US3447230A - Art of making seamless hollow bodies from sinterable powders - Google Patents

Description

June 3, 1969 BARGMNMER ET AL 3,447,230

ART OF MAKING SEAMLESS HOLLOW BODIES FROM SINTERABLE' POWDERS Filed Jan. 5, 1967 INVENTORS. ROGER B. BARGAIN/WEI? WILL/AM SCHE/THAUER, JR.

TTORNEY GLENN A SHA FFER United States Patent 3,447,230 ART OF MAKING SEAMLESS HOLLOW BODIES FROM SINTERABLE POWDERS Roger B. Bargainnier, William Scheithauer, Jr., and Glenn A. Shaffer, Towanda, Pa., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Jan. 5, 1967, Ser. No. 607,510 Int. Cl. B221? 3/ 24; B21c 1/24 U.S. Cl. 29--420.5

ABSTRACT OF THE DISCLOSURE A method of producing a tube shell by pressing, ma-

18 Claims chining and sintering a metal powder is disclosed. This tube shell is then further fabricated by a drawing operation at an elevated temperature.

This invention relates to the art of making seamless hol- 0 low bodies including wrought seamless tubing from sinterable powders and, more particularly, from sinterable metal powders including refractory metal powders, e.g., tung sten, molybdenum, tantalum, niobium, and alloys of such refractory metals with other elemental metals. It is particularly concerned with the production of the thin-walled seamless tubing and wherein the thickness of the wall is, for example, less than 10% of the diameter of the tubing. The scope of the invention includes both article and method features.

Broadly described, the method comprises the steps of pressing, machining, and sintering a sinterable metal powder into a high density hollow body such as a tube shell, and then reducing its cross-section to the desired size by drawing at an elevated temperature. Drawing also swaging does not economically result in uniformly dimensioned articles.

The method of instant invention is a solution to the problem of ecomically manufacturing seamless tubing having uniform dimensions from tungsten, molybdenum and other refractory metals and alloys. For the first time to the best of our knowledge and belief, our invention makes it possible for refractory metals (including refractory elemental metals and refractory alloys thereof) to be directly drawn from the sintered state.

The following United States patents are typical of the prior art: 1,226,470, Coolidge; 2,373,405, Lowit; 2,374,- 747, Hardy; 3,112,165, Davies; and 3,127,641, Pertwee.

Coolidge describes the manufacture of tubular bodies of refractory metal by first filling a tubular mold with the powdered metal, compressing the mass of powder by a uniform pressure in a radial direction applied to the inner wall of the mold, and heating the thusly compacted mass to partially sinter it before detaching it from the outer wall of the mold. The sintering operation is completed by passing an electric current through the detached tube placed in a hydrogen atmosphere.

Lowit discloses a process of making seamless tubing wherein a mold is first half-filled with a metal powder. A forming rod, specifically a glass rod, is centered on the metal powder in the mold after which the mold is entirely filled with the metal powder over the glass rod.

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After compacting the metal powder about the glass rod, the compacted powder is sintered, the glass core melted out, and a metal core is inserted in its place. The resulting assembly is then swaged down until the tubing has been reduced to the desired size after which the metal core is mechanically or otherwise removed. Instead of swaging, the aforementioned assembly may be rolled or forged. The final step may be a drawing step.

The Hardy patent is concerned with the formation of metal linings in tubes and, more particularly, with a technique wherein a plastic core is employed in producing annular linings from metal powder in tubular bearings.

Davies uses a mold having a tubular inner core of a particular construction in producing hollow bodies such as tubing from sinterable, powdered materials. The mold is filled with loose powder about the tubular, yieldable, supporting, inner core, e.g., a sleeve of wire gauze having a diamond weave. Thereafter, the powder is heated to sinter it to a dense non-porous body while providing continuous, yielding, internal support from the aforesaid core during the sintering process.

The Pertwee patent is concerned with a method of manufacturing seamless tungsten tubing by vapor deposition.

The novel features that are characteristic of our invention are set forth in the appended claims. The invention itself, however, will best de understood from reference to the following specification when considered in connection with the accompanying drawing wherein:

FIGURES 1 through 6 are somewhat schematic views, partly in section (and in FIGURES 5 and 6 also partly broken away), showing various stages in the manufacture of a hollow body by the method of the invention; and

FIGURE 7 is a similar view illustrating a modification of the fabrication technique.

The invention will be described with reference, as illustrated in the drawing, to seamless tubes having a circular cross-section. It will be understood, of course, by those skilled in the art that the invention obviously is not limited to the manufacture of only such hollow bodies, specifically tubes of circular cross-section, and that the method is also applicable to the production of tubes or other hollow bodies of elliptical or other noncircular cross-section.

Referring, now, to the drawing.

FIGURE 1 illustrates a mold or mold assembly 10 with the annular void in this assembly filled with a metal powder 12. The mold comprises a smooth-surfaced, rigid, forming core or rod 14 and a thin, pliable, outer shell 16. The core extends through the mold ends 18 and 20 which are formed of rather thick, pliable rings of, for example, rubber or any of the various synthetic rubbers. The'pliable outer shell 16 may be formed of flexible, plastic material such as commercially available pliable, organic, polymer, e.g., polypropylene, plasticized polyvinyl chloride and others known in the art. The thickness of the shell 16 may range, for instance, from about A; to about inch.

The forming core 14 is made of a material, e.g., a metal having sufficient rigidity that it does not deflect during pressing. Adequate rigidity is an especially important consideration when making long, rather smalldiameter tube shells. Molybdenum is a more specific example of a metal that meets this requirement and is preferred for use. Examples of other metals that may be employed under particular circumstances, e.g., when lower molding pressures are used or relatively short-length tube shells of relatively large diameter are to be made, are cold-rolled steel, stainless steel, tungsten or other substantially non-compressible and rigid metal.

When making tube shells of short length it is unnecessary for the core 14 to be tapered. However, in fabricating longer tubes, then cores that are tapered are normally employed as is shown in, for instance, FIG- URES l, 2, and 3. The taper (i.e., difference in crosssection between the two ends of the core) is usually not more than about 0.01 inch per inch. We have found that, for the best results, the surface finish should be 50 noot means square (RMS) or less.

The metal powder employed in practicing this invention may be of any of the particle sizes and ranges of particle sizes commonly employed in powder-metallurgy practice. Preferred ranges are an average of from 1 to microns for Mo and W; and, more particularly, an average of from 3.5 to 4.5 microns for Mo and from 3.6 to 4.4 microns for W.

PRESSING The filled mold is sealed and placed in the compression chamber of an isostatic press. Upon the application of pressure, the pliable outer shell 16 is radially depressed, resulting in compaction of the metal powder 12 about the core 14. The pressure may be varied widely depending, for instance, upon the particular powder that is being compressed and the desired degree of density that is desired in the compressed powder. For example, the applied pressure may range from about 10,000 to about 100,000 p.s.i. and, more particularly, from about 20,000 to about 50,000 p.s.i.

MACHINING After the pressure has been released and the mold has been removed from the compression chamber, the pliable outer shell 16 is stripped from the powder compact 22, which is shown in FIGURE 2 as being still on the core 14. This compact is then subjected to conventional machining operations to provide a machined compact 24 (FIGURE 3) having a uniformly thick wall and a smooth outer surface.

Because the core is tapered (except when short-length tube shells and/or of wider cross-section are being fabricated) and has a smooth surface, it can be manually removed from the compact rather easily as is indicated by the downwardly-pointed arrow and the dotted lines in FIGURE 3.

SINTERING Next the compact is sintered to a density sufficient to impart adequate drawability to the sintered material during subsequent drawing operations and, more particularly, to at least 90% of the theoretical density. Such sintering is effected by, for example, induction or radiation heating. Taking tungsten and molybdenum as illustrative of the metals being sintered, the temperature and time conditions generally range, respectively, from 1800 C. to 2300 C. for from not more than about 48 hours to 2 hours.

The sintered article is a high-density, slightly tapered (in most cases) tube shell with a uniformly thick wall and with smooth inside and outside surfaces. A high degree of wall-thickness uniformity is a very necessary requirement of stock for tube drawing because it dictates to a large extent the wall-thickness uniformity of the final, drawn product. Smoothness is an important factor in reducing frictional forces during drawing. Depending upon the orientation of the tube shell during sintering and upon its wall thickness, the tube shell may be somewhat out-of-round. Neither this condition or the tapered condition are harmful in view of the fact that they are eliminated during the subsequent drawing step.

By the method of this invention sintered molybdenum and tungsten tube shells have been made with nominal dimensions ranging from (0.75 in. to 1.4 in. outside diameter) x (0.060 in. to 0.125 in. wall thickness) x (up to about 40 in, long). These are not necessarily limiting ranges,

4 DRAWING The tube shell 26 (FIGURE 4), which in most cases is tapered, is prepared for drawing by reducing the diameter of one end so that it can pass through at least the first drawing die and extend into the drawing grips 27 (FIG- URES 5 and 6). This operation is called pointing and, in practicing this invention, advantageously is done by swaging. The narrowed end or swaged point 28 is shown in FIGURE 4. The swaged point is fitted with a solid core (e.g., a solid steel core) 30 to prevent it from collapsing under the pressure imparted by the grips during drawing.

If desired, a solid point 32 (FIGURE 7) can be made an integral part of the tube shell by powder molding during isostatic pressing in a compression chamber. Such an integral solid point is swaged after sintering. In other words the hollow body is prepared for drawing by first filling the mold with sinterable metal powder beyond one end of the core so that, after compressing the mass of powder (as previously described), sintering (likewise as previously described) and swaging, a narrowed end or point is formed that is an integral part of the hollow body that results after the core has been removed. FIGURE 7 illustrates the integral structure, consisting of point and main body portion, before machining and removal of the core.

Tube drawing can be done on a conventional or hydraulic drawbench. Facilities must be available for heating the tube just prior to drawing. Any method of heating is suitable provided that it results in a uniformly hot tube at the point of entry into the die. We prefer to use electric batch heating in a furnace located directly in front of the die stand.

We have obtained good results using the dies made from ZrO a lubricant consisting of graphite suspended in a silicone oil, and a drawing speed within the range of from about 20 to about 60 feet, specifically about 44 feet, per minute.

The first draw can be a sink pass (see FIGURE 5). This means that the outside diameter is reduced by drawing through a die 34, immediately after the tube shell has been substantially uniformly heated by suitable means, e.g., an electric-heating furnace 36, and without providing any internal support to the tube wall as the tube is drawn through the die. The purpose of this drawing operation is to eliminate the taper (if the shell is tapered as it is in most cases) and any out-of-roundess (i.e., any undesired abnormalities in circumference) without significantly changing the wall thickness. Subsequent sink passes may be made as desired or as may be required by the particular circumstances.

To reduce the wall thickness, to increase the density, to improve the mechanical properties and to develop a cold-worked structure, the provisions must be made for reducing the wall thickness by providing internal support to the tube wall. In accordance with an embodiment of the present invention, this is accomplished by the use of a fixed-straight plug, i.e., a plug of the kind illustrated at 40 in FIGURE 6.

As shown in FIGURE 6, tooling for carrying out the operation described in the preceding paragraph consists of a die 38, the aforesaid plug 40, and a plug rod 42. The die 38 (advantageously made of ZrO must have an inside diameter (I.D.) that is less than the outside diameter (O.D.) of the entering tube and is the same as the desired CD. of the tube after drawing.

A typical plug consists of a solid cylinder the diameter of which is less than the diameter of the die and is the same as the desired ID. of the tube after drawing. One half of the difference between the diameter of the die and the diameter of the plug must be less than the wall thickness of the entering tube. This provides for reduction of the wall thickness between the die bearing and the plug periphery.

The front end of the plug has a radius or a chamfer.

Suitable means, e.g., a threaded joint, are made at the back end for attaching the plug to the plug rod. The minimum length of the plug should be not less than about 1.5 times the length of the die bearing. The plug should have a very smooth surface. Advantageously it is made of either elemental molybdenum or of a high-strength molybdenum alloy, e.g., a molybdenum alloy containing small amounts of titanium, zirconium and carbon. Elemental molybdenum and high-strength molybdenum alloys are preferred for use as plug-construction materials because plugs made from them have good rigidity and strength and because they are relatively easy to machine.

The plug rod has a diameter which is not larger, and may be equal to or smaller, than the ID. of the entering tube. Preferably the plug rod has a diameter that is smaller than the diameter of the plug. The plug rod must be long enough to reach from the die stand to the mandrel end of the drawbench. Provisions are made at one end of the plug rod for attachment to the mandrel end of the drawbench and at the other end for attachment to the plug. The function of the plug rod is to provide back tension to the plug during drawing so that it will not be pulled through the die. The plug rod can be made of any material that has at least moderate strength at the temperatures to which it will be exposed during drawing. Good results have been obtained'by'using a low-carbon steel rod coated with a high-temperature aluminum paint. Other contemplated materials include stainless steel, and molybdenum and molybdenum alloys.

Referring now more particularly to FIGURE 6, the following is a typical sequence of operations involved in plug drawing by the method of our invention; and, more particularly, when a batch furnace positioned in front of the die stand is employed.

First, point the tube. Lubricate the plug and the outside and inside diameters of the pointed tube. Heat the tube to the drawing temperature. 'Insert the plug, which is attached to the plug rod, to the shoulder 46 of the tube point. Advance the tube point through the die 38 until it can be engaged by the drawing grips 27. Commerce drawing. Quickly advance the plug until its front end is at least just through the die bearing. Preferably, the plug should be centered relative to the die bearing. Then restrain the plug from significant additional advance by applying back tension to the plug rod. Advantageously this is done by anchoring the plug rod to the mandrel end of the drawn bench. Under no circumstance should the back end of the plug be allowed to advance into the die bearmg.

We have found that it is desirable to stress-relieve the tube after each draw pass by heating it for a short time to a temperature which is at least as high as the drawing temperature. This sequence is repeated with progressively smaller tooling until the desired tube size (outside diameter and wall thickness) has been achieved. The drawing temperature of the tube can be lowered with successive draws. We have obtained good results by using initial drawing temperatures of about 900 C. for molybdenum and about 1200 C. for tungsten. A typical sequence of temperatures and reductions used for molybdenum is shown in Table I that follows.

TABLE I.TYPICAL PLUG DRAWING SCHEDULE FOR MOLYBDENUM TUBES 1 W'I=Wall Thickness. 3 OD= Outside Diameter.

In order that those skilled in the art may better understand how the present invention can be carried into effect the following examples are given by way of illustration and not by way of limitation.

Example 1 This example illustrates the fabrication of a molybdenum tube in accordance with the present invention.

The mold employed is made of plastisol, and has the following dimensions: 0.250 in. wall thickness (WT) x 2 in. outside diameter (OD) X 30 in. long. The forming core is made from a molybdenum rod having a center diameter of 1.125 in., a taper of 0.001 in./in., and a surface finish of 35 RMS. The pliable end rings, which are made of rubber, have the following dimensions: 1.12 in. inside diameter (ID) x 1.75 in. OD x 1 in. thick.

To press the tube, the forming core is centered in the pliable mold, and the bottom end ring is put in place and sealed with conventional elastic tape. The mold assembly is filled with molybdenum powder, which has a purity of 99.95%, an average particle size of 4 microns, and a bulk density of 1.4 g./cc. The assembly is vibrated and more powder is added until the mold is filled. The top end ring is then put in place and sealed. The whole assembly (see FIGURE 1) is placed in a rack and lowered into the compression chamber of an isostatic press. A pressure of 30,000 p.s.i. is applied to the assembly. This pressure is reached in a period of 4 minutes and held for /2 minute. The pressure is released and the assembly is removed from the compression chamber.

The pliable outer shell and end rings are stripped from the compacted powder and forming core whereby an assembly such as that shown in FIGURE 2 remains. The stripped assembly is placed in a conventional lathe and machined. The motion of the tooling is in the direction of positive taper on the forming core. After completion of the machining operation, the forming core is removed by sliding it out. It is slid out in the direction of the positive taper as indicated in FIGURE 3. In this specific example the pressed tube is machined so that its dimensions are 0.08 in. WT X 1.3 in. OD x 27 in. long, The tube is then sintered by radiation heating for 6 hours at 1800 C. in a hydrogen atmosphere. After sintering the tube is, typically, of the theoretical density and has the following dimensions: 0.07 in. WT x 1.1 in. OD x 25 in. long. This completes the processing steps for obtaining a sintered tube shell per se.

The next series of steps involve drawing. The tube shell is first pointed by swaging. The end of the tube is heated to 1200 C. and then, by using conventional swaging techniques, its diameter is reduced 0.05 in. per pass until a diameter of 0.85 in. is reached. The tube is reheated to 1200 C. between each reduction. The length of the tube point is from 5 to 6 inches. A molybdenum plug is then fitted into the pointed tube end as shown in FIGURE 4. This supplies radial support for the tube during subsequent draw passes.

The tube is now ready for the first draw pass, which is a sink pass. The tube is heated to a temperature of 900 C. in a furnace which is located directly in front of the die holder (see FIGURE 5 After the tube has been held at temperature for from 2 to 3 minutes it is pushed from the furnace, and the point is placed through the die and gripped by the drawbench gripper. The tube is drawn at a speed of 44 ft. per minute. Immediately after drawing the tube is stress-relieved by heating at 1000 C. for 3 minutes. The resultant tube is now 0.07 in. WT x 0.875 in. OD x 26 in. long.

The next series of draw passes involves drawing with a fixed-straight plug (see FIGURE 6). Since all passes are essentially the same with the exception of the change in tooling sizes, an outline description of only one pass will be given. (Data for subsequent passes are given in Table H.)

A. The tube is repointed at 1200 C. so that its diam- 7 eter is 0.025 in. less than the diameter of the die to be used for the pass.

B. The OD and ID of the tube are coated with a thin, uniform layer of lubricant. The lubricant consists of graphite suspended in a silicone oil.

C. The tube is then placed in the drawing furnace for preheating. The tube point is extended slightly from the front of the furnace so that its temperature is about 200 C. cooler than the tube body.

D. Next, the proper drawing plug is selected and attached to the plug rod. The die, preheated to 400 C., is placed in the die container. After the tube has been at the drawn temperature for about 2 minutes it is ready to draw.

E. The tube is pushed from the furnace and into the die by use of the plug and plug rod. During the initial push-up the plug rod is positioned so that the draw plug is from about 1 to 1 /2 inches from its working position. The tube is gripped and the draw is started. As soon as the draw starts, the plug is pushed up and fastened in its working position. A draw speed of 44 ft./min. is used.

F. Immediately after drawing the tube is stress-relieved for 5 minutes at 1000 C.

G. The above operations are repeated until the desired final tube size has been obtained.

Table II lists data pertinent to draw passes that have been made by starting with a molybdenum tube having the following dimensions: 0.07 in. WT x 1.1 in. OD x 24 in. long.

TABLE II.DATA PERTAINING TO THE DRAWING OF A MOLYBDENUM TUBE STARTING WITH A TUBE OF A PARTICULAR SIZE Tube Size, in. Point Plug Die Drawing Diameter Diameter Diameter Temp. WT OD Length (in.) (in.) (in) C.)

0. 07 1. 1 24 0.850 Sink Pass 0.875 000 0. 07 0. 875 30 0.775 0. 670 0.800 000 0. 065 O. 800 3G 0. 725 0. 634 0. 750 750 0. 058 0. 750 42. 8 0. 075 0. 504 0. 700 750 0.053 0.700 50. 5 0. 625 0. 556 0. 650 750 0. 047 O. 650 00. 7 0. 575 0. 516 0. 600 750 0. 042 0. 600 73. 0. 525 0. 476 0. 550 750 0. 037 0. 550 89. 4 0. 475 0. 436 0. 500 1 RT 0. 032 0. 500 114. 9 O. 425 0. 396 0. 450 RT 0. 027 0. 450 149. 0. 375 0. 354 0. 400 RT 0. 023 0. 400 196. 4 0. 325 0. 310 0. 350 RT 0.020 0.350 254. 7

1 RT= Room Temperature.

Example 2 Essentially the same procedure is followed in the fabrication of a tungsten tube as that described in Example 1 with reference to the fabrication of a molybdenum tube with the exceptions hereafter described.

Instead of filling the mold with molybdenum powder having the specified characteristics, the mold is filled with tungsten powder having a purity of 99.95%, an average particle size of 3.5 microns, and a bulk density of 4.0 g./cc. The pressure at which the tungsten powder is isostatically pressed is 35,000 p.s.i., and the sintering time is 36 hours at 1800 C. as compared with 6 hours at the same temperature that is employed in sintering the machined, pressed, molybdenum tube of Example 1. After sintering, the tungsten tube is typically 95% (94% in the case of molybdenum tube) of the theoretical density.

In pointing the tungsten tube shell by swaging, the end of the tube is first heated to 1350 C. and is reheated to this same temperature between each reduction. 111 Example 1 this temperature of heating-treating the molybdenum tube is 1200 C. In preparing the tube for making the first sink pass, the tungsten tube is heated to a temperature of 1200 C. as compared with the 900 C. temperature to which the molybdenum tube is heated. Also, the tungsten tube of this example is stressrelieved by heating at 1200 C. versus 1000 C. employed in similarly stress-relieving the drawn molybdenum tube in Example 1.

The draw passes of tungsten tubing that are directed to drawing with a fixed-straight plug involve the same manipulative procedures described in Example 1 with reference to similarly drawing molybdenum tubing. However, variations in reduction ratios and temperatures employed from those set forth in Table I and in Example 1 (including Table II) with reference to the drawing of molybdenum tubing are made as desired or as may be required by the different, known, inherent characteristics of tungsten as compared with molybdenum. For example, unlike the RT (room temperature) drawing of molybdenum tubing that is shown in Table II of Example 1, no RT drawing is done with a fixed-straight plug in this example of means for practicing the present invention.

It will be understood, of course, by those skilled in the art that the present invention is not limited to the use of tungsten and molybdenum powders employed in the specific examples as the sinterable metal powders that are employed in making seamless hollow bodies therefrom. Thus, in addition to tungsten, molybdenum, tantalum, niobium, and alloys of these metals with each other and with other elemental metals and/ or metalloids, one may use other sinterable materials in finely divided or powdered state as rhenium, osmium, iridium, ruthenium, boron, rhodium, zirconium and titanium. One may also employ powdered alloys of two or more of the aforementioned elemental substances with each other and/or with other metals and metalloids; as well as powdered carbides, nitrides, oxides and other compounds (especially refractory compounds) of one or more of the aformentioned elements alone or in combination with one or more of the elemental substances above set forth.

What is claimed as new is:

1. The method of making a seamless hollow body from sinterable metal powders which includes the steps of (A) filling a mold having a pliable outer wall with loose sinterable metal powder about a rigid forming core;

(B) compressing the mass of said powder against the forming core by means of uniform pressure in a radial direction applied to the pliable outer wall of said mold;

(C) removing the core with its layer of solid compressed powdered mass thereon from the mold;

(D) machining the compressed layer to form a structure having a smooth outer surface finish;

(E) removing the core from the compressed and machined structure leaving a seamless hollow body;

(F) sintering the said layer in situ to densify the compressed powdered mass;

(G) preparing the hollow body for drawing by a technique that includes reducing the diameter of one end so that it can pass through at least the first drawing die and extend into the drawing grips;

(H) removing from the hollow 'body any taper and any undesired abnormalities in circumference by one or more sink passes of the substantially uniformly heated body through a die; and

(I) reducing the wall thickness, thereby increasing its density and improving the properties of the hollow body, by drawing the substantially uniformly heated hollow body through a restricted opening while providing internal support to the Wall of the said hollow body as it passes through the said opening.

2. The method as in claim 1 wherein the hollow body is seamless tubing.

3. The method as in claim 2 wherein the seamless tubing has a substantially circular cross-section.

4. The method as in claim 1 wherein the sinterable metal powder is powdered refractory metal.

5. The method as in claim 4 wherein the powdered refractory metal comprises tungsten.

6. The method as in claim 4 wherein the powdered refractory metal comprises molybdenum.

7. The method as in claim 1 wherein the hollow body is seamless tubing and the sinterable metal powder consists essentially of tungsten powder.

8. The method as in claim 1 wherein the hollow body is seamless tubing and the sinterable metal powder consists essentially of molybdenum powder.

9. The method as in claim 1 wherein the rigid forming core is tapered.

10. The method as in claim 9 wherein the rigid forming core has a taper that is not more than about 0.01 inch and not less than about 0.001 inch.

11. The method as in claim 1 wherein in Step I internal support to the wall of the hollow body, as it passes through the restricted opening, is provided by a fixed-straight plug.

12. The method as in claim 1 wherein the layer of solid, compressed, powdered mass is sintered to a density sufiicient to impart drawability to the sintered material during subsequent drawing operations.

13. The method as in claim 12 wherein the solid, compressed, powdered mass is sintered to at least 90% of its theoretical density.

14. The method as in claim 1 wherein the hollow body is prepared for drawing by (a) swaging one end to reduce the diameter thereof so that it can pass through at least the first drawing die and extend into the drawing grips; and (b) fitting the narrowed end with a solid core adapted to prevent the said end from collapsing under the pressure imparted by the grips during drawing.

15. The method as in claim 1 wherein the hollow body is prepared for drawing by first filling the mold with sinterable metal powder beyond one end of the core so that, after carrying out Steps B through F, followed by swaging, a narrowed end is formed that is an integral part of the hollow body that results after the core has been removed.

16. The method as in claim 1 wherein:

(a) the hollow body is seamless tubing having a substantially circular cross-section;

(b) the restricted opening through which the substantially uniformly heated seamless tubing is drawn to reduce its wall thickness is a substantially circular die opening having a diameter that is less at least at its exit end than the outside diameter of the seamless tubing as it enters said opening and is the same as the desired outside diameter of the said tube after drawings;

(0) the internal support which is provided to the wall of the said seamless tubing as it passes through said die opening has a substantially circular periphery, and a diameter that is less than the diameter of the aforesaid circular die opening and is the same as the desired inside diameter of the said tube after drawing, and one-half of the dilTerence between the diameter of the said die opening and the diameter of the said internal support being less than the Wall thickness of the entering seamless tubing, whereby the wall thickness of the said seamless tubing is reduced as it passes through the said die opening; and

(d) the said internal support is maintained in position by applying back tension thereto sufiicient to elfect this result.

17. The method as in claim 16 wherein the drawn seamless tubing is stress-relieved after each draw pass by heating it for a short time to a temperature which is at least as high as the drawing temperature.

18. The method as in claim 17 wherein a plurality of draw passes are carried out with progressively smaller tooling and as many times as necessary to produce seamless tubing of the desired outside diameter and wall thickness; and the drawing temperature of the tube is lowered with successive draws.

References Cited UNITED STATES PATENTS 519,086 5 1894 Larson 72----287 2,983,660 5/1961 Loeb. 2,992,172 7/ 1961 Blainey et a1. 29-4205 3,124,875 3/1964 Takahashi et al. 3,300,848 1/ 1967 Leitten ct al. 29'420.5 3,344,508 10/1967 Stohr 29-4205 JOHN F. CAMPBELL, Primary Examiner.

PAUL M. COHEN, Assistant Examiner.

US. Cl. X.R.

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