US3689614A

US3689614A - Centrifugal molding of ceramic tubes containing metal fibers

Info

Publication number: US3689614A
Application number: US6435A
Authority: US
Inventors: Seymour A Bortz; Linden E Shipley; Lefferd B Haughwout
Original assignee: Abex Corp
Current assignee: PepsiAmericas Inc
Priority date: 1970-01-28
Filing date: 1970-01-28
Publication date: 1972-09-05
Anticipated expiration: 1989-09-05
Also published as: DE2048358A1; DE2048358B2; FR2065324A5; GB1327050A

Abstract

A sound strong ceramic body of tubular or pipe shape is obtained by centrifugally casting a mixture of ceramic powders and fibrous metal, the metal having a coefficient of expansion higher than that of the ceramic.

Description

CENTRIFUGAL MOLDING OF CERAMIC TUBES CONTAINING METAL FIBERS This application is a continuation-in-part of application Ser. No. 719,977, filed Apr. 9, 1968, and now abandoned.

This invention relates to the production of ceramic tubes reinforced with fibrous metal and in particular large tubes having a diameter of 6 to 8 inches, a wall thickness of one-half inch and a length upto l feet.

Transmission of fluids at high temperatures, particularly in the instance of the petrochemical industry, demands tubing of such composition as to be resistant to both the high temperatures involved and the corrosive atmosphere prevailing. The mechanical stresses that may be imposed also demand strong tubes, both from the standpoint of extermal loading and pressures that prevail internally.

A ceramic tube will satisfy the demand of resistance to heat and corrosive influence, but ceramic products are susceptible to brittle failure. A mere fissure or surface defect per se could be tolerated in a cermic tube as involving at the worst a detectable leak, but there can be no allowance in overhead systems for catastrophic failure under stress or impact loading.

The primary object of the present invention is to develop a composite ceramic tube presenting acceptable strength from the standpoint of resistance to external and internal loading and to accomplish this by casting the tube centrifugally from a mixture of a ceramic and metal fibers. Another object of the present invention is to pre-stress the ceramic matrix by selecting the fibers of a metal having an expansion coefficient greater than that of the ceramic.

In the practice of the present invention, any finely divided ceramic may be used in conjunction with fibrous metal having the necessary coefficient of expansion. We define a ceramic as any inorganic non-metallic such as to include silica, alumina, titania, zircon, silicon carbide and so on. The requirements are basically physical, not chemical, and many equivalents can obviously be used. The only other limitation on the metal is that its melting point be above the temperature at which the tube will be cured and the temperature to which it will be exposed in actual service. For most purposes, any ferro-alloy or nickel-chromium alloy (IN- CONEL or Nl-CHROME) may be used. However, it should be distinctly understood that for severe service conditions we can also use ceramics such as aluminum silicate (e.g., mullite) and zirconium silicate for the ceramic matrix, and highly refractory metals such as tungsten and molybdenum.

Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings, disclosing the preferred embodiment of the present invention and the principle thereof and what we now consider to be the best mode contemplated for applying these principles. Other embodiments of the invention embodying the same or equivalent principles may be used and changes may be made as desired by those skilled in the art without departing from the present invention.

In the drawings:

F IG. 1 is a view in perspective of centrifugal molding equipment;

FIG. 2 and FIG. 3 are cross sections of tubing molded in accordance with the present invention; and

FIGS. 4, 5, and 6 are cross sections of tubing.

Under one mode of practicing the present invention, a ceramic slurry is prepared and molded centrifugally while incorporating metal fibers in the slurry before or during molding. While any castable ceramic can be used, the metal fibers, to develop superior strength, must have a higher coeicient of expansion than the ceramic. If this precaution is not observed, cracking may occur in the molded tube'due to residual stresses in the ceramic occurring during the cooldown period from the firing temperature used to cure or harden (sinter) the ceramic. If the expansion phenomenon is observed, a form of reinforcement is introduced into the ceramic matrix so that we develop high resistance to cracking due to impact.

The slurry containing the metal fibers is introduced into a circular metal mold rotating at high speed. The mixture spreads itself with uniform thickness on the mold wall due to centrifugal force developed in the spinning mold. Free water gravitates toward the axis of the mold where it is easily drained, while the metal fibers are forced toward the outer limit of the spun tube where they align themselves uniformly concentric to the wall of the cast tube. After the cast is completed, the green tube is stripped from the mold, dried in air, and fired to harden the same.

Tests demonstrate a strength almost five times greater, with only about 5 percent (volume) metal fiber addition, compared to a tube solely of the ceramic. The metal fiber content, however, may be as low as about 3 percent, but no advantages are realized when the metal fiber content exceeds 30 percent by volume (dry basis).

Diametrical compression under expected loads may produce fissures, but not disintegration, and any cracks or fissures which do appear close up when the load is removed, manifesting an unexpected advantage which appears to be one of pre-stressing. The present invention is particularly suited to the production of strong, thin-walled cylinders impossible to produce by extrusion techniques.

The work hereinafter described in detail was directed toward the manufacture of cast tubes 6 to 8 inches in diameter with a wall thickness of one-half inch and at least 10 feet long, with expected operating conditions of almost 2,700F and gas flow of 2,500 feet per second under an internal pressure of 40 pounds per square inch (gauge).

While a castable slip may be used, one eminently satisfactory system is an aqueous slurry of' fused silica containing chopped stainless steel fibers 0.003 inches diameter and from one-eighth to one-half inch in length. The metal fibers preferably have a length greater than l0 times the diameter. However, we are not limited to such specifications, especially since the water content serves only to expedite compaction by reducing the centrifugal force necessary to produce compaction. Thus, the water content may be progressively less with progressively higher centrifugal forces during molding. The specific materials, silica and stainless steel, present the prerequisites of low elastic modulus for the matrix, high strength and high elastic modulus for the metal fibers, and a coefficient of expansion for the metal higher than that of the ceramic:

TABLE l Fused Property Silica Stainless Steel Coefficient of expansion (in,-/in./F) 1.0 X 10-6 8.2 10-6 Elastic Modulus (Young, lpsi) 10.5 28.0

A series of experiments were initially performed without fibers to evaluate the feasibility of centrifugally casting a ceramic slurry, using the equipment illustrated in FIG. 1 typical of centrifugal casting apparatus wherein a cylindrical mold of tubular construction 20 is secured to a rotatable hub 21 in turn driven by a motor 22. The end of the centrifugal mold opposite the hub is suitably meshed with a ring gear 23 which establishes uniformity of rotation. In this initial feasibility determination, the castable ceramic slurry was introduced into the mold through a manually manipulated conduit, and the conduit was moved along the axis of the mold interiorally thereof to spread the slurry along the length of the mold. The mold was rotated at 1,500-2,000 rpm and of course the mixture spread itself with uniform thickness. The spinning forces cause free water to gravitate to the inside surface of the casting where it is free to drain, and in this connection it will be appreciated that after the tube is formed the mold may be tipped slightly to allow the water to drain by gravity.

From the initial determination, it was established that a centrifugal mold is indeed capable of spin casting a ceramic slurry to produce a sound, dense tube that can be hardened at high temperature. It was also determined that removal ofthe casting is facilitated by using a split mold (two 180 segments) having quick release capabilities and that the addition of ball clay to the ceramic mixture will produce shrinkage of the casting after standing for some time, contributing to easy separation of the casting from the centrifugal mold. We can use Tennessee No. 5 ball clay, or any equivalent form.

Further experimental investigations, using stainless steel fibers one-half inch long and 0.003 inches diameter, established that metal fibers cannot be easily mixed into the ceramic slurry; they matted for the most part into a ball, and spin casting could not be accomplished. This difficulty was overcome by introducing the slurry separately into the mold with the metal fibers thereafter introduced into the ceramic matrix while spinning the mold with the slurry in a wet state. The metal fibers were introduced into the slurry in the spinning mold simply by feeding the metal fibers through a conduit while maneuvering the discharge and of the conduit to produce uniform sprinkling of the metal fibers.

The silica, ball clay, and water are mixed to a homogeneous state outside the mold and then introduced as above described into the centrifugal mold spinning only at a speed sufficient to spread the slurry uniformly outward along the length of the mold, say about 500 rpm. The metal fibers are thereafter introduced into the slurry while continuing to spin the mold at about the same speed. The fibers may be so introduced by spilling them from the discharge end of a conduit, moving from one end of the mold to the other, the rates being such as to produce a substantially uniform sprinkling of the metal fibers onto the exposed surface of the slurry.

Thereafter the mold is spun at about 1,800 rpm for about 6 minutes, expunging nearly all the water and causing the metal fibers to arrange themselves concentric to the wall of the cast tube throughout its thickness. The mold halves are disassembled toexpose the green tube which is strong enough to be handled, the tube being allowed to dry in air for 24 hours. Thereafter the green tube is fired, preferably in a non-oxidizing atmosphere (e.g., percent nitrogen, 5 percent hydrogen) to avoid oxidation of the steel fibers. To inhibit promotion of transformation products of the ceramic content the firing temperature should not exceed 2,000 F. The air-dried tube is slowly brought up to temperature, starting at room temperature, in increments of 250 F per hour, requiring 8 hours; the tube is then held at 2,000 F for 3 to 10 minutes, whereafter it is removed from the furnace and allowed to cool in air overnight, or at about F 1 hour. Other schedules may be used since there is no criticality in this regard.

If desired, the metal fibers may be mixed with the slurry before introducing the slurry into the mold as above described, but if this is done then a small amount of a binder should be employed in order to prevent matting of the metal fibers when mixing. The binder may be about 3 to 5 grams of bentonite or as little as half a gram of methyl cellulose, or a mixture of the two.

During firing, which cures and hardens the tube, manifest in sintering of the ceramic particles, there is some diffusion bonding between the metal and the ceramic. The metal, during cooling, will shrink more rapidly than the ceramic; the metal goes into tension, pulling the ceramic into compression and the latter becomes pre-compressed.

Tubes prepared from a ceramic slurry alone, without metal fibers, and otherwise processed almost exactly like the tubes of Example l showed much less strength under diametral compression loading, namely, an average stress of 245 psi at failure compared to an average of 1,030 psi at failure for tubes made in accordance with Example I. Thus, the tubes for comparison had the following composition: *325 mesh fused silica, 3,160 grams (98.8 vol. percent); ball clay (Tennessee No. 5), 50 grams (1.2 vol. percent) and 420 grams water, centrifugally cast and fired as specified under Example l.

FIGS. 2 and 3 are perspective views of shapes contemplated under the present invention and serving as comparisons for what is shown in FIGS. 4, 5, and` 6. Thus, evidence of the benefits of reinforcement is exhibited in FIG. 4. A green or unfired section of an Example l pipe just separated from the mold was accidentally dropped. Instead of shattering into small pieces, only the edge where the pipe hit the floor cracked as shown. Even here the broken pieces could not easily be detached from the pipe section. While this accident was not intentional, it does indicate the improved toughness of the spin-cast reinforced pipe.

Additional evidence of improved behavior is shown in FIGS. 5 and 6. Unreinforced pipe sections will fail in four sections under diametral compressive loading. The fiber reinforced section cracked, but did not separate into sections. In fact, when the load was released the sections resumed their original circular shape, FIG` 5. FIG. 6 is a close-up of the crack which does not penetrate to the inside diameter of the pipe section, establishing that pre-stress has been introduced due to the metal having a higher coefficient of expansion than the vceramic matrix. The parts shown in FIGS. 5 and 6 were cast and fired in accordance with Example I.

There is nothing critical about spinning speed, so long as it is high enough to throw the slurry against the mold wall. Thus, spinning speed only determines (inversely) how long the mold must be rotated to align the fibers and expunge enough water to produce a dense tube which can be easily handled when it is removed from the mold. Likewise, there is nothing critical about air drying the tube removed from the mold since the purpose is to have the tube dry enough so that it will not crack when subjected to the firing schedule.

Hence while we have illustrated and described a preferred embodiment of the invention, it is to be understood that this is capable of variation and modification.

We claim:

1. A method of producing a ceramic tube reinforced by metal and comprising: presenting to the interior of a centrifugal mold of cylindrical form a mixture of ceramic particles and metal fibers dispersed in water, said mixture including a binder selected from the group consisting of bentonite and methyl cellulose in an amount sufficient to prevent matting of the metal fibers in the mixture, the metal fibers having a coefficient of expansion higher than that of the ceramic and a length greater than ten times their diameter, and said mold consisting essentially of a tubular member having an inside diameter representing the outside diameter of a green ceramic tube to be formed therein; rotating the mold to cause the metal fibers uniformly to align themselves concentric to the wall of the centrifugal mold and to separate water from said mixture, producing a green ceramic tube containing the fibers; draining the water from the mold and removing the green tube from the mold, drying the green tube in air,l and then heating the tube to sinter the ceramic particles as a matrix surrounding the metal fibers, the metal melting above sintering temperature.

2. A method according to claim' l in which the ceramic is silica and the metal is stainless steel, the metal fibers being present in an amount of at least about 3 percent but not more than about 30 percent by volume based on the ceramic and the metal.

3. A method according to claim 2 in which the parameter for molding is about 1,800 rpm for about 6 minutes, and in which the air-dried tube is brought gradually up to a temperature of about 2,000F where it is held for about 3 to l0 minutes and then allowed to cool in air.

4. A method of producing a ceramic tube reinforced by metal and comprising: presenting to the interior of a centrifugal rn ld of cylindrical form a mixture of ceramic partic es dispersed in water, said mold consisting essentially of a tubular member having an inside diameter representing the outside diameter of a green ceramic tube to be formed therein; rotating the mold containing said mixture and while the mixture is being so rotated adding metal fibers thereto, the metal fibers having a coefficient of expansion higher than that of the ceramic and a length greater than ten times their diameter, rotation of the mold causing the metal fibers uniformly to align themselves concentric to the wall of the centrifugal mold and causing water to separate from said mixture resulting in a green ceramic tube containing the fibers; draining the water from the mold and removing the green tube from the mold, drying the green tube in air, and then heating the green tube to sinter the ceramic particles as a matrix surrounding the metal fibers, the metal melting above sintering temperature.

5. A method according to claim 4 in which the ceramic is silica and the metal is stainless steel, the metal fibers being present in an amount of at least about 3 percent but not more than about 30 percent by volume based on the ceramic and the metal.

6. A method according to claim 5 in which the parameter for molding is about 1,800 rpm for about 6 minutes, and in which the air-dried tube is brought gradually up to a temperature of about 2,000F where it is held for about 3 to l() minutes and then allowed to cool in air.

Claims

2. A method according to claim 1 in which the ceramic is silica and the metal is stainless steel, the metal fibers being present in an amount of at least about 3 percent but not more than about 30 percent by volume based on the ceramic and the metal.
3. A method according to claim 2 in which the parameter for molding is about 1,800 rpm for about 6 minutes, and in which the air-dried tube is brought gradually up to a temperature of about 2,000*F where it is held for about 3 to 10 minutes and then allowed to cool in air.
4. A method of producing a ceramic tube reinforced by metal and comprising: presenting to the interior of a centrifugal mold of cylindrical form a mixture of ceramic particles dispersed in water, said mold consisting essentially of a tubular member having an inside diameter representing the outside diameter of a green ceramic tube to be formed therein; rotating the mold containing said mixture and while the mixture is being so rotated adding metal fibers thereto, the metal fibers having a coefficient of expansion higher than that of the ceramic and a length greater than ten times their diameter, rotation of the mold causing the metal fibers uniformly to align themselves concentric to the wall of the centrifugal mold and causing water to separate from said mixture resulting in a green ceramic tube containing the fibers; draining the water from the mold and removing the green tube from the mold, drying the green tube in air, and then heating the green tube to sinter the ceramic particles as a matrix surrounding the metal fibers, the metal melting above sintering temperature.
5. A method according to claim 4 in which the ceramic is silica and the metal is stainless steel, the metal fibers being present in an amount of at least about 3 percent but not more than about 30 percent by volume based on the ceramic and the metal.
6. A method according to claim 5 in which the parameter for molding is about 1,800 rpm for about 6 minutes, and in which the air-dried tube is brought gradually up to a temperature of about 2,000*F where it is held for about 3 to 10 minutes and then allowed to cool in air.