US2872363A - Method of working beryllium - Google Patents

Description

nite States 2,872,363 METHOD OF WORKlNG BERYLLIUM 11 Claims. (Cl. 148-115) are Thisinvention pertains to the fabrication of beryllium metal into various shapes and forms and more particularly to a method whereby beryllium metal is fabricated into sound rods, sheet, and foil having predetermined favorable, physical and mechanical properties.

The process of this invention is particularly useful in the fabrication of beryllium destined for use as a moderator in a chain-reacting neutronic reactor Where it is of primary importance that the metal used therein have certain physical and mechanical properties as well as the necessary purity.

It is an object of this invention to provide a method for the fabrication of beryllium metal whereby fabricated shapes having predetermined favorable physical and mechanical properties such as texture, hardness, ultimate strength and reduced cross-sectional area may be produced.

It is a further object of this invention to provide a method whereby beryllium metal can be deformed to a state wherein substantial reduction of cross-sectional area can be obtained without sacrifice in the quality of surface structure or mechanical properties.

It is a further object of this invention to provide a method for the fabrication of beryllium metal into sheets and foil having a bright finish and a fine texture free of cracks.

It is a further object of this invention to provide a method whereby berylliummetal can be measurably reduced in cross-sectional area to form rods or tubes which are unmarred by cracks, scams or scores.

It is a further object of this invention to provide a method whereby beryllium can be protected during the deformation process from the effect of contact with the deforming apparatus.

, Other objects and'advantages will become apparent'to those skilled in the art upon further examination of this specification.

. Heretofore, beryllium metal has beensuccessfully deformed only at temperatures exceeding the recrytallization temperature. For beryllium metal, the lower limit of recrystallization is about 700 C., but may be higher in the absence of prior working.

I have discovered that beryllium metal, preferably that having a refined grain obtained by prior working above the recrystallization temperature, can be successfully fabricated at a temperature between 300 and 400 C. by such processes of plastic deformation as extrusion, rolling, forging, swaging, and drawing. Beryllium metal mechanically worked according to the process of this invention within the 300 to 400 C. temperature range to eifect plastic deformation has a texture, preferred orientation, and physical and mechanical properties equal or superior to those of metal worked at temperatures substantially higher than the recrystallization temperature.

Studies of the ultimate strength, elongation and reduc tion in area as a function of temperature show that a maximum ultimate strength of approximately 38,000 p. s. i. is attained at about 200 C. from which point the ultimate strength decreases linearly with an increase in temperature. However, the values fortensile strength at temperatures between 300 and400" C. do not go below about 28,000 p. s. i. Within the 300-400 C. temperature range beryllium metal is suificiently ductile for operations such as extrusion or rolling. The elongation reaches a value of 24 to 26 percent in the 300 to 400 C. temperature range and drops off with an increase in temperature above 400 C. Although reduction-inarea values reach a maximum at temperatures above 400 C., this effect is offset by the disadvantage of necking down, that is, the contraction area tends to concentrate at one point on the bar because at this temperature range the plastic flow occurs only locally. However, at 300 to 400 C., stretch and plastic flow extend over the entire gauge length and no necking down is observed, while as much as 24 to 32 percent reduction can be achieved by working within the 300-400 C. temperature range.

Hardness values of extruded material show that this property is a function of the extrusion temperature in that the hardness of the deformed metal varies inversely with the temperature of extrusion. Moreover, studies of the torsion properties of beryllium as a function of temperature show that the strength of the beryllium decreases approximately linearly with increase in temperature. In addition, the observed type of break which occurs between room temperature and 200 C. is helicoidal, while the break occurring in metal treated at temperatures of about 300 C. and above is the transverse type.

In addition to the improved mechanical and physical properties observed in beryllium metal fabricated at temperatures between 300 and 400 0, working beryllium metal within this relatively low range has the added advantage that at such temperatures beryllium is relatively inert to atmospheric oxidation so that the use of an inert or non-oxidizing atmosphere, such as helium, argon or hydrogen, is not necessary. Moreover, by working within this relatively low temperature range, the tensile strength of the deforming apparatus, for example a mandrel, is only slightly affected.

in practicing my invention, beryllium metal is heated to a temperature between 300 and 400 C. after first cleaning it in hot nitric acid and cutting it to a size suitable for rolling. After the-initial heating in a suitable container, the heated billet or sheet is passed through rolls, the distance between the top roll and the bottom roll being capable of adjustment after each pass according to an arbitrary amount, such as 002", until the thickness desired is attained.

If the rolls are not heated to the temperature necessary for the rolling of the metal, the metal can be restored to the desired rolling temperature by reheating between passes. About 3 minutes are required to reheat a jacketed billet as jacketing' (as set forth in another portion of this specification) tends to preserve the rolling temperature. In rolling billet-s through heated rolls some cooling may be necessary following each pass since the temperature of the metal-may rise as the result ofworking in the rolls. However, in any event the Working of beryllium undergoing the deformation process should be effected as nearly as possiblewithin the 300 to 400 C. temperature range.

Rolling within this low temperature range of 300 to 400 C.'may also be combined with the steps of periodically annealing at a temperature above the recrystallization point. However it must be kept in mind that in using unjacketed beryllium, the annealing and subsequent cooling to the working temperature must be carried out in a non-oxidizing atmosphere, or, if oxide is formed during annealing, it must be removed prior to reworking at 300 to 400 C.

In the fabrication of beryllium at temperatures between 300 and 400 (3., the, use of a protective metal coating may beomitted. In one embodiment of this 'invention unjacketed beryllium is fabricated into sheets by heating to a temperature between 300 and 400 C. and then immediately passing it through rolls heated to about 400 C. and annealing periodically in a nonoxidizing atmosphere, such as hydrogen, at a temperature of about 800 C. or at some temperature above the ternperature of recrystallization. The metal is then cooled to at least below 600 C. in a non-oxidizing atmosphere or it may be water-quenched from the annealing temperature. Any subsequent working is carried out at 300 to 400 C. The subsequent annealing at high temperatures of metal previously worked at temperatures between 3004-00 C. is not absolutely necessary as better than 50 percent reduction in area can be attained solely by fabrication within the BOO-400 C. temperature range.

In processes using rolling between heated rolls at 400 C. followed by periodic annealing at 800 C. in the presence of hydrogen, the reduction per pass has usually been from 5 to percent, and up to 50 percent reduction in area can be attained between annealing periods Without adversely affecting hardness values. In addition to attaining greater reduction in area and strain-hardening by periodic annealing as set forth herein, the annealing subsequent to cold-working at 400 C. materially increases the ductility, ultimate strength, and elongation in the metal.

As shown in Example II, beryllium metal can be extruded Without the use of a protective jacket. However, for the same reasons set forth in the discussion of metal coverings in another section in this specification, a protective metal covering is more commonly used in the extrusion processes.

A typical extrusion operation, according to a preferred embodiment of my method, comprises encasing a billet of beryllium, having an improved grain refinement over the cast ingot, with a low-carbon steel jacket which is drawn to size and in which the wall thickness on all sides is suitably from about 0.1" to .02". The encased beryllium billet is then placed in a soaking furnace, e. g. for about one-half hour for billets measuring from 2 to 3" in length and .945" in outside diameter. The container and stem are also preheated to a temperature within the temperature range used for working according to the process of this invention, namely 300 to 400 C. The die is suitably lubricated and fitted With a copper nib machined to fit the conical portion of the die. The billet is then inserted and extruded under a pressure preferably of about 50 to 100 tons per square inch. Such a pressure is suitable where extrusion is made through a conical die having about a 120 included angle and a diameter of at least .335". A nominal reduction in area of about 8:1 is achieved by extrusion within this temperature range under these conditions of pressure and type of apparatus. However, it will be apparent to those skilled in the art that the combined factors of extrusion, pressure, rate of flow and deformation defined by the reduction in area all contribute to the fabrication of beryllium metal, and variations in these factors which affect the ultimate factor of the extrusion temperature are intended to be included within the scope of this invention.

It will be noted as shown in Example II that the above process for extrusion is applicable to the fabrication of tubes as well as rods. The only alteration necessary for the extrusion of rods is that a hole is drilled in the beryllium billet prior to jacketing. A sleeve consisting of a suitable protective metal such as copper is fitted into said hole in order to provide lubrication between the mandrel and the beryllium metal undergoing deformation.

In a further embodiment of this invention, a protective metal coating for the beryllium metal undergoing fabrication is used. In this embodiment, beryllium is enclosed within a metallic covering or jacket which serves as a buffer material between the deforming apparatus and the refractory beryllium metal being subjected to treatment. The use of a metallic jacket extends the life of the die, especially where numerous passes through the deforming apparatus are required. The protective metal jacket also serves as a lubricant and preserves the billet from galling and scoring during extrusion or edgecracking from rolling. In addition the metal jacket reduces the coefficient of friction developed between the die surface and the beryllium metal thereby affording a means for controlling the temperature of the billet and also preventing undue heat losses by direct exposure of the billet to cooling surfaces.

The jacketing material should be of such metal composition as to be ductile and formable when subjected, as .a composite with the beryllium metal, to the deformation processes. The enclosing metal should also not alloy with the beryllium at the fabrication temperature so that it is easily removable from the billet. However, at the temperatures used in the process of this invention, the tendency for beryllium to alloy with the jacketing material, even copper, is virtually negligible. By rolling at 300400 C., the stainless steel jackets do not require as frequent replacements as in rolling at temperatures of 700 C. or above.

Coating materials may be of either the ferrous or nonferrous type. Jacketing material which have been successfully used in the process of this invention are copper, low-carbon steels, stainless steel containing nickel and chromium, e. g. Monel and 18-8. For rolling purposes, jacketing of beryllium may be effected by slipping the beryllium into a formed steel tube having a wall thickness of about A3" or less and welding steel caps of about the same thickness to each ,end. A beryllium slab or sheet may also he slipped between a folded sheet of protective metal jacketing material for rolling foil or the beryllium billet or sheet may be spray-coated with the protective metal.

The following examples will illustrate the preferred embodiments of this invention.

EXAMPLE I Rolling of beryllium Beryllium stock measuring .004" x 2.5 x 2" which had been previously hot-rolled at 800 C. from one inch rod extruded at 1000 C. and had been reduced in crosssectional area by a ratio of 16:1 was first pickled in hot nitric acid. The clean beryllium sheet was then slipped between a folded sheet of 18-8 stainless steel of hi thickness and was heated in an electric furnace to 350 C. The diameter of the roll was 8.75" and the speed was 40 R. P. M. with a reheating time of three minutes following each pass, seven passes in all being necessary to reduce the beryllium from a thickness of .004 to .002", thereby forming beryllium foil which was free from holes and cracks and having a bright surface. The rolling data are as follows:

Gauge, Reduc- Beryllium Pass inches tion, Thickness,

inches inches 1 Distance from top of bottom roll to bottom of top roll. 2 Amount of reduction in gauge. Does not take into consideration spring-back in metal or spring in rolling mill.

The .002"-foil produced by the rolling treatment just outlined was enclosed in a sheet of stainless steel .065" thick and subjected to rolling under conditions identical with those used for the first seven passes just described.

Gauge, Reduc- Beryllium Pass inches 7 tion, Thickness,

i inches inches EXAMPLE II Extrusion of beryllium- Billets of beryllium previously extruded at 1000" C. with a 16 to 1 reduction to refine the grain were reextruded into rods and tubing at 350 C. in a l00-ton Verson press. Both jacketed and unjacketed billets were tested. Copper and low-carbon steel were used as jacketing. materials.

The dimensions of billets used in fabricating rods ranged from 2 to 3" in length and the core outside diameters were from .925" to 0.945. These billets were jacketed in copper or steel having a wall thickness of about .010. Rods were extruded through a 120 included angle conical die, the die being at least .355" in diameter. 7

For tube fabrication, holes measuring .025" (inside diameter) were drilled in billets 3" long and .930" wide and these billets were then fitted with a copper sleeve of- .016" thickness and an inside diameter of .215 to accommodate the mandrel, which measured .200" in. diameter with a taper of 0.002" per inch. Tubing was formed by using a .500"-diameter die.

The container and stem were preheated to 350 C. and the billets were heated to 350 C. and permitted to soak for one-half hour prior to extrusion. The container was; loaded for extrusion in the following order: The die was lubricated with heavy oil; a copper nib machined to fit the conical portion of the die was placed on the die; the

EXTRUSION OF RODS billet was "inserted and a graphite cut-oii'measuring approximately one inch was placed on top of the billet after'whichthe billet was extruded. In order to prevent fracture of rods subsequent to their emerging from the press, a substantial number of asbestos sheets was placed in the pit beneath the press to break the shock as the rod falls into the pit.

In general it may be said that the processes disclosed in the present application areillustrative rather than limiting in scope and that all of the numerous equivalents and modifications which would-naturally occur to those skilled in the art will be included in the scope of the present invention. Only such limitations as are indicated in the appended claims should be imposed on the scope of this invention.

What is claimed is:

1. A method of working beryllium metal comprising subjecting beryllium metal to plastic deformation at a temperature between 300 and 400 C.

2. The method of claim 1 wherein the plastic deformation is effected at a temperature of about 350 C.

3. A process of working beryllium metal comprising extruding beryllium metal at a temperature between 300 and 400 C.

4. A process of working beryllium metal comprising rolling said metal at a temperature between 300 and 400 C.

5. A process of fabricating beryllium metal comprising covering said beryllium with another metal thereby efiecting a jacketed billet, said jacketing metal being formable at temperatures between 300400 C. and non-alloying with beryllium at said temperature range, and subjecting said jacketed billet to plastic deformation at temperatures between 300 and 400 C.

6. The process of working beryllium metal comprising jacketing said beryllium with copper thereby forming a unitary billet and extruding said copper jacketed beryllium at a temperature between 300 and 400 C.

7. The process of Working beryllium metal comprising jacketing said beryllium with stainless steel thereby forming a unitary billet and extruding said jacketed beryllium at a temperature between 300 and 400 C.

8. A process for working beryllium metal comprising jacketing said metal with stainless steel and rolling said jacketed billet at a temperature between 300 and 400 C.

9. A process for working beryllium metal comprising subjecting beryllium metal to a process of plastic deformation at a temperature of 300400 C. and subsequently annealing periodically said deformed metal at a temperaturein excess of the recrystallization temperature.

10. The process of claim 9 wherein annealing takes place at a temperature of at least 700 C.

Billet Container Diameter Diameter Jacketing Material Temp, Temp, of Die, of Reduc- Pressure, Remarks Used 0. 0. inches Mandrel, tion lbs./sq.in.

inches Copper 350 350 .500 4:1 153, 000 Jacket was continuous and smooth. Only slight shallow seams visible on extruded rod.

Steel 350 350 .500 4:1 154,000 Steel jacket ruptured during extrusion but beryllium surface showed only shallow seams.

N Ja ket 350 350 .355 7. 9:1 191, 000 Leading third and trailing third "rattlesnaked" but middle third was sound. Die surface showed scores.

Steel 350 350 .355 7. 9:1 229, 000 Steel jacket ruptured during extrusion.

EXTRUSION OF TUBES Copper (20 mils) 350 350 .500 .200 4. 6:1 153, 000 Satisfactory-m0 scoring observed.

Copper (18 mils) 350 350 .500 .200 4.621 153,000 Tube surface satisfactory-no scoringtube extruded easily.

Copper (15 mils) 350 350 .500 .200 4. 6:1 153,000 Tube extruded easily, but outer surface was slightly scored in one area.

11. A process for working beryllium metal comprising subjecting an ingot of beryllium to heat treatment at a temperature above the recrystallization temperature, cooling said recrystallized metal and reheating said recrystallized metal to a temperature between 300 and 400 C. and subjecting said metal to plastic deformation at a temperature within,300 and 400 C.

References Cited in the file of this patent UNITED STATES PATENTS 1,547,395 Hoyt July 28, 1925 3 1,597,189 Gero Aug. 24, 1926 2,384,351 Slagle Sept. 4, 1945 FOREIGN PATENTS 5 550,110- Great Britain Dec. 23, 1942 OTHER REFERENCES Raynor: Journal of the Royal Aeronautical Society, vol. 10 50, pp. 390-415 (1946).