EP0025818A1

EP0025818A1 - Method of casting shell molds

Info

Publication number: EP0025818A1
Application number: EP19800102627
Authority: EP
Inventors: Colin Taylor; Roger A. Hayes; James M. Allison; Donald B. Anslow
Original assignee: DEPENDABLE-FORDATH Inc; Dependable Fordath Inc
Current assignee: DEPENDABLE-FORDATH Inc; Dependable Fordath Inc
Priority date: 1979-09-10
Filing date: 1980-05-12
Publication date: 1981-04-01
Also published as: JPS5641044A

Abstract

A method of casting shell molds is disclosed, wherein the process of casting, reclaiming resin coated sand, and heat treating the resulting castings are combined. A bed of granular refractory material (18) is fluidized sufficiently to allow a plurality of vertical shell molds (30) formed of silica sand bonded with organic resin binderto be immersedtherein. The fluidized bed is then collapsed and molten steel is poured into the rigidly supported shell molds. Thereafter air is introduced into the bed (18), the rate of delivery of the air being controlled so that the heat of the hot steel castings is utilized to burn away substantially all of the resin binder in the molds without refluidizing the bed. Finally, heat treating of the steel castings is accomplished while they are still immersed in the bed by incrementally cooling the castings to a pluralityof preselected temperatures and maintaining the temperatures for a plurality of corresponding preselected time intervals through the controlled introduction of a spontaneously igniting combustible gas mixture.

Description

THIS INVENTION relates to a method of casting shell molds, and in particular concerns a method of casting shell molds in a fluidized-bed while thermally reclaiming resin coated sand and heat treating the metal castings.
A widely used method of casting steel where castings of particular configurations are desired involves the use of vertical shell molds consisting of silica sand bonded with cold setting organic resin binders. Frequently, cores of similar composition are suspended inside such shell molds to provide voids within the castings. It is desirable that the walls of shell molds be relatively thin in order to minimize the amounts of silica sand and resin binder utilized in the casting operation and thus minimize overall operational costs.
Generally it is necessary to provide support for shell molds of this type while the molten metal is being poured - into them in order to prevent breaking or distortion of their relatively thin walls. This necessity increases as the size and weight of the castings to be produced increases. Heretofore it has been known to provide such support by placing the shell molds into a container and then pouring into the container a granular refractory material, for example, gravel or sand, compacted if necessary, to support the molds. After the castings have been poured and have solidified, they are removed from the container. The granular support material must then be removed from the container in order that the process can be repeated.
Heretofore it has also been known to fluidize a bed of silica sand by introducing compressed air through the bottom of the bed with a controlled distribution and velocity so that a plurality of vertical shell molds can be immersed therein. The delivery of compressed air is thereafter terminated and the sand settles to rigidly support the molds in position. Thereafter molten metal is poured into the shell molds. Each shell mold is typically supported in an open-mesh basket during movement into and out of the bed and during pouring. The bed may be compacted prior to pouring by vibrating the container. The thermal conductivity of the bed and thus the rate of cooling of the molten metal may be varied by using different types of sand, for example zircon sand, or other materials such as chromates. The thermal conductivity of the bed may also be varied by changing the granular size. It has been found that the fluidized bed method of supporting shell molds is superior.
When pouring molten metal into shell molds a considerable amount of smoke and flame will often obscure the ladle operator's vision. The heat of the molten metal causes the organic resin binders present in the innermost portions of the shell mold walls to combust, thus producing toxic fumes. These fumes are normally removed by ventilation which is not entirely satisfactory since individuals are exposed to the fumes until such time as the room air is purged of the same. Another method of removing the fumes resulting from the combustion of the organic resin binders involves transportation of the shell molds into an extraction tunnel. However, such tunnels are undesirable both because they are costly and because in many cases they remove heated air which might otherwise be used to keep the foundry plant warm for the workers or used in other foundry operations.
When the solidified castings are removed from the shell molds, the molds are partially disintegrated. The same is true of the cores if any have been utilized. The partially disintegrated shell molds and cores as well as the disintegrated portions thereof generally contain significant amounts of uncombusted organic resin binder. Disposal of sand having significant quantities of resin binder adhered thereto creates both economical and ecological problems. Therefore, it is desirable to upgrade and reclaim this scrap sand for reuse.
Reclamation of resin coated sand by mechanical attrition has been practiced for a number of years. Although this method results in a certain amount of resin in the form of a carbonaceous coating remaining on the grains of sand, the presence of such a coating is beneficial as it ensures that the reclaimed sand has an affinity for the resin used in rebonding. There is a consequent economy in resin addition rates and a better casting finish.
However, sand to be used in the formation of shell molds cannot be reclaimed by mechanical attrition. This is due to the way in which shell molds are produced. Sand is precoated with resin, e.g. four percent by weight, before being bonded with more resin in a pattern to form a shell mold. Mechanical attrition leaves a small amount of carbonaceous coating on this sand. Repeated use results in sand grains with an excessive amount of coating. The shell molds formed from such excessively coated sand are unacceptably weak. Therefore, thermal reclamation, in which all of the residual resin is burned away, is the best method for recovering sand to be used in the formation of shell molds. However, the high cost of this method has prevented widespread adoption thereof.
Recently the growing cost of dumping used foundry sand and concern about the possible leaching of toxic chemicals, particularly phenol, into water supplies has resulted in the industrial use of a number of thermal sand reclamation apparatuses. One type utilizes a rotary, refractory lined kiln with an oil or gas burner. Maintenance costs associated with this type of apparatus are particularly high. Another type of apparatus in use is a multiple rotating hearth furnace. The capital cost of this apparatus is very high. A third type of apparatus in use has a shaft furnace, similar to a gas-fired cupola. This apparatus is thermally very efficient, but the quality of the reclaimed sand is variable and the capital cost of the apparatus is high.
Fordath Ltd. of Brandon Way, West Bromwich, West Midlands, B708JL, U.K., sells a thermal sand reclaimer under the trademark FORDATH FLUIDFIRE. A gas/air mixture is used to fluidize a bed of resin-coated sand through a porous refractory membrane. Combustion occurs in the bed itself, raising the temperature of the sand from room temperature to about 800°C. The bed is contained in a mineral wool insulated tank made of a special grade of heat-resisting steel. The tank is fitted with expansion joints. When the bed is at working temperature disintegrated shell molds are fed into the same. Lumps up to about two inches in size are acceptable. Excess air is provided in the fluidizing air/gas mixture for combustion of the resin, most of the heat from the resin combustion being available to heat incoming sand. The good heat exchange characteristics of the fluidized bed cause a rapid breakdown of the sand lumps and complete removal of all combustible residues. After leaving the bed the sand may enter a FORDATH fluidized bed cooler where the heat is extracted by water-cooled pipes.
It is common when casting steel to heat treat the same in order to produce uniform metallurgical conditions within the casting so that the steel is adequately hardened while still retaining sufficient ductility for easy machining.
As used herein, the term "heat treating" will refer generally to one or more steps of heating and/or cooling the steel to certain temperature levels which may be maintained for predetermined time intervals. Included within the meaning of heat treating are the various methods of tempering steel. A detailed discussion of the different methods of heat treating steel can be found in Elements of Physical Metallurgy by Albert G. Guy and John J. Hren, Third Edition, Copyright 1974.
U.S. Patent No. 3,557,867, Krzyzanowski, discloses a method and apparatus for casting combustible shell molds in a fluidized-bed of granular material. Hot combustion gases are withdrawn from the interior of the bed container through a gas-permeable circumferential wall. This patent does not address itself to either thermal reclamation of sand or heat treating of the metal castings.
U.S. Patent No. 3,683,995, Zifferer, discloses a method of producing a composite shell mold having an inner core made of facing sand bonded with a resin binder. Surrounding the inner core is a layer of coarse sand bonded with sodium silicate cured by reaction with CO₂. Column four, lines 9-10 indicate that the fine facing sand may be recovered by removing the resin with a suitable thermal reclaimer.
U.S. Patent No. 3,741,276, Fallows et al, discloses a method of casting shell molds in a fluidized-bed of granular material such as sand. The shell molds are initially placed in bags of polythene before being inserted into the fluidized-bed of sand. The apparent function of the bags is to ensure that the fluidizing gas does not pass in any substantial amount through the shell molds. During the molten metal pouring operation, the bags burn away so that combustion gases can pass through the walls of the shell molds. Also disclosed (see Figure 3) is a turntable apparatus adapted to carry a plurality of fluidized-bed containers which are indexed through a series of stations at which the various shell mold casting operations are performed. This patent also does not address itself to either thermal reclamation of sand or heat treating of the metal castings.
U.S. Patent No. 4,130,436, Hauser et al, discloses a method of thermally reclaiming sand from sand moldsbonded with sodium siliate, organic resin binders, or both clay binders and a fine carbon additive. The disintegrated sand molds are incrementally heated, first to a temperature in the range of 1400°F. to 1600°F., and then to a temperature in the range of 1800^oF, to 2200°F. Thereafter the disintegrated sand molds may be cooled or may be further heated to a temperature in the range of 2200°F to 2500°F and thereafter cooled. Column four, lines 37-41 indicate that the method of the patent may be performed in a rotary kiln, in a fluid bed calciner, or in a hearth-type roaster.
Japanese patent No. 48-15772 (according to the translated abstract) discloses a method of thermally reclaiming sand in a fluidized bed. This patent does not appear to be any more pertinent to the present invention than the FORDATH FLUIDFIRE apparatus described above.
An article entitled "Potential Improvements in Shell Mould Casting Practice" appearing in the June, 1978 issue of the JOURNAL OF RESEARCH published by the British Steel Castings Research and Trade Association (SCRATA) also discloses a method of casting shell molds in a fluidized-bed. In accordance with this article a bed of silica sand is fluidized by introducing compressed air through the buttom of the bed with a controlled distribution and velocity so that a plurality of vertical shell molds, loaded in wire mesh baskets, can be immersed therein. The shell molds are made of silica sand bonded with a resin binder. The delivery of compressed air is thereafter terminated and a plastics sheet is used to cover the bed to form an airtight seal. Molten metal is poured through the plastic sheet into the shell molds. Simultaneously with the pouring operation air is extracted from the bottom of the bed in order to remove the fumes from the resin binder burned by the molten metal.
The above-noted SCRATA article, on page 2, indicates that to reduce atmospheric contamination and to assist in the cooling of the bed, extraction is maintained during removal of the wire mesh baskets containing the castings and pieces of the shell molds. The article further indicates that subsequent fluidization of the bed for the next batch of molds has a self-cleaning effect in that "fines" separate to the top of the bed. Thermal reclamation of resin coated sand and heat treating of metal castings is not suggested. A French patent no. 2,366,078 appears to have been obtained by SCRATA.
Heretofore the casting, heat treating, and sand recovery operations have been performed independently. Significant amounts of time have thus been required to perform all three separate operations. Thermal recovery of sand typically requires large amounts of combustible gases such as butane, methane, propane, etc. Ever increasing energy costs have made it desirable to reduce the energy consumption of thermal reclamation operation.
It has been common practice to extract the castings from the bed after they have cooled to a temperature of around 1100⁰F. At this time the castings are somewhat brittle. Thereafter the castings have been placed in an oven and reheated so that they can be incrementally cooled to produce the desired tempering. Significant amounts of energy have been consumed during the reheating step.
Finally, significant amounts of scale form on the surfaces of the hot castings when they have been exposed to air. The scale is formed as a result of a reaction between oxygen and steel, is brittle and inhibits easy machining of the castings.
The present invention provides a method of casting shell molds made of sand bonded with a combustible binder, and thermally reclaiming the binder coated sand, comprising fluidizing a bed of granular refractory material sufficiently to allow the shell molds to be immersed in the bed to a predetermined depth suitable for pouring a molten metal into the shell molds; immersing the shell molds in the fluidized-bed to the predetermined depth; collapsing the fluidized-bed so that the shell molds are firmly supported by the granular refractory material; pouring the molten metal into the shell molds to form hot metal castings; and introducing oxygen-containing gas into the bed so that the heat of the hot castings is utilized to burn away substantially all of the combustible binder.
The main advantage provided by the invention is that substantially all of the combustible binder can be burnt away while the shell molds are still in the casting bed. Heat from the hot castings is thus utilised in the thermal reclamation process, allowing the amount of energy required to reclaim the sand for further use to be reduced.
In a preferred method according to the invention, a bed of silica sand is fluidized by introducing compressed air through a porous ceramic plate at the bottom of the bed with a controlled distribution and velocity so that a plurality of vertical shell molds can be immersed therein. The shell molds are made of silica sand bonded with a resin binder. The delivery of compressed air is thereafter terminated causing the fluidized-bed to collapse, i.e. the sand settles and compacts about the shell molds and rigidly supports them. Next, a plastic sheet is placed over the bed to form an airtight seal. Molten metal is poured through the plastic sheet into.the shell molds. Simultaneously with the pouring operation, air is extracted from the bottom of the bed in order to remove the fumes from the resin binder burned by the molten metal. The extraction of air is thereafter terminated and the plastic sheet is removed.
While the castings are still red hot, air or oxygen is introduced into the bed so that the remaining uncombusted resin binder in the shell molds can burn. Upon complete combustion of the resin binder the delivery of air or oxygen is terminated. Next, the sand and castings are allowed to cool to a first preselected temperature at which time a combustible gas mixture is introduced and ignited in the bed. By controlling tb_P rate of delivery of the gas mixture the first preselected temperature is maintained for approximately fifteen minutes after which the delivery of the gas mixture is terminated. The sand and castings are then allowed to cool to a second preselected temperature, at which temperature the combustible gas mixture is again introduced into the bed and ignited in order to maintain this temperature for approximately fifteen minutes. Thereafter the castings are removed from the bed and allowed to rapidly cool in ambient air. Thus, in this preferred embodiment the castings are heat treated while still immersed in the bed and, as a result, less scale is produced on the castings than if the castings were removed from the bed to carry out the heat treatment process. Moreover, the incremental cooling of the castings allows the desired degree of tempering to occur without requiring reheating of the castings as would be required if the castings were extracted from the bed for heat treatment.
In order that the invention may be readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which :

FIGURE 1 is a simplified vertical cross-sectional view illustrating a fluidized-bed apparatus for use in the method of the invention and having a plurality of vertical shell molds supported therein; and
FIGURE 2 is a simiplified flow diagram of the method of the present invention.

The preferred embodiment of the method of the present invention will be more readily understood by way of reference to Figure 1 which illustrates one form of a fluidized-bed apparatus 10 which may be used to perform the method. The apparatus includes a relatively large, generally rectangular tank having an upper portion 12 and a lower portion 14 separated by a horizontal porous refractory membrane 16. The tank is preferably made of plates of a special grade of heat resisting steel which are fitted together with expansion joints. The bed is preferably insulated with mineral wool. The upper tank portion 12 is filled with a suitable granular refractory material such as silica or zircon sand to form a bed 18. The type and size of the granular material must be carefully selected so that the bed has the desired fluidizing capabilities and thermal conductivity. One suitable depth for the bed, when not fluidized, is approximately thirty inches (76.2 cms), although the depth necessary will depend on the vertical height of the shell molds.
The lower tank portion 14 incorporates vents or ports for providing both positive and negative pressures inihe bed 18. The design of the vents or ports must be such that the extraction vents do not interfere with the fluidizing vents and vice versa. One suitable construction for the lower tank portion 14 is illustrated and described in the above-noted SCRATA article. A corrugated sheet of steel 19 is abutted against the underside of the porous membrane 16. In Figure 1 the sheet 19 is shown in longitudinal section so that its corrugated shape and the manner in which it abuts the membrane 16 are not visible. The corrugated shape provides an alternating series of flat-topped ridges and troughs. The sheet 19 further has a plurality of small fluidizing vents (not shown) along the flat upper surfaces of its ridges which provide for the correct distribution of the fluidizing air through the porous membrane 16 uniformly over the bottom area of the bed 18. These fluidizing vents communicate with a common chamber 20 having an inlet pipe 22. The troughs such as 24 of the corrugated sheet 19 form large extraction vents and are manifolded together in a separate end chamber 26 having an outlet pipe 28.
The bed 18 can be fluidized by introducing air through the inlet pipe 22 with a controlled velocity or pressure. The fluidized vents of the sheet 19 uniformly distribute the air so that it passes through the porous refractory membrane 16 into the underside of the bed 18 causing the sand grains to separate and the bed to behave as a fluid. A bed of sand of approximately thirty inches in depth will rise a couple of inches when so fluidized. Therefore the upper tank portion 12 must have a correct amount of sand therein so that displacement of the sand by the shell molds and by fluidization will not cause it to spill from the tank. Air can be delivered at a lesser velocity or pressure so that it will permeate through the bed 18 without causing the same to be fluidized.
Turning now to a description of the preferred embodiment of the method of the present invention, the first step is to introduce air through the inlet pipe 22 so that the bed 18 is fluidized sufficiently to allow a plurality of vertical shell molds 30 loaded in a wire supporting rack (not shown) to be readily immersed into the bed 18. The shell molds consist of silica sand bonded with a cold setting organic resin binder. Each is formed about a pattern which is suitably configured to yield the desired casting void 32 (shown in phantom lines) within the mold. The upper end of each of the shell molds 30 is provided with a funnel-shaped pouring bush or down-gate 34 which facilitates pouring of the molten metal into the shell mold. Preferably the shell molds 30 are immersed to a predetermined depth so that their downgates 34 extend a couple of inches above the surface of the bed 18. As will become more,apparent hereafter, it is desirable to have the upper edges of the downgates at the same level as the upper edges of the upper tank portion 12.
Next, the fluidized-bed 18 is collapsed by terminating the delivery of air through the inlet pipe 22. Air no longer flows through the bed between the sand granules and the sand settles and compacts about the vertical shell molds 30. This causes the molds to become rigidly held and supported by the sand of the bed 18. By this technique even the re-entrant contours on the underside of horizontal shell molds can be packed with sand. This is a result which is not readily achieved merely by pouring the sand bed into the upper tank portion 12 after the mold loaded rack has been positioned therein. It may be preferable to vibrate the bed 18 to make the fluidization more uniform. When the fluidization is terminated, the maintenance of vibration ensures that the bed collapses and compacts rapidly. Vibration is typically stopped within a short time, for example two to four seconds after fluidization ceases.
A horizontal plastics sheet 36 is then placed over the bed 18 atop flanges 38 which extend along the four upper side edges of the upper tank portion 12. The sheet 36 covers the bed 18 and forms a substantially airtight seal. Preferably the vertical shell molds 30 are positioned so that the upper edges of their downgates 34 contact or are closed to the sheet 36 when it is placed over the upper tank portion 12. As discussed later on, this reduces the amount of fumes that escape into the work area when the molten steel is poured. The sheet 36 may be made of any relatively thin, lightweight material such as polyethylene which will rapidly burn away upon contact with molten metal but will not readily support combustion on its own. In other words, molten metal poured onto the sheet will burn a hold therethrough but the entire sheet will not burn up.
The shell molds are now covered and in position in the fluidized-bed apparatus ready for the molten metal pouring operation. Molten steel at +2600°F., and preferably at a temperature of approximately 3000⁰-3100°F. is poured from a ladle onto the plastics sheet 36 immediately above each of the downgates 34 of the shell molds until each mold is filled to the desired level. The plastics sheet immediately vaporizes or burns away when contacted by the molten steel to form holes therein directly above the downgates 34. Typically, spaces between the edge portions of the sheet defining these holes and the upper edges of the downgates will be created. As later explained, fumes can be drawn through these spaces downwardly from above the sheet 36 and into the bed 18.
As soon as the molten steel contacts the shell molds the organic resin binder present in the innermost portions of the mold walls begins to burn. Toxic fumes are produced and preferably these are extracted from the bed so that the ladle operator will not be exposed to the same. This is accomplished by establishing a negative pressure within the bed via a suction pump or other suitable air extraction mechanism connected to the outlet pipe 28. Any fumes which rise above the down-gates 34 tend to be drawn back into the bed 18 through the spaces between the upper edges of the downgates 34 and the edges of the holes burned through the sheet 36. Such fumes are sucked downwardly through the bed, through the porous membrane 16, through the troughs, into the end chamber 26, and through the outlet pipe 28. Fumes are also sucked through the permeable walls of the shell molds 30 and out through the outlet 28.
It should be noted that while the resin binder in the innermost portions of the walls of the shell mdds immediately burns away upon contact with the molten steel the dimensional integrity of the casting voids 32 is maintained. This is because the resin binder in the walls of the shell molds burns progressively outwardly and the steel castings begin to solidify before the structural stability of the shell molds is inadequate. Solidification of the steel typically occurs at approximately 2600⁰F. At the conclusion of the pouring operation the plastics sheet 36 is removed.
Shortly after the steel castings are poured the burning of the resin binder in the shell molds will terminate because there is insufficient oxygen within the bed 18 to support further combustion. The depth to which the burning will penetrate toward the exterior surface of the shell molds 30 depends upon such factors as the size and shape of the castings, the thickness of the shell mold walls, the type of granular material utilized in forming the shell molds, and the type and percentage of resin binder utilized. Without the introduction of additional oxygen to complete the burning process, substantial portions of the shell molds will remain intact and significant quantities of the sand will still be resin coated.
In order to burn away substantially all of the resin binder air is again instroduced through the bottom of the bed 18 through the inlet pipe 22. Other gases containing sufficient amounts of oxygen may also be utilized, but for convenience air will suffice. Preferably the rate of delivery of the air is controlled so that there is sufficient oxygen to complete the burning away of the resin but without sufficient pressure to refluidize the bed. The bed must not be refluidized because the shell molds would no longer be rigidly supported which might result in damage to the castings. The air is introduced into the bed while the castings are still red hot, and preferably when the castings are at a temperature greater than 1850 F., so that the heat of the castings is utilized to burn away substantially all of the combustible resin binder. The temperature of the castings within the bed may be monitored with suitable sensing devices such as thermocouples. The shell molds must be formed with sufficient permeability to permit adequate amounts of the air to pass through their uncombusted wall portions. This facilitates the burning away of the resin progressively from the innermost portions of the walls of the shell molds to the outermost portions thereof.
It is desirable to extract fumes produced by the initial burning of the resin which occurs during the pouring operation. However, it is necessary to terminate the extraction before introducing air to complete the burning operation. Otherwise the air introduced through the fluidization vents would be immediately pulled out from the bed through the adjacent extraction vents and would not reach the molds. Conceivably, the illustrated fluidized-bed apparatus could be redesigned so that fumes produced by the burning resin during the introduction of compressed air could also be extracted. Otherwise it may be desirable to collect the hot combustion gases in a thermally insulated steel hood. Fumes collected in the hood can be directed to a high efficiency cyclone or wet collector for dust removal. However, such a redesigned fluidized-bed apparatus may be unnecessary since the majority of the smoke and flame occurs during the initial pouring operation when the extraction is performed.
The introduction of air or oxygen when the castings are still red hot results in the formation of a certain amount of scale on the surface of the castings. However, as later explained, the process which produces this scale is effectively reversed and the amount of scale is reduced by controlled burning of a combustible gas mixture within the bed.
The method includes further steps for heat treating the metal castings while they are still immersed in the bed. The castings are incrementally cooled to a pluarlity of preselected temperatures which are maintained for a plurality of corresponding preselected time intervals. Thus, it is unnecessary, as has previously been done, to reheat relatively brittle castings from a temperature of approximately 1100⁰F. in an oven so that they can be incrementally cooled. Thus, the method results in substantial energy savings. In addition, since the castings are heat treated while they are still immersed within the bed of sand less scale is formed on the surface of the castings. Significantly greater amounts of scale are formed if the castings are removed from the bed and exposed directly to air during the reheating and incremental cooling process heretofore used to accomplish heat treating. When immersed in the bed of sand the castings are not directly exposed to air and as explained hereafter a blue reducing flame is used to pull oxygen away from any scale that has formed on the surface of the castings.
After compressed air has been introduced into the bed for a time sufficient to burn away substantially all of the resin binder, the heat treating of the metal castings is performed as follows. The steel castings are allowed to cool to a first preselected temperature of approximately 1550^o to approximately 1850°F., and preferably to approximately 1650°F., while the castings are still immersed in the bed. When the castings reach the first preselected temperature a combustible gas mixture is introduced into the bed through the inlet pipe 22 which spontaneously ignites. The rate of delivery of the combustible gas mixture is controlled in order to fluidize the bed while at the same time maintaining the first preselected temperature. Fluidization ensures that the gas mixture burns uniformly throughout the bed, thus more uniformly heating the steel castings.
The combustible gas mixture may include air or oxygen along with natural gas such as butane, propane, or methane in appropriate percentages so that the mixture will burn within the bed. The ratio of air to natural gas is preferably controlled to produce a blue reducing flame within the bed. This blue reducing flame has the effect of pulling oxygen from any scale that has formed on the surface of the castings, thus reversing the process by which the undesirable scale is formed.
It is desirable that the membrane 16 which separates the upper and lower tank portions 12 and 14 be made of a porous refractory material such as ceramic tiles instead of, for example a wire mesh screen, since the former will prevent the combustible gas mixture from burning within the lower tank portion. The combustible gas mixture will generally ignite when it comes into the contact with the very hot steel castings. However, the upper tank portion 12 may be fitted with pilot burners to ensure ignition within the bed.
The first preselected temperature is preferably maintained for approximately fifteen minutes after which the delivery of the combustible gas mixture is terminated. Thereafter the steel castings are allowed to cool further to a second preselected temperature of approximately 900° to approximately 1450°F., and preferably to approximately 1250°F. The castings are still immersed in the bed at this time. When the castings reach the second preselected temperature the combustible gas mixture is again introduced into the bed and its rate of delivery is controlled so that the bed is again fluidized and the second preselected temperature is maintained. Again, the air/ natural gas ratio is controlled to produce a blue reducing flame to promote scale reduction.
The second preselected temperature is preferably maintained for approximately fifteen minutes after which the delivery of the combustible gas mixture is terminated. Thereafter the castings are removed from the bed and are allowed to rapidly cool in the ambient air. If desired, the sand within the upper tank portion 12, now virtually free of organic resin binder, may be dumped for rapid cooling. Alternatively, the bed of sand 18 may be allowed to cool within the upper tank portion 12 prior to the commencement of the next succeeding casting operation.
The invention has been described in connection with the making of steel castings, but it is obviously useful in the making of castings of other metals. Also, sands other than silica sand can be utilized in the making of the shell molds. The method may be performed with other types of fluidized-bed apparatus. Automatic temperature controls may be utilized. The number of incremental cooling steps may be varied, and in addition, the temperature levels and the length of the time intervals can be adjusted according to the metal being cast and the heat treatment desired. Other types of combustible gas mixtures can be utilized.

Claims

1. A method of casting shell molds made of sand bonded with a combustible binder, and thermally reclaiming the binder coated sand, comprising fluidizing a bed of granular refractory material sufficiently to allow the shell molds to be immersed in the bed to a predetermined depth suitable for pouring a molten metal into the shell molds; immersing the shell molds in the fluidized-bed to the predetermined depth; collapsing the fluidized-bed so that the shell molds are firmly supported by the granular refractory material; pouring the molten metal into the shell molds to form hot metal castings; and introducing oxygen-containing gas into the bed so that the heat of the hot castings is utilized to burn away substantially all of the combustible binder.

2. A method according to claim 1, wherein the rate of introduction of the oxygen-containing gas is controlled to prevent refluidization of the bed.

3. A method according to claim 1 or 2, comprising extracting from the bed fumes produced by the initial - burning of the combustible binder before introducing the oxygen-containing gas into the bed.

4. A method according to any one of claims 1 to 3, comprising incrementally cooling the metal castings while still immersed in the bed to a plurality of preselected temperatures and maintaining the temperatures for a plurality of corresponding preselected time intervals in order to heat treat the metal castings.

5. A method according to claim 4, wherein the heat treatment of the metal castings comprises: allowing the hot metal castings to cool to a first preselected temperature while still immersed in the bed; introducing and igniting a combustible gas mixture in the bed and controlling its rate of delivery to fluidize the bed and maintain the first preselected temperature for a first preselected time interval and thereafter terminating the delivery of the combustible gas mixture; allowing the castings to further cool to a second preselected temperature while still immersed in the bed; introducing and igniting the combustible gas mixture in the bed and controlling its rate of delivery to fluidize the bed and maintain the second preselected temperature for a second preselected time interval and thereafter terminating the delivery of the combustible gas mixture; and removing the castings from the bed so that they can cool rapidly.

6. A method according to claim 5, wherein the composition of the combustible gas mixture is controlled to produce a blue reducing flame within the bed to reduce the amount of scale formed on the castings.

7. A method according to claim 5 or 6, wherein the first preselected temperature is approximately 1550⁰ approximately 1850°F.

8. A method according to claim 7, wherein the first preselected temperature is approximately 1650 F.

9. A method according to any one of claims 5 to 8, wherein the second preselected temperature is approximately 900° to 1450°F.

10. A method according to claim 9, wherein the second preselected temperature is approximately 1250 F.

11. A method according to any one of claims 5 to 10, wherein the first and second preselected time intervals are each of approximately fifteen minutes duration.

12. A method according to any one of claims 5 to 11, wherein the combustible gas mixture comprises air and natural gas.

13. A method according to any preceding claim, wherein the oxygen-containing gas is air.

14. A method of casting shell molds and heat treating the metal castings, comprising the steps of: fluidizing a bed of granular refractory material sufficiently to allow the shell molds to be immersed in the bed to a predetermined depth suitable for pouring a molten metal into the shell molds; immersing the shell molds in the fluidized-bed to the predetermined depth; collapsing the fluidized-bed so that the shell molds are firmly supported by the granular refractory material; pouring the molten metal into the shell molds to form hot metal castings; and incrementally cooling the metal castings while still immersed in the bed to a plurality of preselected temperatures and maintaining the temperatures for a plurality of corresponding preselected time intervals in order to heat treat the metal castings.