US3616842A - Continuous centrifugal casting of tube using liquid mold - Google Patents

Abstract

Continuous centrifugal casting of metal tube on a centrifuged lining of a heavier liquid metal mold, (as lead, tin, or lead-tin alloy). Both the liquid mold material and the molten metal, to be cast to tube, are continuously introduced into the starting end of the centrifuge and continuously exit from the opposite end where the still liquid mold material flows into a suitable catchring for recirculation and the semisolidified or solidified centrifugally cast tube exits axially from the centrifuge for subsequent use as tube or as a basic hollow cylinder for conversion to longitudinal structural items by the technique of collapse deformation.

Description

United States Patent [72] Inventor George R. Leghorn [56] Referen e Cited t? f' g'g gg Santa UNITED STATES PATENTS 3 1,831,310 11 1931 Lindemuth 164/81 Appl' 768983 2 940 143 6/1960 Daubers m1 164/5 22 Filed Oct. 21, 1968 i i Y 45 Patented Nov. 2, 1971 FOREIGN PATENTS Continuation-impart of application Ser. No. 22,708 1 H1896 Great Britain 164/81 ir' g k zgg Primary Examiner-R. Spencer Annear 8 ay Att0rr1eyLawrence Fleming ABSTRACT: Continuous centrifugal casting of metal tube on a centrifuged lining of a heavier liquid metal mold, (as lead, [54] gg gi aiifggg CASTmG 0F TUBE tin, or lead-tin alloy). Both the liquid mold material and the g I Q 3 D molten metal, to be cast to tube, are continuously introduced 1C almsl rawmg into the starting end of the centrifuge and continuously exit [52] U.S.Cl 164/84, from the opposite end where the still liquid mold material l6 4/ 6 4 flows into a suitable catch-ring for recirculation and the semis- [51] Int. Cl B22d ll/00, olidified or solidified centrifugally cast tube exits axially from B22d 13/02 the centrifuge for subsequent use as tube or as a basic hollow [50] Field of Search 164/82, 84, cylinder for conversion to longitudinal structural items by the technique of collapse deformation.

solidified casting \\\\\\\Q Q a5 liquid mold PATENTEU 2 SHEET 30F 6 PATENTEUNDV 2 Ian 3. 6 1 6 .842 SHEET 86F 6 INVENTOR.

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GzmeczE. LEG/10m B MW% CONTHN UOUS CENTMFUGAL CASTING F TUBE USING LlQUllD MOLD This application is a continuation-in-part of my application Ser. No. 538,506, filed Feb. 11, 1966, now U.S. Pat. No. 3,445,922 issued May 27, 1969.

BACKGROUND OF INVENTION A great many techniques for the casting of tubes are known and used in the metal casting industry and most .of these techniques have long been in public domain. More than this, it has long been obvious that a means of continuously casting such tubing would permit great economy to be realized in the manufacture of such hollow-ware.

One of the earliest attempts for the casting of tubing on a continuous basis is exemplified in British Pat. No. 15,912, issued to Lane and Chamberlain in 1891. This invention attempted to continuously cast tubing by the expedient of continuously pouring the molten metal (to be cast) into one end of a solid-wall centrifugal mold and continuously removing the solidified tube from the other end. The technique, while meritorious in conception, failed primarily as a result of the high frictional contact between the mold bore ID. and the cast tube 0D.

A number of other patents teach the continuous centrifugal casting of tube in a solid wall mold by the basic technique of Lane and Chamberlain. These include U.S. Pat. Nos. 777,559 to 777,562 issued to Stravs and Jager in 1904 (this series of patents disclosed both horizontal and vertically downward extraction of the tubes so cast); U.S. Pat. No. 950,884 issued to Winner in 1910 (in this method, a superimposed slinging action was utilized to continuously force the centrifugally cast tube from the mold bore); U.S. Pat. No. 1,223,676 issued to De Lavaud in 1917 which teaches the use of a rotary mold and a roller disposed within said mold as well as a means for continuously ejecting the casting as formed; U.S. Pat. No. 2,752,648 issued to Robert in 1956 (this is essentially a repeat of the methods of Stravs and lager as taught in their patent disclosures in 1904 and utilizes canted rolls to extract the centrifugally cast tube downwardly from a vertical mold); and, lastly, British Pat. No. 984,053 issued in 1963 which teaches the downward extraction of a centrifugally cast tube from a vertical centrifuge having an internal offset and tapered rotating core mold.

Whereas the foregoing processes have been made to work and produce tubing in a continuous manner, they have the drawback of exceptionally high frictional forces between the mold wall LD. and the cast tube OD. as a result of the outward forces on the molten and solidified tube metal clue to the centrifugal action. Conventionally horizontal centrifugal casting is done between rotational speeds which produce from 50 to 100 gravities of centrifugal force (a 1 pound mass of metal would effectively weigh 50 pounds when centrifuged at the rotational speed of 50 G's necessary to produce a dense sound casting and to prevent raining and sloshing of the molten metal) and, as a result, the extraction of the continuously centrifugally cast tube from the bore of the solid wall mold is extremely difficult. With metal wall molds, the exceptional wall friction causes circumferential splits in the tubing so cast and such splits have resulted in the exiting tube being pulled out of the bore of the mold as a broken off length instead of continuously. To correct this defect, one aspect of the Maxim patent of 1895 (British Pat. No. 22,708) pertained to the use of slippery refractory materials such as axially aligned asbestos fibers compacted with plumbago (graphite). Such slippery and refractory linings greatly increase the workability of solid wall centrifugal molds for continuous casting; however, these same centrifugally created frictional forces cause exceptionally high wear rates on such soft materials. Once an annular circumferential depression has been worn into the l.D. of the centrifugal casting mold at the starting end, a tube is cast having too large a diameter to permit extraction from the exit end. In practice, this is a steady wear process and the bore keeps opening up the solidified tube is extracted from the mold. For the casting of steel, the wear rates can be extremely rapid and economically disadvantageous.

Due to the foregoing detrimental aspects of solid wall continuous centrifugal casting molds a number of nonrotating methods for the continuous casting of tube have been conceived. These are invariably based on the use of concentric inner and outer solid mold walls that are cooled by various means. Such devices are best exemplified by U.S. Pat. No. 2,473,221 issued to Rossi in 1949 (wherein a cast tube is withdrawn vertically downwards from such a mold) and U.S. Pat. 3,022,552 issued to Tessman in 1962 (wherein the tube cast between such concentric molds is horizontally forced out of the casting apparatus by the hydrostatic pressure of the molten metal being cast). Both of these processes utilize nontapered internal (I.D.) molds and as :such, are extremely difficult to operate on a continuous basis due to the cast tube shrinking inwardly (thermal contraction) onto the solid mold in the bore. Shrink fits are used to prevent concentrically assembled items from slipping and, in the case of continuously cast tube, the shrink fit can cause complete stoppage or rupture of the cast tube. In order to obviate the foregoing problem, the inner concentric mold has been tapered so that the cast tube moves to a smaller diameter portion of the [.D. mold as it contracts of the concentric molds are made as short as possible. Such tubing is being successfully continuously cast by utilizing either tapered or very short [.D. molds. However, the output rates (withdrawal rates) are fairly slow and must be carefully controlled to prevent either shrinkage binding (too slow a withdrawal) or molten metal seepage (too fast a withdrawal).

The foregoing processes, where used, are of economical value due to the increased cost of tube and pipe as an end item.

The obvious commercial advantage of being able to continuously cast tubing by the centrifugal method (no internal mold required; and with resultant high-integrity pressure cast metal) led to the invention of the liquid wall centrifugal mold by Hiram and Hudson Maxim. The process was disclosed in 1895 in British Pat. No. 22,708. Essentially, the Maxim invention consists of a horizontal centrifugal mold (as in the Lane and Chamberlain process of 1891) with the exception that the bore [.D. at the back part of the mold is greater than at the exit orifice LB. and the resulting shallow annular depression is filled with a centrifuged lining of liquid lead which extends to the exit end of the rotating mold. The ID. of the liquid lead lining is substantially equal to the ID. of the exit orifice. Molten steel is centrifugally cast onto the liquid lead lining of the apparatus and forms a molten steel cylinder thereon. As the molten steel solidifies to a hollow cylinder (due to heat extraction by and through the liquid lead), as it is forced towards the exit end of the mold, it shrinks diametrically due to the ther' mal contraction of the steel and is thus capable of being withdrawn from within the solid exit lip of the rotating mold. The Maxim process (British Pat. No. 22,708) is innovated to greatly increase its product range and rate of output as, also, is that of Daubersy et al. (U.S. Pat. No. 2,940,143). The Maxim process is a perfectly valid one but 1 have discovered that it has severe limitations as to the ratio of wall-thickness to diameter of tubing which it can produce, as explained hereinafter.

The knowledgeable detail of the Mlaxim patent disclosure attests to the extensive developmental work carried out on the process. As an example, the Maxims provided a hot zone at the starting end by surrounding the rotary mold in that area with a furnace. One of the drawbacks to the successful operation of such a device results from the: fact that molten steel, when poured directly onto liquid lead even when it is heated to a fuming temperature, will chill so rapidly that the hardened steel interface on the liquid lead is rough and knobby and, as such, can effectively increase the diameter of the cast tube so as to prevent its removal from the exit end of the centrifugal tube caster. This fast chilling effect depends on the thermal conductivity of the steel (whether it is less or greater than that Density of solidifying steel of liquid lead) and, also, on the thickness of the steel layer. For a steel having a considerably higher thermal conductivity than lead and where the wall (layer of steel) is fairly thick, the solidified steel skin will remelt and smooth out. This product area (tubes having fairly thick walls), however, is primarily denied to the Maxim process by the D=65T limitation of formula 1, hereinafter set forth.

The patent also discloses pouring on the down-going sidewall of the caster and the use of vanes to bring the poured in steel into rapid rotation. Such techniques help to obviate the knobby surface caused by too rapid chilling of the pouredin steel by sluicing the steel onto the liquid lead.

it is quite probable that this process was ahead of its time as far as availability of suitable engineering structural materials was concerned. it should be noted that the heated steel (the structural material then available) walls at the starting or hot end of the apparatus would soon fail by creep under the high G(as 70 Gravities) forces and an internal load of layered liquid lead and molten steel.

In a static casting (such as one made in a conventional sand mold), the total contraction depends on the solidification contraction and the thermal contraction. In a centrifugal casting (operating at the high G," gravitational, forces necessary to produce a dense casting) the solidification shrinkage is nonex istant since, as the denser solid grains grow from the molten matrix of the surrounding liquid steel, they are centrifuged to the outer surface and form a solid ring of welded particles which have already undergone their solidification contraction prior to uniting into a solidified ring. More than this, the thin solidified ring is in a highly pliable condition at a temperature just below its melting point and is readily stretched to its maximum equilibrium diameter under the centrifugal forces involved. solidification contraction occurs; however, it is evidenced as a decrease in the wall thickness of the solidifying tube while the outside diameter remains essentially unchanged. From then on the only contraction is the thermal contraction of the solidified ring as its temperature is lowered, by heat abstraction, from the solidification temperature of about 1,500 C. to just above the melting point of lead (330 C. which is the minimum allowable cooling for such a system using lead as the liquid mold.

The distance that the nearly solidified steel (a steel of about 0.20 percent carbon is used for illustrative purposes since this is the range of greatest commercial output) will sink into the liquid lead will depend on the density of the steel (7.30 g./cc.) at the l,500 C. solidification temperature, the thickness of the semisolidified steel layer, and the density of the liquid lead that is being displaced (hotter liquid lead will be less dense and the solidifying steel will sink deeper into it). If, therefore, we take the liquid lead at its greatest density (10.66 g./cc. at just above its melting point or 330 C.) we can determine the minimum amount that the steel will sink into the liquid lead as follows:

X 100 equals percent of layer thickness of steel that sinks into the liquid lead Density of cool liquid lead In other words, the solidifying steel will sink into the cool liquid lead by an amount that is equal, at least to two-thirds of its own layer thickness while for hotter, less dense, liquid lead, the solidifying steel will sink in even more.

It is obvious also that, in order for the solid steel tube (which has been centrifugally cast upon the liquid lead mold) to be capable of extraction from the fixed exit diameter of the mold, the tube radius will have to contract by at least the distance which it sank into the liquid lead. The specific volume thermal contraction (the only effective contraction in the centrifugal process) between the l,500 C. solidification temperature of the steel down to just above the melting point of lead or 330 C. is 6 percent as shown in FIG. 1. Since the X 100 equals 68% or just over 2/3.

linear diametrical contraction is one-third of the volume contraction, the diameter of the tube will shrink by one-third of 6 or 2 percent in going from the solidifying tube at l,500 C. to the cool tube exiting from the centrifugal casters exit orifice at 330C. A A

Any tube centrifugally cast by the Maxim process which has a solidifying steel wall thickness of T units and an exit diameter of D" units will be limited (as to the minimum diameter of tube that can be cast relative to the solidification wall thickness) in accordance with the terms of the following formula:

which is derived as set forth hereinafter.

Since the solidifying steel sinks at least two-thirds of its wall thickness T into the cool liquid lead, the diameter of the solidifying steel tube, at the casting or starting end of the liquid mold, will be twice the radial sinkagc of two-thirds T or 4T/3 units greater than the D' units diameter of the exit orifice (which is the same as the ID. of the liquid mold material that overflows the exit orifice weir when displaced by the molten steel).

Since the linear or diametrical thermal contraction (in going from the solidifying state at l,500 C. to the solid state at the minimum exit temperature of 330 C.) is 2 percent, then the 2 percent diametrical shrinkage must reduce the diameter to that of the exit orifice D. for exit thereof.

FORMULA l.

D=65T a tube (so cast) would have to be 5 feet and 5 inches in diameter for a 1 inch wall thickness. in other words, for the Maxim process to work, the minimum diameter of the tube being centrifugally cast must be 65 times greater than the wall thickness of the tube as it forms in the centrifugal casting machine (for a 0.20 percent carbon steel).

There is an important limitation to the lower limit of the temperature of the exiting steel tube and this is due to the phase transformation of the austenite (high-temperature phase) to thermal decomposition products such as ferrite and pearlite which starts at about 700 C. In low carbon, low alloy steels this phase transformation takes place in less than 2 seconds at about 600 C. and is accompanied by a volume growth which counteracts and reverses the shrinkage to the extent that a 3-foot diameter tube will quickly experience a diametrical increase of slightly over l/l6th inch. Such an expansion can cause jamming of the tapered tube into the outlet orifice of the mold with catastrophic results.

The thermal specific volume contraction graph for a low carbon steel is shown in FIG. 1 and clearly illustrates the volume expansion between 700 and 500 C. on cooling.

Due to the danger ofjamming as a result of this sudden expansion (due to phase transformation) of the exiting tube, the minimum exit temperature should be at 700 C. or above for such steels.

With reference to FIG. 1, the scaled volume contraction distances from l,500 C. to room temperature to that in going from l,500 to 700 C. is 0.009l/0.0058 and the 0.009] distance is equal to the 7.2 percent volume contraction. Therefore the percent volume contraction (V) is going from 1,500 to 700 C. is 0.009l/0.0058=%/v% or V=0.0058/0 .0091 (7.2%)=4.58%

The linear or diametrical contraction is one-third of the volume contraction of4.58/3=l .5 3%.

The release of the heat of solidification of steel would raise the temperature of the cool lead to at least (actually to a much higher temperature) 530 C. and the density of liquid lead at 530 C. is 10.420 g./cc.

The solidifying steel (density 7.30 at l,500 C.) will sink into the lead (density of 10,420 at 530 C.) to 7.30/l0.420X l00=70% or 0.07 T (where T is the layer thickness of the solidifying steel tube.

Therefore, by a rederivation of formula 1, we have the following: expression reads: (D+2X0.7OT) l(D+2 0.7OT)-l.53(D+2X0.70T)=1OOD 100D +l40Tl.53D 2.l4T=l00D 1.5 3D=l40T-2.l4T 1.53%13186T FORMULA 2 It can readily be realized by this ratio of tube wall thickness to diameter (to just let the tube clear the exit orifice of the mold at 700 C.) that the Maxim process is subject to some very severe limitations of product output. It may well be that this reexpansion of steel, which began at 700 C. on cooling, was unknown in 1895 and it may have been too severe a hurdle for the Maxim process in its developmental state.

FIG. 2 illustrates the limitation on product output due to the diameter of the low-carbon steel tube, produced by theMaxim process, having to be 65 or 90 times as great as the wall thickness of the cast metal as it solidifies.

An even more restrictive limitation on the rate of output derives from the fact that the wall of the tube must bealmost completely solidified before any product at all can be extracted from the centrifuge exit orifice since, under the high G forces of a centrifugal caster, the solidification contraction (in going from liquid to solid at the solidification temperature) is practically nil. In order to increase the output rate of such a continuous tube caster, it would be highly desirable to have the tube exit from the apparatus with a solid outer shell and an inner shell of still molten steel. In this manner, the outer shell could be immediately chilled with multiple sprays or jets of a cooling liquid (as cool liquid lead or mixed hydrocarbons and water as in the Maxim system whereby the inventors attempted to correct this limitation to some extent by internal spray cooling) for more rapidtheat extraction andgreatly increased product output.

Another process in which a steel tube is cast on a centrifuged mold of liquid lead is the subject of US. Pat. No. 1,831,310 issued to Lindemuth in 1931. Basically, it is disclosed in one portion of the Maxim patent but includes some slight improvements thereon such as glass additions to the l.D. surface of the centrifugally cast tube.

The latest patent for the continuous centrifugal casting of tube in contact with a liquid lead lining is U.S. Pat. No. 2,940,143 issued to Daubersy and Schlemmer in 1960. Basically, this Daubersy et al., system uses continual small additions of lead to the system so that the solid centrifugally cast tube is permitted to exit from the bore of the caster on'a thin lubricating film oflead.

Analysis of the Maxim Process as Disclosed in British Pat. No. 22,708:

The Maxim patent (line 5 of page 2) states The dam at the forward or exit end of the cylinder has a height or depth approximating the thickness of the fluid bed in order to prevent the latter from flowing out at that end and to prevent an excess of waste of the fluid bed over the surface of the dam. The fluid bed may be replenished or maintained as fast as it becomes deteriorated or waster, by feeding into the rearward end of the cylinder an additional quantity of lead or other substance of which the bed is composed. The lead thereby supplying the fluid bed with fresh material.

From the Maxim patent starting at line of page 2The iron or other pipe formed upon the fluid bed is preferably solidified by cooling at a point considerably to the rear of the dam at the exit end of the cylinder, so that it shall contract sufficiently upon cooling to give it clearance in order that it may pass freely over the dam at the forward or exit end of the cylinder."

With respect to the foregoing prior art, it is evident that dragout of liquid lead occurred in the system (and this would have attendant lubricating characteristics) and that such Lil losses, along with those from other sources, were continually made up by lead additions. The Maxims attempted to avoid such dragout by solidifying the cast tube considerably to the rear of the exit dam whereas, in the Daubersy patent, the escape (leakage flow) ofa small amount of lead is encouraged by the continuous additions of small amounts of lead to the system so as to form a lubricating film of liquid lead between the darn lip and the outside surface of the exiting tube.

The Maxim patent includes means for controlling the rate of extraction of the tube (this permits solidification and subsequent thermal contraction considerably to the rear of the dam so as to avoid any possibility of the solidified tube jamming against the exit orifice dam by too fast or uncontrolled withdrawal) as in lines 50 to 53 of page 2 as follows: We also provide means for drawing the pipe from the rotating cylinder as fast as it is formed, that is, as fast as it becomes solidified, these means preferably consist of friction rollers set at a suitable angle. Given a constant speed of rotation, a constant speed of charging with molten metal, and a constant means for cooling, the thickness of the pipe or tube formed will be governed by the rapidity with which it is drawn from the forming cylinder."

The process of the Maxim patent is operable within the restrictions of the D=65Twall thickness to tube diameter ratio for steel not exhibiting a phase change and D=9OTfor low carbon lowalloy steels which undergo a rapid phase change expansion. The method is entirely workable when use is made of the more advanced structural material available today.

It should be noted, however, that inadvertent changes in liquid levels, or too rapid chilling of the inpouring molten casting metal, or too rapid (or too slow, if the resulting increased wall thickness exceed the D to T ratio) a withdrawal of the tube will result in jamming at the exit orifice which will result in stoppage at the very least.

Analysis of the Daubersy Process as Disclosed in U.S. Pat. No. 2,940,143:

The Daubersy patent is essentially a variation of the Maxim process by the method of liquid lead additions and by the interior design of the solid portions of the centrifuge mold wall in an attempt to create a self-regulating effect over inadver' tent diamctrical changes of the centrifugally cast molten metal.

Essentially, the Daubersy patent combines a shortened dry wall mold (this is a technique for cutting down on the excessive frictional forces that attend a complete centrifugal dry wall mold) with an extended liquid wall mold of the Maxim type. A hot, thin, malleable shell of solidified steel forms on the interior surface of the dry wall mold and is then forced off of the drywall mold by the head of molten steel and onto a liquid mold which then acts as a heat extracting medium for solidifying the balance of the molten steel of the centrifugally formed tube. From this point on, the process is the same as the Maxim process except that the lubricating film of lead is maintained by continuous additions of small amounts of cool liquid lead at the starting end. The head of inpouring molten steel forces the solidified steel tube out of the bore of the casting machine and is facilitated in doing so by the lubricating film of lead which lines the bore of the caster. The casting machine has no positiveimeans of extracting the solidified tube as in the Maxim process but depends on the pushing action of the head of molten steel at the starting end. Further, and as clearly stated in thepatent, the exit orifice of the mold conforms closely to the outside diameter of the cast tube after the piece has accomplished its shrinkage. (Lines 22-24 of column 2 of the Daubersy patent, and which refer to the dam or exit orifice lip of the patent drawings, notes the r is the shrinkage when the centrifuged annular piece has accomplished its shrinkage.

in all four patent drawings, the dam of the centrifugal tube caster is shown as being inwards from the CD. of the just solidifying tube (at the starting end) by the amount of shrinkage r and except for a thin lubricating flow of liquid lead, the dam l.D. is in close contact with the CD. of the tube (Annular piece" as it is called in the patent) which has accomplished its shrinkage.

It can be seen from the foregoing that the dam at the exit orifice extends to within lubricating contact of the tube after shrinkage has been accomplished and is, therefore, the same as in the Maxim process. The Maxim and the Daubersy processed depend on thermal shrinkage of the tube to permit its egress from the system. In this manner, the primary mode of escape of the cast tube is by thermal shrinkage and in this respect, the process is subject to the requirement of complete solidification of the tube wall prior to its exit from the system.

RESUME OF THE INVENTION The present invention. provides for introduction of moderate (approximately to 25 percent by weight of the metal being cast to tube) to large (over 25 percent by weight of the tube metal) amounts of liquid mold material and the maintenance of the outside diameter of the molten metal cylinder equal to, or less than, the exit orifice diameter of the centrifuge, which permits the cast tube to float out of the bore of the centrifuge on an axially flowing stream of liquid mold material. Due to the freedom inherent to the floating action, such exceptional rates of output are permissible that the process is superior, on a tonnage per hour basis, to the currently used continuous casting processes.

THE CONTINUOUS CENTRIFUGAL CASTING PROCESS OF THE INVENTION IN GENERAL The process of my invention is designed to greatly extend the limited range of tube product output inherent in the Maxim process. It has become apparent to me that the ratio of wall thickness to diameter of tubing that can be produced by this process is severely limited due to the fact that solidifying steel at its solidification temperature (about 1,500 C. for a 0.2 percent carbon steel) will sink into the liquid lead until it has displaced its own weight of the liquid mold material (the Archimedes principle).

This limitation of product output range (for a low-carbon, low-alloy steel) which is expressed by the general formulas D=65T and D=9OT, and is shown in FIG. 2, is not applicable to my process. As an example, the Maxim process cannot produce a mild steel tube having a one inch thick wall and a diameter of less than five feet. On the other hand, my process can produce such tubes of one inch wall thicknesses having diameters of less than one foot. This ability to cast fairly heavy walled tube in small diameters is particularly important where the tube is to be used as a basic starting point for the manufacture of longitudinal structural items (by collapse deformation thereof) such as I-beams channels, angles, etc.

This advantage of my process derives from the technique of restricting the outside diameter of the molten tube so that it does not exceed the diameter of the exit orifice of the centrifugal continuous casting machine. In this manner, the cast tube can float out of the bore of the casting machine on an axially flowing cylindrical stream of liquid mold material. In fact, the exiting speed of the flowing liquid mold material (under the action of the high G, psuedogravitational, leveling force of the centrifuge) can be so rapid that the casting machine must be exceptionally long in order to permit sufficient time for solidification of the molten metal; or, the flow of the liquid mold material and the speed of exit of the solidified tube must be restricted to allow sufficient time for heat extraction and solidification. In actual practice, the exiting speed of the centrifugally cast tube is controlled by conventional means (such as a modification of that used in the Maxim process) and the flow of liquid mold material is restricted (by appropriate downstream location of the annular overflow dam or weir) so that advantageous casting rates are obtained with a moderate length casting machine. At the same time, the casting machine is segmentized so that it can be readily extended to greater lengths so as to greatly increase the rate of output as demanded. More than this, the casting machine is unitized so that a centrifugal casting cylinder, permitting outputs of larger or smaller diameter tubes, can be readily exchanged and still utilize the same basic mechanisms or rotation, extraction. cutoff, molten metal and liquid mold introduction and exit and the like. Segmented molds and unitized construction have long been known in batch-type centrifugal casting.

The restriction of the outside diameter of the molten metal tube so that it does not exceed the exit orifice diameter of the centrifugal casting machine (whether an annular weir is present or not) has a very important advantage since it obviates the danger of a solidified tube (having a diameter that is greater than the exit orifice, as in the Maxim and Daubersy processes) jamming the exit port and creating an expensive stoppage or a catastrophic failure. These casting machines, of Maxim and Daubersy, can effectively and continuously produce centrifugally cast tube (when the outside diameter of the molten tube is greater than the diameter of the exit orifice) providing that the exiting speed of the solidified tube is carefully controlled. However, a slippage or other uncontrolled extraction failure can result in an expensive stoppage or hazardous condition and it is for this reason (as well as to increase the scope and speed of product output) that the diametrical restrictions of the disclosed process are imposed.

It should be noted that the segmentized and unitized construction of the machine permits a rapid and easy change of product output (as from a large diameter thin-walled tube to a small diameter moderate-walled tube) with the same basic mechanism and it is a primary intention of this process to produce such tube variations as basic items for the more economical production of other longitudinal structural shapes (as plate by the collapse of large diameter thin-walled tube or railroad rails by inwardly collapsing moderate-wall small diameter tubes and roll-welding the contiguous interior surfaces while sizing the collapsed structure to the final desired item of longitudinal structure).

It should be noted further that my process permits the continuous centrifugal casting of tubes having smaller diameters than is now feasible by batch type solid wall centrifugal casting. In batch type centrifugal casting there are certain limitations as to the length of tube that can be cast for particular diameter and this is particularly true for tubes having small diameters (as less than two inches OD). The longitudinal contraction of such tubes, in cooling down from the just-cast to the extraction temperature is sufficiently great that the circumferential rupture will occur if this shrinkage is unduly restrained. Such restraint is produced by minor ovalness, or out-of-line of the bore of the centrifuge, end sticking, or surface roughness. In small diameter tubes, the diametrical shrinkage is insufficient to obviate (shrink away from) such restraining mechanisms and the large amount of rejections due to such circumferential rupture makes such production uneconomical. My process can produce such small diameter tubes on a continuous basis and without ruptures since the only restraint to longitudinal shrinkage would be the shearing forces in the liquid mold material and these are reasonably small. Such small tubes can contract both longitudinally and diametrically without damaging hinderance.

As stated previously, one of the primary features that differentiates my process from those of Maxim and Daubersy is the fact that the CD. of the just solidifying contrifugally cast tube is maintained equal-to or less-than the ID. of the exit oriflce of the centrifugal caster. In order to accomplish this criterion, certain physical requirements must be met. As a specific example of the operation of the present invention, in that the disclosed process can continuously centrifugally cast mild steel tubes having a wall thickness of one inch and an OD. of less than one foot, the following example is presented:

The mild steel tube being cast has an CD. of 10 inches and an ID. of 8 inches (wall thickness of 1 inch) and is centrifugally cast at a rotational speed which is equivalent to 50 Gs (gravities). At the solidification temperature of l,500 C., the density of the just-solidifying steel is 7.30 g./cc. or 0.264 lbs/cu. in. However, since the casting is being carried out at 50 G's. the effective weight of a cubic inch of the steel (at l,500 C.) is 50X0.264 or 13.2 pounds. But, if we project a square inch area of surface on the CD. of the tube onto the tube axis, we have a truncated wedge removed from the tube wall which has an exterior surface area of 1 sq. in. and an interior surface area of 0.8 sq. inches, along with a radial (wall thickness) depth of 1 inch. The volume of this truncated wedge is 0.9 cubic inches and this volume of semimolten metal bears on the onesquare inch of outer surface due to the centrifugal action. Since one cubic inch of the metal weights 13.2

pounds, then the 0.9 cubic inches of the truncated wedge will have an effective weight of 0.9X1 3.2 or 11.9 pounds at 50 Gs and this weight is exerted against the one square inch of area on the OD. surface and creates a pressure of l 1.9 p.s.i.

Therefore, in order to prevent the 1 inch semimolten steel layer from sinking into the liquid mold material (as liquid lead), aback pressure on the liquid lead (in excess of its normal pressure at 50 GS) of 11.9 p.s.i. must be accomplished. Such a back pressure of 11.9 p.s.i. will counter-balance the weight of the 1 inch steel layer and reduce the outside diameter of the semisolid steel tube to that of the annular overflow weir (exit orifice l.D.).

SPECIFIC EXAMPLES OF METHOD This necessary back pressure can, and is, created by any one or any combination or permutation of the following five species of my method (which will herein be designated as Method 1, Method 2, Method 5.

Method 1. By restricting the liquid mold exit orifice (the annular gap between the solidified tube OD. and the the centrifuges exit orifice l.D.

Method 2. By extending the length of the weir (exit orifice) lip to a sufficiently great extent that the required down-stream line pressure drop of l 1.9 p.s.i. is experienced.

Method 3. By creating a vacuum within the steel tube that counterbalances the weight of the 1 inch thick layer of steel at 50 Gs.

Method 4. By raising the atmospheric pressure (exterior'to the tube and the exit orifice or at the entrance end and exterior to the vacuum seal means) by the desired amount over that of the ambient atmospheric pressure (14.7 p.s.i. is the average sea level atmospheric pressure and 14.7+l 1.9 or 26.6 p.s.i.a. would be required under normal conditions). This is actually done by surrounding the exiting steel tube and the exit-end of the centrifuge with a suitable enclosure and introducing an inert dry gas therein under the desired 26.6 p.s.i. of pressure.

Method 5. By reducing the rotational speed of the centrifuge so as to decrease the centrifugal force (number ofGs). Actually, 50Gs, while not the lower limit of rotational speed necessary to prevent raining and sloshing of the liquid contents of the centrifuge, normally considered the lower limit for the production of a dense defect-free casting.

With respect to Methods 1 and 2, it can be noted that these methods are entirely feasible. However, a large amount of liquid mold material must be introduced into the system to maintain the desired back pressure at an equilibrium value. As a approximation (depending on restriction of the exit orifice and the length or line drop of the weir lip) the through-put weight of the liquid mold material must be equivalent to the casting output. Since this can be greatly in excess of 100 tons of steel per hour, it can be realized that a large amount of liquid mold material is required. Even where Methods 1 and 2 are used in combination and a minimum rotational speed of 50 Us (for the desired density of casting) is used, moderate amounts (such as percent by weight of the cast metal through-put) of liquid mold material must be continuously introduced into the system. For this purpose, the Methods of l and 2 are not the preferred means of producing the desired back pressure even though they are present to some extent in any system where the liquid mold material exits continuously from the system.

Method 3, the creation of a partial vacuum on the interior of the tube, is the preferred method since it is the most forceful means of accomplishing the desired back pressure and introduces other beneficial effects as well. In a prior US. patent application, Ser. No. 538,506, the use of an internal vacuum within the centrifugally cast tube has been described in con junction with the continuous collapse forming the tube to longitudinal items of structure. In such a process, the tube cavity is sealed at the exiting end by the inward collapse of the tube walls and the welding together of the inner contiguous surfaces of the tube wall. The seal at the starting (pouring) end of .the centrifuge is created by a nonrotating end plate, or disc,

then applying the suction, the gases given off by the molten metal are of a-reducing or inert nature (as carbon-monoxide, hydrogen and nitrogen) and these gases maintain the inner surfaces of the tube in abright oxide-free condition which permits and facilitates the pressure-welding of the contiguous interior surfaces of the tube one to the: other. It may be mentioned thatthese same gases create porosity or blowholes in ingots cast-the the old ingot-mold process and that these gas cavities are collapsed to a defect-free solid condition by subsequent rolling which welds the clean oxide-free inner surfaces of the pockets together. This is but one ofthe majoradvantages from the use of a partial vacuum internal to the tube being cast.

b. Where the internal vacuum is sufficiently great that a positive force must be exerted by the axially aligned pullout mechanism the tension on the tube aids in preventing axial warpage thereof.

C. The liquid mold material has less chance of oxidation since no airis internal to the casting chamber.

d. The internal partial vacuum materially aids the collapse forming operation.

e. The internal partial pressure of reducing gases can be maintained interior to the tube, as will be revealed in the teachings of this inventions disclosure, for as long as desired and the internal surfaces of the tube will remain bright and oxide free for subsequent reheating of the tube and collapse deformation thereof or, if desired, as a precleaned surface for subsequent application of an internal oxidation resistant coating of enamel, plastic, rubber, zinc, tin, aluminum, lead or the like.

In the Method 4, the volume external to the exit end of the centrifuge is enclosed to afford an effective seal which permits the applicationof a higher than ambient gas pressure which forces the liquid mold material to back up in the centrifugal caster until the ID. of the liquid mold is equal-to or less-than the ID. of the centrifuges orifice. This pressurization is accomplished with adry, inert gas such as nitrogen, argon, helium, or the like. it is preferred to use this method in conjunction with the Method of number 3, since, by this combination, the wall thickness of the 10 inch (0 .D.) tube can be considerably in excess of one inch.

As an example: A partial vacuum of 12 p.s.i. (2.7 p.s.i. absolute) less than the ambient air pressure 14.7 p.s.i.) internal to the tube and an excess pressure of 6 p.s.i. (20.7 p.s.i. gage) external to the tube would create an additive effective pressure (for decreasing the OD. of the molten metal tube) of 18 p.s.i. Since, at the 50 Gs used for centrifuging, 1 cubic inch of the solidifying metal (mild steel) would weigh 13.2 lbs./in and, therefore, the 18 p.s.i. would support a thickness of l8/l3.2 or 1.36 inches of the steel. Actually, due to the truncated wedge section, the layer thickness of molten steel sup ported by the additive l8 p.s.i. would be calculated as follows:

If the layer of molten steel was flat, the 18 p.s.i. would support a layer thickness of 1.36 inches. A truncated wedge section (FIG. 3) of such a 10 inch diameter tube would have considerably less volume (for the same layer depth) bearing on the square inch of the tubes OD. and, for the same volume of 1.36 cubic inches, would have a greater radial depth of molten metal. If we let this layer thickness of the tube be designated by T, then the radius of the ID. of the tube is inches T.

If we let L designate the length of the ID. are intersected by the projection of the one inch long are (the 1 inch long circumferential side of the square inch of area on the tubes O.D.), then l/L =S/S-T and L=5T/5 But, the volume of the truncated wedge is equal to L+1l2 T l and this is equal to the 1.36 cubic inches of circumferentially layered steel. Therefore, L l

X T= 1.36 or (L+1)T=2.72

In other words, an 18 p.s.i. excess pressure due to a combination of an internal vacuum and an external positive pressure will counterbalance a 1.62-inch layer thickness of mild steel tube having a inch O.D.

This same excess of pressure (18 p.s.i.) can also be used to counterbalance a less thick layer at a higher G rotational speed (as 1.08 inch at 75 Gs).

Aluminum, with a density of about one-third that of steel, can be produced as a 10 inch O.D. tube having a wall thickness some three times that of steel under the same, forementioned, conditions of centrifuging. In the case of such metals as aluminum and copper (which, unlike steels and irons, have a considerable solubility in the liquid mold materials in the molten state), it is preferred to minimize the layer thickness and employ higher rotational speeds (G forces) since such high G forces promote the layering effect and greatly ameliorate the tendency for intermixing and allowing between the molten metal being cast and the liquid mold material. This technique, of minimizing the wall thickness and utilizing high G forces to accentuate the layering effect, permits the continuous centrifugal tube casting of such metals as titanium and zirconium onto a liquid mold of tin. Due to the reactiveness of such metals with refractory conduit materials, however, solid rods of these metals are arc-melted in the interior of the inert-gas-purged and evacuated tube cavity to produce the desired molten metal.

Method 4 has the further advantage of preventing any oxidation of the liquid mold material (as lead, lead-tin) since the liquid mold material is protected by the inert gas of the external enclosure. Also, the higher than ambient pressure of the inert gas helps to suppress the vaporizing tendency of the liquid mold at the exit or overflow-end of the centrifuge.

Method 5 has already been discussed. Less than 50 6'5 can be used but it is not particularly desirable unless a very heavy wall thickness is mandatory.

It should be noted that batch-type centrifugal casting is an old and well-established art. Such parameters as the rotational speed necessary to produce a specific G force for a specific mold diameter are well known as, also, are the lower and upper practical limits of G forces (rotational speeds) utilized. it is sufficient to note herein that the supporting action of the liquid mold material on the outer surface of the tube being centrifugally cast (and, also, the use of Method 3 and/or Method 4) permits the use of much higher rotational speeds (G forces) than is permissible with a conventional dry-wall centrifugal mold.

With respect to the methods 3 and 4, it is preferred to utilize higher internal vacuums (Method 3) and lower external positive pressure (method 4) where tubes having a smaller diameter and heavier wall thickness is concerned. Conversely, in the production of large diameter tubes of thinner wall section, it is preferred to utilize a much lower internal vacuum (Method 3) and higher external positive pressures (Method 4 in combination. The reason for this preference is that the ambient pressure of the air (as standard 14.7 p.s.i.) creates a back pressure on the tube which is directly proportional to the crosssectional area of the tube and, also, to the pressure differential between the ambient atmospheric pressure and the internal vacuum. As an example, a tube having a 10 inch O.D. (crosssectional area of 78.5 sq. inches) and an internal vacuum of 4.7 p.s.i. (pressure differential of 14.74.7=l0 p.s.i. with regards to a standard atmospheric pressure) would experience a backward thrust of 78.5 inFXlO p.s.i. or 785 pounds. in other words, it would require a force of 785 pounds on the tube to counteract the internal suction and pull the tube out of the bore of the centrifugal casting machine. On the other hand, a large diameter thin-walled tube (30 inches in outside diameter as an example) would have a cross-sectional area of 709 sq. inches and, if the pressure differential (between the interior vacuum and the ambient pressure) was 10 p.s.i., a force of 7.090 pounds would be required to get the tube out of the bore of the casting machine. If the 30 inch diameter tube had a inch wall thickness and was centrifugally cast at 50 G's, the pressure differential necessary to counterbalance the steel would be one-fourth of 13.2 p.s.i. or 3.3 p.s.i. In this case, the required 3.3 p.s.i. could be made up entirely by application of a positive external pressure (method 4) of l4.7+3.3 or l8 p.s.i. and the internal pressure of the 30 inch diameter tube would be 14.7 p.s.i. or the same as the ambient pressure. By this technique, a very small force (supplied by the liquid mold flow) would be required to extract the tube from the bore of the casting machine since the external pressure (of Method 4) acts on the periphery of the tube to just counterbalance the weight of the steel tube at 50 G's and does not act on the end (cross-sectional area) of the tube to create a backforce which must be overcome (as in Method 3) to get the tube out of the casters bore.

lt is readily apparent from the foregoing examples that a very wide range of latitude is available to the operator, in the application of an internal vacuum (Method 3) and an external positive pressure (Method 4), for ready extraction of a tube from the centrifugal casting machine. A judicious (readily calculated) selection of internal and external pressures is available for all practical casting requirements.

All of the foregoing examples have been predicted on the use of an internal vacuum (Method 3), or an external positive pressure (Method 4), or a combination thereof just counterbalancing the centrifugal weight of the layer of metal being cast and, under these circumstances, any slight thermal contraction as in cooling from the l,500 C. solidification temperature of mild steel down to a collapse deforming and rollwelding temperature of about l,l 15 C.) or slight back pressure (as is normally attendant to such a system by Method l and/or Method 2) is sufficient for free exit of the tube from the exit orifice.

Actually, by increasing the through-put of liquid mold material for any fixed conditions of Methods 1 and/or 2, such back pressure quickly asserts itself and the molten part of the metal tube being cast is squeezed in to a decreased equilibrium O.D.v Due to this combined action, of Method 1 and/or 2 in combination with Method 3 and/or 4, the action of Methods 3 and/or 4 can be considerably less than that necessary to make the CD. of the molten tube equal to the ID. of the exit orifice of the centrifugal casting machine. The action of Methods 1 and/or 2 can be utilized to further decrease the CD. of the cast tube to the amount desired for purposes of exit from the system.

It is preferred to utilize Method 3 and/or 4 only to the extent that the OD. of the tube is somewhat greater than the ID. of the exit orifice of the casting machine since, by so doing, the tube stays in heat transfer contact with the liquid mold material for a longer period of time instead of moving out of contact with the liquid mold due to thermal contraction. The Methods of 1 and/or 2 are utilized to the small extent necessary to back up the liquid mold and thus decrease the tube OD. and, at the less steels as an example) having a lower thermal conductivity than the liquid mold material (as lead) can exhibit a considerable molten interior lining on exit from the centrifugal casting system whereas those tube metals (aluminum, copper, or lowealloy low-carbon steels as examples) having a thermal conductivity that is greater than that of the liquid mold material must be at least in a semisolid state on the tubes interior (and solid on the OD.) for successful processing.

In the case where the pressure differential of Method 3 and/or 4 is sufficiently great to more than just counterbalance the centrifugal weight of the metal being cast, it might be expected that the tube would decrease in diameter (which it does) to the extent that it would lift away from the liquidmold and permit ingress of air or inert gas into the vacuum of the tube's interior via bubbling through the molten zone of the tube. This can and does happen, but not immediately beyond the point where the pressure differential overbalances the zero point.

A stable-state condition exists for pressure differentials in excess of the zero point and this is due to the wetting action (attraction) of the liquid mold material (especially where tin is present) and the surface tension of the molten metal being '50 cast. This operating area (pressure differential beyond the zero point) is not actually used since the stable-state condition is not that broad and can readily be destroyed by any out-ofbalance or other vibration producing condition of the rotating system. It does, however, afford a usable margin of safety for the condition of exact counterbalance.

It is one of the important features of this invention to utilize the advantageous system of a vacuum internal to the tube being cast (Method 3) in the instance wherein tube itself is the end'item instead of a longitudinal structure formed by inwardly collapsing the tube walls over its entire output length. In the practice of making tube for its own use, a tube (having a capped or crimped vacuum sealed exit end) is used as the starting tube so that the desired vacuum (depending on the wall thickness of the tube, the densities of the molten tube metal and the liquid mold, the G force of the centrifuge, and

-the ambient pressure of the atmosphere) can be drawn on the tube interior. The machine then continuously produces a long length of solidified rotating tube which exits into an axially aligned cradle which permits such combined egress and rotation. Such a cradle can rotate with the tube by virtue of the same drive mechanism as that which rotates the centrifugal casting machine. A multiplicity of axially aligned rollers supports the periphery of the tube and, at the same time, can either permit or cause the tube to move axially away from the casting machine. In the case where axial movement is permitted, the rollers are mere idlers which are attached to and rotate with the cradle. In the case where they cause the tube to move axially, the rollers are spring or piston loaded onto the outer surface of the tube to give a friction drive contact which pulls the tube from the bore of the centrifuge as is necessary where an internal vacuum (Method 3), which causes a suc tion, must be opposed. The rollers, in this instance, are suitable driven by sun gears (via a suitable gear cluster system for such power transmission) and are activated or deactivated by a suitable clutch mechanism. Such mechanisms are well known to those practiced in the art of rotary coupling and uncoupling. At the same time, there is an axial gap in this cradle system, near the exit end of the centrifuge, with appropriate torch reheating means and rotating opposed swaging or forging hammers which move in axial synchronization with the exiting tube and swage or pinches a reheated section of the tube to a vacuum tight closure after any desired length has been produced. The pinch or swage closing mechanism then returns to the initial starting place where its operation is recommenced after another appropriate length of vacuum-sealed tube has been produced. Along with the swaging mechanism, and axially further away from the centrifugal caster by any appropriate length (a 2 foot long swaged section and a 200 foot length of tube between swages would limit the loss of tube due to swaging to one percent), is located an appropriate cutoff device which travels in axial synchronism with the exiting tube and cut off the tube at the middle of the swaged or forged down closure so as not to destroy the integrity of the internal vacuum. After cutting the tube in the axial center of the swaged section, the cutoff returns to its starting point for recoupling to the axial travel mechanism and cutoff of the tube section at the appropriate time. By this synchronized and discretely repeatable sequence of swaging-down and subsequently cutting offthe exiting tube, the integrity of the internal vacuum (with its manifold advantages) is maintained dur ing and after the tube casting operation.

It is convenient to forge-flatten the exiting tube (just as a soda straw can be pinch-flattened in a selected area between thumb and forefinger) at the separating point. However, even though this serves as a simple means of sealing and maintaining the integrity of the internal vacuum, it is the preferred method of this invention to swage or peripherally hammer forge such separation points to a solid round having its forge welded centerline coincident with the axis of the tube. These end closures (after separation of the tube lengths at the midlength of the solid swaged-down closure) can be cut from the tube ends with an integral portion of the tube length as long as desired. Such cutoff closure lengths are conveniently used to fabricated pressure bottles or tanks for oxyacetylene, propane storage and the like. In this manner, the closure part of the tube is not subject to remelt but affords great economies in the manufacture of pressure tanks and storage vehicles.

My preferred means for extracting (pulling the tube out of the bore of the caster in opposition to the suction of the internal vacuum) is to power the rotating swaging apparatus so that, once it has swaged down the tube to a vacuumtight solid round, the swaging apparatus remains gripped to the solid reduced tube closure and pulls the tube out of the bore. The axial travel of the apparatus can be powered by any convenient means (such as a chain drive, cogwheel, worm screw, etc.) and can be geared to or be separate from the rotational means as desired. The system utilizes two such swaging down and pullout mechanisms so that, while one mechanism is pulling out the tube, the second mechanism can be swaging down a tube closure some 200 feet closer to the centrifugal caster. Once the 2nd mechanism has swaged-down and gripped the tube closure for powered pullout, the first mechanism (axially further away from the centrifugal caster) the seversthe tube lengths from each other at the midlength of the swaged-down closure so as not to destroy the vacuum seal. The first mechanism is then returned to the starting point to restart as the second mechanism. The two mechanisms thus continually replace each other at the starting point. Alternately, by way of decreasing the axial floor-space requirements, the swage-down and pullout mechanism can grip the swaged' down end of the tube being pulled out and, at the same time, sever the completed length which then is released from the accordion pleat cradle (d series of idler supports which pull out at regular intervals to support and align the rotating tube sections between the swage-down mechanisms) and rolled off at right angles for storage of processing. This is not the preferred means since a grip slippage would result in the tube being sucked back into the bore of the caster with attendant destruction of the internal vacuum, increase in the molten metal tube OD. and stoppage of output for repairs. In the preferred means (using some length, as the 200 foot example, more floor space, depending on the tube lengths produced), any slippage of the grip merely brings the pullout mechanism into contact with the belled-down part of the tube and creates a positive and safe pullout.

In the foregoing manner, long sections of tube (like straight sausage links) are produced which have an internal vacuum of partial nature. The internal surfaces of these tube lengths are clean and bright (due to the inert or reducing nature of the gases contained therein) and this permits the collapse deformation thereof to longitudinal structure (at an appropriate reheat temperature) with roll-welding of the clean contiguous interior surfaces. The partial interior vacuum, along with the clean bright interior surfaces, are very effective in promoting the application of interior coatings to the tube since (by clipping the tube end while immersed in the fluid coating media and replugging the opening once the exact amount of coating has been sucked into the interior of the tube'length) the tube can then be rotated-in-place to evenly coat the tube's interior surface while the coating is being heat-cured, catalytically cured or solidified in place as suits its nature (whether organic, nonorganic or metallic). The clean interior surfaces accept such coatings with excellent adhesion.

In the collapse-deformation and roll-welding of such tube, the tube section can be collapse-formed partially (over its entire length) or completely collapse-deformed (over a part of its length), with appropriate preheating, so that a positive internal pressure (above ambient) is built up inside the tube. The back end of the tube is then perforated to permit escape of the internal gases for continued hot collapse-deformation and sizing to a completed item of longitudinal structure. In this manner, the internal vacuum does not suck in moist air which could contaminate the bright-clean interior surfaces to the detriment of their being roll-welded together.

The long lengths of tube (they can readily be made as milelong lengths by exiting the tube onto a body of water, such as a bay or down a stream or river, which floats the tube and acts as the support cradle), having an internal vacuum as a result of both ends being swaged close, can then be cut up into desired lengths for use (or for sizing and/or grain refinement since the ends are appropriately capped) or they can remain unchanged for float shipment to any desired shoreline location on earth by bundling into appropriate rafts. Such lengths can then be extended inland (by means of bag rollers and use of the already laid pipe or pipes as a rail line) for end cutoff and weld or other attachment as mile-long lengths. The savings in transportation costs and decreased welding for pipeline fabrication is readily apparent.

It is a purpose of this invention to improve the invention of the Maxim (British Pat. No. 22,708 and that of Daubersy and Schlemmer (U.S. Pat. No. 2,940,143) by application thereto of Method 3 (a vacuum internal to the tube being cast) or Method 4 (a positive external pressure exterior to the tube and the exit orifice or at the entrance of the centrifuge) and combinations of Methods 3 and 4 In the Maxim process, as improved by the foregoing means, a static (not axially flowing) centrifuged cylinder of liquid mold material has its interior diameter (adjacent to the exit orifice annular weir) substantially equal to the [.D. of the exit orifice of the centrifuge. No liquid mold material overflows the exit orifice weir except the dragout that naturally occurs with the Maxim process. Small additions of liquid mold material are added to the system by any convenient means so as to continually make up the liquid level and compensate for any losses due to dragout, vaporization, etc. The application of Methods 3 and/or 4, as taught in this invnetion's disclosure, may be utilized to decrease the CD. of the semisolidified tube (being cast) to a slight extent, or to its greatest possible extent, or to any in-between extent as desired. Due to the Maxim process not having available an exciting volume of liquid mold material which can be restricted to build up an aiding back pressure by the restriction to flow methods of l and 2, the

present invention must depend to a slight or a large extent (depending on the amount of application of the Methods of 3 and/or 4) on the diametrical shrinkage of the tube OD. as it cools to the desired exit temperature. The exiting rate of the tube is controlled, as in the Maxim process, so that the CD. of the tube thermally shrinks to a less value than the ID. of the exit orifice considerably prior to passage through the annular exit orifice in order to preclude jamming.

The Methods of 3 and/or 4 are also applied as an improvement to the process of Daubersy and Schlemmer as a positive and practical means of reducing the CD. of the tube (being cast) to one which is equal-to or less-than the exit orifice ID. The amount of application of Methods 3 and/or 4 extends from the minimum to the maximum range as desired. By this means, small amounts of liquid mold material (normally less than 5 percent of the throughput weight of the molten metal being cast to tube on a timed basis) are continuously circulated through the system in order to maintain a lubricating flow of liquid mold material between the outside surface of the tube and the face of the annular exit orifice of the centrifuge. I also apply, as an improvement to the Daubersy and Maxim processes, the means shown herein for the positive extraction of the centrifugally cast tube so that a controlled rate of output of the centrifugally cast tube can be effected and thus preclude the danger ofjamming the tube into the exit orifice of the casting machines. I also apply, to the teachings of Maxim and Daubersy, the methods of vacuum sealing at the exiting end (as by continuous collapse-deformation of the exiting tube to items of longitudinal structure or by intermittent vacuum seal closures at specific intervals of length of the tube) so that the Methods of 3 and/or 4 can be effectively applied to theses older processes.

By the foregoing means (the vacuum sealing of the starting end and closure sealing of the exiting tube by collapsing to a solid shape or section and the application of Methods 3 and/or 4 of my invention) the Maxim and Daubersy processes are improved upon to the point where very high rates of casting output can be obtained and the product limitations of the Maxim process (as expressed by the formula 1, D=T, which is given as an example for the system of liquid lead mold and a lowcarbon, low-alloy steel being cast thereon to tube) are removed.

My continuous centrifugal process not only produces a wide range of tubular products for use as such but it produces this variety of tube at such high rates of output (on a hundreds of tons per hour basis) that the tube can be economically and very advantageously used as a basic item for the production of other items of longitudinal structure. It is therefore a bona fide continuous casting process that is highly competitive when compared to the current continuous casting of solid billets and slabs. More than this, the collapse deformation of such continuously cast tube (as a basic starting item of production) into other longitudinal structural shapes can be readily and much more economically done than by current techniques and this can be accomplished by the use of very light mills (as light rolling mills) and with very few passes. Capital investment is thus greatly reduced and thus augments the other economies of the process.

The foregoing advantages apply also to the Maxim and Daubersy processes once they have been improved by application of the teachings of this invention.

OBJECTS OF THE INVENTION lt is an object of this invention to continuously centrifugally cast metal tube on a centrifuged and axially flowing layer of liquid mold material, consisting of molten lead or tin and alloys thereof, wherein the amount ofliquid mold material flowing through the system is equal to or greater than five percent by weight of the molten metal being cast to tube in the same time interval.

It is a further object of this invention to maintain the CD. of the solidifying metal tube equal-to or less-than the diameter of the exit orifice of the centrifugal casting machine so as to permit a rapid but controlled egress of the cast tube without danger ofjamming at the exit orifice.

Another object of the invention is to utilize a vacuum seal at the entrance or starting end of the centrifugal casting machine for purposes to be subsequently noted.

Another object of the invention is to continuously collapse the tube to a longitudinal structural solid shape so as to form a vacuum tight seal for the tube at the exiting end.

Still another object of the invention is to collapse a limited portion of the tube, as it exits from the machine, to form vacuum tight closures at specified intervals along the length of the tube.

Another object is to cut off such lengths of tube at the midlength of the closure so as to maintain the integrity of the vacuum internal to the tube and to obtain long useable lengths oftube having such closures at both ends thereof.

A further object of the invention is to introduce a vacuum internal to the tube, as it is being cast, as a Method of reducing the CD. of the molten metal tube to a diameter that is lessthan that which would result from normal shrinkage due to the Archimedes principle. (Method 3).

A still further object is to maintain a positive pressure (above the ambient atmospheric pressure) of inert or reducing gas, external to the tube, at the exit end of the casting machine asla Method of reducing the OD. of the molten metal tube to a diameter that is less-than that which would result from nor mal sinkage clue to the Archimedes principle (Method 4).

An alternative object is to maintain a positive pressure of inert or reducing gas at the entrance end of the centrifugal. caster, and exterior to the vacuum seal at that end as an alternate Method of reducing the CD. of the molten metal tube to a diameter that is less than that which would result from normal sinkage due to the Archimedes principle.

An additional object is to utilize the methods of a vacuum internal to the tube and a positive pressure external to the tube in a desired combination for the purpose of reducing the CD. of the solidifying tube to a less value than would result by normal sinkage due to the Archimedes principle.

A still further object is to decrease the exit aperature between the tube OD. and the exit orifice l.D. so that a sufficient back pressure may be built up within the liquid mold material to maintain the CD. of the molten metal tube within the caster equal-t or less-than the ].D. of the exit orifice (Method 1).

A still further object of the invention is to utilize all com. binations and permutations solidifying the Methods (listed herein as l, 2, 3, and 4) to maintain'the CD. of the moltenmetal tube within the caster (centrifugal casting machine) equal-to or iess'than the ID. of the exit orifice.

Another object of the invention is to utilize an extended: hot-zone at the starting end of the caster in order to accentu ate the effects of gravity segregation to obtain a useful result such as a lower carbon surface on steel sheet for use in the au v tomotive industry.

Another object of the invention is to introduce a small percentage of liquid mold material into the bottom of an annular molten steel trough, at the starting end of a centrifuge, so that the molten steel being poured into the trough preheats the restricted amount of liquid mold material to an elevated nonchilling temperature which efi'ects an initial hot-zone (for leveling or accentuated gravity segregation) prior to the molten steel coming into contact with the major and colder amount of liquid mold material further down the bore of the centrifuge.

It is a further object of this invention to provide acontinuous centrifugal casting machine having no lip or weir at the exit end and which uses a liquid lining (denser than the metal being cast) to float the cast tube out of the bore.

A further object of the invention is to utilize a liquid mold material of lead, tin, and alloys of lead and tin for the purposes of floating the cast tube out of the bore of the centrifugal tube caster.

Another object of the invention is to improve the Maxim process (British Pat. No. 22,709 by application of the novel Methods herein disclosed.

Still another object of this invention is to improve the process of Daubersy and Schlemmer (U.S. Pat. No. 2,940,143) by application of the novel methods herein disclosed.

A further object of this invention is to so increase the casting rate and versatility of continuous centrifugal tube casting machines, utilizing a liquid mold, that the tube product can be used as a basic continuously cast item for economical conversion into other items of structure on a continuous or noncontinuous basis.

SPECIFIC PROBLEMS AND ADDITIONAL OBJECTS Gravity Segregation One of the limitations encountered in centrifugal casting concerns the centrifuging of denser constituents towards the outside surface (and, conversely, lighter constituents towards the interior surface) by the high G" centrifugal forces. Under normal, fairly rapid solidification this is no problem but it is sufficiently severe in some alloy systems as to obviate or limit the use of centrifugal casting. The variation of composition from the interior to the exterior surface of a centrifugal casting is termed gravity segregation" and has been considered as either a limitation or a nuisance by centrifugal casters.

It is a purpose of this invention, and one of its features, to enhance and utilize gravity segregation to a useful purpose.

The specific method of accomplishing or enhancing gravity segregation to effect a useful purpose is to introduce as entended-hot-zone at the starting end of the continuous centrifu' gal casting system herein disclosed. The Maxim process has a hot zone at the starting end of the caster for the purpose of preventing a knobby surface (to enhance the leveling or smoothing action) and another invention, US. Pat. 2,754,559 issued to Fromson in 1956, utilizes an initial hot-zone to enhance layering or smooth spreading out of the molten metal to be solidified on top of a flat liquid mold of lead. in the present process, the hot-zone is appreciably extended, (where desired to enhance gravity segregation and only in this instance is the hot-zone so extended, beyond that required for effective leveling or layering of the molten steel) so that segregation will be-emphasized and can be utilized in a very worthwhile manner as will be explained in detail later on.

Automotive sheet steel (used for the exterior body covering) is normally made from rimmed-stcel ingots even though it would be considerably cheaper, if the desired properties were present, to utilize continuously cast slabs or billets instead of remaining with the old ingot process. The reason for this is that rimming-steelexhibits a vigorous boiling action onlpouring into the ingot mold and this creates a scrubbing action at the solidifying surface of the ingot. The result is that rimmedsteel ingotsihavea fine grained exterior layer of fairly low carbon contenLWhen such ingots are rolled, the surface of the sheet is smoother and takes a better polish than:steel made by other processes. It also has a better deep drawing qualities. The spattcring (which creates a rim on the ingot mold and is the basis for the term rimmedsteel) caused by the release of gases, with resultant vigorous boiling action, is the main reason that rimmed steel cannot be effectively cast by current continuous casting processes.

Rimming-steel can be cast in the centrifugal process using a mold having a fairly large diameter (as 3 feet) since any spattering merely ends up on the opposite interior surface of the tube. The scrubbing action is absent, however, since the released gases are directed inwardly by the centrifugal forces. C'entrifugally ,cast steel does, however, have the required density since it is pressure cast under optimum conditions.

If, however, an extended-hot-zone is used, either with,

rimming steel or with semior fully-killed low carbon steel, the delta ferrite (essentially pure iron) solidifys first, and being solid and denser than the balance of the molten metal, centrifuges to the exterior surface. The resultant centrifugally cast tube is characterized by having an exterior layer of dense, fine grained, low-carbon steel. Such a tube can be collapsed to a plate and rollwelded on its interior contiguous surfaces to yield a product capable of being rolled to sheet stock which exhibits all of the properties (smooth surface, high polishability, and deep drawing characteristics) required of automotive sheet stock. Such a tube can also be slit longitudinally and flattenedto plate stock, by prior art processes, and rolled to sheet havingthe desired properties on one, the tubes exterior, surface.

It can be appreciated that such automotive sheet stock can also be produced from batch-type centrifugally cast cylinders of steel by the expedient of an extended (slow) cooling action using preheated or low heat conductivity molds of a solid wall nature.

The extended-hot-zone is basically a means of slowing the solidification rate over a specific temperature range. With low-carbon steel this range coincides with the delta-ferrite region of the iron-carbon phase diagram which encompasses the temperature range of about I ,500 to l,475 C.

The extended-hot-zone (slowed solidification range) can, by intentional varying of the length of the hot-zone or utilizing higher G forces create a wide variation of surface properties in collapse-formed sheet products made from such tube. Ordinarily, the extended-hot-zone is used only where an end product of uniquely advantageous properties is created (as automotive sheet stock). The hot-zone is restricted to that necessary for leveling or smoothing of the molten steel or other metal layer under all other conditions. This is especially true where the tube is to be longitudinally collapsed -formed to a structural item (as l-beam or railroad rails) where a lower carbon surface could result in a loss of fatigue resistance.

Other alloys can be advantageously processed by the technique of using an extended-hot-zone. Cast iron pipe continuously centrifugally cast from gray or nodular irons can be produces with a gradient metallurgical structure (from the exterior to interior surface of the pipe) of varying carbon content which exhibit advantageous properties under certain conditions of use.

Lead-Tin Alloy Liquid Mold Whereas the Maxim process utilizes lead and some alloys thereof for a liquid mold material, as does my process herein disclosed, it is an object of this invention to improve the liquid mold material by additions of tin to the lead and the use of lead-tin alloys and tin as liquid mold materials is claimed when used in conjunction with this invention.

Tin is particularly used as an addition to the liquid lead mold material when it is desired to retain the exterior lead film on the tube as a corrosion resistant barrier both for collapseformed items of structure and, in particular, for use in pipelines since tin greatly increases the adhesion of lead to other metal surfaces. Both lead, tin, and lead-tin alloys are very corrosion resistant and have been historically used for this purpose. Other advantages of tin additions to the lead include lowering the melting point and raising the boiling point beyond that of lead alone and this extends the usable liquid range of the liquid mold system. Tin additions also increase the fluidity and heat conductivity of the liquid mold and, more important, tend to suppress the vaporizing tendency of lead at elevated temperatures. It thus helps to prevent lead losses, due to vaporization, and reduces the danger of toxic lead vapors escaping from the system. The foregoing advantages outweigh the extra cost incurred by tin additions to the liquid lead mold.

It can be realized that, by continuously casting metal tube on an axially flowing ring of liquid mold material (the ID. of which is maintained equal-to or less-than the ID. of the exit orifice of the centrifuge) the solidified or semisolidified tube will float out of the bore on the axially flowing liquid mold material without danger of jamming in the exit orifice. More than this, the wall thickness to diameter restrictions of the tube output are largely obviated and much smaller diameter tubing can be continuously produced.

The liquid mold material, used in conjunction with the continuous centrifugal casting systems herein disclosed, embodies the following characteristics: (1 has a solidification temperature lower than that of the material being cast; (2) is substantially immiscible with and nonreactive to the molten material being cast (except where alloying is desired for a corrosion preventive surface coating such as tin on iron); (3) has a boiling point which is substantially higher than the melting point of the material being cast under the rotational forces involved (high G rotation suppresses the boiling tendency); and (4) has a density which is greater than the material being cast to tube. Liquid lead and lead-tin alloys are generally used in the casting of light metals such as titanium, aluminum, magnesium, etc.

The novel features which are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the description when read in connection with the accompanying drawings.

IN THE DRAWINGS FIG. 1 is a graphical representation of the change in specific volume ofa solidifying and cooling steel;

FIG. 2 is a graphical representation of the formulas l and 2 respectively which show the limitations of product output of liquid mold centrifugal tube casting machines which depend on diametrical shrinkage of the solidified tube to accomplish extraction thereof;

FIG. 3 is a diagram of a unit volume section of tubing wall, in the form of a truncated wedge with radial sides, used in computing the pressure differential required for counterbalancing the expansion effect of centrifugal force on rotating tubing being cast by my process;

FIG. 4 is a partial sectional view of a simplified centrifugal, liquid-mold continuous casting machine wherein no exit orifice lip (reduced diameter annular orifice weir) is used;

FIG. 5 is a more sophisticated axial sectional view of a liquid-mold continuous centrifugal casting machine adapted to the floating of the tube out of the bore;

FIG. 6 is an axial sectional view of one embodiment of this invention depicting vacuum sealing means at the entrance (pouring) end of the centrifuge and a means of vacuum sealing the tube subsequent to the exit end;

FIGS. 6A, 6B and 6C are partial axial sectional views depicting other embodiments of the entrance end vacuum sealing means;

FIG. 7 is an axial sectional view of an embodiment of the exit end of a centrifugal casting machine which depicts means of enclosure to effect a positive pressure (above ambient) external to the exiting tube;

FIG. 8, 8A and 8B are partial axial sectional views depicting various means of layering the molten metal onto the liquid mold material in a smooth continuous manner.

DETAILED DESCRIPTION Referring now to the drawings in detail, and in particular to FIG. 1 (redrawn from Wulffs Metallurgy for Engineers), I have shown, by way of example, that a centrifugally cast mild steel tube will experience a diametrical shrinkage of about 2 percent in cooling from the solidification temperature of about 1 ,500 C. to a temperature just above that of the melting point of a liquid lead mold material or 330 C. I have also shown that the diametrical shrinkage of a centrifugally cast mild steel tube in cooling from 1,500" C. down to 700 C. is about 1.53 percent.

By using these percent shrinkage values and the densities of the axially flowing centrifuged molten tube of mild steel and the liquid lead mold at temperatures involved, I have derived formulas l and 2, given earlier which illustrate the minimum diameter of a mild steel tube for any given wall thickness, in order to satisfy the displacement requirements of the Archimedes principle and the diametrical contraction requirements for withdrawal of the tube from the exit orifice of the centrifugal casting machine where such tube shrinkage is the means by which such exit is accomplished.

The limitations of formulas l and 2 (D=65T and D=T respectively) are graphically illustrated in FIG. 2 wherein, for any wall thickness of mild steel being centrifugally cast to tube on a liquid mold of lead, the tube diameter, necessary to permit sufficient contraction of the tube so that it can just escape out of the systems exit orifice, can readily be determined. It should be realized that these are merely examples formulas and graphical figures which are applicable to the continuous centrifugal casting of a mild steel tube on a liquid lead mold. Similar formulas and graphs can readily be derived for other systems of casting materials (as aluminum, copper, nickel, etc.) when used in conjunction with other liquid mold materials (as lead, tin, and leadtin alloys).

Reference is now made to FIG. 4 which is an axial cross-sectional view of a simplified version of a continuous centrifugal tube caster (casting machine) or centrifuge utilizing a liquid mold and having an exit orifice diameter which is equal to or greater than the CD. of the tube being cast. In FIG. 4 the centrifugal caster is rotatable about its axis 1 by means of suitable trunnions, and drive mechanisms not shown. At the entrance orifice 2 a liquid mold material 3 is poured upon the rotating annular refractory and thermally insulating part 4 of the centrifuge via spout 5. At the same time, the molten metal 6, to be cast to tube, is poured onto the refractory part 4 of the centrifuge by way of spout 7. The refractory part 4 of the centrifuge extends to a point 8 (towards the exit end 9) so as to form a hot-zone 10 wherein solidification of the metal tube is retarded and where the molten metal 6 and the liquid mold material 3 have time to layer into over-and-underlaying cylindrical shells in the liquid state. The refractory part 4 is enclosed in a structural shell 11 which supports the refractory part 4 and then extends to the exit end 9 as the solid wall 12 of the centrifuge. The solid wall 12 is cooled on its exterior surface by multiple peripherally arranged jets of Water (not shown) or other cooling material so as to remove heat from the molten metal 6 through the liquid mold lining 13and solidify the molten metal to a solid tube 14. The solidified tube 14 continues out of the centrifuge into an axially aligned and rotating cradle (not shown) and is intermittently cut off to desired lengths by any desired mechanism such as that shown in the Maxim patent. The liquid mold material 3 cascades at 15 from the annular exit end 9 of the centrifuge into an annular trough (not shown) such as that used in US. Pat. No. 2,866,703 issued to Gross in 1958 and wherein an axially flowing molten metal effluent is spun out of the exit end of a centrifuge into an annular catch basin. The liquid mold material 3 is then recirculated back to the pouring spout 5 by any convenient means such as that of U.S. Pat. No. 2,6l7,l48 issued to Ryan in 1952 and wherein a metallic liquid mold material is recirculated from the exit end of a casting machine back to the entrance end via suitable heat exchangers (coolers) and a suitable pump.

It should be noted that the continuous centrifugal tube casting machine of FIG. 4 utilizes a long bore so as to accentuate the shearing action (resistance flow) in the liquid mold material. A positive pullout mechanism of any type (such as that used in British Pat. No. 22,708 issued to Maxim) is used to control the rate of exit of the cast tube so that it is sufficiently solidified prior to exiting from the end 9. The refractory part 4 of the centrifuge is preferably made of pyrolytic boron nitride or pyrolytic graphite with the C" planes (the plane of low heat conductivity) being perpendicular to the axis 1 of the bore and the "A plane (the plane of greatest heat conductivity) being parallel to the axis ofthe bore. In this manner, the inside (I.D.) of the hot-zone 10 is at a high and uniform heat that prevents solidification in that area. By greatly extending such a hot zone, an extended-hot-zone results which permits the accentuation of gravity segregation to a useful extent.

This system has the virtue of extreme simplicity; however, due to the high G forces involved the liquid mold material has a higher exciting flow than the cast tube with its controlled pullout. This flow differential can cause wrinkling (shirt-sleeving) of the tube surface at the point of incipient solidification and this surface roughness anchors the liquid mold material and results in excessive dragout.

FIG. 5 is illustrative of a more sophisticated system for the continuous centrifugal casting of metal tube on an axially flowing lining of liquid mold material. A criterion of the apparatus of FIG. 5 is that the CD. of the molten metal tube (within the centrifuge) be equal to or less than the exit orifice I.D. In FIG. 5, the molten metal 6 pours into an annular trough 16 which is similar to the annular distributing chamber used by Stravs and Jager in US. Pat. No. 777,559 of 1904 and serves to take up the impact of the inpouring molted metal 6 and to evenly distribute the molten metal, via the refractory annular shelf 17, as a molten cylindrical tube within the bore of the centrifuge. The refractory part 4 of the centrifuge is extended towards the exit end 9, as shown, so as to form a hotzone 10 whereon the cylinder of molten tube metal 6 becomes leveled or layered into a smooth cylindrical tube 26 on top ofa thin cylindrical layer 18 of hot liquid mold material.

The liquid mold material 3 is poured into an annular sump l9 and moves (via multiplicity of longitudinal holes 20 peripherally spaced around the base of the refractory part 4) downstream in the centrifugal caster via the main series of flow-holes 20 to the maine exit 21 what the main part of the cooler liquid mold material flows into a heat extracting ring 22 of liquid mold material which both supports and solidified the ring of molten metal to an exiting soli-d tube 14. The ring 22 of liquid mold material may be in contact with a finned wall portion of the centrifuge, which is cooled by sprays of water or other coolant from nozzles 81, supplied via piping 82 and a pump P.

Upstream from the main exit 21 of the liquid mold material is another series of annular liquid mold flow-holes 23 via which a restricted (quite small) amount of liquid mold material forms a thin lining 18 of very hot liquid mold material which extends downstream for the length of the hot-zone l0 and permits rapid and effective cylindrical layering and leveling of the molten'6 and liquid 3 materials. The cylindrically layered ring 26 of molten metal substantially solidifies to a solid tube 14 on the ring 22 of heat-conducting liquid mold material which flows axially down the bore of the centrifugal tube caster towards the exit end 9 and becomes a thin ring 24 of restricted flow (in accordance with Method 1 for creating a back pressure on the liquid mold material 3) as it passes over the exit orifice weir 25 having an axially extended surface area, adjacent to the periphery of the solidified tube 14, which creates a line pressure drop along its length (in accordance with Method 2 detailed in this disclosure) which accentuates the back pressure on the liquid mold material 3 to the extent that the CD. of the molten metal tube is maintained equal to or less than the ID. of the exit orifice weir. The rotating solid tube 14 exits axially from the centrifuge for cutoff, seal crimping, or continuous collapse deformation as desired while the liquid mold material 22 spins offas a tangential stream 15 into a suitable annular catch-ring 83 and is recirculated by conventional means not shown. These means, along the the rotational mechanisms and spray cooling method, are indicated but are not detailed since they are a part of the prior art and well understood by those versed in such techniques. Here also, the hot zone as at 10 may be extended in length so that slow cooling of the molten metal can be accomplished. In this manner, when desired, accentuated gravity segregation results (as delta ferrite being centrifuged towards the outside surface of a mild steel tube which is later to be converted to automotive sheet steel).

In FIG. 5, the ring of axially flowing and heat extracting liquid mold material 22 (between the downstream end of the hot zone 10 and the upstream end of the exit orifice weir 25) has a preferred thickness approximating the thickness of the tube wall being solidified thereon. In this manner, the molten and solidified metal 26 and 14 of the tube flows axially in approximate synchronization with the axial flow of the liquid mold material and this results in a smoother exterior surface on the solid tube and less retention of the liquid mold material thereon.

FIG. 6 is illustrative of a vacuum seal means at the entrance end 2 of the liquid mold continuous centrifugal tube casting machine wherein a solid nonrotating disc 30 has it periphery 31 immersed into the liquid mold material 3 which is con-. tained in the annular rotating trough l9. Passing through and vacuum sealed to the nonrotating end plate 30 are the liquid mold circuit 5, the molten metal conduit 7, a dry inert gas purge tube 32, and a vacuum suction outlet 33. The purge solid 32 (or other sealed entrance conduit) may be used as a plasma torch entrance for the purpose of heating up the refractory part 4 prior to startup. In this instance, the inert gas (as helium, argon, nitrogen, etc.) from the plasma torch also acts as an initial purge of the centrifuge cavity and the torch melts down the starter blank which has solidified within the bore of the centrifuge from the prior shut down operation. The suction tube 33 is fairly large and connects to a vacuum pumping system (not shown) so that the interior cavity of the centrifuge can be continuously pumped down to any desired vacuum.

Exterior to the exit end 9 of the centrifugal casting machine is a set of opposed forging rolls 34 and 35 which travel axially and in synchronism with exiting tube 14. At the same axial location and at right angles to the plane between the axis of the forging rolls (34 and 35) are two opposed banks of burners (as not shown plasma torches) which maintain the heat of the exiting tube 14, or bring it to a desired forge welding temperature. These forging rolls 34 and 35 move synchronously and axially along with the hot tube and gradually come together with sufficient force to collapse a small portion of the tube (as a 2 foot length) to a solid round having a forge welded interior 36 which is vacuum tight. Such collapsed sections of the tube can be as far apart as desired (as every 300 feet of solid tube length) and provide the vacuum seal to the tube at the exit end of the centrifugal caster. Further on, and after another seal has been so forge-closed, the solid section 36 can be cutoff at its midlength 37 for removal of the discrete length of the vacuum sealed sausagelike tube lengths, for use as previously described. It can be appreciated that other conventional means, as swaging flat-crimping, etc. can be used to form the discrete collapsed section for vacuum closure, beyond the exit end 9, of the hot tube. Also, the axial travel of the sealing rolls (34 and 35) can be extended (as to 300+ feet) so that they act as pullout grips for the tube so cast.

FIG. 6A is a partial sectional axial view of another configuration of the entrance end 2 vacuum seal means wherein the stationary seal disc 30 is peripherally immersed in an annular trough 40 of a low melting liquid metal such as Woods Metal or molten tin. It has the advantage of permitting the seal to be at a lower temperature and obviates oxidation losses of the seal fluid. In this case, both the molten metal and the liquid mold material are subjected to the internal vacuum at the entrance end 2.

FIG. 6B is representative of another such configuration wherein the annular seal trough 40 is intermediate between the molten metal trough l6 and the liquid mold material trough 19. By this means, the liquid mold material is not subject to the internal vacuum but to the ambient atmospheric pressure and this helps to raise the level (decrease the O.D. of the molten tube 26) of the liquid mold material within the bore of the caster.

FIG. 6C is yet another variation of the vacuum seal means at the entrance end 2 wherein the method of FIG. 6B is further enhanced by use of another end plate 41, exterior to the end plate 30, which is peripherally immersed into an annular rotating trough of liquid sealing metal 42. This system permits the liquid mold material 3 in annular trough 19 to be pressurized via inert gas tube 38 while this interior cavity of the centrifuge is subjected to vacuum. The system of FIG. 6C is even more effective in reducing the O.D. of the molten metal tube 26 to the desired size.

FIG. 7 is illustrative of a means for applying a positive pressure of inert gas 50 to the outside of the solidified tube 14 at the exit end 9 of the continuous centrifugal tube caster. The inert gas 50 is introduced into the end closure 51 via the high pressure gas tube 52 and the pressurized gas 50 acts on the liquid mold material 3 at the point of tangential spinoff so as to produce a greater than normal back pressure on the liquid mold lining 22 within the centrifuge. This back pressure (Method 4) causes the heat extracting ring of liquid mold material at 22 to push inwardly and decrease the OD. of the molten metal tube to any desired limit.

The end closure 51 is sealed at the annular area 53 (exterior to the exit end 9 of the centrifugal tube caster) by means of an iris ring of carbon or graphite blocks 54 which are contained within the annular holding-rings 55. An annular pressure cavity 57 is behind the iris blocks 54 so that, by pressurizing this annular cavity 57 by means of the high pressure inert gas line 58, the iris blocks 54 are forced against the O.D. area 53 of the centrifuge to form a pressure seal. Alternately, the seal at the area 53 can be of the liquid metal type as designated by trough 40 of FIG. 6A.

A similar inert gas pressure seal exists at area 60 on the opposite side of the end-closure closure 51 so as to prevent undue gas leakage around the tube periphery. This iris of carbon ploughs or blocks 61 also act as scrapers to remove any excess liquid mold material from the periphery of the tube. Alternately, a carbon iris block 62 can be used which has a multiplicity of small radial holes 63 leading from the annular pressure cavity 64 to the ID. of the blocks 62 are the area 60. Passage of high pressure inert gas (as nitrogen) through the holes 63 onto the periphery of the tube 14 at area 60 causes a gas bearing action which wipes back any excess liquid mold material into the closure 51 and, at the same time, maintains the desired inner gas pressure therein. As a still further alternate, the pressure cavity 64 may be pressurized with relatively cool liquid mold material 3 so that a liquid bearing seal is formed. This alternate would only be used where a maximum amount of liquid mold material was desired as an exterior coating to the tube so produced.

Referring to FIG. 8, this partial axial sectional view of the entrance end 2 of the centrifugal continuous tube caster illustrates a simplified means of sluicing the molten tube metal 6 onto the 1D. surface of the axially flowing ring 22 of liquid mold material 3. In FIG. 8, the peripheral flow-holes 20 for the liquid mold material 3 terminate downstream at a point 27 and the refractory part 4 continues downstream and tapers to an annular feather edge at point 28. At point 28 the axial flowing annular rings of molten tube metal 26 and of liquid mold material 22 cone into heat exchange contact with a layered laminar flow. The shelf 17 of the refractory part 4 acts as a hot zone for layering and leveling of the molten metal 26. This is the simplest technique, but not the preferred one, for introducing the molten metal layer 26 onto the liquid mold layer 22.

FIG. 8A represents an improvement of the method for sluicing the molten ring of axially flowing metal 26 onto the axially flowing ring of cool liquid mold material 22 via an interposed thin ring 18 of axially flowing hot liquid mold material which is introduced onto the shelf 17 of the refractory part 4 by way of small inclined flow-holes 23 to produce a hot zone or extended hot zone 10 as desired.

The preferred technique for producing a hot or extended hot zone 10 and for bringing the axially flowing annular streams of molten metal and hot and cool liquid mold materials into laminar contact is illustrated in FIG. 8B. In this technique, an annular trough I6 is filled with a small flow of liquid mold material 3 by way of the small ducts 23 which lead from the liquid mold trough 19 to the bottom of the molten metal trough 16 from whence it flows internal to the ledge I7 of the refractory part 4 as a hot relatively thin lining which supports the molten metal ring 26, The molten metal 6 pours onto the surface of the liquid mold material which fills the trough l6, and heats the liquid mold material to a temperature above the melting point of the tube metal. The annular trough 16 serves the purpose of decreasing the impact of the molten metal input 6 and of creating a very effective layering and leveling zone even prior to the downstream hot-zone represented by the relatively thin hot liquid mold lining 18. At the downstream sharp edge 28 of the refractory part 4, the hot liquid mold lining I8 continues downstream. for a short distance and acts as a buffer between the axially flowing cool liquid mold ring 22 and the molten tube metal 26 and prevents too rapid chilling of the metal tube. It is preferred that all three annular rings (the molten metal ring 26, the hot liquid mold l8, and the cooler liquid mold 22) have an approximately synchronized axial flow rate at the point 29 where solidification of the molten tube metal begins.

All of the systems illustrated in FIGS. 8 to 88 can be used in conjunction with the entrance end vacuum seal means of FIGS. 6 to 6C.

Rejuvenation f the Liquid Mold Material Regardless of the use of internal and external inert atmospheres, the liquid mold (whether of lead, tin, or lead-tin alloys) will gradually build up an oxide content which, being lighter than the liquid mold material, will centrifuge to the LD. surface of the liquid mold and adhere to the CD. of the metal tube being cast. Normally, this concentration is not large enough to cause problems but, at heavier concentrations, it can cause excessive dragout of the liquid mold material and, in extreme cases, clogging of the conduits and flow-holes of the system. This can be corrected either by continuous or occasional passage of the liquid mold material through a bath of molten cyanides (asthose of sodium, potassium or barium or mixtures thereof). By such treatment, an oxidation-reduction reaction takes place that produces a reduced liquid mold material that is completely rejuvenated (oxide free).

Start and Stop Procedures ln stopping the process, a clutch system, not shown, is released that stops the pullout of the tube 14, as in figs. 5,6,7, from the exit orifice 9 of the centrifuge while, at the same time, permitting the tube to continue rotating along the with centrifuge. Coincidental with stopping the axial pullout of the tube, the input of molten metal 6 is terminated. The centrifuge is allowed to rotate until the ring of molten metal 26 on the inside of the centrifuge has solidified. Normally the centrifuge rotation is continued and the liquid mold material circulation is maintained (but bypassed through an external heating system to keep it in the liquid state) until the next startup.

Where complete shutdown is made (as for repairs), the solidified tube within the centrifuge is withdrawn by re-applying the pullout clutch (not shown), the input of liquid mold material 3 is terminated, and the centrifuge is then braked to a stop. The liquid mold material is allowed to cascade out of the system via the annular catch-ring and drained (by means of suitably located drain-plugs) into a suitable holding tank where it can be heated and maintained as a liquid until further use. Said holding tank can also contain the molten eyanides which float on the surface of the liquid mold material and removes any accumulated oxides as well as shielding the liquid mold material from contact with the air, during storage thereof.

On startup (under normal nondrained conditions), the internal vacuum is released by input of a dry inert gas via the purge tube 32. An inert gas plasma torch is then introduced through any convenient and resealable aperture (not shown) in the stationary end plate 30. The torch flame is directed onto the solidified tube overlaying the hot zone 10 and the annular trough 16 until the solid tube again becomes the molten metal layer 26. The torch is then quickly removed, the required vacuum redrawn on the tubes interior, and pullout of the tube 14 is recommenced. Coincident with the tube pull out, the input of molten metal 6 is started up.

Where complete shutdown is used, the start up (previously solidified internal tube) blank is reintroduced into the bore of the centrifuge, rotation is started, and the starting end over the hot zone 10 and trough 16 is preheated by the plasma torch. The flow of liquid mold material is restarted. The plasmatorch then melts down the hot but still solid tube at the starting end, the seal trough (s) is filled, the internal vacuum is reestablished and pullout is recommenced along with input of molten metal 6.

It should be noted that plasma torch can be made integral (normally not removable) part of the end-seal since such torches are usually water cooled and not subject to heat damage.

It should be further noted that the starting blank will always have the external end forge-closed at a point 36 in any case where an internal vacuum (Method 3) is utilized in the various systems of this inventions disclosure.

Grain Refinement In general, centrifugally cast metal tube is characterized by columnar grains extending radially inwards from the exterior surface. Such grain type is an advantage where the tube is used at elevated temperatures and pressures since a coarse-grained structure inhibits creep deformation. However, for most purposes, a fine grained material is desired due to its more favorable mechanical properties. Where the tube is collapsed and roll-sized to structure, such grain refinement can be accomplished due to the hot-working recrystallization. In the instance where the tube is to be used, as such (as for oil line pipe, etc.), grain refinement can be accomplished either during the continuous centrifugal casting process or subsequent to its cooling to room temperature.

In the first instance (grain refinement coincident with tube casting), a shearing action can be set up between the external shell of already solidified metal and the interior layer of still molten metal (as downstream from point 29 of FIG. 8). This can be done by mechanical or magnetic means and the layer of still molten metal can be either slowed down or speeded-up rotationally so that the still molten metal has a circumferential speed that is different from that of the already solidified exterior shell metal. ln this manner, the shearing action at the solid-liquid interface destroys the columnar grain growth and creates an equiaxed fine grained structure in the solid tube metal.

Such differential rotational speed between the solid exterior shell and the inner still molten layer of metal can be caused by an interior refractory drum (of light, hollow construction and having an CD. which is less-than the ID. of the molten metal wall 26) which rotates either faster or slower than the centrifuge and is driven by a cooled shaft extending through the stationary vacuum-seal end plate 30. Rotating skimmer blades can also be used. Such differential solid-liquid interface shear can also be created by a rotating magnetic flux internal to the centrifuged tube by an adaption of the method of Pestel as disclosed in US Pat. No. 2,963,758 of 1960.

Grain refinement of the tube metal once it has exited from the casting machine can be accomplished by pulling the hot exiting tube through a rotating sizing bell or by drawing the tube, in the cold state, through nonrotating internal and/or external sizing dies which cold-work the tube metal while sizing it. Where discrete lengths of tube, having the ends sealed by forged closures, are made, a high pressure aperture can be made in one end and the tube length can be hydroforged as per the teachings of US. Pat. No. 2,931,744.

In both cases where cold working is done on the tube metal, grain refreshment is accomplished by subsequent reheating to its recrystallization temperature.

It is a primary purpose of this invention to so increase the continuous casting rates (especially for steels and cast irons) of the metals that the tubular output will be used as a basic item for the production of longitudinal structural shapes by collapse deformation of such tube. The primary method of utilizing such tube would be to cut off discrete lengths as the tube exits from the continuous casting machine and then to deform the tube to other structure while still hot or, alternatively, by cooling to room temperature and subsequently reheating the stockpiled tube to the forming temperature desired. Such collapse forming of tube to a desired cross section (as l-beam plate, channel, etc. and rolling to final size, with accomplishment of roll-welding of the contiguous inner surfaces of the tube, requires that the interior surfaces of the tube be clean and, preferably, in a fluxed condition.

I propose to protect and flux the interior surfaces of the tubes by blowing a powdered material (anhydrous borax- Na B 07in particular) onto the interior surface of the tube at a point where the temperature of the tube is less than the decomposition temperature of the flux and well previous to the point where the tube interior cools below the melting temperature of the flux. In this manner, the tube interior remains coated with a thin layer of protective flux which is squeezed out during any subsequent roll-welding of the interior and can be collected for reuse. Alternatively, the flux can be introduced into the tube interior by spraying in the molten condition by any conventional means.

Alternatively, molten salts such as the higher melting and boiling alkali and alkaline earth chlorides and fluorides (or

Claims (20)

1. 2. The method defined in claim 1 wherein said diameter control is effected by pressurizing said liquid lining within the mold so as to increase its constricting pressure against said tubular body over the normal buoying effect determined by the displacement of the tube materials''a own weight of the liquid lining.
2. 3. The method defined in claim 2 wherein said pressurizing is effected by providing said mold outlet with an annular lip which is extended axially sufficiently to create a substantial frictional resistance to flow of said mold lining to counterbalance the weight of the molten casting material.
3. 4. The method defined in claim 2 wherein said pressurizing is effected by providing said mold, having no lip, with an axially extended bore of sufficient length to create a substantial frictional resistance to flow on said liquid lining.
4. 5. A method for continuously casting tubing of a material of predetermined specific gravity and melting temperature, comprising the following steps: rotating on a generally horizontal axis, an elongated tubular mold having an inlet at one end and an outlet at its other end; injecting into the inlet of the rotating mold a liquid substance of higher specific gravity and lower melting temperature than said casting material and causing said substance to acquire the form of a cylindrical lining within said mold in response to the rotation of the mold and resulting centrifugal force, and thereby providing a cylindrical casting chamber within the mold; injecting said molten casting material into said cylindrical casting chamber and causing it to assume the form of a cylindrical tube in response to the rotation of said lining, and causing said tube of liquid casting material to be cooled by said lining and to thereby be congealed to a state within the range including semisolid and solidified states; causing said lining to flow from the inlet to the outlet of the mold and to discharge from said outlet while simultaneously causing said tube to exit from the outlet while enveloped in the outflowing lining of liquid substance; and causing controlled exit of said tube and aiding in controlling the diameter of said tube to permit clearance through said outlet by subjecting said tube to a gas-induced pressure differential while it is still in a relatively plastic state.
5. 6. The method defined in claim 5 wherein said aid to diameter control is effected by creating a vacuum within said tube.
6. 7. The method defined in claim 5 wherein said aid to diameter control is effected by creating a back pressure on the liquid lining by applying supra-atmospheric pressure to the exterior of the discharging tube.
7. 8. The method defined in claim 5 wherein said aid to diameter control is effected by creating a pressure on the liquid lining by applying supra-atmospheric pressure to the liquid lining material at the entrance end of the casting machine.
8. 9. A method for continuously casting tubing of a casting material of predetermined specific gravity and melting temperaTure, comprising the following steps: rotating on a generally horizontal axis, a bulbous tubular mold having an inlet at one end and an outlet at its other end; filling the bulbous cavity of the rotating mold with a liquid substance of higher specific gravity and lower melting temperature than the casting material and causing said substance to acquire the form of a cylindrical lining within said mold in response to the rotation of the mold and resultant centrifugal force, and thereby providing a cylindrical casting chamber within the mold having a liquid lining which is substantially equal to, but does not overflow, the exit orifice weir; injecting said molten casting material through said inlet into said cylindrical casting chamber and causing it to assume the form of a cylindrical tube in response to the rotation of said lining, and causing said tube of liquid casting material to be cooled by said lining and to thereby be congealed to a solidified state; causing controlled withdrawal of said tube from the exit orifice; and applying a gas pressure differential between the inside and outside of said tube in substantially its molten state to facilitate its withdrawal from said mold.
9. 10. The method defined in claim 9 wherein said pressure differential comprises a partial vacuum within said tube.
10. 11. The method defined in claim 9 wherein the tube within the mold is partially supported by a supra-atmospheric pressure exterior to the tube at the exit end of the casting machine.
11. 12. A method for continuously casting tubing of a material of predetermined specific gravity and melting temperature, comprising the following steps: rotating on a generally horizontal axis, an elongated tubular mold having an inlet at one end and an outlet at its other end; injecting into the inlet of the rotating mold a liquid substance of higher specific gravity and lower melting temperature than said casting material and causing said substance to acquire the form of a cylindrical lining with said mold in response to the rotation of the mold and resultant centrifugal force, and thereby providing a cylindrical casting chamber within the mold; injecting said molten casting material into said cylindrical casting chamber and causing it to assume the form of a cylindrical tube in response to the rotation of said lining, and causing said tube of molten casting material to be cooled by said lining and to thereby be congealed to a state within the range including semisolid and solidified states, creating a partial vacuum inside said tube sufficient to aid substantially in reducing its diameter for clearance through said outlet; causing controlled output of said tube; and closing said tube at selected positions along its length in order to seal said tube and to create a vacuum seal at the outgoing end of the tube.
12. 13. A method for continuously casting tubing of a casting material of predetermined specific gravity and melting temperature, comprising the following steps: rotating on a generally horizontal axis, an elongated tubular mold having an inlet at one end and an outlet at its other end; injecting into the inlet of the rotating mold, a liquid substance of higher specific gravity and lower melting temperature than said casting material and causing said substance to acquire the form of a cylindrical said said mold in response to the rotation of the mold and resultant centrifugal force, and thereby providing a cylindrical casting chamber within the mold; injecting said molten casting material through said inlet into said cylindrical casting chamber and causing it to assume the form of a cylindrical tube in response to the rotation of the lining, and causing said tube of liquid casting material to be cooled by said lining and to thereby be congealed to a state within the range including solidified and semisolidified states; causing said lining to flow from the inlet to the outlet of the mold and to discharge from said outlet While simultaneously causing said tube to exit from the mold outlet while enveloped in the outflowing lining of liquid substance; causing controlled output of said tube; maintaining an annular hot-zone in said casting chamber near said inlet and extending toward said outlet; bypassing the major proportion of said liquid mold lining substance in a bypass flow externally around said hot-zone and then directing it inwardly to the diameter of said lining; and directing a ducted small flow of said liquid mold lining substance diagonally forwardly and inwardly to said hot hot-zone from said bypass flow, to promote flow in the hot zone.
13. 14. The tube-casting method defined in claim 5, including the step of maintaining an annular hot-zone in said casting chambers near said inlet and extending toward said outlet, for the enhancement of gravity segregation.
14. 15. The tube-casting method defined in claim 5 wherein a lead-tin alloy is utilized as said liquid mold lining substance.
15. 16. A liquid mold lining for centrifugal casting of tube, consisting of molten lead-tin alloy.
16. 17. The method defined in claim 5, wherein said diameter control is effected by a combination of a partial vacuum inside said tube and a supra-atmospheric pressure outside said tube.
17. 18. The method of claim 5, wherein said mold has not exit orifice lip.
18. 19. A method as in claim 6, wherein the exit end of said tube is sealed by capping.
19. 20. A method as in claim 10, wherein the exit end of said tube is sealed by capping.
20. 21. A method for continuously casting tubing of a casting material of predetermined specific gravity and melting temperature, comprising the following steps: rotating on a generally horizontal axis, an elongated tubular mold having an inlet at one end and an outlet at its other end; injecting into the inlet of the rotating mold, a liquid lining substance of higher specific gravity and lower melting point than said casting material and causing said substance to acquire the form of a cylindrical lining within said mold in response to the rotation of the mold and resultant centrifugal force, and thereby providing a cylindrical casting chamber within the mold; injecting said molten casting material through said inlet into said cylindrical casting chamber and causing it to assume the form of a cylindrical tube in response to the rotation of said lining and causing said tube of molten casting material to be cooled by said lining and to thereby be congealed to a state within the range including solidified and semisolidified states, causing said lining to flow from the inlet to the outlet exit orifice of the mold and to discharge from said orifice while simultaneously causing said tube to exit freely through said orifice while enveloped in the outflowing lining of liquid substance; causing controlled exit of said tube by mechanism external to said orifice moving said tube at a predetermined speed; and controlling the outside diameter of said tube substantially back of said orifice by means of back pressure in said liquid lining to a diameter smaller than said orifice when said tube is congealed to to state, said back pressure being determined by the rate of introduction of said liquid lining substance and the resistance to flow thereof in said mold

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