug. 1, 1933 2,250,009 Cable July 22, 1941 2,276,671 Rentschler et al Mar. 17, 1942 2,462,255 Charman Feb. 22, 1949 2,514,793 Pletsch July 11, 1950 FOREIGN PATENTS 449,713 Great Britain July 2, 1936 OTHER REFERENCES Clays, by Ries, Third Edition, pages 1 and 185. Refractory Minerals Their Manufacture and Uses,

by A. B. Searle, Third Edition, pages 21-23.