US 3926248 A
Orifice structures comprised of high density, polycrystalline thorium oxide have been found to be highly resistant to dimensional changes when employed in the extrusion of corrosive metal melts to form fine diameter wire.
Description (OCR text may contain errors)
United States Patent English 5] Dec. 16, 1975  ORIFICE STRUCTURE FOR EXTRUDING 3.645.657 2/1972 0mm et al. 164/66 x MOLTEN METAL o FORM FINE 3,658,979 4/1972 Dunn et a1. 164/82 X DIAMETER WIRE Primary ExaminerR. Spencer Annear Attorney, Agent, or FirmRussell E. Weinkauf  ABSTRACT Orifice structures comprised of high density, polycrystalline thorium oxide have been found to be highly resistant to dimensional changes when employed in the extrusion of corrosive metal melts to form fine diameter wire.
3 Claims, 1 Drawing Figure  Inventor: Jerome J. English, Cary, NC.  Assignee: Monsanto Company, St. Louis, Mo.
 Filed: Oct. 11, 1973  App]. No.: 405,388
 US. Cl. 164/273 R; 164/82  Int. Cl. B22D 11/00  Field of Search 164/66, 82, 273 R; 264/ 176 F  References Cited UNITED STATES PATENTS 3,516,478 6/1970 Dunn et a1. 164/82 X U.S. Patent Dec. 16, 1975 fl///7//A a J \\\\\\\\\\\\\\\Y 7 ORIFICE STRUCTURE FOR EXTRUDING MOLTEN METAL TO FORM FINE DIAMETER WIRE BACKGROUND OF THE INVENTION This invention is generally directed to an improved apparatus for fabricating filamentary products by a continuous extrusion of very low viscosity melts. More particularly, the invention pertains to an improved orifice assembly for extruding molten metals and metal alloys to form fine diameter wire.
Methods by which fine diameter wire can be formed by spinning molten metals through an orifice as afree molten filamentary stream are known and have been described in US. Pat. No. 3,216,076 and US. Pat. No. 3,658,979, among others. According to these methods, a low viscosity melt (e.g., molten metal or metal alloy) is extruded at an appropriate velocity into a selective atmosphere. When the hot jet issuing from the extrusion orifice contacts this atmospherea reaction occurs which results in the formation of a film about the jet surface. This film, referred to as the stabilizing film functions as a protective sheath in that it stabilizes the filamentary jet or stream against break-up from the forces of surface tension until sufficient heat can be transferred to effect a phase change to the solid state.
The apparatus employed in the practice of these methods is comprised essentially of a crucible having an orifice element in its base, either as a part of the base, or preferably, as an orifice insert. The crucible is further provided with heating means to melt the charge and maintain it in the molten condition. In addition, means are provided for applying a positive head pressure to the molten charge to force it through an extrusion orifice at appropriate jet velocities.
From the nature of the process, it is readily evident that severe demands are imposed upon the materials which comprise the orifice. Among other requirements, the materials must be highly resistant to thermal shock and have sufficient high temperature strength to withstand the mechanical stresses imparted throughout the course of extrusion. Moreover, for successful operation, the orifice material must be chemically compatible with the molten charge being extruded. That is, the orifice material should be corrosion and erosion resistant to chemically reactive melts at high temperatures in order to assure dimensional stability of the orifice during operations.
Although the science of materials has advanced at a rapid pace in recent years, information and data are not yet available for directing the art to a substance having the properties which would fully meet the stringent requirement as set forth hereinabove. Consequently, considerable time and effort has been expended in the search for a suitable material. Materials such as alumina, sapphire, zirconia, and other ceramics which have been successfully employed to contain molten metals, such as steel, in metallurgical procedures have been investigated but failed in performance as orifice defining members through which molten metals can be extruded as fine diameter filamentary jets. The cause of such failures can be variously attributed to insufficient strength, low thermal fracture resistance and chemical incompatibility with the charge under the conditions of temperature and pressure required for extruding molten metals through orifices having diameters below about 35 mils. Failures were particularly aggravated when processing the highly corrosive melts of various 2 ferrous alloys where process temperatures exceeding l500C. are often required.
In a relatively recent development, as disclosed in US. Pat. No. 3,584,678, orifices of high-density beryllia have been used in apparatus for extruding molten metals to form fine diameter wire. Although this development represents a clear advance over previous prior art, it has been found that when such beryllia orifices are employed in the processing of certain molten metals, e.g., various steel alloys, dimensional changes occur in the orifice which exceed acceptable limits.
It is therefore, an object of this invention to provide an apparatus for extruding molten metals and alloys thereof with an orifice member which can withstand the thermal, mechanical and chemical stresses imposed by the extrusion operation without change in the structural integrity of the orifice.
It is a further object of the invention to provide an apparatus for extruding molten metals and alloys thereof with an orifice member having a high degree of resistance to dimensional changes in the orifice when highly corrosive melts, such as alloys of steel, are processed at temperatures exceeding 1500C.
SUMMARY OF THE INVENTION It has now been found that the above objects are achieved when an extrusion apparatus for forming fine diameter wire from molten metal contains an orifice structure which is composed of polycrystalline thorium oxide (ThO having a density of from 9.5 to 10.0 gm/cm and a purity of at least 99.5 percent. The thoria may be in the completely pure form or contain up to 0.5 percent by weight of a sintering agent. Although not an essential requirement, when used the sintering agent or densification aid may be selected from any one of a large number of compounds known to perform this function. As illustrative examples, there may be mentioned calcium oxide, yttrium oxide, ammonium chloride, cerium oxide and magnesium oxide among others.
DESCRIPTION OF THE DRAWING The FIGURE represents a schematic cross-sectional view of a typical extrusion apparatus wherein molten metals are extruded as filamentary jets to form fine diameter wire. The molten metal charge is contained in crucible 2, having a base plate 3, with the crucible and base plate being supported by pedestal 4. Insulating cylinder 5 and susceptor 6 enclose the crucible 2 and its base plate 3. The heat required for conducting the process is provided by induction heating coils 7. An extrusion head pressure is provided by a pressurized inert gas (source not shown) supplied through gas line 8, which communicates with the interior of the unit through the unit head 9. Sealing rings 10 serve to maintain the pressure within the enclosure and prevent leakage past base plate 3. The molten metal 1 is forced through orifice 11 in orifice plate 12 by the applied head pressure to form a filamentary shaped molten jet. Upon emerging from orifice 11, the nascent jet passes through a film-forming atmosphere contained within cavity 14 of pedestal 4. The film-stabilized molten jet then passes through a cooling column (not shown) where sufficient heat is removed for conversion to the solid state.
DETAILED DESCRIPTION OF THE INVENTION As previously noted, the orifice structures of this invention are constituted from a polycrystalline tho= rium oxide material which has a density of from 9.5 gm/cm to 10.0 gm/cm and a purity of at least 99.5 percent. A number of processes are presently known to the art by which thoria can be densified to the levels required in the practice of this invention. One such method, commonly referred to as hot-pressing, is fully described at pages 183-230 of the volume titled, High Temperature Oxides, edited by Allen M. Alper and published by the Academic Press of New York and London. Briefly stated, powder-like particles of the ceramic are pressed together and compacted at high pressures and temperatures. The pressures normally employed are in the order of l000l0,000 p.s.i. with the temperatures being generally in the 1500-2000C. range. Another suitable process is disclosed in U.S. Pat. No. 3,574,645. In accordance therewith, a quantity of fine grained thorium oxide having a particle size of about 0.05 to 2.0 microns is first die pressed at about 10,000 p.s.i. to yield a compacted green body having a density of about 40 to 50 percent of the theoretical density. The green body is then sintered to substantially theoretical density at a temperature of at least about 2000C. in a hydrogen-water vapor atmosphere having a dew point between about 25C. to +25C. The sintering operation is conducted over a time span of about 7 hours.
As previously noted, the thoria orifice may consist of 100 percent pure thoria or a composition consisting of thoria and up to 0.5 weight percent of a sintering agent, for example, calcium oxide (CaO). When employing the optional densifying aid, it may be blended with the thoria particles prior to the pressing operation.
Plates or discs of high density, polycrystalline thoria can readily be machined to desired shapes and sizes by methods known to the art. Fine diameter orifices are usually produced by machining a countersink in the feed face of the orifice plate and thereafter drilling and polishing an orifice of the desired diameter concentric with the countersink. The orifice may also be radiused if desired. There may be more than one orifice in any given plate or disc.
As above indicated, the orifice defining members may also serve as the base plate for the crucible. However, it is generally preferred to use orifice plates inserts in the base plate of the crucible as is shown in the FIGURE. The orifice plate insert is generally of a circular configuration and may, if desired, be secured in the base plate by using a clamp or hold-down ring. Multiple orifice insert discs may, of course, be employed in the base plate of the crucible. The orifice should have an aspect ratio of between 1 and 20, preferably less than 10, exclusive of the countersink.
The following examples illustrate the superior performance of the orifice elements of this invention when used in an apparatus for extruding molten metals under severe environmental conditions.
EXAMPLE I An extrusion apparatus of the type illustrated in the FIGURE was fitted with an orifice insert disc having a composition consisting of polycrystalline thorium oxide and 0.125 percent by weight of calcium oxide as a densification aid. The orifice capillary entrance was cone-shaped with a vertex angle of about 30.
In order to ascertain the performance characteristics of the afore-described orifice structure, a test run was made under the severe environmental conditions imposed by the production of fine diameter wire from a 4 highly corrosive steel-silicon alloy charge. In addition to steel and from 1 to 2 weight percent of silicon, the charge employed also contained from about 0.25 to 0.75 percent by weight of manganese.
In conducting the test operation, a solid bar of the steel alloy was first placed in the crucible of the apparatus before heating was initiated. The crucible of the apparatus was then heated to a temperature which ranged between about l510-1550C. At this time, a second steel alloy bar was slowly fed into the heated crucible through an O-ring seal placed in the head of the pressure chamber (i.e., the bar melted at its tip, with the drippings being directed into the crucible).
After the crucible was filled with molten metal charge, the chamber was pressurized to about 20 p.s.i.g. This produced a pressure gradient across the thoria orifice insert which forced the molten steel alloy through the capillary orifice. By maintaining this pressure differential, and a continuous supply of charge by dripping molten metal into the crucible from the feed rod, the apparatus continued to stream over a period of 91 hours. This is an exceptionally long spinning run without experiencing orifice failure under such severe conditions.
Following completion of the test run, the apparatus was cooled down and the thorium oxide orifice insert was removed for microscopic examination of its microstructure and to take measurements on the extent of dimensional change in the orifice capillary. No cracks or other evidence of structural deterioration from mechanical stress were detected. Moreover, it was found that the diameter of the orifice capillary increased only 0.0007 of an inch over this long spinning run under extreme environmental conditions. This indicated an extraordinary resistance to the highly corrosive effects of molten steel alloys as well as resistance to erosion from the flow of these materials.
EXAMPLE II A series of various thoria orifice inserts were tested in the same apparatus and in like manner as described in Example I. The compositions of these inserts upon which test runs were made included: (1) pure thorium oxide and (2) a composition of thorium oxide and 0.25 percent by weight of calcium oxide and (3) a composition consisting of thorium oxide and 0.50 percent by weight of calcium oxide. The densities of the test orifice discs ranged from between 9.90 to 9.97 gm/cm The diameter of the orifice capillaries in all instances was 0.010 inch.
A molten steel alloy charge, such as employed in Example I, was streamed through the orifices under the conditions as set forth in Example I for periods of time ranging from between 50 and hours. Following the test runs, measurements were made on the extent of dimensional change in the diameters of the orifice capillaries. It was found that the diameter increases were limited to the range of from 0.0005 inch to 0.003 inch. Upon microscopic examination, there was no indication of structural deterioration as a result of mechanical stress.
EXAMPLE III The object of this Example is to provide the test results obtained with an orifice insert of the prior art against which the performance of the high-density thoria inserts of this invention as set forth in Examples 1 and II above were compared.
A high density beryllium oxide (BeO) orifice insert was fitted in the crucible plate of the same extrusion apparatus as was employed in Examples I and [1, above. A molten steel alloy charge having a composition as described in the previous Examples was then streamed through the orifice for a period of 3 hours under extrusion conditions corresponding to those as described in Examples I and II. Following the test run, a measurement was made on the dimensional changes in the orifice capillary. It was found that the diameter had increased from an original 0.006 inch to 0.011 inch. This corresponds to an orifice corrosion rate of 0.0015 inch per hour of operation compared to a rate of from 0.00001 to 0.0001 inch per hour for the high density thoria orifices tested under the same conditions in Examples I and II. Thus, it is seen that there is a to 150 fold difference in the corrosion rates between the orifice structures of this invention and those which are constituted from high-density beryllium oxide.
In order to assess the relative chemical compatibility of high-density thoria with highly corrosive steel alloy melts as compared with a large number of other refractory ceramics, a series of bench-scale tests were conducted. The procedure employed consisted of cutting a small slot of known dimensions into the side of a disc formed from the test materials. The disc was then immersed into a heated bath of a molten steel alloy. Bath temperatures ranged between 1500-1600C. After 6 to 100 hours in contact with the hot molten steel composition, the discs were withdrawn, and the dimensional change of the slot was recorded. The extent of corrosion experienced by the ceramic was indicated by an increase in the slot dimensions.
The test results demonstrated that the corrosion resistance of thoria ceramics with densities in the 9.5 to 10.0 gm/cm range was vastly superior to any other refractory material tested. This was especially evident when molten steels which contained silicon and manganese as alloying additions were used as the corrosive media. Even single crystal materials, such as sapphire (A1 0 were severly attacked by this corrosive melt. The slot widths cut in the thoria disc test samples showed dimensional changes of less than 0.001 inch under all test conditions and time spans employed. All other refractory materials showed slot dimensional changes ranging from 0.002 inch in 6 hours to complete dissolution of the disc material in less than 6 hours.
In addition to the high density thoria, samples of high density ceramics of the following compositions were subjected to the test procedure as outlined above: Al- O (single crystal and polycrystalline); BeO; MgO; ZrO 2Al O 'SiO ZrO 'SiO Y O CeS; M081 and BN.
1. In an apparatus for extruding molten metal to form fine diameter wire, said apparatus being characterized by a crucible assembly for containing a molten metal charge and having an orifice defining element as an essential part thereof, means for forcing molten metal contained in the crucible through an orifice in said orifice defining element and heating means for maintaining said charge in the molten state, the improvement which comprises: an orifice defining element which is composed of polycrystalline thorium oxide having a density of from 9.5 to 10.0 gm/cm and a purity of at least 99.5 percent.
2. The improved apparatus in accordance with claim 1, wherein said orifice defining element is composed of polycrystalline thorium oxide having a density of from 9.5 to 10.0 gm/cm, and wherein said polycrystalline thorium oxide is percent pure.
3. The improved apparatus in accordance with claim 1, wherein the composition of said orifice defining element consists of polycrystalline thorium oxide having a density of from 9.5 to 10.0 gm/cm and up to 0.5
percent by weight of a sintering agent.