US3700419A - Smooth high tolerance porous tube and process for making - Google Patents

Abstract

SEVERAL BENEFICIAL PROPERTIES ARE ACHIEVED BY CENTRIPETALLY MECHANICALLY WORKING AS BY ROTARY SWAGING OF POROUS METAL TUBING PARTICULARLY PREPARED FROM SINTERED POWDERED METALS. THESE INCLDE EXCELLENT DIMENSIONAL TOLERANCE, EXCELLENT SURFACE FINISH, CONTROLLED DENSIFICATION DEPENDING ON THE AMOUNT OF REDUCTION, INCREASED STRENGHT AND YET THE OVERALL POROSITY OF THE TUBE IS RETAINED TO A VERY CONSIDERABLE EXTENT. THE INTERNAL SURFACE RETAINS ITS INITIAL ROUGH STRUCTURE.

Description

United States Patent 3,700,419 SMOOTH HIGH TOLERANCE POROUS TUBE AND PROCESS FOR MAKING Frederick J. Sorgenfrei, Lake Elmo, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn.

No Drawing. Original application Aug. 28, 1969, Ser. No. 853,972. Divided and this application Jan. 25, 1971, Ser. No. 109,696

Int. Cl. B22f 1/00 U.S. Cl. 29182.2 3 Claims ABSTRACT OF THE DISCLOSURE Several beneficial properties are achieved by centripetally mechanically working as by rotary swaging of porous metal tubing particularly prepared from sintered powdered metals. These include excellent dimensional tolerance, excellent surface finish, controlled densification depending on the amount of reduction, increased strength and yet the overall porosity of the tube is retained to a very considerable extent. The internal surface retains its initial rough structure.

This is a division of application Ser. No. 853,972, filed Aug. 28, 1969, now Pat. No. 3,626,744.

This invention relates to porous metallic tubes and particularly to porous metallic tubes having highly finished metallic outer surfaces, and sintered internal surfaces.

It is generally not possible to produce smooth accurately dimensioned uniform porosity tubing by normal powder metallurgy techniques. Short lengths of smooth tubing can be produced by die pressing and sintering. It is not possible, however, to produce long lengths of tube by die pressing and other powder metal techniques must be used. The sintered surface of materials made by these processes are usually very rough and it is diflicult to hold machine tolerances. There is also difliculty in that they tend to smear and thereby lose surface porosity when machined as a result of the tangential application of forces. This problem becomes increasingly more diflicult as the porosity (micronic rating) of the tube decrease. Several techniques have been proposed to facilitate the machining of porous materials without smearing the surface. All of these involve filling of the pores with a substance that can be removed after machining. For example, certain salts may be used to impregnate porous materials and, after machining or grinding is performed in the impregnated condition, the impregnant is removed. These are cumbersome high cost and multiple step processes and are very diificult to control, especially for long tubes. The present invention has as an object to finish and dimension the outer surface of porous sintered powdered metal tubing while maintaining uniform porosity to a substantial proportion of that of the unfinished tubing. The process of the invention provides a very inexpensive simple method of achieving these desirable results. The inner surface remains grainy or rough and the outer is dimensioned and finished to an extent determined by the extent to which the outer diameter is reduced.

The prior art shows that porous metallic tubes can be made and it has been suggested by Mott, U.S. Pat. No. 3,313,621, that by insertion of a suitably shaped mandrel the tube can be hammered or swaged on the mandrel to shape the inside and outside. This process tends to reduce porosity quite sharply because both the outer surface and inner surface are subject to deformation. The mandrel in effect exerts a centrifugal force in opposition to the hammering.

It has now been found that centripetally mechanically working the outer surface simultaneously in two to three ice or more longitudinal zones simultaneously, as illustrated particularly by rotary swaging, is advantageously performed on porous sintered metallic tubing formed from metal powders until the diameter has been reduced to a desired extent. No mandrel is used. Unexpectedly the porosity is not greatly reduced when the outer or inner diameter is reduced by up to about 20-50%. This may be because there is no outward or centrifugal deformation of the inner surface. The increase in length usually associated with swaging operations is minimized by the process of the invention. Furthermore reduction in wall thickness is much lower than when a mandrel is employed.

The tubes used in this invention are preferably made from sintered powdered metals using conventional powder metallurgy techniques. Centripetal mechanical forming is then used on the outer surfaces at 2 to 4 positions or zones without applying opposite or centrifugal force directly to the inner surfaces. Thus swaging using rotary swaging procedures applies centripetal force and avoidance of a mandrel omits a centrifugal force. An excellent description of this metallurgical process (without respect to an internal mandrel) is found in Review of the Powder Metallurgy Process, July 1966, published by the U.S. Army Production Equipment Agency, Manufacturing Technology Division, Rock Island Arsenal, Ill. Also see Mott, U.S. Pats. Nos. 2,792,302 and 3,313,621. Rotary swaging is described in Metals Handbook, T. Lyman, editor 8th ed. (1969), volume 4, pp. 333 et seq.

The preferred powdered metals used in this invention are alloys such as austenitic chromium-nickel stainless steel. These alloys generally containing 16.0 to 26.0 weight percent chromium, 6.0 to 22.0 weight percent nickel, 0.03 to 0.25 weight percent carbon, and occasionally some other elements are added to develop certain specific properties, such as 1.75 to 4.00 weight percent molybdenum or small amounts of titanium, tantalum, and niobium to minimize formation of chromium carbides, especially in welding. Standard types of these steels have been assigned numbers and specifications by the American Iron and Steel Institute. These are generally known in the art as stainless steels of the AISI series, types 301, 302, 304 and 305 generally referred to as 18-8 stainless steel, and the workhorse type 316 generally referred to as 18-8 Mo. All of these AISI stainless steels of the 300 series are applicable in the practice of this invention. Of course, other ductile or malleable powdered metals can be used in fabricating the tubes used in this invention, such as nickel, iron, cobalt, copper, and the like, and alloys of such metals, including bronze, monel, etc.

Filters are made from powdered metal which may vary widely in coarseness from as low as 20 or 35 microns up to about 1 mm. selected so that, upon sintering the resulting shaped article, the desired permeability, porosity or micronic rating is obtained. For purposes of making filter elements, it is preferred to use mesh sizes in the range of 20 +325 (40-800 microns), such as -200 +325 (40-72 microns), +200 (92 to microns)- --50 +100 (150 to 300 microns), 20 +50 (300 to 800 microns) or blends thereof, suitably selected to produce the desired micronic rating or bubble point, and to that end small amounts, e.g., 1-20 weight percent, or +325 mesh 40 microns) or even -400 mesh 30 microns) powdered metals are blended with the coarse powder, i.e., with the 50 +325 mesh (40-300 microns). The term rnes referred to herein means mesh size according to U.S. Standard Sieve. Approximate closest sizes in microns are indicated parenthetically. The use of powdered metal with these mesh ranges will enable one to make tube structures which can be swaged in accordance with this invention with various micronic ratings, e.g., maximum beads passed in the range of 1 to 150 microns.

In fabricating each of the filter component layers, the powdered metal of desired mesh is blended with an organic heat-fugitive binder, such as those disclosed in US. Pats. Nos. 2,593,943 of Wainer; 2,709,651 of Gurnick et al.; and 2,902,363 of Joyner; the preferred binder is methyl cellulose with which the lubricants used by Mott in US. Pat. No. 2,792,302 are unnecessary. Various solvents can be used in conjunction with these binders, such as water, as Well as various plasticizers, such as glycerin. The blending can be carried out in a conventional manner in various types of commercially available mixers, blenders, tumblers, and the like, care being taken to insure that the blend is homogeneous and the components well dispersed. The resulting blend will be in the nature of a plastic mass or dough and will be similar in consistency to that of modeling clay. It is extruded by conventional methods.

Sintering atmosphere, temperature, and duration of sintering depends upon the particular powdered metals used and the selection of these conditions is within the skill of the art. In the case of the austenitic stainless steels mentioned above, a hydrogen or dissociated ammonia atmosphere with a dew point of -40 F. or lower and sintering temperatures in the range of 1200 to 1400 C., preferably 1250 to 1350 C., is suitable, and the duration of sintering is usually from 10 minutes to 2 or 3 hours.

As is evident from the above, the porous tube is made entirely from powdered metals without requiring or emloying wrought metal components or welding. Swaging is carried out on a rotary swaging machine of conventional type for example, the 2 die type illustrated in FIG. 4, page 334 or a 4 die type illustrated in FIG. 7 page 335 of the above Metals Handbook Article. In general the swaging operation is used as a finishing operation to provide close outside diameters as well as for the usual purpose of decreasing sizes. The surprising feature is that in this operation it is found that no internal mandrel is desirable and wall thickness is not greatly affected. Also with moderate amounts of swaging or percentage decrease in diameter, porosity is decreased to much less extent than when a mandrel is used and there appears to be no tendency for partial plugging of pores so that additional etching steps are not needed.

The desired surface finish and porosity are produced by suitable combinations of mesh size of the starting material, green forming and sintering parameters and the amount of reduction during swaging. The formation of the initial tubes is not part of this invention and tubes having calibrated porosities (bubble points) are obtained directly. An example of formation is included solely for convenience to readers hereof.

The final size and shape of the tube is determined by the size of the swaging die. Various shapes are illustrated in FIG. 8 of the Metals Handbook Article so that tapers, contours or points may be introduced if desired. Single or multiple reductions can be made with or without an intermediate annealing step if desired. All these will be within the skill of the art from the present disclosure.

The articles produced by the process of the invention, i.e., high surface quality, close tolerance, porous tubes have many applications, for example, as frictionless air turns, as film de-curling bars, as web or film coaters, as air clamps, etc. The tubes can be used for air bearings, e.g., for handling yarns or textiles or for applying lubricant to yarn after spinning. The lubricant can be forced through the porous tube and applied to the yarn. The smooth surface of the porous tube avoids damage to the yarn. Other applications are in places where low friction tube or rod sliding is involved, filters having fine micronic ratings microns absolute), flow controllers, flow restrictors and diffusers.

Although the practice of this invention is described with respect to stainless steel, it is applicable to any porous malleable material such as the copper based alloys, especially brass and bronze. Other applicable metals are nickel and nickel alloys, especially superalloys as cupronickels and monels, cobalt and alloys thereof, other iron alloys including low alloy steels, precipitation hardening stainless steels and ferrous superalloys. Ductile reactive metals and alloys can also be used such as titanium, zirconium, niobium, tantalum and their alloys. Swaging of Group VI metals is difiicult and must usually be done above room temperature. Aluminum is difficult to sinter, especially into tubular configurations but products made of it and its alloys can also be improved by this invention.

Porosity of tubing such as here described may be measured by ASTM Test E128-61 or it may be estimated as to the largest pores by the Bubble Point test described in the report of Micro Metallic Corp., Development of Filters for 400 F. and 600 F. Aircraft Hydraulic Systems, WADC TR 56-249. Pressure drop across the porous surface measured in suitable units at various rates of flow is subject to the difficulty that the capacity of a long porous tube may not be reached at feasible flow rates.

Tubes may be open at both ends if desired or closed at one end. The following shows how such a tube may be made to be swaged in the process of the invention. Preferably porosities will be in the range of from about 1 to microns with pressure drops less than 50 cm. of mercury.

A clay-like mass is produced by first dry-blending 3.0 kg. of 316 L stainless steel powder of 100 to 200 mesh (92 to 150 microns) size and 150 grams of methyl cellulose and then blending with 600 cc. of 10% by volume glycerine in water for about 1 hour in suitable apparatus such as a Braeblender Sigmablade mixer. The clay-like mixture is extruded by conventional techniques using standard dies for the purpose. A suitable apparatus is a ton Loomis extrusion press. Pieces are extruded up to about 1.2 meters (4 feet) in length having outside diameters of about 0.51 inch (13 mm.) and internal diameters of about 0.31 inch (7.7 mm.). The extruded pieces are air-dried for 12-15 hours and prefired at 2150 F. (1170 C.) for two hours in dissociated ammonia. A second quantity of the clay-like mass is extruded through a .327 inch (8.3 mm.) die as a rod. The rod is dried overnight and prefired at 2150 F. (1170 C.) for two hours. The rod or plug is isopressed at 35,000 psi. and is then inserted into one end of the tube. The assembled structure with suitable steel mandrel as a filler to prevent collapse is similarly isopressed, the mandrel is withdrawn and the tube is then sintered for two hours at 2460" F. (1350 C.) in a dissociated ammonia atmosphere. Variations in sizes of particles together with conditions of pressing and firing give tubes having various porosities.

Tubes of stainless steel having various porosities and lengths are reduced from about 0.410 inch (10.8 mm.) outer diameter to 0.376 inch (9.53 mm.) outer diameter using a rotary swaging machine to provide centripetal mechanical working and inch (-9.5 mm.) long dies with a partial taper. Similar results are attained using rotary swages with four dies affecting different longitudinal zones. The finished tubes are characterized by micronic rating and bubble point determined as described in the above-mentioned ASTM procedure and WADC report. Accurate measurements of internal and external diameters, are made and air is forced through the tubes at various rates and pressure drops are measured in centimeters of water or mercury depending on relative areas of flow involved. The data are summarized in Table 1. In tubes of the lengths and porosities of Examples III and VI closed at one end it is found that about 98% of the flow occurs in the proximal 12 inches and about 84% in the proximal 6 inches. The tubes of all examples with the possible exception of Example I may therefore be considered as being of approximately 12 inch effective length in the pressure drop tests. The relatively rough internal surfaces are characteristics of tubes prepared by this process. A deburring operation may be employed when ends have been cut.

TABLE I Micronic rating from Pressure drop (cm. H Percent bubble point Inner Outer 811' flow" of theo- (p) (B-before) diameter diameter retical Ex. (A-after) (inch) (inch) 40 81 121 162 density B 4.8 l .210 421 '3. 8 '8. 12. 7 '16. 6 74 A 3.0 .205 376 21.6 "37.5 '43 '46 80 B 7.6 5 .237 402 3.7 9.6 12 7 16.7 63

6.5 .217 377 6.7 14.5 7 '27. 1 19.0 20. 8 60 15.0 I 41. 4 70 38.0 4 22. 7 55 36.0 30. 0 58 48.0 4 20. 0 53 38.0 6 2 61. 5 B 19.0 VI {A 14.0 47. 7

1 Tube increases 78.0 cm. (before) to 81.6 cm. after rotary swaging.

1 About 12 inches long initially.

3 34.5 inches long at end of swaging operation.

4 40 inches long initially.

5 43 inches long initially.

In cm. of Hg. where marked: otherwise in cm. of 11 0.

"Air flow cubic feet per hour per tube; effective length about 12 inches.

EXAMPLE VII A filter tube was fabricated from 325 mesh 40 micron) fraction of 316 L stainless steel as described above and about 150 cm. long. The bubble point of this tube was 10 cm. of Hg. After swaging,'the bubble point of the tube was 16 cm. Hg. A profilometer was used to determine the surface roughness. Ignoring the holes or pores, the resulting surface was about 4 micro inches /2 thereby eliminating the need for a subsequent step to open pores. Because bubble point is a measure of the largest or maximum pores an increase means reduction in size of the largest pores but not necessarily proportional dimensional changes in all pores. The porosity is lower at higher bubble points and doubling thus corresponds approximately to halving overall porosity.

The finer porosity tube when swaged on a mandrel loses nearly all of;its surface porosity (greater than RMS, yet the porosity was open and uniform.

EXAMPLE VIII A filter tube about 150 cm. long with a bubble point of 12.5 cm. H O was swaged and the result was a tube with a bubble point of 15.8 cm. H O. The ultimate tensile strength went from an average of 13,200 p.s.i. before swaging to 20,500 p.s.i. after swaging. The density changed from 53% to 61.5% of theoretical, yet porosity remained open and uniform.

EXAMPLE IX The diameter of a piece of open-ended porous tubing 52 inches long with a bubble point of 16 cm. Hg was measured at 2 inch intervals along the tube. The total deviation from nominal along the tube length was .0005 inch. A single point measured .0005 inch less than the rest of the tube. Porosity remained open and uniform. The diameter after swaging was .37475 $00025 inch.

EXAMPLE X This example was performed in part according to US. Pat. No. 3,313,621 with swaging on a mandrel. Porous stainless tubing in about 12 inch lengths of borth coarse and fine micronic ratings were swaged and without mandrels. Table II summarizes the results.

cm. Hg pressure) and must be etched to be reopened. Those pores that still remain open are erratic as to their position. They are too few to permit of any reasonable gas flow.

EXAMPLE )6 Tubes with coarse and fine micronic ratings as in Example X are reduced in diameter in a different embodiment of the process of the invention by rotating a section of the tube in a lathe so that 25 mm. (1 inch) diameter 0.7 cm. (0.25 inch) wide steel idler rollers are forced against it simultaneously and with about equal force thereby centripetally applying mechanical force in three longitudinal zones. The. rolls are mounted on the tool post which is mechanically traversed so that the rolls are moved slowly axially along the rotating tube. Reduction in porosity is effected with no visible smearing of the surface due to tangential forces.

What is claimed is:

1. A porous sintered metallic tube having a rough sintered inner surface and a mechanically worked dimensioned outer surface.

TABLE II Outer Inner Percent Percent Bubble Pressure diameter diameter wall retheoretical point drop 1 (inch) (inch) duction density (cm. H O) (cm. H 0) Coarse material:

As sintered 435 294 53 16 4 Swaged without a mandrel. 377 238 1. 4 64 21. 5 8 7 Swaged on a mandrel 382 284 31. 0 70 38 42. 7 Fine material:

As sintered 454 278 70 1 5 54 Swaged without a mandrel 376 182 +11 80 2 10 623 Swaged on a mandrel 385 250 24 93 2 50 (3 1 gressfire drop measured at an air flow of cubic feet per hour. I m. g. I Substantially impermeable to gas flow.

It is evident that swaging on a mandrel results in 25- 50% wall reduction and drastically reduces the air flow through the tube. Swaging without the use of a mandrel has the effect of only decreasing the air flow to some extent while still sizing the outside diameter and retaining an open porosity as measured by bubble point of about 2. A porous tube according to claim 1 composed of stainless steel.

3. A porous tube according to claim 2 having one end closed.

(References on following page) 8 References Cited 697,291 9/1953 Great Britain 75222 707,512 4/1954 Great Britain 75 222 UNITED STATES PATENTS 714,560 9/1954 Great Britain 75-222 213;; gfi 23 717,034 10/1954 Great Brita n 75-222 66 Holtsclaw 2 X 5 832,317 4/1960 Great Bntam 75222 g? BENJAMIN R. PADGETT, Primary Examiner 1 t B2326 R. E. ASSlStaDt EXammer FOREIGN PATENTS 10 U.S. Cl. X.R. 7/1956 Canada 7s 222 203, 222