US3605859A - Continuous centrifugal tube casting with dry mold and gas pressure differential - Google Patents

Abstract

THE INVENTION RELATES TO A RAPID AND IMPROVED MEANS FOR THE CONTINUOUS CENTRIFUGAL CASTING OF METALLIC AND NON-METALLIC TUBE ON TEH INTERIOR SURFACE OF A ROTATING SOLID-WALL BY UTILIZING NOVEL TECHNIQUES TO GREATLY REDUCE THE SIDE-WALL FRICTION.

Description

Sept. 20, 1971 Q R, LEGHORN 3,605,859

CONTINUOUS CENTRIFUGAL TUBE CASTING WITH DRY MOLD AND GAS PRESSURE DIFFERENTIAL Filed Oct. 21, 1968 3 Sheets-Sheet 1 m R m@ l m WP m e A f G ab am.

Nl TlnI-I-T- MQQMMHN I I NSU! l- Das "auf,

G. R. LEGHORN Sept. 20, 1971 CONTINUOUS CENTRIFUGAL TUBE CASTING WITH DRY MOLD 3 Sheets-Sheet 2 Filed OCT.. 2l, 1968 m w \&. \\mv` w. W.. z mv( QN 7/// H/f/ W L m I l c o f n T m V. r V hw, I I MW ww G m mw m` Wm, \nm.. Wm.

III- MK W\ hx *n L Il i I I l l MI vw\l\\\\\\\\\ \,`\\\`0/% W ,i1 i No, www n m.. n 1S www Y, mw A sept 20 1971 G. R. LEGHORN CONTINUOUS CENTRIFUGAL TUBE CASTING WITH DRY MOLD AND GAS PRESSURE DIFFERENTIAL 3 Sheets-Sheet 5 Filed Oct. 21, 1968 INVENTOR. George l? eghorn 0;. z/QZZZ: 74`7'0P/Vff- Z2 PIG.. 64

l Pla.

United States Patent O 3,605,859 CONTINUOUS CENTRIFUGAL TUBE CASTING WITH DRY MOLD AND GAS PRESSURE DIFFERENTIAL George R. Leghorn, 1423 Washington Ave., Apt. 1, Santa Monica, Calif. 90403 Continuation-impart of application Ser. No. 538,506, Feb. 11, 1966, now Patent No. 3,445,922, dated May 27, 1969. This application Oct. 21, 1968, Ser. No. 769,017

Int. Cl. B22d 11/00, 13/02, 27/16 U.S. Cl. 164-62 13 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a rapid and improved means for the continuous centrifugal casting of metallic and non-metallic tube on the interior surface of a rotating solid-wall by utilizing novel techniques to greatly reduce the side-wall friction.

This application is a continuation-in-part of my application Ser. No. 538,506, filed Feb. 11, 1966, now 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 Patent 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 I.D. and the cast tube O D.

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. Pats. 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. 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. Patent 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. Patent 2,752,648 issued to Robert in 1956 (this is essentially a repeat of the methods of Strauss 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. 984,053 issued in 1963 which teaches the downwards 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 I.D. and the cast tube O D. as a result of the outward forces on the molten and solidified tube metal due to the centrifugal action. Conventionally, horizontal centrifugal casting is done between rotational speeds which produce from 50 to 100 gravities of cen- 3,605,859 Patented Sept. 20, 1971 trifugal force (a one pound mass of metal would effectively weight 50 pounds when centrifuged at the rotational speed of 50 Gs) 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 oif length instead of continuously. To correct this defect, one aspect of the Maxim patent of 1895 (British Pat. 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 I.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 as 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 exemplied by U .S. Pat. 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 non-tapered internal (LD.) molds and as such, are ditiicult to operate on a continuous basis due to the cast tube shrinking inwardly (thermal contraction) onto the solid mold in the bore. Shrink iits are used to prevent concentrically assembled items from slipping and, in the case of continuously cast tube, the shrink t 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 I.D. mold as it contracts or the concentric molds are made as short as possible. Such tubing is being successfully continuously cast by utilizing either tapered or very short I.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.

RESUME OF INVENTION The invention pertains to the continuous centrifugal casting of metallic and non-metallic tube onto the I.D. surface of a rotating hollow cylinder which acts as the initial mold of the tube forming operation. Molten material, to be cast to tube, is continually introduced into the entrance end of the centrifuge and the solidified centrifugally cast tube is continuously extracted from the exit end. The process is greatly enhanced as to casting rates and ease of extraction of the solid cast tube by the techniques of utilizing a vacuum on the interior of the tube, in its molten and solid state, and/or a positive pressure (above ambient) external to the solidified or partially solidified tube at the exit end of the centrifugal casting machine.

THE INVENTION IN GENERAL Analysis of requirements for the continuous centrifugal casting of tube The inventor has analysed the prior art and the machanics thereof and has concluded that the extremely lim ited industrial use of the known processes for the continuous centrifugal casting of tube (in spite of the manifold economic advantages which would certainly accrue to an easily workable process of this nature) is primarily due to the exceptionally high side-wall forces and attendant friction created by the centrifugal action. In this respect, reference is made to FIG. l which shows the volume contraction of a mild steel on cooling from a pouring temperature down to room temperature. As shown, the volume solidification contraction amounts to 7.2 percent.

In a static casting (such as one made in a conventional sand mold), the total contraction depends on the solidification contraction and the thermal (solid) contraction. In a centrifugal casting (operating at the high G, gravitational, forces necessary to produce a dense casting), the solidification shrinkage is nonexistant 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 l500 C. to room temperature.

By way of demonstrating the side-wall forces involved, and as an illustrative example, we can considered a solidwall mold having a l0-inch I.D. and the continuous centrifugal casting of the mild steel tube thereon which has a wall thickness of one inch. The mild steel tube, so cast,

would have a lil-inch O D. and an LD. of 8 inches. Since,

in centrifugal casting the solidification contraction is practically nil, it can reasonably be expected that, from the point where a solid skin first starts to form on the molten tubes O.D. surface, the solidifying tube will typically stay in Contact with the mold wall for an axial distance of at least 24 inches. After this point, thermal contraction of the more solid cast tube wall causes the tube to shrink away from the mold wall with attendant alleviation of friction. The contact area of tbe steel, from the axial point where it just starts to solidify to the point 24 inches towards the exit where it shrinks out of contact with the mold wall, is 24 inches times the outside circumference of the tube or 24 3.14 l0=756 square inches. If the centrifuge is operated at 50 Gs (a rotational speed whereat one pound of weight would actually exert a side-wall force of 50 pounds) the weight of one cubic inch of justsolidifying steel (the normal density of mild steel at the solidification temperature of 1500n C. is 0.264 lb./cubic inch) would have an effective weight of 50 0.264 or 13.2'

lbs/in.a and this is the force that the cubic inch of steel would exert on a square inch of surface area of the mold wall. Actually, the amount of molten metal pressing on the one square inch of surface area is not a full cubic inch since the volume is that of a truncated wedge as shown in FIG. 2. The inner surface of this truncated wedge is 0.8 sq. inch and the volume of the wedge is 2 Xl or 0.9

cubic inch. The effective weight of this volume is therefore 0.9 13.2 or 11.9 lbs. at the 50G rotational speed involved. Since there are 756 such volumes in contact with the mold wall, the total side-wall force is 756)( 11.9 lbs. or 9000 lbs. Assuming a coefficient of friction of 0.25, then the force required to pull the tube out of the bore of the centrifuge would be 0.25 X9000 lbs. or 2,225 lbs. Such a pull-out force can create stresses in the cross-section of the solidified steel outer layer that exceeds the hot tensile strength (which is very low at temperature just below the solidication temperature) of the steel and cause circumferential rupturing of the tube with attendant discontinuous pull-out. Static friction is considerably higher than sliding friction and the critical period in continuous centrifugal tube casting would be at start-up when the extracting pull is first being applied to the tube. Such circumferential fracturing of the newly cast hot tube is much more likely to occur at this time.

Solid-wall molds for the continuous centrifugal casting of tube would best be constructed of a material exhibiting the following characteristics. It should be:

(l) Refractory and preferably, have a melting or softening point considerably in excess of the pouring temperature of the molten material being cast to tube. Therefractory requirement is primarily important where the thermal conductivity of the mold wall material is less than that of the material being cast.

(2) Highly heat conductive so that the heat of solidification of the molten tube material can be conducted away with sufiicient rapidity to prevent overheating and welding. In this respect, it should be noted that such highly heat conducting metals as silver, copper, and aluminum can be utilized as mold walls for the casting of materials having solidification temperatures in excess of their melting points. As examples, steel has been continuously noncentrifugally cast in water cooled molds of copper and gray iron has been cast (non-continuously) in Water cooled molds of aluminum. Generally, such mold materials are coated on the contact (with the molten metal to be cast) surface with a fairly thin layer of a more refractory and non-galling material such as chromium plate in the case of copper molds and a heavy anodized surface in the case of aluminum molds. Such high heat conductivity materials can be used for solid-wall centrifugal casting molds but are not the preferred ones due to danger of softening at any localized hot spot.

(3) Non-seizing or slippery. Such materials as dense graphite, molybdenum disulfide, or boron nitride exhibit such characteristics. More than this, they are refractory and (with respect to many casting materials) are non-wetting as well. Graphite has long been available for such use but it has only been in recent times (due to current technological demand) that the denser structural graphites have been made available. Even so, dense graphites, molybdenum disulfide, and boron nitride are relatively soft and exhibit rapid wear under the excessive pullout frictional forces inherent to continuous centrifugal casting as now practiced.

ICurrent engineering materials such as the dense graphites, molybdenum disulfide, and boron nitride; the highly refractory metals such as tungsten, molybdenum, tantalum, and columbium; and such refractory nonmetallics (in highly impervious form) as -alumina and beryllia exhibit some of the foregoing desirable characteristics and can be used to advantage as mold materials for the continuous centrifugal casting of tube. The primary drawbacks, however, are the excessive wear rates for the soft slippery materials and the excessive wall friction of the other materials which limits tube extraction.

It is apparent from the foregoing analysis that the sidewall frictional forces, attendant to such casting machines now known to the art, must be drastically reduced in order to fully realize the economic advantages of continuous centrifugal tube casting.

Methods by which this can be accomplished are given below as Methods 1-5, and hereinafter are referred to by number, as Method l, etc.

Method l.-By use of solid-wall mold materials having low coefficients of friction. This was shown in 1895 by the Maxims of British Pat. 22,708, wherein the use of such slippery materials as axially aligned asbestos fibers and compacted graphite was disclosed.

Method 2.-By centrifugally casting at lower rotational speeds to reduce the G forces and consequently the frictional force. Actually, the G force limits for centrifugal casting are well established for conventional prior art methods and are in the general range of from 50 to 100 Gs. The 50 G rotational speed is not the lower limit for avoiding raining and sloshing of the molten metal but is considered necessary for the production of a sound dense casting. Above 100 Gs, the circumferential stresses produced in the cast tube can cause longitudinal rupture of the tube.

Method 3.-By use of a centrifuged liquid mold as disclosed in the Maxim (Br.) Pat. 22,708.

The foregoing methods are a part of the prior art and can be utilized to the point where tube can be produced as an end item due to its increased cost as such.

THE METHOD OF THE INVENTION- SPECIFIC EXAMPLES It is a purpose of my invention to provide a novel method, using a gas pressure differential, for reducing the side-wall frictional forces to an extent that tube can be continuously centrifugally cast at such high rates of output that the tube, so produced, can be used, not only as tube, but also, as a -basic starting item for fabrication into other longitudinal items of structure such as plate, angle, railroad rails, I-beam, channel, etc.

Two variants of the novel method herein disclosed are listed as follows:

Method 4.-By creating a vacuum, internal to the tube being cast, so as to partially or completely (depending on the wall thickness of the tube) counterbalance the centrifuged weight of the tube wall by the suction of the vacuum.

Method 5.-By raising the atmospheric pressure (exterior to the tube and the exit orifice of the centrifuge) by a desired amount over that of the ambient atmospheric pressure (14.7 p.s.i. is the average standard atmospheric pressure at sea-level) to create a counterbalancing force on the exterior of the tube to partially or completely counteract the centrifuged weight of the tube wall.

Method No. 4, the creation of a partial vacuum on the interior of the tube, is the preferred method since it is the more effective means of accomplishing the desired counterbalance and introduces other beneficial effects as well. In my above identified prior patent application, Ser. No. 538,506, the use of an internal vacuum within the centrifugally cast tube has been described in conjunction with the continuous collapse-forming of the tube to longitudinal items of selected cross-section. 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 non-rotating end plate, or disc, the periphery of which is immersed into an annular trough of a heavy liquid material (such as molten Woods metal, lead, or tin).

By continuous heavy suction (via a conduit extending through and sealed to the non-rotating end plate) a moderately high vacuum (an internal partial gas pressure of a few pounds per square inch) can be maintained. This system exhibits the following advantages, all of which are objects of the invention:

(a) After an initial purge with an inert gas, at start-up, and 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 a bright oxidefree condition which permits and facilitates the pressure welding of the contiguous interior surfaces of the tube one to the other. It might be mentioned that these same gases create porosity or blow-holes in ingots cast by 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 sur-faces of the pockets together. This is but one of the major advantages from the use of a partial vacuum internal to the tube being cast.

(b) The centrifugal load on the external structural shell of the centrifuge is greatly decreased since the effective weight of the metal being cast to tube is essentially reduced to nearly zero.

(c) The molten metal (being cast to tube) is effectively degassed by the internal vacuum during its entrance into the centrifuge via a conduit extending through and sealed to the non-rotating seal plate. As a matter of note, it is preferred to vacuum-degas the molten casting metal prior to its introduction into the continuous centrifugal tube casting device so as to cut down the amount of gas given off by the molten metal.

(d) The internal partial vacuum materially aids any subsequent collapse-forming operation.

(e) The internal partial pressure of reducing gases can be maintained within the tube, as will be revealed in the teachings of this invention, for as long as desired and the internal surfaces of the tube will remain bright and oxidefree for subsequent reheating of the tube and collapsedeformation thereof or, if desired, as a pre-cleaned surface for subsequent application of an internal oxidationresistant coating of enamel, plastic, rubber, zinc, tin, lead or the like.

(f) The primary advantage, of course, is the counterbalancing of the centrifuged effective weight of the molten and solidifying tube wall so as to minimize the side-wall force and attendant friction.

(g) Due to the supporting action of the internal vacuum, greater rotational speeds (higher than the usual upper limit of Gs) can be tolerated and optimum densification of the tube metal can be achieved. Such higher G forces can also be utilized to enhance gravitysegregation (to be discussed later) where so desired. Prior calculations of this disclosure have shown that a 10- inch diameter tube having a wall thickness of one inch (8- inch I.D.) wil create an effective side-wall pressure of 11.9 p.s.i. when centrifuged at 50 Gs. By Method 4, I can reduce this side-wall pressure to zero, if desired. Assuming a standard ambient pressure of 14.7 p.s.i., a suction of 11.9 p.s.i. interior to the tube Will just counterbalance the centrifuged Weight of the tube wall. The absolute pressure of the partial vacuum would be 14.711.9 or 2.8 p.s.i.

In Method 5, the volume external to the exit end of the centrifuge and the exiting tube is enclosed, by appropriate means to be disclosed, so as to afford an effective seal which permits the application of a higher than ambient gas pressure that forces the tube metal inwardly. This external pressure can be used to counterbalance some or all of the effective weight of the tube (depending on the wall thickness and the G forces involved) in the same manner as the internal vacuum of Method 4.

This external pressurization is preferably accomplished with a dry inert gas such as nitrogen, argon, helium, or the like. It is preferred to use Method 5 in conjunction with Method 4 since, by such a combination, greater tube wall thicknesses or operation at higher G forces can be accommodated. It should be realized that the illustrative calculations are based on a mild steel having a density of 0.264 lbs/in.3 at its solidification temperature of 1500 C. Less dense metals (such as magnesium, aluminum, titanium, etc.) can be counterbalanced even more eifectively;

Besides the supporting action, Method 5 has the further advantage of maintaining an inert atmosphere which protects the exterior of the exiting tube and the interior mold wall of the centrifuge from oxidation. Due to this, the refractory metals (such as tungsten, molybdenum, tantalum and columbium) which are prone to catastrophic oxidation, can be used as mold wall materials, provided that the exterior mold surface is jet cooled with suitably reducing fluids that prevent oxygen attack.

With respect to Methods 4 and 5, it is preferred to utilize higher internal vacuums (Method 4) and lower external positive pressures (Method 5) where tubes having a smaller diameter and heavier wall thickness are concerned. Conversely, in the production of large diameter tubes of thinner wall section, it is preferred to utilize a much lower internal vacuum (Method 4) and higher external positive pressures (Method 5) 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 cross-sectional 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 l O.D. (cross-sectional area of 78.5 sq. inches) and an internal Vacuum of 4.7 p.s.i. (pressure differential of 14.7-4.7=10 p.s.i. with regard to a standard atmospheric pressure) would experience a backward thrust of 78.5 in.2 l0 p.s.i. or 785 lbs. In other words, it would require a force of 785+ lbs. 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 7090 lbs. would be required to get the tube out of the bore of the casting machine. If the 30" diameter tube had a M1 wall thickness and was centrifugally cast at 50 Gs, the pressure differential necessary to counterbalance the steel would be 1A: 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 of 14.74-33 or 18 p.s.i. and the internal pressure of the 30" diameter tube would be 14.7 p.s.i. or the same as the ambient pressure. By this technique, a very small force would be required to extract the tube from the bore of the casting machine since the external pressure (of Method 5) acts on the periphery of the tube to just counterbalance the weight of the steel tube at 50 Gs and does not act on on the end (cross-sectional area) of the tube to create a `back-force which must be overcome (as in Method 4) to get the tube out of the casters bore.

It 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 4) and an external positive pressure (Method 5), 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.

It is preferred to utilize Method 4 and/ or Method 5 to the extent that a slight side-wall pressure exists since, by such contact, more rapid heat extraction takes place. In fact, it is fortunate (in the use of Method 4 and/ or Method 5) that solidilication shrinkage of the tube is practically nil under the centrifugal forces involved since such a lack of shrinkage is necessary for maintenance of side-wall contact. Once a solid outer shell has formed on the tubes outer surface, the thermal contraction takes place with great ease (only the slight, allowed, uncounterbalanced weight of the tube material exists to oppose such shrinkage) and a gap then exists between the O.D. surface of the tube and the I.D. surface of the mold wall. In this 8 non-contact area, heat loss from the tube is due largely to radiation and this is facilitated by blackening the interior surface of the mold wall in the non-contact area to absorb the radiated heat.

As an illustrative example (again using a mild steel tube of lil-inch O.D. and a one inch Wall thickness being continuously centrifugally cast at 50 Gs). I will allow an uncounterbalanced side-wall pressure of 0.2 p.s.i. to exist (the balance of 14.7-0.2 or 14.5 p.s.i. being counterbalanced by a combined 14.5 p.s.i. from Method 4 and Method 5, as 10 p.s.i., by Method 4 and 4.5 p.s.i. by Method 5). Due to the paucity of stretch in the incipiently formed solid skin, the contact area of the mold wall will be less than the axial 24 inches previously assumed and would now be closer to 16 axial inches. The surface contact area would then be 1rD l6 or 3.l4 l0 l6 or 502 square inches and the total Weight of the steel tube wall in this area would be 502 sq. in. 0.2 p.s.i. (the uncounterbalance weight) or 100 lbs. This weight of 100 lbs. times the coeicient of friction of 0.25 gives a total pull-out force of 25 lbs.

Such pull-out forces can be made considerably less than those that now occur in the conventional (non-centrifugal) continuous casting of solid billets and, due to this, the extraction rates can be greatly increased.

The length of such a mold (the increased length permits maintenance of the exterior pressure of Method 5 over the entire axial length) can be increased to an extent that an extraction rate of ft./min. is entirely feasible. The cross-sectional area of a tube (10 O D. and 8" LD.) is 1r (Z5-16) or 28.3 sq. in. A one inch length of such a tube would weigh 28.3 cu. in. 0.264 lb./cu. in. or 7.5 lbs. An extraction rate of 40 ft./min. equals 480 in./min. or 28,800 in./hr. and, since one in. of tube length equals 7.5 lbs., the casting rate would be 7.5 lbs./in. 28,800 in./hr. or 216,000 lbs/hr. or 108 tons/hr. For the same wall thickness, the casting rate is proportional to the tube diameter and a -in. diameter tube would have a casting rate of 5 108 or 540 tons/hr. Such outputs are entirely feasible if the mold is made sufficiently long or, alternatively, annular gas and liquid bearing cooling rings (to be disclosed below) are utilized. Where both techniques are used, even higher casting rates are entirely feasible providing such a large amount of molten casting material is continuously available.

By use of the foregoing combination of methods, the tube can egress from the system at a temperature only Slightly less than its solidication temperature (practically no thermal shrinkage whatsoever) and, in this instance, the interior of the tube can still be in a semisolid or even molten state depending on the rigidity and thickness of the exterior (solid) portion of the tube wall. Cooling external to the exit end of the casting machine can then be accomplished with attendant greater production rates or, if desired, decreased length of the centrifugal casting mold.

Production-wise, such a 50-in. diameter steel tube with one in. thick walls would be collapse-formed to a two in. thick plate having a width-of 1rD/2 in. or 78.5 in. or 6%. ft. No process in existence today can produce such a product at anywhere near the casting rates noted.

It should be noted that the disclosed process permits the continuous centrifugal casting of tubes having smaller diameters than are 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 a particular diameter and this is particularly true for tubes having small diameters (as less than two inches O.D.). The longitudinal contraction of such tubes, in cooling down from the just-cast to the extraction temperature, is sufficiently great that 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 diametral shrinkage is insuicient to obviate (shrink away from) such restraining mechanisms and the large amount of rejections due to such circumferential rupture makes such production uneconomical. The present process can produce such small diameter tubes on a continuous basis and without ruptures since the only restraint to longitudinal shrinkage would be the side-wall friction due to the allowed un-counterbalanced tube wall weight which can be made practically nil.

kIn the case where the pressure differential of Method 4 and/ or 5 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 to the extent that it would lift away from the mold and permit ingress of air or inert gas into the vacuum of the tubes interior via bubbling through the molten zone of the tube. This can and does happen, but not suddenly beyond the point where the pressure differential overbalances the zero-point.

A stable-state range exists for pressure differentials in excess of the zero-point and this is due to the surface tension of the molten material being so 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-of-balance 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 inventions disclosure to utilize the advantageous system of a vacuum internal to the tube being cast (Method 4) 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 out-put 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 density of the molten tube material, 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 device mechanism as that which rotates the centrifugal casting machine and 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 4) which causes a suction, must be opposed. The rollers, in this instance, are suitably driven by sun gears (via a suitable gear cluster system for such power transmission) and are activated or de-activated by a suitable clutch mechanism. Such mechanisms are well known to those practiced in the art of rotary coupling and un-coupling. At the same time, there is an axial gap in this cradle system, near the exit end of the centrifuge, with appropriate torch re-heating means and rotating opposed swaging or forging hammers which move in axial synchronization with the exiting tube and swage or pinch a re-heated 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 re-commenced 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 two 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 cut-off device which travels in axial synchronism with the exiting tube and severs the tube at the middle of the swaged or forgeddown 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 cut-off returns to its starting point for recoupling to the axial travel mechanism and cut-off of the tube section at the appropriate time. By this synchronized and discretely repeatable sequence of swaging down and cutting olf the exiting tube, the integrity of the internal vacuum (with its manifold advantages) is maintained during and after the tube casting operation.

It is convenient to forge-atten the exiting tube (just as a soda-straw can be pinch-flattened in a selected area between thumb and foreiinger) at the separation 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 center-line coincident -with the axis of the tube. These end closures (after separation of the tube lengths at the mid-lengths 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 cut-off closure lengths are conveniently used to fabricate pressure bottles or tanks for Oxy-acetylene, propane storage and the like. In this manner, the closure part of the tube is not subject to re-melt but affords great economies in the manufacture of pressure tanks and storage vehicles.

My preferred means of 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 vacuum-tight 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, cog-wheel, 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 pull-out mechanisms so that, while one mechanism is pulling out the tube, the second mechanism can be swaging down a tube closure sorne 200 `feet closer to the centrifugal caster. Once the second mechanism has swaged down and gripped the tube closure for powered pull-out, the iirst mechanism (axially lfarther away from the centrifugal caster) then severs the 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 pull-out 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 (a 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 or 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 side-wall friction and stoppage of out-put for repairs. In the preferred means (using some length,

as the 200 foot example, and more floor-space, depending on the tube lengths produced), any slippage of the grip merely brings the pull-out mechanism into Contact with the belled-down part of the tube and creates a positive and safe pull-out.

In the foregoing manner, long sections of tube (like straight sausage-links) are produced which have an internal vacuum of a 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, is very effective in promoting the application of interior coatings to the tube since (by clipping the tube end while immersed in the iiuid coating medium 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 tubes interior surface while the coating is being heat-cured, catalytically cured or solidified in place as suits its nature (whether of or ganic, non-organic 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 mile-long lengths by exiting the tube onto a body of Water, such as a bay or down a stream or river, which tloats the tube and acts as the support cradle), having an internal vacuum as a result of both ends being swaged closed, 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 oat-shipment to any desired shore-line 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 cutoi and Weld or other attachment as mile-long lengths. The savings in transportation costs and decreased welding for pipeline fabrication is readily apparent.

Whereas the example calculations of this disclosure have been based on a mild steel tube product, it should be realized that this is by way of an illustration of convenience. The innovations herein disclosed are applicable to a Wide range of tube materials of a castable and deformable, when hot, nature and include such organic materials as plastics and rubbers; inorganic materials such as glasses, and most of the industrial metals now used, the only criterion being that the materials be meltable (either exterior to the casting machine or as electrodes or powders which can be conveniently melted by arc methods internal to the machine without degradation due to the partial vacuum of inert atmosphere thereinsuch metals conveniently electric or plasma arc melted internal to the system would be the highly reactive ones such as titanium, zirconium, beryllium, etc.) and, subsequently, deformable.

The mold Walls of the centrifuge can be conveniently made of slippery and generally non-wettable materials such as boron nitride, graphite, molybdenum disulfide, etc. which (under the conditions of minimized side-wall pressure resulting from application of Methods 4 and/0r 5 to the system) exhibit entirely satisfactory surface life with minimized wear. Due also to this minimized sidewall pressure, the mold walls can be made of various metals (Without attendant danger of welding or seizing between the mold Wall and the tube material being cast). Also, due to the greatly reduced loads of the mold walls resulting from the counterbalancing action of Method 4, the .mold Walls can oftentimes be conveniently made of such refractory inorganic materials, having good heat conductivity, as alumina and beryllia. The range of castable tube materials and mold wall struc- 12 tural materials is exceptionally broad when used in conjunction with the methods herein disclosed.

My continuous centrifugal casting process not only produces a Wide range of metallic and non-metallic tubular products for use as such but it produces this variety of tube at such high rates of output (e.g., hundreds of tons per hour) that the tube can be economically and very advantageously used as a basic item for the production of other items of longitudinal structure. lt is therefore a 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.

OBJECTS OF THE INVENTION lt is an object of this invention to reduce the side-wall forces and attendant friction of solid-Wall continuous centrifugal tube casting machines by application thereto of a vacuum internal to the tube being so cast (Method 4) and/ or a positive pressure, external to the exit orifice of the caster and the tube O.D. (Method 5).

Another object of this invention is to utilize a vacuum seal at the entrance or starting end of the centrifugal cast ing machine for the purpose of reducing the pressure internal to the tube being cast.

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 or section of the tube, as it exits from the casting machine, to form vacuum tight closures at specified intervals along the length of the tube.

Another object is to cut ott such lengths of tube at the mid-length of the closure (limited collapsed section) so as to maintain the integrity of the vacuum internal to the tube and to obtain long useable lengths of tube having such closures at both ends thereof.

A still further object is to produce and maintain a vacuum internal to the tube being cast (Method 4) as a means of reducing the side-wall forces and attendant friction as well as for other advantageous reasons.

Another object is to produce and maintain a positive pressure of an inert or reducing gas external to the exit orifice of the caster and the tube 4O D. (Method 5) as a means of reducing the side-wall forces and attendant friction as Well as for other advantageous reasons.

Another object is to utilize an extended-hot-zone at the starting end of the caster in order to accentuate the effects of gravity segregation to obtain a useful result such as a lower carbon surface on steel sheet for use in the automotive industry. Normally, an unextended hot zone is used for layering of the material being cast to tube in the teachings of this invention.

Another object of this invention is to utilize an annular section of pyrolytic material (such as pyrolytic graphite, pyrolytic boron nitride, graphfoil, etc.) in the hot zone or extended hot zone section of the caster in such a manser that the c direction (the direction of high thermal insulation) is perpendicular to the axis of the centrifuge and the a direction (the direction of high thermal conductivity) is parallel to the axis of the centrifuge in order to enhance and equalize the heat distribution in that area.

Another object of the invention is to utilize annular irises of carbon or other refractory materials at the exit end of the centrifuge as means of sealing the enclosure of Method 5.

An additive object is to utilize a multiplicity of small radial holes penetrating such annular seal rings whereby either an inert or reducing gas or a stream of liquid can be forced therethrough onto the rotating surfaces to act as a gas or liquid bearing (under high pressure) with attendant cooling and sealing action.

A still further object of this invention is to so increase the casting rate of continuous centrifugal tube casting machines, utilizing a solid-wall 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 non-continuous basis.

The novel features which are considered characteristic of this invention are set forth with particularly 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 foregoing description when read in connection with the accompanying drawings, in which:

FIG. 1 is a graphical representation of the change in specic volume of a solidifying and cooling steel;

FIG. 2 is a geometrical diagram of a truncated wedge section hypothetically removed from the cast tube for illustrative purposes;

FIG. 3 is a partial axial sectional view of a horizontal centrifugal solid-wall continuous tube casting machine with seal means at the entrance and exit ends thereof;

FIG. 4 is an axial sectional view of an embodiment of the exit end of a solid-wall centrifugal tube casting machine which depicts means of enclosure thereat to effect a positive pressure (above ambient) external to the exiting tube as per Method 5;

FIG. 5 is a partial axial sectional View of a vertical centrifugal solid-wall continuous tube casting machine with seal means at the entrance end thereof;

FIG. 6 is a partial axial sectional view of a plasma torch arrangement, utilizing a bellows vacuum seal, in the retracted position;

FIG. 6A is a similar view of the plasma arrangement in the extended position.

.DETAILED DESCRIPTION-THE PROCESS Referring now lto the drawing in detail, and in particular to FIG. 1 (redrawn from Wullfs Metallurgy for lEngineers) it can be seen that a centrifugally cast mild steel tube will experience a volume shrinkage of about 6% or a diametrical shrinkage of 2% in cooling from the soliditication temperature of about 1500C. to a tempera- -ture of about 330 C. under centrifugal casting conditions. It can also be derived that the diametrical shrinkage of a centrifugally cast mild steel tube in cooling from l500 C. down to 700 C. is about 1.5%. The specific volume contraction curve of FIG. 1, illustrates the amount of shrinkage attendant to the cooling of a mild steel casting under normal or static conditions and is reproduced herein for information purposes.

FIG. 2 is illustrative of a solid geometrical configuration wherein a square inch area on the periphery of a -in. diameter tube, having a l-in. wall thickness, is radially projected inwardly onto the axis of the tube to form a truncated wedge within the contines of the radial projection lines and the exterior and interior surfaces of the tube wall. The projection of the l-in. sq. area on the exterior surface of the tube onto the tubes axis cuts out a rectangular area on the interior surface of the tube that is l-in. long and has a circular length of 0.8-in. on the adjacent side. The inner rectangle has an area of 0.8X1 or 0.8 sq. in. The volume of the truncated wedge is, therefore, 0.9 cu. in. and this volume bears on the 1 sq. in. of exterior tube surface under the influence of the centrifugal action. The geometrical configuration is used to illustrate the decrease of volume bearing on the O.D. surface of a tube as the diameter becomes smaller and the corresponding decrease in bearing pressure (p.s.i.).

In the following drawings pertaining to continuous centrifugal casting machines and devices, such means as cooling of the mold, tube withdrawal techniques, trunnions, bearings, rotational mechanisms, and the like which are well known to the prior art, are not shown and have been omitted for the sake of brevity.

Reference is now made to FIG. 3 which is an axial cross-sectional view of a horizontal solid-wall continuous centrifugal casting machine which rotates about its axis 1. The molten material 2 to be cast to tube, is continuously introduced into the entrance end 3 via the conduit 4 and pours into the annular distributing trough 5 of the refractory part 6. The refractory part 6 is encased in a structural metal housing 7 which extends towards the exit end 8 of the centrifuge as the solid mold wall 9 the exterior surface of which may be cooled by a multiplicity of peripherally spaced jets of cooling liquid (not shown). The molten material 2 overflows the ledge 10 which is lined with an annular ring 11, of axially aligned pyrolytic material for rapid axial heat conduction and radial insulation and constitutes a hot zone 16 and forms an axially flowing ring of molten material 12 which freezes to a solid tube 13 by heat conduction to the mold wall 9 in area 14 and by radiation to the blackened mold wall interior in area 15. At the entrance end 3 of the centrifuge and axially external to the refractory part 6 is an annular trough 20 which is partially filled with a centrifuged heavy liquid 21 of a high boiling nature (as Woods metal, molten tin or lead, etc.). A non-rotating end plate (disc) 22 has its outer periphery 23 immersed in the annular trough uid 21 and constitutes a vacuum seal for the casting machine at its entrance end 3. The end seal disc 22 has circumferential gutters 24 which collect any cascading fluid 21 and return it to the trough 20 at the bottom side. The molten material conduit 4 as well as an inert gas purge tube 25 and a vacuum suction line 26 extend through the end plate 22 and are attached thereto by leakproof seals. By means of the purge tube 25, the cavity 30 of the tubes interior is purged with an inert gas and a vacuum is then drawn on the interior cavity 30 via vacuum tube 26. The diameter of the trough 20 is considerably greater than the diameter of the centrifuge and the diameter of the liquid level of the sealing material is also quite large. By this means, greater access area (via sealed but removable port holes) is available in the end plate 22 for insertion of required mechanisms such as plasma torches, rotary skimming devices, etc. as needed. The trough 20 is deep enough to contain all of the seal fluid 21, without overow, when rotation is stopped.

Exterior to the exit end of the centrifugal casting machine may be a set of opposed forgin rolls 34 and 35 which travel axially and in synchronismwith existing tube 13. At the same axial location and at right angles to the plane between the axis of the forging rolls (34 and 3S) may be two opposed banks of burners such as plasma torches (not shown) which maintain the heat of the exiting tube 13, 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 ft. length) to a solid round having a forge welded interior joint 36 which is vacuumtight. Such collapsed sections of the tube can be as far apart as desired (e.g. every 300 ft. of tube length) and comprise 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 cut off at its mid-length for removal of the discrete length of vacuum sealed sausage-like tube lengths, for use as previously described. It can be appreciated that other conventional means, such as swaging. flat-crimping, etc. can be used to form the discrete collapsed section for vacuum closure at point 37 of the hot tube. Also, axial travel of the sealing rolls (34 and 35) can be extended (as to 300 plus feet) so that they act as powered pullout grips for the tube so cast. The tube can also be sealed at the exit end by continuous collapse-deformation thereof to longitudinal items of structure in accordance with the teachings of my prior patent application Ser. No. 538,506.

By means of the forged tube closure 36 and the end plate seal 22, a vacuum can be drawn (via conduit 26) on the tube cavity to an extent that it partially or completely counterbalances the side-wall force and resulting friction of the tube being cast in accordance with Method 4.

It should be noted that the hot zone 11 can be lengthened beyond that necessary for ring layering 12 so as to create an extended-hot zone 16 so that slow cooling of the molten tube can be accomplished. In this manner, when desired, accentuated gravity segregation results (eg. delta ferrite being centrifuged towards the outside surface of a mild steel tube which is later to be converted to automotive sheet steel).

FIG. 4 is an axial sectional view of the exit end 8 of another horizontal solid mold centrifugal tube caster according to the invention and is illustrative of the annular exit end closure 40 utilized in the application of Method 5.

In FIG. 4, the collapsing and forge Welding rolls 34 and 35 have already been described as a means for sealing the tube 13. Whereas the tube 13 and the mold 9 are rotating, the annular end closure is stationary. An inert or reducing gas 41 (inert is a relative term since a gas such as carbon dioxide, which is oxidizing to hot steel, is practically inert to hot aluminum and can be so used in the casting of aluminum tube) is introduced into the end closure 40 via the high pressure gas tube 42 and the pressurized gas 41 acts on the outer surface of the tube (both exterior to the exit end and in the shrinkage gap between the tube and the mold wall of the centrifuge) and supports it (counteracts the centrifugal weight of the tube wall) to a desired extent. The end closure 46 is sealed at the annular area 43 (on the OD. of the centrifugal caster at its exit end 8) by means of an iris ring 45 of carbon, graphite, or boron nitride leaves 46 which overlap each other (as camera iris blades do) to form an annular ring 45 of such blocks (leaves) in friction contact with the 0.D. of the mold wall at area 43. The iris ring 45 is contained within an annular groove 47, the opening of which faces inwardly, and this groove encompasses a pressure chamber 48 (radially exterior to the iris ring 45) which is pressurized by an inert gas 49 introduced via conduit 50. A multiplicity of iris leaves 46 make up the iris ring 45 and these leaves are each attached at one end to the groove 47 by means of pivot pins 51.

At the other side of the pressurized enclosure 40 and at area 53 on the O.D. of the tube 13 is'another similar iris ring 55 which seals the enclosure 4t) at the surface of the exiting tube 13. Such iris rings as described, are not the preferred means of sealing the enclosure 40 since they exert a considerable wiping force and wear at a fairly high rate. The preferred means is to utilize an annular iris seal ring 45A which is much the same as that of 45 except for a multiplicity of small radial holes 56 which exist over the entire iris ring 45A and conduct a high pressure inert gas 49A onto the outer surface of the mold wall 9 at area 43A. In this manner, the iris ring 45A acts as a gas bearing and does not actually contact the rotating surface of the mold. Due to this, Wear of the iris ring 45A face areas is eliminated and the escaping gas of the bearing face maintains the desired pressure of the enclosure 40. The iris ring 55A which seals against the rotating tubes O.D. surface at area 53A can also utilize the gas bearing technique; however, it is sometimes preferred to use a liquid bearing at the area 53A for the following listed purposes:

(1) A heat extractive coolant of a non-oxidizing nature (as a mixture of water and methyl alcohol). Such liquid bearings can also be used to cool the exterior surfaces of the mold wall 9,

(2) As a quenchant (as a brine plus a suitable reductant) for the purpose of hardening the tube for use as heat-treated pipe. In this instance, the forge welded closures at point 37 would be normalized and subsequently removed from the pipe. The balance of the quench hardened pipe would be tempered to a desired hardness and strength level prior to removal of the forge welded ends and breaking of the internal vacuum.

3. A liquid bearing of lead, tin, zinc, aluminum, etc., or desired alloys thereof (as lead-tin) would be used (in the molten state where an exterior coating of such metals is desired for corrosion protection of the pipe. Coincidentally, a steel tube could be heat-treated by austempering with such molten metal liquid bearings.

Where liquid or gas bearing irises 43A or 53A are used for sealing the end closure 40 and centering the exiting tube 13, the iris blocks (leaves) 45A and 55A can be made of other materials such as copper, steel, alumina, or other non-metallic materials, etc. since they do not have a frictional contact with the outer surfaces of the mold 9 or the tube 13.

FIG. 5 is a representation of a starting end 3 vacuum end seal for a vertical continuous centrifugal tube casting machine (as of the type depicted in British Pat. 984,053 and elsewhere and is presented as a partial axial crosssectional View.

In FIG. 5, the axis 1 of the centrifuge is vertical and the molten material, to be cast to tube 13, is introduced into an annular distributing trough 5 via conduit 4. The molten material 2 is sluiced horizontally so that its direction of flow has tangential coincidence with the rotational motion of the molten material 12 in the distributing trough 5. The cavity of the molten and solidied tube 13 contains a partial vacuum (Method 4) of inert gas by virtue of being sealed beyond its exit end (not shown) by the inward collapse and forge welding of a discrete section of the exiting tube 13 and, at its entrance end 3, by a non-rotating seal plate (dish) 22 which has its periphery 23 immersed in a dense high-boiling liquid 21 (as Woods metal, cadmium, lead, tin and alloys thereof) contained in an annular trough 20. The convex side of the seal plate (dish) 22 has an annular gutter 24 which inhibits access of air to the molten metal 21 of the seal and, also, prevents any inadvertent escape of iiuid from the trough. Under non-rotating (stop periods) conditions, the liquid levels of the fluid 21 are as shown by dotted lines 27 while, under the centrifugal forces of casting, the liquid levels of the uid 21 assume the positions shown by the vertical lines 28. It can be appreciated that the periphery 23 of the dished end plate 22 is always immersed in the uid 21, whether the centrifugal caster is operating or not, to form an effective vacuum seal. Such orifices in the end plate 22 as the purge tube 25 and the suction tube 26 are the same as in FIG. 3 and the other numbered points (not discussed herein) are the same as in FIG. 3 except for the vertical attitude. The molten material 2 enters the conduit 4 by way of a conventional trap 29 as a means of maintaining the vacuum Within the internal cavity 30.

The central area of the end plate 22 is reserved for other entrance ports as required in such a system (as the vertical water-cooled shaft, which is an extension of the rotary mandrel used in the vertical centrifugal tube casting machine disclosed in (British Pat. 984,053; or, the bellows encased plasma torch of the following FIGS. 6 and 6A).

The exit end 8 end closure 40 as shown in FIG. 4 is also used in the application of Method 5 to the vertical system although not shown since it would vary but slightly from that already disclosed.

It can be realized that the straight sausage-like links of vacuum sealed tube, or item of longitudinal structure formed by the continuous collapse of such exiting tube,

would have to be of fairly limited length due to height restrictions. Due to height restrictions, the vertical sys- 17 tems are not preferred over the horizontal continuous centrifugal tube casting machines herein disclosed.

The vacuum seal means of FIG. can also be used in conventional, non-centrifugal, vertical continuous casting of solid -billets and can be used as means of applying Method 4 (a vacuum above the pool of molten metal beting cast to billet) to this older continuous casting means. By application of such a vacuum, the hydrostatic forces on the mold side-wall can be reduced and the extraction rate speeded up.

The .great advantage of the continuous centrifugal casting process disclosed herein concerns the rapid continuous casting of thinner tubular walls of high density material that has optimum integrity with a minimum of cross-sectional reduction by subsequent working (as rolling to structure).

FIG. 6 is a partial axial sectional View of a retractable plasma torch 71 confined within a vacuum sealing bellows 72 and located at the area 70 of the end plate seal 22. The purpose of the torch or torches is to preheat the refractory part 6 (of FIGS. 3 and 5) prior to start-up of the tube casting machine; and the bellows 72 is merely a means of maintaining the vacuum within the tube cavity 30 during extension for use and subsequent withdrawal (as shown in FIG. 6) out of the hot area of the cavity 30.

In FIGS. 6, the annular ange 73 seats against the orifice lip 74 of the end plate 22 and acts as a heat-shield to prevent overheating of the bellows 72.

FIG. 6A shows the plasma torch 71 in the extended or use position and, in this case, the annular fiange 7S seats against the orifice lip 74 and acts as the heat barrier. The plasma torch is encased in a refractory material 76 such as alumina.

Whereas the mechanism is shown in connection with the vertical casting system, it is also applicable to the horizontal systems of this disclosure.

Stopping and starting procedure In stopping the machine, the axial travel of the tube extraction device (as gripped forging rolls 34 and 35 of FIGS. 3 and 4) is stopped by a suitable clutch mechanism (not shown) and the tube is allowed to rotate along with the centrifugal caster. Coincidental with the extraction stoppage, the input of molten material 2 is terminated and the tube 13 is allowed to solidify within the bore of the casting machine. Normally, the machine is kept rotating in any normal interval between stopping and starting of tube casting. Once the tube extraction is stopped and the tube has solidified overall (including the heavy material section filling the annular trough 5), the positive pressure of inert gas (from the exit end 8 enclosure 40) will seep into the tube cavity 30 (as in FIGS. 3, 4) via the crevice between the contracted tube wall O D. and the mold wall I.D. Alternately and preferably, once the seepage begins, the vacuum can be gradually broken by an inert gas purge via purge tube 25, the suction via tube 26 being stopped.

In starting up, from a normal rotating interim stop, the plasma torch 71 is inserted into the cavity 30= to its predetermined full extension and turned on so that its flame melts the solidified tube material in the annular trough 5. Once this has been accomplished, a desired vacuum is drawn on the cavity 30, a desired positive pressure is created in the end closure 40, the axial extraction is recommenced, and the appropriate amount of molten material 2 is continually introduced to the system. The plasma torch is then turned off and withdrawn as shown in FIG. 6.

If the centrifugal casting machine requires repairs in `areas not covered by the tube 13, the rotation of the centrifuge may be stopped once the tube material within the bore of the centrifuge has completely solidified. After repairs have been made the start-up sequence is as previously noted.

In the instance Iwhere repairs or replacements have to be made to the mold 9 or the components of the refractory part `6, the cast tube is allowed to completely solidify in the bore of the machine while rotating, but without extracting the tube or adding molten material 2. See FIGS. 5-6a. Once solidification has been completed and the vacuum broken, the plasma torch 71 is inserted into the cavity 30 and turned on to quickly remelt the surface material 2 in the annular trough 5 and the torch is then turned off and withdrawn. The tube extracting mechanism is then brought into action 'and solidified tube is pulled out of the bore of the casting machine for subsequent use as a starter-blank. Once the tube blank is clear of the bore of the machine, rotation is stopped and the necessary repairs are made.

On start-up, the cast starting-blank is moved back into the bore of the caster (by any suitable reversing mechanism), an inert gas purge is made in the cavity 30, rotation is started up, the material in the trough 5 and part of the inserted end of the starting-blank is melted down with the plasma-torch, a vacuum is drawn on the cavity, a positive inert gas pressure is created in the enclosure 40, the plasma-torch is shut off and withdrawn, molten material 2 is continuously introduced via spout 4 and extraction is simultaneously commenced.

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 in area 14 of FIG. 3). This can be done by mechanical or magnetic means and the layer of still molten metal can be either sloweddown 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. In 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 O.D. which is lessthan the I D. of the molten metal wall 12) which rotates either faster or slower than the centrifuge and is driven by a cooled shaft extending through the stationary end seal plate 22. 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 U.S. Pat. 2,963,758 of 1960 when metal tube is being produced.

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 non-rotating 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 taught in U.S. Pat. 2,931,744. In both cases, where cold working is done on the tube metal, grain refinement is accomplished by subsequent reheating to its recrystallization temperature.

Gravity segregation One of the limittaions 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 the teachings disclosed herein, to enhance `and utilize gravity segregation to a useful purpose.

yThe specific method of accomplishing or enhancing gravity segregation to effect a useful purpose is to introduce an extended-hot-zone at the starting end of the continuous centrifugal 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, U.S. 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 process disclosed herein, 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 Iwill be emphasized and can be utilized usefully as will be explained in detail later on.

Automotive sheet steel (used for the exterior body covering), is normally made from rimmed-steel 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-steel exhibits a vigorous boiling action on pouring into the ingot mold and this creates a scrubbing action at the solidifying surface of the ingot. The result is that rimmedsteel ingots have a fine grained exterior layer of fairly low carbon content. When 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 better deep drawing qualities. The spattering (which creates a rim on the ingot mold and is the basis for the term rimmed-steel) 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.

Rimrning-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 and is not oxidized due to the internal inert vacuum. The scrubbing action is absent, however, since the released gases are directed inwardly by the centrifugal forces. Centrifugally cast steel does, however, have the required density since it is pressure cast under optimum conditions.

I-f, 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) solidifies 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 roll-welded 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 polish-ability, and deep drawing characteristics) required of automotive sheet stock. Such a tube can also be slit longitudinally and flattened to plate stock, by prior art processes, and rolled to sheet having the 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 pre-heated or low heat conductivity molds of a solid wall nature.

The extended-hotzone is basically a means of slowing the solidification rate over a specific temperature range. With low-carbon steel this range coincides with the deltaferrite region of the iron-carbon phaseV diagram which encompasses the temperature range of about 1500 to 1475o C.

The extended-hot-zone (slowed solidication range) can, by intentionally varying 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 material layer under all other conditions. This is especially true where the tube is to be longitudinally collapse-formed to a structural item (as I-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 produced 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. Silicon steels can be so treated to produce a highsilicon interior surface on the centrifugally cast tube.

I claim:

1. A method for continuously casting tubing from molten casting material comprising the following steps:

rotating on a generally horizontal axis, an elongated tubular mold having an inlet at one end, an outlet at its other end, and a solid cylindrical casting charnber between said inlet and outlet;

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 mold, and causing said tube of liquid casting material to be cooled and to thereby be congealed to a state within the range including solidified and semi-solidified states;

causing controlled output of said tube from the mold;

and controlling the sidewall pressure of the casting material within the mold so as to restrict the sidewall frictional forces between the casting material and the mold wall which normally oppose the exit of the tube from the mold;

wherein the restriction of said sidewall frictional forces is effected by subjecting said tube to a differential of external gas pressure over internal gas pressure.

2. The method defined in claim 1, wherein said pressure differential is developed by applying vacuum to the interior of said tube while subjecting the exterior of the tube to atmospheric pressure.

3. The method defined in claim 2, including the step of injecting an inert purge gas into said tube while applying said vacuum thereto.

4. The method defined in claim 1, wherein said restriction of sidewall frictonal forces is effected by applying a vacuum to said tube internally thereof while subjecting the tube externally to atmospheric pressure;

and collapse-sealing the exiting portion of the tube to maintain said vacuum.

5. The method defined in claim l, wherein said restriction of sidewall frictional forces is effected by application 21 of supra-atmospheric pressure to said tube externally thereof.

6. The method defined in claim 1, wherein said restriction of sidewall frictional forces is effected by application of supra-atmospheric pressure to the exterior of said tube within said mold;

sealing said pressure within the mold around said inlet;

and sealing the exterior of the exiting tube to said outlet.

7. A method for continuously casting tubing of a material of a predetermined specific gravity and melting temperature, comprising the following steps:

rotating on a generally vertical axis, an elongated tubular mold having an inlet at the top end and an outlet at its bottom end; providing a cylindrical casting chamber within the mold; injecting said casting material in molten form into said cylindrical casting chamber and causing it to assume the form of a cylindrical tube in response to the rotation of the mold, and causing said tube of liquid casting material to be cooled to a solidified state;

restricting the sidewall frictional forces of said tube against the mold wall by subjecting said tube to a gas pressure differential operative to decrease said frictional forces;

and causing controlled output of said tube;

the restriction of said frictional forces facilitating said output.

8. The method defined in claim 7 wherein said restriction of sidewall frictional forces is effected by creating a vacuum within said tube.

9. The method defined in claim 7 wherein said restriction of sidewall frictional forces is effected by applying supra-atmospheric pressure to the exterior of the discharging tube.

10. The method of claim 1 wherein:

said pressure differential is effected by causing a partial vacuum in the interior of said tube.

11. The method of claim 1, wherein:

said pressure differential is effected by applying supraatmospheric pressure to said tube externally thereof and a partial vacuum internally thereof.

12. The method of claim 7, wherein:

the restriction of said sidewall frictional forces is effected by applying supra-atmospheric pressure to the exterior of the exiting tube and creating a partial vacuum within said tube.

13. A method for continuously casting tubing from molten casting material comprising the following steps:

rotating a generally tubular mold having an inlet and an outlet,

introducing said molten casting material into said inlet,

said molten casting material assuming the shape of a tube in response to the centrifugal forces produced by the rotation of said mold,

causing said tube to exit from said outlet in a relatively solidified state,

producing a gas pressure inside said tube sufficiently lower than the pressure outside to contract said tube enough to reduce substantially the sidewall frictional forces tending to oppose exit of said tube from said mold,

sealing the inlet end of said mold by means of a nonrotating end plate whose periphery is immersed in a centrifugal annular trough containing a relatively dense liquid, and

sealing the exit end of said tube,

whereby said sealing steps serve in maintaining said gas pressure.

References Cited UNITED STATES PATENTS 777,559 12/1904 Straus et al 164-84 2,940,143 6/1960 Daubersy et al 164-5 3,367,400 2/ 1968 Hathorn 164-84 FOREIGN PATENTS 22,708 ll/l896 Great Britain 164-81 R. SPENCER ANNEAR, Primary Examiner

Images