CA1189284A - Ingot casting - Google Patents

Abstract

INGOT CASTING

Abstract of the Disclosure Disclosed is a mold structure for casting large metallic parts such as ingots so as to avoid the formation of the "shrinkage pipe" which normally develops on the exposed top sur-face of such castings during their solidification. The structure comprises a cap for placement on a mold which includes a diaphragm having a front surface located to intercept radiation emitted from the top surface of the cooling metal mass and to re-emit radiation back thereto, and a means for returning radiation emitted from the back surface of the diaphragm such as an infrared reflector or a stack of baffles. The cap may include means for cooling the reflective surface and suitable ports which allow the interior of the mold and cap to be evacuated. Portions of the mold adjacent its open top may be provided with insulation to prevent heat loss from the metal by conduction. Various means for providing make-up heat to the top surface of the metal mass may also be included in the structure.

Description

2~3~

1 Background of the Invention This invention relates -to the castiny of large metal parts such as ingots.
When ingots are cast in a mold, the exposed top of the ingot cools much more rapidly than its main body, and as a consequence, the upper part of the metal mass adjacent the open top of the mold contracts more rapidly than the main body of the ingot. This results in a cavity or "pipe"
which extends from the top surface of the ingot down into its interior. The shrinkage pipe forms early in the process of solidification. Because of oxidation and other chemical effects on the surface of the shrinkage pipe, the material surrounding the pipe cannot be rolled successfully because the pipe forms an unbonded crack within the rolled ingot.
The most commonly employed method of overcoming this problem involves cutting away the top portion of the ingo-t and recycling the material surrounding the shrinkage pipe.
The loss of this material severely affec-ts the metals industry. For example,~n steel making the loss of ingot tops due to shrinkage pipe formation reduces manufacturing ylelds by 20 to 25 percent. While this portion of the metal may be recycled, it nevertheless represents a substantial cost in steel making since in order to produce an ingot for rollincJ
o e.g., 100 tons, approximately 120 tons of steel must be cast.

1 To overcome this problem some steel producers burn thermite or similar ineendiary material directly on top of the ingok to prevent the rapid cooling which produces the shrinkage pipe. This approach is eostly and only partially effective in reducing shrinkage. It is also characterized by a significant environmental problem because metallic particulates and gases are evolved as the incendiary material burns. These are difficult and costly to eontrol.

Summar of the Invention The instant invention is based on the realization that the formation of the shrinkage pipe is caused by a more rapid rate of cooling of the cast metal at its exposed top surfaee as eompared with its sides'whieh are eooled as they give up heat to the body of the mold'by eonduction. More particularly, the invention is based on the realization that the dominant meehanism of heat loss from the exposed top of the ingo-t is through the emission of thermal i.eO, infraredd radiation. It has been diseovered that ingots ean be cast in, a manner to lnhibit or avoid the formation ox a shrinkage pipe by returniny the emitted thermal radiation baek to the top surface of the ingot.
, One aspeet of the invention provides a molcl structure for easting metal ingots having an upper surfaee shrinkage pipe of redueed size relative to ingots cast in an open top mold, The structure eomprises a mold body having a top opening and a mold eap for placement over the opening. The cap comprises a dia-phragm of material having a melting point higher than the metal to be east, whieh diaphragm has a bottom surfaee disposed to intereept radiation emitted from and'to re-emit radiation baek toward the top surfaee oE the liquid metal mass placed in the 1 ¦ mold body, and means for returning radiation emitted from the top surface of the diaphragm back hereto In one embodiment, radiation is returned to the diaphragm by a reflector having a reflectivity of at least 0.5. In another embodiment, a plurality of baffles stacked in parallel on the back side o the diaphragm are employed for this purpose.

In combination with the cap structure, insulation may be positioned on the inside of the mold cavity adjacent its top surface to inhibit conductive heat transfer from the cast metal 1 mass into the told body. Further, the structure may include means for sealing the cap to the mold and means for producing a subatmospheric pressure within the structure on both sides of the diaphragm. When a roughing vacuum is created within the mold structure, convective heat losses from the top of the ingot are essentially eliminated, the reflective surface is protected against degradation, and chemical effects which produce scale on the top surface of the ingot are minimized.

The invention al50 contemplates the use of means for heating regions adjacent the top surface of the ingot ugh as pair of electrodes for passing alternating current therethrough or an induction heating toil. Advantageously, high frequency alternating current passed through the region is substantially confined to surface layers of the metal as a consequence of the well Xnown skin effect, The mold cap may also include cooling means and a dome-shaped housing for supporting the atmosphere when the interior of the mold is evacuated.

In another aspect, the invention provides a cap for use with a conventional open top mold which is e!ffective to reduce the size of the shrinkage pipe. The cap includes a housing 1 ¦ having a portion for mounting on the mold, a diaphragm desiyned to intercept radiation emitted from the top ~rface of the liquid ¦
metal and to re-emit radiation back thereto, and a reflector or series of baffles for returning radiation emitted from the back surface of the diaphragm kack thereto.

In another aspect, the invention provides a process for inhibiting the production of a shrinkage pipe on the top surface 11 of a metal mass cooling in a mold comprising the steps of inhi-¦¦ biting conductive heat transfer from a region of the metal adja-¦! cent the top of the mold into the mold, and returning radiation ¦! emitted from the top surface of the metal back thereto.

Accordinglyl it is an object of the invention to pro-vide a method of casting large metal parts such as ingots so as to inhibit the formation of a shrinkage pipe. Another object is I to provide a told structure and a mold cap which allow the ¦I casting of shrinkage pipe-free ingots and other large metal I castings. Yet another object is to imprvve the yield of metal ! produced by casting by reducing or eliminating oxidation and other chemical effects which occur on exposure of the metal to the atmosphere.

These and other objects ancl eatures of the invention will be apparent from the hollowing description of some preferred embodiments and from the drawing.

Brief Description of the Drawing Figure 1 is a cross-sectional view of a conYentional ingot mold containing a mass ox liquid metal;

Figure 2 is a crGss-sectional view of the mold of Figure 1 showing a shrinkage pipe which forms on cooling of the Imetal mass;
I

_5_ ~19~

1 Figure 3 is a cross-sectional view of a mol.d structure embodying the inven-tion;
Figure 4 is a cross-sectional view of a mold cap embodying -thy invention in place on a conventional ingot mold;
Figure 5 is a cross-sectional view of a second embodi-:. ment of the mo'.~ structure of the invention;
Figure 6 is a cross-sectional view of an ingot mold for use with a mold cap of the invention showing means for heating surface regions of an ingot;
10 Figure '7 is a cross~sectional view of the mold of Figure 6 taken at line 7-7, and Figure 8 is a cross-sectional view of an ingot mold for use with a mold cap of the invention showing an induction coil for heating surface regions of an ingot.
Lilce reference characters in the respective figures indicate corresponding parts.

Description of ,khe Preferred Embodiment .
. . . .
The phenomenon xesponsible for ingo-t shrin~acJe is r(.l,p.i~
~0 cooling of the ingot from its exposed top surace. 'I'n control shrinkage it is necessary to control the rate of coolincJ. us solidification and cooling proceed, the surface of a molten steel ingot is at a temperature 2200 F to 2400 F. Since the intensi-ty of thermal radiation ~.rom a surface is proportional to the 4-~h power of the temperature of that surfacej whereas heat t.ransfer by convection is proportional to the first power of the surface temperature, at temperatures near the melting point of steel, virtually all of the heat kransferred from the exposed top sur-1 llface of an ingot occurs by thermal radiation. Thus if a surface at absolute temperature having an emissivity E radiates to an environment at temperature To, with emissivity the net -radiant flux (qr) from each unit area of the surface to the lenvironment is given by the equation:

(T4 - To4) qr = -1/E + 1/EO-1 1 Iwhere 6 = 0.1713 x 10-8 BTU/ft2 hr ~F)4. On the other hand, the ¦rate of heat transfer by convection (~) from such a surface is given by the eauation: !

a h (T-To) where h is the film coefficient which, for hot horizontal sur- !
faces facing upward eauals about 1.5 BTU/ft2 hr OF. When E =
0.95, Eo = 1, T - 2860R (2400F), and To - 537~R ~77F) it may ¦be seen that qr has a value of 108,74~ BTU/ft2 hr. whereas qc has a value of only 3484 PT~/ft2 hr. Thus as an approximation, one may neglect all but the radiant loss of heat froTn the top ~urace of an ingot and assume that that surface will radiate appr~xi- ¦
mately 110,000 B~'U/hr. ft2 into the atmosphere. As the surface of the ingot cools to about 2200F~ this radiant flux w.ill decrease to approximately 86,000 BTU/hr. it Broadly, the invention contemplates reducing the net radiative heat loss from the ingot surface, and thus reducing the rate of tooling at the ingot surface to a level more in confor-nity with other portions of the metal casting, by returning the omitted radiant energy back to its source. By itself this approach is effective to reduce significantly the tendency of a tooling ingot to form a shrinkage pipe. However9 the return of 1 radiation may be combined with other means of reducing the net rate oE surface cooling so as to result in a substantially flat topped solid ingot. Thus,the mold body may be provided with insulation adjacent its open top to reduce conductive heat transfer into the mold body. The interior of -the structure may be evacuated to xeduce convective heat loss. Also, it is possible to provide heat directly to top surface layers of the solidifying ingot with radiant heat lamps, an induction coil, or a pair of electrodes which pass a current through the layers.
Various combinations of these approaches to retarding the cool-ing rate of the exposed ingot surface are contemplated. us used in this specification and the appended claims, the term "ingot"
refers collectively to large metal castings.
One method for returning thermal radiation to the surEace of the ingot involves the use of the heat recuperators disclosed in U.SO Patent NOSN 4,082,414 and 4,160,577~ The xeflective devices disclosed .in these patents are capable oE
returning as much as 90 percent of the radiation em~.tted fxom a source and may be placed at a distance E.rom the top of l.lle ingot. Such heat recuperators may be placed, or examp:le, the cei:ling in a room where casting is done.
However khe preferred method o:E retuxnin-J -the radicl.nt heat is by providing a cap structure which :Eits over the top o:E
the mold and comprises, as essential elements, a diaphragm con-structed of a material having a melting point above that of the molten metal to be cast disposed parallel to the top surface of the casting so as to intercept thermal radiation emitted therefrom, and a means for returning radiation emitted from the .;~ ,~;
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1 opposite surface of the diaphragm back thereto. In one embodi-ment, radiation is returned by a reflector integral with the cap.
In another embodiment, the radiation is returned by a stack of baffles arranged in parallel. Such cap structures may be used with a conventional casting mold or preferably with a specially designed mold featuring either or both insulation to re-tard con-ductive heat losses in regions adjacent the top of -the casting and means for supplying heat to top surface layers of the casting.
l Referring to the drawing, Figure 3 illustrates an ingot mold having a mold body 10 within which a mass of metal 12 has partially solidifiedO Integral with the mold body is an insular tion layer 14 which inhibits transmission of heat by conduction from liquid layer 16 of the ingot into the mold body 10. Layer 16 extends about the inside surface of mold body 10. A mold cap 18 rests on the top surface 20 of mold body 10. on O-ring 2~ or other conventional scaling device seals the cap to the mold body so that a "roughing".vacuum, on the order of 10 2 atm., may be produced within the cap and mold by vacuum pump 2~ vla su.itab:le ports and ducts 26O The cap comprises a domed housinc3 2~, a thin, lightweight diaphxagm 30, and an infrared xeflctor 32 disposed to face the back side oE the cliaphragm 30. The .reflec-tor preferable is character:ized by a reElectivit~ o:E at least 0.5. A cooling loop 34 circulates a fluid coolant in heat exchange relation with the reflector 32 to maintain the tempera-ture of the reflector at levels below that at which the re-flector may be degraded, eOg., less than about 1,000F.
The material from which the cap is made should be capable of service at high temperatures. In particular,

3~

1 diaphragm 30 and its associated support structure 36 should be capable of service at temperature in excess of the temperature of ¦
the liquid metal 16. The vacuum is drawn via eonventional ducts to produce a subatmospheric pressure simultaneously on both sides of diaphragm 30, and preferably also in the space 35 within the housing 28~ This arrangement insures that reflector 32, its associated cooling loop 34, and especially the thin diaphragm 30 remain free of potentially destructive pressure differentials at l all times. the housing 28 preferably takes the form of a dome or l arch lfor example, a hemisphere) Jo that it can support more easily the external pressure of the atmosphere. With this housing design, one can employ, for example, a brittle ceramic material for the housing 23. Once the interior of the structure l has been evacuated, the pressure of the atmosphere is carried Il entirely by the housing 28 and its înternal parts are free of ¦l load. Auxiliary cooling (not shown) may be incorporated either in the cap 18 or adjacent the top of the mold body 10 so that conventional roughing vacuum seal materials such as a polymeric or a metallic Oaring 22 can be used without danger of degrada-~0 tion. Non limiting materials which may be used in fabricating the cap structure, and especially diaphragm 30, are listed in the table below.

TABLE

Maximum Temperature at Materialwhich material May Be Used Thoria-dispersed nickel~300DF
Tungstengreater than 3000F
Lithium-alumina-silica glass 2200F
Alumina 3300F
Silicon carbide 3000F
Silica 2B00F
Tungsten carbide 5000F
Platinum 3000F

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1 The lithium-alumina silica (LAS) glass is available commercially under the trademark CERVIT. Vari4us combinations of these ¦¦ materials and others may be used as desired In the situation ¦¦ where diaphragm 30 is constructed of a material that could be damaged when exposed to the temperature of the molten metal, auxiliary cooling may be employed to maintain its temperature at suitable levels. however, in all cases, the diaphragm should l! have a melting point above that of the cast metal.

The reflector 32 preferably takQs the form of a specu- ¦
¦ lar reflective coating applied by vapor deposition or other con- !
¦ ventional techniques comprising gold, aluminum, or copper. A 11 ¦ protective oxide film may be deposited upon the reflecting sur- ¦
lo face. Materials 6uch as TiO2, ZrO2, MgO, or A12O3 as well as ! various proprietary glass ceramics may be employed9 The ¦~ thickness of the protective coating should be approximately l,OOOi Il Angstroms in order to avoid undesirable interference effects and ¦¦ to attain high transmission levels in the infrared.

Referring to Figure I, a second embodiment of the in-vention is shown. It comprises a zap structure 18' for use with ¦
a conventional casting molt 40. Cap 18' may also be used in Jon-nection with the molds illustrated in Figures 3, 5, 6, 7 or 8.
Cap 18l comprises a domed housing 29 having a plurality of integral fins 42 which serve to increase the exterior surface area of the dome, and an interior infrared reflective surface coating 32. A diaphragm 30 is disposed in face-to-face relation with the top surface 17 of the partially solidified ingot l As with the embodiment of figure 3~ both tides of diaphragm 30 within the structure may be evacuated simultaneously through ports 26.

1 The operation of the embodiment of Figures 3 and 4 are similar. Once a subatmospheric pressure (roughing vacuum range) ¦
has been produced within the structure, the only mechanisms through which heat can be transferred out of the molten metal at ¦
the top of the told are conduction through the mold body 10 and the ~truc~ure of the cap and by radiation from the molten metal to the diaphragm. Convection is substantially eliminated because¦
of the low pressure within the mold. In the embodiment of Figure ¦ 3, the effect of con~uctlon through the mold body 10 is l.0 I controlled by insulation layer 14 and through design of the structure of the cap- For example, the use of materials such as CERVIT, which has a relatively low thermal conductivity, is advantageous in this respect. Even without insulation 14 (see ure 4) the contribution which conduction makes to cooling of ¦ the molten metal it minor compared with the effect of radiation, but this small contribution can be further reduced with the use of insulation as shown at 14 in Figure 3. Thus, in both embodi- ¦
ments, the diaphragm 30 in effect s2rves as an ambient atmosphere to which the metal radiatesi thermal radiation emitted from ingot ¦ surface 17 impinges upon and is absorbed by diaphragm 30. it the outset, the net radiative flux between the metal surface and I diaphragm is heavily toward the diaphragm. However, as the tem-¦ perature of the diaphragm increases, it increasingly emits ¦ radiation back toward surface 17 For example, a freshly cast steel ingot or other large petal part at 2700F, if radiating to the ambient atmosphere it 77~F, would give up approximately 171,000 BTU/ft2 hra Bowever, if the molten metal is facing diaphragm 30p which is at a tem-perature of 2500F, the rate of transf2r from the top of the ¦ metal would be approximately 40,000 ~TU/ft2~hr, iOe~ f about 23 percent of the rate that would occur if the molten metal were radiating directly into the atmosphereO If the diaphragm were at 2300F; the rate of radiant heat loss from the metal to the diaphragm would be approximately 72,000 BTU/ft2 hr. This i8 approximately 42 percent ox the loss that would occur if the molten metal were to radiate freely to the atmosphere Diaphragm 30, when heated emits radiation both from its front surface facing the ingot 12 and its back surface. In the embodiments of both Figures 3 and 4~ radiation emitted upwardly from the diaphragm 30 is reflected at 32 and returned to the diaphragm 30~ To obtain the low net radiative flux in the first example above with the diaphragm at 2500F, the reflector would have ko return to the diaphragm approximately 70 percent o the heat radiated from -the diaphragm. In the second example, with the diaphragm at 2300F, the reflector must return approximately 27 percent of the radiation emitted by the diaphragm. Reflective materials of the type mentioned above can easily attain the required reflectivities. :Cn fact, commercially available coatings can provide reElectivities ox JO 95 to 97 percent in the range of wavelengths of importance here One is required to prevent the temperature ox most reflective materials from raising to very high levels. For example, certain very effective coatings can be used only to temperatures of approximately 1,000F and suffer damage at temperatures above this level. For this reason, auxiliary cooling loop 34 is provided in the cap structure illustrated in Figure 3 and fins 42 axe provided in the structure of Figure 4. Thus, as reflective coating 32 in Figure 4 increases in temperature, a temperature gradient develops 3~ across the dome 29 and fins 42~

l . l 1 I Heat may be dissipated through the wins and dome surface, possibly with the aid of a fan to promote convection. When insu-I lation is employed as shown in Figure 3, the jeans or cooling ¦ the refleotor 32 comprises essentially the only path for removal !
of heat from the surface layer of the cast petal If no heat whatever were to escape to the atmosphere from the upper surface of the diaphragm, the diaphragm would come Il to thermal equilibrium with the molten metal and thus would reach ¦ the same temperature as the metal. Thus, by controlling the net 1 rate at which the diaphragm i8 cooled, one can cause the tem-perature of the diaphragm to approach, as closely as may be desired, the temperature of the molten pool of metalD In doing I, so, it is advantageous to use a diaphragm which Jan be safely ¦' heated to a temperature that exceeds that at which the metal I, being cast solidifies. The major function of the diaphragm is to tl protect the reflector from damage it might suffer it exposed directly to the molten pool of metal in the top of the ingot i mold. For example, hot gases in the convection pattern that develop over the mo.lten pool of metal immediately after costing ~0 I could sweep metal particles slag, or other worms ox dirt up against the reflector. Also, the same hot gases could heft the I reflector to temperatures approaching that of the molten metal ¦l itself. Rowever, the diaphragm protects the reflector from ¦¦ direct contact with gases, particulates, etc. which could be swept up above the molten metal as the cap is installed, posi-tioned and evacuated. The vacuum system protects the reflector prom convective heating by the 9ases in the space between the diaphragm 30 and reflector 327 In addition, it e3iminates con- ¦
I vection above the molten petal pool. This helps ~ubstan~ially to , reduce the rate of heat loss rom the molten metal vnce the com-'.

l l l ll 1 ¦ bination of the diaphragm and the reflector have reduced the heat 1 loss by radiation. The vacuum alto reduces the amount of scale ¦ foxmed on the ingot surface.

In view of the foregoing it will be apparent that if I' the diaphragm is constructed from thin, lightweight material, its li heat capacity will be low and it will not introduce a delay in 'I heating the gases between the diaphragm and the reflector 1 However, in Rome embodiments it may be useful to use a diaphragm ¦~ with a somewhat greater mass so that its heat capacity may be I, u6ed to delay or buffer heating of the gases behind it, thereby providing time to install and evacuate the cap.

It thus may be appreciated that if e.gO, steel, is cast at a temperature of 3000F, so long as the diaphragm of Figures 3 and 4 can ye used safely at this temperature, it can serYe as a therrnal image of the molten steel throughout its thermal history after casting. Thus, cooling of the ingot can be retarded so that no shrinkage pipe is formed.

: Referring again to Figure 4, and assuming that the rev flector 32 must be prevented from rising to a temper3ture above I about 1000F, it may be teen that the difference in temperature I between the surface of reflector 32 and associated ins 42 and the ambient air (77F) is approximately 900F. If, as i illustrated, toe reflector were formed directly on the interior ox dome 29, if a temperature gradient of 100F were allowed to , develop across the thickness of the dome, and if the exterior li surface area ox the dome were twioe as great as that of the ¦l liquid metal pool atop of the ingot, then the outer surface of ¦i the dome would have to dispose of approximately 4,270 BTU/ft2 hr i; while being at a temperature of 800F greater than the airO This I. -15-Il i 1 cooling flux could be attained by a convection coefficient (h3 within the 5-6 range. co~fficien~ of this magnitude can be Il attained easily in air by using mildly forced convection, e.y. a ! simple fan. In fact, it may be possible Jo achieve this heat exchange without the use of fins 42.

Referring to Figure 5, still another embodiment of the I
invention is illustrated. It comprises a mold body 10 furnished i with insulation 14 identical in construction to the insulation of Il Figure 3O The zap 44 comprises a domed housing 31 equipped with , suitable ducting 26 for producing a subatmospheric pressure within the told structure and a diaphragm 30 disposed in face-~o-face relation with the top surface 17 of a solidifying sass of metal 12. However, in place of the reflector 32 of the embodi-ments of Figures 3 and 4, the cap employs a plurality of stacked ¦I baffles ~6. The top baffle 43 of the stack 46 is serviced by a cooling loop 50 which circulates cooling fluid in heat conducting relation to baffle 48.

When thin baffles, each havinq eMissivity I, are placed between a radiating surface with emissivity E at a temperature T
1 and the environment with emissivity E at temperature To the rate of radiant heat transfer from the surface to the environment is reduced from:

` (T14 To4) E(T14 - To4) I qr (no baffles) _ =
lye l~E-l 2-~
to:

E (T14 - To4 ) qr (n baffles) -I, (nil) (2-E) ! (See, eJg., Handbook of teat Transfer, Rohensow and ~artnet, I

McGraw Hill 1973~ pp 3-lOQo ) Thus, qr (n baffles) . . . _ =
qr (no baffles) n-~l A somewhat different result obtains if the emissivities of the radiant sources, the baffles, or the environment differ, but the effect of such diferences is minor unless the differences in emissivities becomes pronounced.
If the radiant losses are to be reduced by 90 percent by using baffles, then l/(n must eqùal 0.10 and n must equal 9r i.eO D g baffles must be usedO To reduce xadiant losses by 95 percent requires that in 1) equal 0.05, and 19 baffles are necessary For a 97 percent reduction, 33 baffles are required. Thus, as the desired reduction of radiant loss increas4s, the number of haffles which must be employed increases sharply. Thus, where it is sought to reduce radiant heat losses from the back surface of diaphragm 30 only moderately, eOg., on the order of 90 percent or less, the use of baffles may be more practical and less ~xpe~si.v~ an the use of a reflector However, when, for example, a 97 percent reduction is sought, attempts to use bafEles may lead to an overly complicated or fragile st.ructure, in which case a singly high quality reflector is preferred.
One of the major advantages ox using baffles as opposed to a reflector is that the baffles are far less succeptible to damage by hot gases .Thus a system for evacuating the cap is a less critical component of this embodiment than of the reflector embodiments. Auxiliary cooling is still required to extract heat from the cap..

Such means for cooling cay be applied either to D

1 Ithe top baffle, as shown in Figure 5, to the dome itself, as ¦shown in Figure 4 (with the omission ox reflective surface 32), lor to another suitable location.
-.. I
Il oven though it is not necessary to protect the surfaces !l of the baffles from the type of contamination what could hart a i reflective surface, it is nevertheless advisable but not required to use a vacuum system, as this would substantially eliminate the convective heat losses from the liquid metal to the mold and 1.
I minimize chemical effects on the inqot surface. With the liquid l metal at, for example, 2700F, he convective loss of heat to the ¦
atmosphere even taking into account the impedance offered by the I
¦ ~truc~ure of the told cap, will be on the order of 3~000 l BTU/ft2 hr. If baffles were used to reduce the rate of heat loss I from the metal to percent of that which would occur if the ¦ rnetal could radiate freely to the enYirvnment, the heat loss by radiation would be reduced from about 171,~00 BTU/ft2 hr to about 8,550 BTU/ft2 hr. The additional loss by convection of 3,000 BTU/ft2 hr would have two effects: it would increase the total ~Iheat loss by about 35 percent (increasing it from 5 to 7 11 percent); and it could cause the distribution of telnpera-ture in the baffle stack to depart substantially from that which would obtain in the absence of convection. This, in certain cir-¦ cumstances, can increase the rate oE radian heat transEer through the baffle QtaCk and can in act undo the effect intendedO

As a minimal measure to suppress the effects of oonvec-lion it is advisable to insure that diaphragm 3D is sealed suf-!ficiently well to the baffle support structure 47 to prevent the !flow of gas from the space directly above the liquid metal up I'.

I; ' Il 2~

l l l 1 I' through the baffle assemblyO Also, it is possible to select a ,spacing between the ~ul~iple baffles Jo as to control convection :in the spaces. By making the spaces sufficiently small, it is possible to suppress convection entirely and to confine héat kransfer through the gas between the baffles to the conduction mechanism. To achieve this result, both highly effective ~ealin~ !
of the edges of the baffle plates and control of thermal distor-tion of the plates are required. If, through thermal distortion, the plates were to vouch over significant areas, heat conduction ~Idirectly through the plates could undo the effect sought. The sealing of the edges of the plates and the control of thermal ,distortion can both be accomplished through conventional design techniques. Thus there is a design tradeoff between employing a vacuum system to control heat transfer and fabricating a sealed stack of baffles which must be resolved on economic considera- ;
tions of material costs, fabrication costs, and reliability of ;performance.

,~ The baffles may be constructed of a material similar to I
I those set forth above for making diaphragm 30. One poten~i~lly , attractive fabrication technique is to construct the entile I baffle plate assembly of thin pure aluminum foilt and then to i anodize the foil structure to convert its aluminum content ko A1~03, which can serve at temperatures as high as 3,300 I Referring to Figures 6 and 7, a modified mold structure I or use with the mold caps of Figures 3, 4 or 5 is shown9 This design features, in addition to insulation layer 14 about the !linterior walls of the upper regions ox the mold body 10, a pair ! f refractory electrodes 52 for passing a current through surface !
¦ regions of the cast metal mass lo Electrodes 52 are positioned l --lg ' ;
Il :~1139284 1 I on opposite sides of the mold body 10 and are separated from each other by transverse insulating plates ~4. Each electrode is electrically isolated by suitable nonconducting mountings 56 and is serviced by leads ~8, 60 an their associated contacts 62.
Preferably, as will be explained more fully below, the electrodes I
I receive high frequency alternating current supplied by high fre- ¦
!quency generator 64~ i I I
This embodiment of the structure of the invention may 1, be employed in situations where the liquid metal pool 17 cools li too rapidly despite the use of insulation layer 14 and a cap of the type disclosed above. Since, in accordance with the inven-tion, loss of heat from the surface area 17 of liquid metal mass 16 occurs substantially entirely by thermal radiation from the surface of the metal, one can supply energy to this area such that only the surface of the metal is heated. The net radian I heat loss may then be offset to the degree required to reduce the ;rate of cooling and solidification. To provide makeup heat to j the ingot surface 17, it is possible to employ high temperature ¦lelectric heaters or infrared heaters located directly above the I metal surface and mounted, for example on the cap support struc-ture. However it is preferred to employ electrodes as shown in Figures 6 and 7 ! the application of a high frequency electric field Jo a metal results in a current which tends to flow Yery close to the surface of the metal The depths (I to which current penetrates iferrous metals, at various frequencies (~3 ore given in the table below.

, 20- ;

1 winches) Heft . . . _ 2.60 ~i 0 . 39 3 000 0 . 03 450 r 000 thus, it is apparent that the application of high frequency fields to the liquid metal pool can produce electrical dissipa-tion heating that is confined -to a very thin layer (e.g. 0.20 to 0.03,inches -thick). This creates a heated "skin" on the metal and the power dissipated in the skin can compensake for the radiational cooling of the metalO Furthermore, it the tem-perature of the.surface of the metal is not uniform, the elec-tri-cal current tends to concentrate in those zones at lower temperature because these zones would also have a lower electri-cal resistance. Thus if the metal in the top of the mold had begun to.solidify from the boundary of khe mold inward, a high frequency electrical current would concentrate in the cooler solidi:Eied zones and would tend to.induce melti.ng. Thi.s effec-t would of course tend to prevent the development o:~ a shr.inkaga pipe.

Suitable materials :Eor fabricating insulators 56 in-clude alumina;:platinum sheet mounte.d on alumina o.r some o-ther substrate to.provide structural support can be used for the plate electrodes. In any event, the materials for the nonconducting plates and for the electrodes must be selected so that they do ` not react with the molten metal Other non-limiting examples of materials for use in fabricating the electrodes include osmium (mp 5477F, electrical resistivity 9Q/cm) or molybdenum imp 4750F, electrical resistivity 5. 2Q/Cm~ .

1 By monitoring the temperature of the me~a~ at the top f the mold, one can determine whether its solidification is pro-~ceeding as desired. If not, e.g., if a Rignificant shrinkage or the development of a pipe is detected, electric current may be !idischarged through the material by energizing high-frequency ¦Igenerator 64 to retard the rate of cooling and thus prevent I shrinkage.

The disclosure set forth above demonstrates that heat loss from molten metal at 2700F can be reduced to less than I about 1700 BTU/ft2 hr~ By applying 5 kw/f~2 of high-frequency ¦ power to the metal through the electrodes described in Figures 6 ¦'and 7, one can entirely balance this heat loss. Thus, a relay vely small high frequency power supply (5 kw/ft2 of ingot cross I section) will be sufficient to adjust the rate of cooling of the l ingot as may be necessary to prevent shrinlcage.

., Figure 8 illustrates another mold design which includes ¦ means or heating surface regions of the ingot 12. This struc-l ture includes an induction coil 66, which may comprise a thin !Imetallic tube through which cooling fluid is circulate e coil 66 is disposed within a refractory electrical insulation 68 which is jacketed by a ceramic liner 70. Upper regions oF the l casting l may be heated to achieve the same effects discussed I above by passing alternating current through the coils The invention may be embodied in other specific forms ¦so without departing from the spirit and scope thereof. accord-ingly, other embodiments are within the following claims, ,i What is claimed is: I
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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for inhibiting the production of a shrinkage pipe on the upper surface of a liquid metal mass cooling in a mold having a top opening, said process comprising the steps of:
A. placing over said opening a diaphragm of material having a melting point higher than the metal mass, said diaphragm having a bottom surface and a top surface, said bottom surface being essentially non-reflective, and being disposed to intercept radiation emitted from and to re-emit radiation toward said upper surface;
B. placing adjacent the top surface side of said diaphragm means for returning radiation emitted from said top surface back thereto;
C. allowing radiation emitted from said upper surface to be absorbed by said diaphragm;
D. allowing radiation emitted from said top surface to be returned back thereto by said means for returning; and E. allowing radiation emitted from said bottom surface to be absorbed at said upper surface.

2. The process of claim 1 further comprising the step of inhibiting conductive heat treansfer from the metal mass into said mold.

3. The process of claim 1 further comprising the step of providing a housing for said diaphragm and means for returning, sealing said housing to said mold, and producing a subatmospheric pressure within said mold.

4. The process of claim 1 further comprising the step of heating a portion of the metal mass in said mold in a region adjacent said upper surface to retard cooling.

5. The process of claim 1 wherein said means for returning placed in step B comprises an infrared reflector and means for cooling said reflector.

6. The process of claim 5 wherein said reflector has a reflectivity of at least 0.5

7. The process of claim 1 wherein said means for returning placed in step B comprises a plurality of baffles.

8. A mold structure for casting metal ingots characterized by an upper surface shrinkage pipe or reduced size relative to ingots cast in an open-top mold, said mold structure comprising:
a mold body having a top opening; and a mold cap for placement over said opening, said cap including a housing having a portion for mounting said cap on said mold body comprising:
a diaphragm of material having a melting point higher than the metal to be cast, said diaphragm having a bottom surface and a top surface, said bottom surface being essentially non-reflective, and being disposed, when said mold cap is positioned on said mold body, to intercept radiation emitted from and to re-emit radiation toward the top surface of a liquid metal mass placed in said mold body; and means for returning radiation emitted from the top surface of said diaphragm back thereto.

9. A cap for use with a mold for casting metal ingots, said cap being effective to reduce the size of the shrinkage pipe which forms during solidification of an ingot on the top surface thereof, said cap comprising:
a housing including a portion for mounting said cap on a mold;
a diaphragm of material having a melting point higher than the metal to be cast, said diaphragm having a top surface and a bottom surface, said bottom surface being essentially non-reflective, and being disposed, when said cap is positioned on a mold, to intercept radiation emitted from and to re-emit radiation toward the top surface of a liquid metal mass placed in the mold; and means for returning radiation emitted from the top surface of said diaphragm back thereto.