US2870907A - Forming tubes and rods of uranium metal by extrusion - Google Patents

Description

E. C. CREUTZ Jan. 27, 1959 FORMING TUBES AND RODS OF URANIUM METAL BY EXTRUSION Filed July 22, 1944 3 Sheets-Sheet 1 F'IE-1- 'wdm Jan. 27, 1959 E. c. CREUTZ 2,

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FORMING TUBES AND RODS OF URANIUM MEIT'AL BY EXTRUSION Filed July 22, 1944 3-Sheets-Sheet 5 E'essure F I E -4- 7'enz mruzare .improved apparatus for extruding FORMING TUBES AND RODS OF URANIUM METAL BY EXTRUSEON Edward C. 'Creutz, Chicago, 111., assignor to the United States of America as represented by the United States Atomic Energy Commission Application July 22, 1944, Serial No. 546,177

2 Claims.- (Cl. 207-10) My invention relates to methods and apparatus for the extrusion of uranium metal into various shapes and particularly to the formation of rods, tubes and other elongated shapes from metallic uranium.

Uranium is an unusual metal inthat it is relatively hard, of very great density, has a high melting point, oxidizes at moderate temperatures and oxidizes quite rapidly at elevated temperatures, and becomes alloyed with the materials of machines tools. Many commercial applications are arising for uranium in rod and tube form. Machining the uranium metal into rod and tube form is not Wholly satisfactory because uranium work .hardens very readily so that conventional cutting tools become dull with very little use. The metal is very pyrophoric and turnings formed by machining create a fire hazard. A uranium oxide coating that forms on the metal when exposed to air is a great hazard to health when it is released as dust by working the metal. Furthermore, the relatively high cost of production of the metal limits the waste that can be tolerated in machining processes. These and other limitations imposed by the physical properties of uranium necessitate improved working processes that are not limited by the factors involved in machine working.

While attempts to forge uranium have been made, it is found that the metal in general becomes brittle at low temperatures, very brittle at higher temperatures, and too soft at still higher temperatures. For example, at temperatures up to 500 C. the metal is very hard under the hammer and of low ductility so that the metal cracks badly. At 700 C. the metal is substantially as hard as at 500 C. and exceedingly brittle, while at 900 C. the metal is found to be so soft and mushy that the lightest blows of the forge hammer cause great deformation and crumbling of the metal.

It is an object of this invention to provide a method of working uranium metal into various elongated shapes without cutting the metal.

It is a further object to provide a method of forming uranium into various shapes without materially oxidizing the metal and without rendering it brittle.

It is a still further object of this invention to provide a method for extruding uranium into rods and tubes.

It is a further object of this invention to provide an uranium into rods and tubes. More particularly, it is an object of this invention to extrude uranium metal while controlling the temperature of the uranium, the temperature of the extrusion apparatus and the rate of extrusion.

These and other objects and features of my invention will be apparent from the following detailed description of a preferred embodiment taken with the accompanying drawings in which:

Fig. 1 is a graph which shows the effect of temperature change on uranium by plotting temperature against electrical resistance;

Patented Jan. 27, 1959 ice Fig. 2 is a vertical sectional view of an extruding die and plunger;

Fig. 3 is a front elevational view of a piercing member; and

Fig. 4 is a graph showing the etfectof temperature change on extrusion of uranium in which temperatures are plotted as abscissae against extruding pressures plotted as ordinates. I

Uranium appears to have three crystal forms which are referred to hereinafter as the alpha, beta and gamma phases, the beta and gamma phases being formed in succession as metal in the low temperature alpha form is increased in temperature.

It is found that uranium is very brittle when worked in the beta phase. Thus, Whileextrusion in the beta phase is possible, the extruded forms are brittle and will stand little mechanical shock even when an otherwise satisfactory extrusion is obtained. In the majority of cases, however, extrusion in the beta phase region produces a cracked or checked product. Consequently, it is desirable to extrude uranium While at a temperature within its gamma phase. Inasmuch as this temperature is high, and at such temperatures the uranium readily forms an alloy with the metals with which it comes in contact, and becomes oxidized to a great degree, it is necessary to observe certain precautions in extruding uranium.

The temperature of formation of the three metal phases referred to above may be determined in several ways, such as by reference to electrical resistance, to inertia in temperature rise with constant heat input, or by microscopic examination. The first method is probably the most accurate in determining changes in crystal form with temperature. There are shown in Fig. 1 a number of curves representing change in electrical resistance with temperature. Referring to Fig. 1, the curves A, B, C, D, and E are for representative .uranium ingots showing the phase transition points derived by measuring the electrical resistance of specimens from said ingots with increasing temperature from 0 to 900 C. and with decreasing temperature from 900 C. to 0 C. The arrows to the right and downward are representative of the curve with increasing temperature while those to the left and upward are for decreasing temperatures.

From a consideration of the curves shown in Fig. 1, it is evident that a discontinuity in electrical resistance exists in uranium over the temperature ranges of 640 to 670 and 760 to 780 C. These discontinuities are presumably due to changes in the crystal structure of the uranium and are apparently characteristic of the metal. Thus while various masses of uranium may be prepared differently by different processes, the changes in crystal form and the temperature ranges over which these changes occur are substantially the same. It will be noted from these curves that the temperatures at which the changes in crystal form occur are slightly different for increasing and decreasing temperatures, and that the change from the gamma form to the beta forms occurs at a lower temperature on the decreasing temperature curve than on the increasing temperature curve. This is a definite advantage in extending the extrusion temperature range since during the extrusion process the metal is worked on a decreasing temperature basis. quently, in accordance with one teaching of my invention I extrude uranium while maintaining it at or within Conserange between 870 C. and the melting point of uranium. However a slight decrease is obtained in the necessary pressures for extrusion by increasing the temperature of uranium through this range. In the case of extrusion of tubes with a small .inner diameter, the additional fluidity obtained by heating the billet to 1,000 C. aids in producing the extruded tube. Decreasing the extruding temperature of the uranium increases the pressure necessary for extrusion. The pressures increase slowly with the decrease of temperature in the range between 870 C. and 820 C. When the extruding temperature has decreased below 820 C., the extruding pressure mounts rapidly, and below 780 C. the extrusion becomes difficult and the extruded product becomes objectionably brittle, and cross-checked on the surface.

The following table shows billet temperature against extrusion pressure for extruding a inch rod from a 1% inch billet:

Fig. 4 shows graphically the data in the above table obtained by extruding uranium through a resistance heated press. The extruding pressure-in pounds per sq. in. is shown on the ordinate and the temperature to which the billet is raised is shown onthe abscissa in both the Fahrenheit and Centigrade scales. Two curves are laid out on the graph, one for the initial pressure on the billet and the other for the pressure during the extrusion. The initial pressure is slightly higher but the shape of the curves is similar. The curves show that minimum extrusion pressures are approached at a billet temperature of 870 C. From the shape of the curves,

only a slight decrease in pressure would be anticipated at higher temperatures. Below 870 C., the pressure increases slowly to 820 C. but below 820 C. the pressure increases rapidly. The pressure during extrusion does not rise between 802 and 788 C. but extrusion below 780 C. is virtually impossible.

I have shown in Fig. 2 an extrusion die and billet chamber that is suitable for extruding uranium, although it should be appreciated that the specific die form shown is selected only for purposes of disclosure and thatmany variations thereof may be made in practicing my inven tion without departing from the scope thereof.

The extrusion apparatus shown in Fig. 2 includes a die block 2 of massive form to better withstand the operating pressures with a central chamber 3. The chamher 3 may be of circular cross-section and encloses an extrusion die 4 having a central channel 5 through which the uranium may be extruded and formed into an elongated shape. The die block 2 is provided with a resistance winding 6 to heat the apparatus. A movable plunger 9 is provided of slightly smaller diameter than the chamber 3 so that it may slide freely in the chamber and force the metal to be extruded through the die channel 5. The die 4 is of the funnel type being characterized by sides 7 sloping toward the die channel 5 at an angle of approximately 30 with the horizontal.

The die 4 is preferably provided with a facing 8 of hot die steels such as the chromium-cobalt-iron-tungsten alloys. An alloy which has been found to be particularly desirable as a die fiacing in uranium extrusion is commercially known as Stoodite-63. Stoodite 63 has approximately the following compositions: 2.48 percent front of the uranium billet to serve as a lubricant.

carbon; 10.1 percent chromium; 0.12 percent nickel; 21.7 percent cobalt; 0.30 percent manganese; less than .0005 percent boron; 40 percent iron; and 21 percent tungsten. Another alloy which is found to be effective is commercially known as Tlantung G and contains tantalum. Tantung G" has approximately the follow ing compositions: 45 to 55 percent cobalt; 20 to 35 percent chromium; 10 to 23 percent tungsten; 0 to 10 percent tantalum columbium carbide; 2 to 4 percent carbon; and 2 to'5 percent iron. The plunger 9 is formed with a sloping conical end at a corresponding angle to the sides 7 of the die 4. The die channel 5 determines the outside form or diameter of the extruded uranium whether it be in rod or tube form. For extruding tubing, I prefer to provide a piercing member afi'ixed centrally of the plunger 9. For example, a threaded recess 10 may be provided in the plunger 9 into which a piercing member 11 shown in Fig. 3 may be fitted. The piercing member 11 forms the interior surface of the extruded tubing and should be of suflicient length to lie within the throat 14 of the die 4 at the start of extrusion. When using the plunger 9 for rod extrusion, the recess 10 should be filled by a screw plug 12 as shown.

The preferred mode of operation for extruding a uranium rod will now be explained. a

The apparatus illustrated is heated by resistance wind ing 6 and maintained at about 870 C. for around two hours in an air atmosphere to produce an oxide film thereon which provides a protection against alloying between the uranium and the internal surfaces of the apparatus. A billet 13-is preheated separately from the apparatus in a furnace in an atmosphere containing natural gas, butane, or hydrogen to a temperature in excess of 780 C., and below the melting point of uranium, which latter appears to be of the order of 1100 C. When the apparatus and the billet 13 have received requisite preheating, the billet 13 is placed in the central chamber 3 of the apparatus and a few minutes are allowed for the apparatus and billet 13 to attain the same temperature. The recess 10 of plunger 9 is filled by plug 12. The extruding pressure is than applied to the plunger 9 and thereby against the billet 13 forcing'it in turn against the funnelled sides 7 and through the throat 14 and into the die channel 5, thereby forming the uranium metal in the shape of a rod. The plunger 9 is forced against the billet 13 to extrude as much of the latter as possible. Since the uranium cools during the process of extrusion, the extrusion takes place on the decreasing temperature gradient and uses the advantage of the lag in the return of the uranium crystalline structure to the beta phase. During the ex trusion, the apparatus may or may not be heated. A hot die in the extrusion of uranium has the advantage of aiding the elimination of cross-checking on the surface of the extruded article. This in turn gives the extruded object greater structural strength and prevents the stretching of the partly extruded uranium' when it hangs by its own weight from a vertical press.

When it is desired to produce a tube of uranium rather than a rod, screw plug 12 is removed from'the threaded recess 10 and the piercing member 11 is fitted into the recess 10. Thus, when extruding a uranium billet, he

piercing member 11 precedes the plunger 9, pierces the.

piercing member 11 moves into the die channel 5 creat-- ing a hollow center in the extruded uranium.

It is found that it is an aid to the extrusion of uranium v to place a copper disc on the face of the extrusion die in The copper adheres to the uranium as it is extruded and may be removed by pickling, leaving a very smooth surface under the coating.

A wide range of sizes may be extruded by the above method and apparatus. As an example, a cast uranium billet 4 /2 inches in diameter and inches long weighing approximately 100 pounds may be heated in an atmosphere of natural gas to a temperature of somewhat above 790 C. When the billet has reached the proper temperature, it is removed from the preheating furnace, brushed to remove scale and placed in a horizontal extrusion press. The extrusion time is 7 to 10 seconds with a die of Stoodite 63. The extruded rod is fed onto an angle iron strip on a table and allowed to cool in air until the alpha transformation temperature is reached, and is then quenched. After being cooled, the rod is straightened in a conventional straightener and is then outgassed to remove hydrogen. One manner of accomplishing outgassing is by placing the uranium rods in tubes. These tubes connected in series, are placed in a furnace and heated at a temperature of 620 to 649 C. for 9 hours. During this heating argon is allowed to flow through the tubes at a rate of 18 cubic feet per ton of uranium per hour. After outgassing, the uranium rod is again straightened and is ready for machining.

While the specific example of an application of the present invention has been illustrated by the extrusion of a rod, the method applies to the extrusion'of tubes and other articles. It will be apparent to those skilled in the art that various modifications can be made without departing from the principles of the invention as disclosed herein, and for that reason it is not intended that it should be limited other than by the scope of the appended claims.

I claim:

1. A method of extruding uranium comprising preheating a billet of uranium in an inert atmosphere to a temperture in excess of 780 C. but below 1100 C., placing the heated billet in an extrusion apparatus wherein the die of such apparatus had been maintained at a temperature of about 870 C. for a suflicient time to produce an oxide film thereon, placing a copper disc between the V billet and die of such character as to act as an extrusion lubricant, and extruding said billet while said billet is at a temperature above 780 C.

2. In the method of extruding uranium metal comprising preheating said metal in an inert atmosphere, placing the heated metal in an extrusion apparatus wherein the die of such apparatus has an oxide film thereon, and extruding said metal, the improvement consisting of maintaining the preheating temperature within the range of 780 C. to 1 C. and maintaining the temperature during the extrusion within the range of 780 C. to

References Cited in the file of this patent UNITED STATES PATENTS 817,085 Moshier Apr. 3, 1906 982,751 Thowless Jan. 24, 1911 1,082,933 Coolidge L Dec. 30, 1913 1,551,333 Schroter et a1 Aug. 25, 1925 1,685,915 Gero Oct. 2, 1928 1,759,454 Heany May 20, 1930 1,771,620 Ehrmann July 29, 1930 1,935,286 Born Nov. 14, 1933 2,036,182 Singer Mar. 31, 1936 2,123,416 Graham July 12, 1938 2,217,802 Koehring Oct. 15, 1940 FOREIGN PATENTS 703,161 Germany Mar. 3, 1941

Claims (1)

1. A METHOD OF EXTRUDING URANIUM COMPRISING PREHEATING A BILLET OF URANIUM IN AN INERT ATMOSPHERE TO A TEMPERTURE IN EXCESS OF 780*C. BUT BELOW 1100*C., PLACING THE HEATED BILLET IN AN EXTRUSION APPARATUS WHEREIN THE DIE OF SUCH APPARATUS HAD BEEN MAINTAINED AT A TEMPERATURE OF ABOUT 870*C. FOR A SUFFICIENT TIME TO PROVIDE AN OXIDE FILM THEREON, PLACING A COPPER DISC BETWEEN THE BILLET AND DIE OF SUCH CHARACTER AS TO ACT AS AN EXTRUSION LUBRICANT, AND EXTRUDING SAID BILLET WHILE SIAD BILLET IS AT A TEMPERATRUE ABOVE 780*C.

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