EP0020536B1 - Annular casings and process for the production of tubes by powder metallurgy and process for the manufacturing of such casings - Google Patents

Abstract

A casing for isostatically pressed pieces, which are intended for the extrusion of metal articles, in particular stainless steel metal pipes. The outer and inner envelopes (302, 304) of the casing (301) are comprised of a thin metal sheet: the outer envelope (302) at least has, along the periphery thereof, resistance characteristic substantially uniform in the axial direction and it consists of, in particular, a spiral welded pipe and is provided, preferably, with a bulge directed outwardly, opposed to the narrowing. It is provided, at least at the front part of the casing an intermediary piece which is made of one or several parts; this piece is obtained from ductile material or from a material obtained by compression of a powder. It is also provided a method for producing such casings and pressed pieces and a method for the extrusion of pipes as well as pipes obtained by such method.

Description

The present invention relates to an annular capsule for compacts for the powder-metallurgical production of pipes, the capsule having an outer and inner jacket made of thin, ductile sheet metal, between which there is an interior with an annular cross section for receiving metal or alloy powders to be pressed isostatically, which can be closed tightly on its end faces by means of an annular cover. The present invention further relates to a method for producing such capsules and a method for the powder-metallurgical production of tubes.

From DE-AS 24 19 014 (German Auslegeschrift No. 24 19 014) a method for manufacturing pipes made of stainless steel is already known, which have a uniform structure and physical and chemical properties and good processability, powder of such steel filled into metallic capsules and the sealed capsules are compressed by means of pressure acting on all sides, and the compact thus obtained is extruded into tubes, and thin-walled capsules made of a ductile metal are used by atomizing melt in inert gas, produced steel powder from predominantly spherical particles , whose wall thickness is a maximum of about 5% of the outer diameter of the capsule, the density of the steel powder filled into the capsule is increased to about 60 to 70% of the theoretical density by vibration and / or ultrasound, and the density of the steel powder by cold isostatic pressing of the capsule by means of a Pressure is increased from at least 1500 bar to at least 80%, preferably 80% to 93% of the theoretical density, the compact is heated and then extruded hot, preferably at temperatures of at least about 1200 ° C., to give the desired semi-finished product.

According to DE-AS 24 19 014, the metallic capsules, which are filled with the steel powder, can be evacuated before closing and / or with a gas, in particular an inert gas, e.g. Argon, fill.

According to DE-AS 24 19 014, metallic capsules are preferably used, the wall thickness of which is less than 3%, in particular less than 1% of the outer diameter of the capsule, and in particular metallic capsules are used, the wall thickness of which is between about 0.1 to 5 mm, preferably is between about 0.2 and 3 mm.

According to DE-AS 24 19 014, composite pipes can also be produced, using thin-walled metallic capsules which are separated into two or more areas by one or more concentric partition walls and the predominantly spherical powder particles of the different steel qualities being in one of these areas at the same time Vibrate filled, then the partitions removed and the capsules closed, whereupon the isostatic cold pressing and the extrusion takes place at elevated temperature. Glass is normally used as a lubricant when extruding the compacts into tubes. Since high demands are made on the lubricant during extrusion, in particular of stainless steel qualities, at higher temperatures, it is necessary to have a substantially flat end face of the compact so that the lubricant given in the form of a glass rondelle on the end face of the compact is really used .

It has now been shown that surface defects were obtained during the extrusion, specifically in the front part of the extruded tube, which was due to the fact that a strong disturbance in the flow course was obtained at the transition between the cover and the jacket, which had an adverse effect on the Was due to welding. This caused a significant loss in yield.

The object of the present invention is to increase the yield, i.e. to reduce the percentage of defective extruded tubes and to increase the quality and dimensional accuracy of the extruded tubes.

Starting from a capsule of the type mentioned at the outset, this object is achieved according to the invention in that at least the outer jacket has a circumference which changes in the direction of the longitudinal axis of the capsule, the jacket end sections adjacent to the end faces of the capsule having the radial dimensions of the finished compact and the jacket center section with one Shrinkage in the isostatic pressing compensating bulge, which is uniformly directed outwards from the capsule axis, and the central jacket section is connected to the end sections via conically widening jacket intermediate sections, and that at least on the front end face of the capsule is a solid material or powder-pressed, a flat outer end face is provided, provided with a central bore for receiving the inner shell, tapered, hemispherical or conically tapered insert, the tapering part of the Inserted into the flared intermediate section of the jacket.

The embodiment of the capsule according to the invention has the advantage that the compact does not show an "hourglass shape" with a constricted central region after the capsule has been cold-isostatically pressed. This so-called "hourglass shape" often arises from the fact that the ends of the capsule, which are closed by means of a lid or the like, show less shrinkage in cold isostatic pressing than that middle area of the capsule. Since a compact is required for the extrusion process, the outer jacket of which is cylindrical as precisely as possible, it is necessary to trim the ends of an "hourglass shape" of the compact, which is a very expensive process, with the risk of cracks occurring. The design of the capsule according to the invention has the advantage that there is no processing or trimming of the compact to achieve a cylindrical shape. In addition, the invention makes it possible to produce compacts whose diameters correspond very precisely to the desired diameter dimensions. Accuracies of ± 0.2%, in particular + 0.1%, can be achieved according to the invention. The diameter dimensions of the compact can be manufactured with an accuracy of ± 0.2 mm, in particular ± 0.1 mm.

In the capsule according to the invention, the outer shell and / or the inner shell are preferably produced in the region of the capsule ends as essentially cylindrical sections, the diameter dimensions of which correspond exactly to those of the desired compact and which continuously merge into a bulged central capsule region.

Advantageously, the shape of the outer and / or inner shell of the capsule is designed according to the invention such that the bulge from each of the cylindrical sections at the capsule ends in the axial direction, as seen in each case towards the center of the capsule, initially gradually in a region having a concave cross-sectional profile and increases steadily, the inclination of the outer and / or inner casing against the capsule axis also gradually and steadily increasing, then preferably follows a conical intermediate region in which the inclination of the outer and / or inner casing remains essentially constant, whereby this conical intermediate area is adjoined by an area in which the outer and / or inner jacket has a convex cross-sectional profile and gradually and continuously merges into an axially parallel central section, which preferably has a substantially constant diameter.

An improvement in the dimensional accuracy of the compact and a reduction in the reject rate can also be achieved according to the invention in that a plate, cone, hemisphere or funnel-shaped insert made of solid material is arranged on the front and / or rear end of the capsule. The provision of such inserts significantly improves the flow properties when extruding the compact and increases the yield of stainless material, since the inserts, which preferably consist of electrically conductive metal, in particular soft iron or a low-carbon soft steel, form the ends of the extruded tubes. that have to be cut off anyway. Furthermore, the inserts, which preferably consist of an electrically conductive metal, in the region of the front and / or rear end face of the capsule considerably facilitate the heating of the compact prior to extrusion by means of inductive heat, since the metal inserts can be easily inductively heated and their heat to the Dispense the remaining parts of the compact, in particular to the powder-filled interior, and thus contribute to the rapid heating of the entire compact.

The combination of the above-mentioned inserts in the area of the front and / or rear end face of the capsule and the formation of the outer and / or inner shell of the capsule with a bulge which is opposed to and outward from the shrinkage during isostatic pressing has proven to be particularly advantageous is such that it is essentially eliminated again by the shrinkage. With this combination, the dimensions of the compact can be maintained even more precisely according to the invention. In particular, it is possible to maintain the dimensions of the compact to ± 0.05% or absolutely to ± 0.1 mm or more precisely, which is of the greatest importance for the faultless manufacture of extruded objects, in particular of extruded tubes.

According to the invention, the inserts can be designed as caps that close the end of the capsule, wherein the inserts can be tightly welded to the outer shell of the capsule and the inner shell of the capsule. Advantageously, sheet inserts can also be arranged between the inserts and the interior of the capsule, which are designed as lids and are tightly connected to the outer jacket and the inner jacket by welding.

According to the invention can be used for capsules for the production of compacts for extruding tubes inserts for the front end of the capsule, which are funnel-shaped and are provided with a central bore, the angle y between the wall of the central bore for the inner shell of the capsule and the conical surface of the funnel-shaped insert is approximately 40 ° to 60 °, preferably approximately 40 ° to 50 °, in particular approximately 45 °.

According to the invention, it can be advantageous for the tube production that at least on the front end face of the capsule an annular insert provided with a central bore is provided, which has a substantially flat end face, and its boundary surface between the wall of the central bore and its largest outer diameter has approximately a circular cross-sectional profile, the center of the circular profile approximately in the area the line of intersection between the flat end face and the central hole.

A further significant improvement of the capsules and the compacts produced therefrom and the extruded objects, in particular the extruded tubes, can be achieved in combination with the inserts designed according to the invention described above or independently of these inserts according to the invention by at least covering the capsule has approximately the same strength properties in the axial direction over the entire circumference. Preferably, at least the outer shell of the capsule according to the invention is formed by a thin-walled, spiral welded or extruded tube. Such a design of the outer shell of the capsule offers the advantage that extruded products, in particular tubes, are obtained in which the error rate and thus the reject are markedly reduced.

The slope of the spiral formed by the weld seam in relation to the length of the capsule is preferably dimensioned such that the weld seam forms approximately one complete turn. An outer jacket provided with such a weld seam has only one weld seam at each point along its circumference in the axial direction and approximately the same strength properties in the axial direction. Alternatively, the weld seam can form two, three or more complete turns.

The present invention is applicable to capsules and compacts for extruding objects, in particular pipes, rods or similarly profiled, elongated, dense, metallic objects, in particular made of stainless steel or high-alloyed nickel steels, in particular heat-resistant steels for heat exchangers, e.g. high-alloy nickel steels with 80% nickel and 20% chromium, powder in the capsule according to the invention made of metal or metal alloys or mixtures thereof or mixtures of powders made of metals and / or metal alloys with ceramic powders. Spherical or predominantly spherical powder is preferably used as the powder, with an average diameter preferably below about 1 mm. According to the invention, spherical powder is used which, in an inert gas, preferably argon, atmosphere from the desired starting material, i.e. the desired metal and / or metal alloy has been produced by atomization. Powder grains with a diameter greater than 1 mm are preferably sieved, at least for the most part, since there is a risk that argon will be enclosed in powder grains with a diameter larger than 1 mm. Such inclusion of argon can be used in atomizing e.g. done by turbulence. Inclusion of argon would cause unfavorable properties of the extruded articles during extrusion and lead to inclusion lines.

According to the invention, the capsule for producing the compacts for the pipes to be extruded is filled with the powder, the density of the powder filled in the capsule being increased by vibration to about 60 to 71% of the theoretical density and the frequency of the vibration preferably being at least about 70 Hz, advantageously 80 to 100 Hz is selected. A density of about 68 to 71% of the theoretical density can be obtained by vibration at 80 to 100 Hz.

After the powder has been filled in and compacted by means of vibration, the capsule is closed, preferably after evacuation and / or filling with inert gas. The density of the powder is then increased by isostatic cold pressing with a pressure of at least 4000 bar, preferably 4200 to 6000 bar, in particular 4500 to 5000 bar, with at least 80 to 93% of the theoretical density.

It has been found that capsules which are generally made of thin sheet metal, preferably about 1 to 2 mm thick sheet, in particular about 1.5 mm thick sheet, are particularly advantageous. Low-carbon soft steel, in particular with a carbon content of less than 0.015%, in particular less than 0.004%, is preferably used as the material for this capsule in order to prevent carburization of the powder during heating and during extrusion.

Due to the all-round pressure during cold isostatic pressing, the capsule is compressed uniformly both in the longitudinal direction and in the radial direction and then forms a compact. As far as possible, this compact should not have any irregularities, since these lead to difficulties in extruding, in particular when extruding pipes.

In order to produce a compact for extruding a tube, a capsule is used which is designed as an annular body, the outer jacket of this annular body being formed by a spiral-welded tube section which, for example, is made from an approximately 1.5 mm thick sheet.

In the interior of this outer jacket, an inner jacket is used, for example in the form of a longitudinally welded tube section, which has a smaller diameter but the same wall thickness as the outer jacket. An annular cover is fastened on one side between the outer and inner jacket and the annular space between the two tubes is closed on one side. Then spherical powder is poured into the annular space and compacted to about 68% of the theoretical density by vibrating at 80 Hz, for example. It is then evacuated and the other end face of the annular body is sealed by a corresponding second cover. This is followed by cold isostatic pressing in a liquid, for example water, at a pressure of, for example, 4700 bar. Through all-round printing a compact with a density of, for example, 85% of the theoretical density is obtained.

The aim of the capsule according to the invention is that the spiral weld seam is as smooth as possible and that the properties of the sheet do not change significantly. Therefore, the weld seam is preferably smoothed by means of rollers and / or by means of grinding. The welding seam can be smoothed by means of rollers immediately after the welding process.

In the case of capsules for the production of pipes, it may be expedient not only to produce the outer casing, but also the inner casing from a pipe which has approximately the same strength properties in the axial direction along its circumference. The inner jacket can either consist of a spiral welded tube or an extruded tube. The use of an extruded or spiral welded tube for the inner jacket is particularly useful with large dimensions. In the case of smaller dimensions, it is generally sufficient if the outer casing of the capsule is produced according to the invention from a tube section which has approximately the same strength properties in the axial direction along its circumference.

• Fig. 1 zeigt eine Ausführungsform der erfindungsgemäßen Kapsel im Längsschnitt;
• Fig. 2, 3 und 4 zeigen Längsschnitte ähnlich Fig. 1 von abgewandelten Ausführungsformen.

The invention is explained in more detail below with the aid of a schematic drawing using an exemplary embodiment.

• Fig. 1 shows an embodiment of the capsule according to the invention in longitudinal section;
• 2, 3 and 4 show longitudinal sections similar to FIG. 1 of modified embodiments.

The capsule is generally designated 101 in FIG. 1. The capsule has an outer casing 102 and an inner casing 104. Inserts 130 and 140 are arranged both in the area of the front face of the capsule and in the area of the rear and bottom face of the capsule, which form the front and rear face of the capsule. The outer jacket 102 consists of a spiral welded pipe section, over the circumference of which the weld seam runs in a spiral, the pitch of which is dimensioned such that the spiral forms approximately a complete turn over the length of the pipe section.

In particular, it has been shown that it is expedient to arrange the spiral weld seam of the outer jacket 102 such that it forms a complete turn between the weld seams 116 and 126, by means of which the covers 110 and 120 are welded to the outer jacket 102. However, it can also be advantageous to choose the slope of the weld seam of the outer jacket so that the spiral weld seam between the weld seams 116 and 126 forms two, three or more complete turns.

In a practical exemplary embodiment, the outer casing 102 and also the inner casing 104 of the capsule 101 consisted of sheet metal with a thickness of 1.5 mm and a carbon content of less than 0.004%. In the exemplary embodiment mentioned, the capsule had a length of 600 mm and an outer diameter of 150 mm. The inner diameter of the inner jacket 104 was approximately 55 mm. The inner jacket 104 consisted of a longitudinally welded pipe section. To produce the compact, powder, which consisted predominantly of spherical grains with an average diameter of less than 1 mm and was obtained from the desired starting material by atomization in an argon atmosphere, e.g. had been made of stainless steel, filled in the bottom sealed capsule 101. After filling, the powder was compacted by vibration at a frequency of 80 Hz to a density of about 68% of the theoretical density. The capsule was then evacuated and closed by means of the lid 110, which was connected to the outer casing 102 of the capsule by welding approximately along the weld seam 116 in FIG. 1. The density of the powder was then increased to about 85% of the theoretical density by cold isostatic pressing at a pressure of 4700 bar. The compact thus obtained was, as described in DE-AS 24 19 014, extruded to the tube.

In the embodiment according to FIG. 1, the front insert 130 has a central bore 132 for receiving the inner shell 104 of the capsule. The insert 130 has an essentially flat end face 134. However, it is chamfered or rounded at its outer edge at 135 and then initially has a cylindrical section 137 which merges into the conical outer surface 136. At 139, the transition from the conical outer surface 136 to the wall of the central bore 132 is rounded. The contour of the cover 110, which is designed as a sheet metal insert, corresponds exactly to that of the adjacent parts of the insert 130. In particular, the cover 110 has a cylindrical section 117 on the outer edge, which ensures that the cover 110 is in good contact with the outer jacket 102. wherein the outer edge of this cylindrical section 117 is connected to the outer jacket 102 by means of a weld seam 116. Also in the inner region, the cover 110 has a short, essentially cylindrical section 119, which bears against the inner casing 104 of the capsule and is sealed at 118 by means of a weld seam to the inner casing 104. The cover 110 also has a rounding corresponding to the rounding 139 of the insert 130.

In the area of the rear end face of the capsule 101, an insert 140 is arranged, which has a central bore 142 and a flat end face 144 pointing outward. This insert 140 is also beveled or rounded at the edge at 145 and has one outer cylindrical portion 147. The shape of the bottom cover or capsule base 120 corresponds to the shape of the insert 140 and also has an outer cylindrical section 127 and an inner cylindrical section 129. The bottom cover 120 is welded tightly to the outer casing 102 and the inner casing 104 by means of weld seams 126 and 128. The inserts 130 and 140 are preferably made of soft iron or low-carbon soft steel.

In the exemplary embodiment according to FIG. 1, the insert 130 provided on the front end face of the capsule has an essentially circular arc-shaped cross-sectional profile 136. The center points of the circular cross-sectional profile 136 lie on a circle which is approximately in the region of the intersection between the flat end face 134 and the wall of the bore 132, i.e. lies in the region of the front boundary line of the bore 132, and is indicated by two crosses 138 in FIG. 1. The approximately circular cross-sectional profile 136 offers the advantage that when extruding the compact, the insert 130 made of soft iron or a similar metal together with the cover 110, the weld seams 116, 118 and the adjacent parts of the outer casing 102 and the inner casing 104 form the first part of the Form tube which is cut off after extrusion or even falls off by itself if the connection to the subsequent tube, which is preferably made of stainless steel and made from the powder filling of the capsule, has no or no sufficient strength. The approximately circular arc of the boundary line 136 of the insert 130 ensures that the dividing line between the front, waste section of the extruded tube and the actual tube, which is made of high-quality stainless material, is formed sharply and as a separating surface which extends essentially perpendicular to the longitudinal axis of the tube . The lid 110 also has an approximately cylindrical section 117, which is welded at 116 to the outer casing 102 of the capsule, and an approximately cylindrical inner section 119, which bears against the inner casing 104 and at 118 by means of a circumferential weld seam tightly to the inner casing connected is. The transition from the wall of the central bore 132 to the circular cross-sectional profile 136 is rounded off at 139.

It may also be advantageous to tightly weld the inserts 130 and 140 directly to the outer jacket 102 and the inner jacket 104, respectively. In this case, cover 110 and base 120 can be omitted.

When using sheet metal inserts as a lid or base, it may be expedient to attach inserts 130, 140 to them by spot welding. In many cases, however, it is also sufficient to fix the inserts 130 and 140 through the flanged ends 115 and 125 of the outer jacket 102.

The use in the area of the front end face of the capsule leads to a kind of tunnel effect when extruding, if this insert is made of ductile material, e.g. ductile iron, soft iron, low-alloy carbon steel or cast iron. The pressure required in the container of the extrusion press to extrude the compact is reduced if the front insert is made of ductile material and this material is easier to flow than the powder filling of the compact. Once the flow process that takes place during extrusion is initiated, it also spreads to the powder filling, even if the flow limit of the powder filling is higher than the flow limit of the ductile material of the insert; so there is a kind of tunnel effect.

In FIG. 1, the outer jacket 102 has a bulge 103 which is opposite to the shrinkage during cold isostatic pressing. In FIG. 1, the insert 140 in the area of the capsule base 120 also has an approximately circular cross-sectional profile 146, which in the area of the central bore 142 merges into the wall of the bore 142 via a rounded region 149. On the outside, the insert 140 has an essentially cylindrical section 147, against which a cylindrical section 127 of the base 120 comes to rest. The cylindrical section 127 is welded at 126 to a substantially cylindrical section 166 of the outer jacket 102. The cylindrical portion 129 of the cover 120 bears against the inner shell 104 and is welded to the inner shell at 128. The outer end face 144 of the insert 140 is planar and rounded or beveled at the outer edge at 145, so that the flanged lower edge 125 of the outer casing 102 can hold the insert 140. The bulge 103 is dimensioned such that the inner surface of the outer jacket 102 shrinks after the cold isostatic pressing up to the line 170, which corresponds to the ideal cylindrical shape. Accordingly, the cylindrical sections 156 and 166 of the outer casing 102 are preferably drawn in by rolling until they are aligned with the line 170.

In order to avoid wrinkling and to achieve a compact that is centered as precisely as possible, it is advantageous according to the invention to essentially limit the change in the diameter of the outer casing 102 to the area of the inserts 130 and 140, respectively. Between these inserts, the outer jacket 102 essentially has a constant outer diameter in the region indicated at 150. It has proven to be particularly advantageous to have a respective area 157 and 167 on the cylindrical sections 156, 166 towards the center of the capsule to connect an outwardly concave cross-sectional profile and to it a frustoconical intermediate region 158 or 168 and to let this be followed by a region 159, 169 which has an outwardly convex cross-sectional profile and merges into the cylindrical, axially parallel central region 150. It is essential that the outwardly concave regions 157 and 167 are approximately mirror images of the cross-sectional profile 136 and 146 of the inserts, the line 170 representing the mirror symmetry axis and the angle of curvature of the outer jacket indicated at β approximately in the ratio of the percentage shrinkage to the angle of curvature 8 of the neighboring mission is reduced.

FIG. 2 shows a modified embodiment, similar to FIG. 1, in which all the same or similar parts are provided with reference numbers increased by a hundred. The main difference is that the inserts 230 and 240 have a cross-sectional profile which is substantially tapered at 239 and 249, so that the appropriately designed lid 210 and the appropriately designed bottom 220 extend directly to the inner jacket 204 and form an obtuse angle with them form a or a '. It has been shown that this training is advantageous for exact centering of the compact. In the embodiment according to FIG. 2, bulging of the inner jacket 204 is not necessary. In contrast, a slight outward bulge of the inner jacket can be advantageous in the embodiment according to FIG. The bulge of the outer and / or inner jacket can be advantageous according to the invention in connection with any insert. The bulge in combination with a spiral-welded outer and / or inner tube can also be advantageous.

FIG. 3 shows a modified embodiment, similar to FIG. 2, in which all the same or similar parts are provided with reference numbers increased by a hundred. The main difference is that the inserts 330 and 340 are provided with tips 339 and 349 and that no sheet metal inserts are provided. The bulge 303 from each of the cylindrical sections 356, 366 in the axial direction, in each case towards the center of the capsule, initially gradually and steadily increases in a region 357, 367 with a concave cross-sectional profile, the inclination of the outer casing 302 towards the capsule axis also gradually and continuously increases, then the inclination of the outer casing 302 remains substantially constant over a conical intermediate region 358, 368 and an area 359, 369 adjoins in which the outer casing 302 has an outwardly convex cross-sectional profile and gradually and continuously merges into an axially parallel central region 350 . The regions with a changing cross section of the outer jacket 302 each form a transition region 355, 365, which is arranged in the region of an insert 330 or 340. The cross-sectional contour 336, 346 of the inserts 330, 340 is approximately a reflection of the contour of the outer jacket in the transition regions 355, 365, which is mirrored on the line 370 of the desired cylindrical shape of the compact, but is stretched in the radial direction, the extent of the stretch being approximately corresponds to the ratio of the difference between the outer and inner diameter of the compact to the shrinkage of the capsule, preferably taking into account the change in the cross-sectional area as the radius becomes smaller.

With 316, 318, 326 and 328, the inserts 330 and 340 are directly welded to the outer and inner jacket, respectively. 337 and 347 are cylindrical portions of the inserts 330 and 340, respectively, which correspond to the cylindrical portions 137, 147 and 237, 247 of Figures 1 and 2, respectively.

Example:

In order to produce a compact with an outer diameter of 144 mm for the extrusion of a tube made of stainless steel with an outer diameter of 50 mm and a wall thickness of 5 mm, a spiral-welded 600 mm long tube with an outer diameter of 154 mm and a wall thickness of 1.5 mm constricted at both ends by rolling or rotary pressing, in such a way that cylindrical sections with an outer diameter of 144 mm, corresponding to sections 156, 166; 256, 266; 356, 366 of FIGS. 1 to 3 were present, which were followed by transition regions, which correspond to transition regions 155, 165; 255, 265; 355, 365 were molded. Then the ends of the outer jacket were ground flat. At a first end, a sheet-metal insert forming a bottom was welded similarly to insert 120 in FIG. 1, on the one hand tightly to the outer jacket and on the other hand tightly to an inner jacket which consisted of a 590 mm long, longitudinally welded tube with a wall thickness of 1.5 mm and an inner diameter of 40 mm.

Until it touched the floor panel, an annular or funnel-shaped insert, similar to insert 140, made of low-alloy carbon steel with approximately 0.004% carbon stock was inserted from the first end of the outer jacket and fastened by means of spot welding.

The capsule was placed upright on a plate, filled with powder and vibrated at 80 Hz and compressed to about 68% of the theoretical density and at the same time provided with a funnel-shaped sheet-metal insert similar to 110 in FIG. 1, which between Inner and outer sheath was pushed in with great pressure from above. Then the sheet insert was tightly welded to the inner and outer jacket, as indicated in Figure 1 at 116 and 118. Then the front ring-shaped or funnel-shaped insert was inserted from above, which was designed similarly to 130 in FIG. 1 and consisted of low-alloy carbon steel with approx. 0.004% C. This ring-shaped insert was advantageously welded to the funnel-shaped sheet metal insert or the inner or outer jacket by means of spot welding.

The capsule was cold isostatically pressed at 4700 bar in water to a density of 88% of the theoretical density. The compact shrank to an outside diameter of 144 mm, i.e. to the same dimension as the retracted cylindrical sections at the ends. The dimension of 144 mm also corresponded to the inner diameter of the container of the extrusion press. This ensured perfect centering. In addition, the inside diameter of the compact was almost exactly 40 mm.

The compact was also completely straight and, after induction heating to 1200 ° C., could be extruded directly into the desired seamless tube made of stainless steel, without further processing being necessary. The front section of the tube made of low-alloy carbon steel was cut off. Nothing was cut from the stainless steel. Due to the fact that the insert is conical, a line of separation between the extruded insert and the stainless steel, which is approximately perpendicular to the pipe axis, was maintained in the extruded tube. The part of the pipe made of stainless material had a flawless surface. The material loss was thereby reduced to a minimum.

In order to achieve a good separation between the front section of the extruded tube, which is made of low-alloy carbon steel, and the desired seamless tube made of stainless steel, a glass layer can be applied according to the invention on the surface of the front insert facing the powder filling 308. For this purpose, it may be expedient to heat the front insert 330 and to sprinkle the surface 336 with glass powder, the temperature of the insert being selected so that the glass powder softens and sticks. Such an intermediate glass layer makes the separation between the low-alloy carbon steel and the stainless steel considerably easier when the extruded tube is obtained, so that the two types of steel are obtained completely separately from one another and without mixing.

In an analogous manner, the surface of the bottom-side insert 340 adjoining the powder filling 308 can also be provided with a glass layer which facilitates the separation of stainless material and low-alloy carbon steel.

The inserts 130, 140, 230, 240, 330 and 340 can also be pressed from powdered starting material. For this, e.g. water-atomized soft iron or water-atomized low-carbon steel can be used, which is cold isostatically pressed to the desired shape of the insert mentioned and then sintered. The pressing of the soft iron powder can be carried out cold isostatically in a plastic mold, the pressure preferably being chosen to be at least as high, if not higher, than the pressure for the cold isostatic pressing which is used for the production of the capsules. Subsequent hot sintering can result in a dense material. Alternatively or additionally, a seal can be obtained by applying an outer glass layer, in this case also on the end faces 134, 234, 334 or 144, 244 and 344 and the peripheral surfaces.

The embodiment according to FIG. 4 largely corresponds to that according to FIG. 3. Only the insert pieces have a modified shape. The front insert 330 'consists of two rings 380 and 381, which are held together by means of several spot welds 382. Instead of two rings 380, 381, three or more rings can of course also be provided, the outer contour of which approximates the ideal contour of the front insert, which is represented by curve 336 of FIG. 3 or circular cross-section 236 of FIG. 2 or 136 of FIG. 1 is given. In the embodiment according to FIG. 4, the bottom-side insert 340 'consists of an annular plate. Here too, if desired, additional rings with a stepped outer diameter and / or stepped inner diameter can be provided in order to approximate the desired ideal profile, e.g. to achieve an approximation to the profile 346 according to FIG. 1.

Claims (10)

1. Annular capsule for blanks for the powder metallurgical production of tubes, the capsule comprising an outer and an inner wall of thin ductile metal sheet between which there is an inner space of annular cross-section for receiving metal or alloy powders to be isostatically pressed which is adapted to be sealingly closed at its end sides by means of annular covers, characterized in that at least the outer wall (102; 202; 302) comprises a periphery varying in the direction of the capsule longitudinal axis, the wall end portions (156, 166; 256, 266; 356, 366) adjacent the end sides of the capsule having the radial dimensions of the finished blank and the wall center portion (150; 250; 350) being provided with a bulge which compensates the shrinkage on isostatic pressing and is directed uniformly outwardly from the capsule axis and the wall center portion being connected to the end portions via wall intermediate portions (155, 165; 255, 265; 355, 365) widening conically from the latter and that at least at the front end of the capsule a funnel-shaped, hemispherical or conical tapering insert (130, 140; 230, 240; 330, 340; 330', 340') is provided which consists of solid material or is pressed from powder, has a planar outer end face and is provided with a central bore (132, 142; 232, 242; 332, 342) for receiving the inner wall (104; 204; 304), the tapering portion of the insert (130, 140; 230, 240; 330, 340; 330', 340') extending into the conically widening wall intermediate portion (155, 165; 255, 265; 355, 365).

2. Capsule according to claim 1, characterized in that the tapering portion of the insert (130, 140; 230, 240; 330, 340; 330', 340') extends into the conically widening wall intermediate portion (155, 165; 255, 265; 355, 365) substantially over the entire axial length of the latter.

3. Capsule according to claim 1 or 2, characterized in that the inserts (130, 140; 230, 240; 330, 340) have a cross-sectional profile (136, 146; 236, 246; 336, 339; 346, 349) which represents substantially a mirror image of the cross-sectional profile of the conically widening wall intermediate portions (155, 165; 255, 265; 355, 365) mirrored at the desired cylindrical form (170; 270; 370) but enlarged in the ratio of the diameter of the blank to the diameter shrinkage taking place in the isostatic pressing in the radial direction.

4. Capsule according to one or more of claims 1 to 3, characterized in that at least the insert (130; 230) at the front end of the capsule (101; 201) comprises between the wall of its central bore (132; 232) and its greatest outer diameter towards the capsule interior a boundary surface having a substantially arcuate cross-sectional profile (136; 236), the center point of the arcuate profile (136; 236) lying substantially in the region of the circular intersecting line between the planar end face of the insert and its central bore (132; 232) or within said circular intersecting lines.

5. Capsule according to one or more of claims 1 to 4, characterized in that the inserts (130, 140; 230, 240) consist of electrically conductive metal, preferably soft iron.

6. Capsule according to one or more of claims 1 to 5, characterized in that the inserts (330, 340) are constructed as covers closing the capsule (301) at the end faces and are welded to the outer wall (302) and inner wall (304).

7. Method of making annular capsules for the isostatic pressing of blanks for the powder metallurgical production of tubes, the capsules comprising an outer and an inner wall of thin ductile sheet metal between which a capsule interior of annular cross-section is present for receiving metal or alloy powders to be isostatically pressed and at least the outer wall comprises a periphery which varies in the direction of the capsule longitudinal axis and the wall end portions of which adjacent the ends of the capsule have the radial dimensions of the finished blank and the wall center portion of which is provided with a bulge which compensates the shrinkage in isostatic pressing and which is directed uniformly outwardly from the capsule axis, the wall center portion being connected to the end portions via wall intermediate portions widening conically from the latter and the capsule interior being sealingly closed at its ends by means of annular covers and at least at its front end a funnel-shaped, hermispherical or conical tapering insert consisting of solid material or pressed from powder, having a planar outer end face and provided with a central bore for receiving the inner wall is inserted in such a manner that the tapering portion of the insert extends into the conically widening wall intermediate portion, characterized in that the outer and inner walls are made from tubes and at least in the case of the tube serving for the production of the outer wall the tube diameter corresponds to the diameter of the bulge of the wall center portion and the two ends of the tube are constricted by rolling or rotary pressing to such an extent that at its ends the cylindrical wall end portions with a reduced diameter corresponding to the diameter of the finished blank and adjoining conically widening wall intermediate portions are formed.

8. Method according to claim 7, characterized in that the covers are coated with a glass layer at their side facing the interior of the capsule receiving the powder filling.

9. Method according to claim 7 or 8, characterized in that at least the tube of the outer wall of the capsule is produced by spiral welding or extruding.

10. Method for the powder metallurgical production of tubes, blanks being made by isostatic pressing of metal or alloy powders in annular capsules which have an outer and inner wall of thin ductile sheet metal between which a capsule interior of annular cross-section is present for receiving the powder to be pressed and in which at least the outer wall comprises a periphery which varies in the direction of the capsule longitudinal axis and the wall end portions of which adjacent the ends of the capsule have the radial dimensions of the finished blank and the wall center portion of which is provided with a bulge which compensates the shrinkage in isostatic pressing and which is directed uniformly outwardly from the capsule axis, the wall center portion being connected to the end portions via wall intermediate portions conically widening from the latter and the capsule interior being sealingly closed at its ends by means of annular covers and at least at its front end a funnel-shaped, hemispherical or conical tapering insert which consists of solid material or is pressed from powder, has a planar outer end face and is provided with a central bore for receiving the inner wall is inserted in such a manner that the tapering portion of the insert extends into the conically widening wall intermediate portion, characterized in that the planar outer end face of the insert provided at the front end of the capsule is provided at its outer periphery with a bevelled edge and for obtaining a good lubrication in the extrusion operation the glass serving for lubrication is placed in the form of a glass disk at the end of the blank in the container or receiver of the extrusion press and by the bevelled edge at the front end of the insert and by exact adaptation of the exactly cylindrical outer diameter of the blank to the cylindrical inner diameter of the container or receiver of the extrusion press during the entire extrusion operation is supplied distributed substantially uniformly in the peripheral direction between the tool and the surface of the extruded tube.

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