CA1050217A - Capsules from which elongate metal objects can be produced by extrusion - Google Patents

Abstract

Abstract of the Disclosure A capsule for producing tubes, bars or similar profiled elongated dense metal objects, preferably in stainless steel qualities, by single or multi-stage extrusion of capsules which are filled with a powder of the group comprising metals, metal alloys, ceramics and mixtures thereof and sealed and which are adapted in their form to the desired object or intermediate product.
As starting material a powder is used which consists at least predominantly of substantially spherical grains and the capsule is filled with the powder, sealed and compressed by means of cold-isostatic pressure acting on all sur-faces until the density of the powder reaches at least 80% of the theoretical density.

Description

~0~ 7 This application is a division of applicant~s ~anadian Patent ~pplication 224,940, filed April 18, 1975.
The parent application relates to a method of producing tubes, bars or similar profiled elongated dense metal objects, preferably in stainless steel qualities, by single or multi-stage extrusion of capsules which are filled with powder of metals or metal alloys or mixtures thereof or with mix-tures of powder of metals and/or metal alloys with ceramic powder and sealed and which are adapted in their form to the desired object or intermediate product.
For economic and production technique reasons it is necessary for for the capsule material to be as thin as possible. This involves the problem that the capsule has a tendency to wrinkle or form creases during the extru-sion operation. In the production of elongated objects such as tubes or the like the ratio of the length to the diameter of the capsule must be greater than one. This further increases the tendency to crease or fold of the cap-sule, especially when the capsule wall is thin.
Various proposals have been made for solving this problem but so far none has provided an economically and technically satisfactory solution.
Thus, for example~ it has been proposed to cold-press the capsule after intro-ducing the powder and sealing the capsule. However, with this technique, because of the frictional forces between the capsule and the mechanical tool : used for the cold-pressing the results are not satisfactory, particularly when the length of the capsule with respect to its diameter has a ratio of more than one. The frictional forces also unacceptably reduce the total reduction which can be achieved and cause it to vary over the length of the blank which inter alia leads to unfavourable conditions on heating the blank prior to extrusion.
The invention of the parent application adopts a completely differ-ent procedure for solving the aforementioned problem.
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According to the method of the parent application this problem is - solved in that the starting material is a powder which consists at least predominately of substantially spherical grains, the capsule is filled with said powder, sealed and compressed by means of cold-isostatic pressure acting on all surfaces until the density of the powder reaches at least 80% of the theoretical density, and the blank thus obtained is heated and extruded in one or more stages to form the desired object. This method has the advant-age that the capsule does not form any creases on extrusion.
The present invention is concerned with the provision of capsules suitable for use in such a method.
According to the present invention there is provided a structure from which tubes, bars or other elongate metal objects can be produced by extrusion, which structure consis~s of a thin ductile sheet metal capsule whose wall thickness is less ~han 3 % of the external diameter of the capsule and which is sealed after having been filled with powder of one or more metals ; or metal alloys or mixtures thereof or of a mixture of metal and/or alloy powder with ceramic powder, said powder consisting at least predominantly of spherical grains and having a density of about 60 to 70 % of the theoreti-cal density.
The sheet metal may be, for example carbon steel or nickel.
The wall thickness of the capsule is preferably less than 1% of the external diameter of the capsule.
The wall thickness of the capsules is preferably between 0.1 and 5mm, advantageously between 0.2 and 3mm.
If compound objects are to be made, various metals are used in powder form. The powder is introduced into a capsule which is divided by one or more separating walls. Said walls may be either of plastic, steel or a similar material. After filling and vibrating the powder the walls are re-moved. The powder is sealed in the capsule with or without evacuation or 3n introduction of an inert gas.

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In use, the capsule is subjected with the enclosed powder to a cold-isostatic pressure of at least 1500 bar ~about 21,800psi), but pre-ferably higher pressures, for example 5000 bar ~about 72,500psi), the den-sity of 60 to 70% being increased to 80 to 90% of the theoretical density, depending on the pressure used. Because the starting density of the powder is so high the capsule does not crease during the cold-pressing and the ex-trusion in spite of the fact that the ratio of the length to the diameter may be greater than one, for example four, and that a thin capsule is used which is very important for economic reasons, as already mentioned. It has been found that the ratio between the outer diameter of the capsule and the capsule wall thickness is critical. This ratio is to be a maximum of 3%
and is advantageously below 1%. The wall thickness of the eapsule is pre-ferably between about 1.0 and 5mm, especially about 0.2 and 2mm. It is pointed out that the high percentages are to be used with relatively small diameters and conversely the low percentages with relatively high diameters.
Due to the pressure on all surfaces in the cold-isostatic compres-sion the blank is given a substantially uniform density over its entire length.
Because the density increases greatly, it is also easier to heat the blank in a short time in an induction furnace or in similar manner.
After the heating the blank is extruded in one or more stages. The capsule material is drawn out as this is done to a very thin layer or skin.
On emergence from the extrusion press the layer or skin may oxidize in the air and partially peel off.. The residue of the capsule material may be re-moved in subsequent annealing, pickling in nitric acid or sand blasting. The ~3 .
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object can then be further processed in the normal manner.
The tubes, rods, or similarly profiled elongated objects made by the method of the parent application and employing the parent capsules have a surprisingly uniform structure and surprisingly consistent physical and chem-ical properties. In particular, the fluctuations regarding the hardness and chemical resistance of the products obtained are substantially smallerO This applies also to compound articles made by the method of the invention. mese properties of the tubes and the like made according to the invention are due to the fact that the segregations which always occur in conventional produc-tion, in particular in streak form, cannot arise.
If desired the capsule may consist of a material giving a high-quality surface finish by providing the extruded tubes or the like with a permanent coating of the capsule material. The thickness of the surface coating or plating may be predetermined by suitable choice of the wall thick-ness of the capsule~ Highly ductile materials are particularly suitable for making such surface layers.
The invention will be explained in detail hereinafter with the aid of examples.
~MP:[E 1 Argon-atomized stai-nless powder of spherical grain form and a grain size of less than 600 /u~ having a low total oxygen content~ was placed in a tubular capsule and vibrated. The capsule was constructed as annular body having an external diameter of about 140 mm and consisted of a steel of low carbon content. The wall thickness was 3 mm and the length 550 mm. The annular capsule comprised a central inner continuous tubular section having about the same wall thickness and the same carbon steel quality as the outer ; casing of the capsule. The low carbon content of the capsule ~aterial was necessary to prevent carburization of the powder during the heating and extrusion.

~)S(~7 The capsule was evacuated and sealed in known manner. Thereafter, the capsule was subjected to a cold-isostatic pressure by l~wering it in a liquid (water in the present case) and subjecting it to an all round pressure of 5000 bar (about 72,500 psi). The capsule shrank and the density of the powder rose from about 68% to about 90% without the capsule material creasing.
To facilitate the explana~ion, an identical capsule to that in example 1 was for comparison subjected to a normal cold pressing instead of a cold-isostatic pressure, i.e. compacted in a mechanical press. A density of the powder of 75% of the theoretical density was achieved although the pressure used was twice as high as that in example 1.
me blank made by cold-isostatic pressure was then heated in a pre-heating furnace to 900C and finally to 1240C in an induction coil, where-after the blank was extruded to form a seamless tube. The tube was cooled in the water bath and the capsule material removed in a nitric acid bath. me tube was faultless.
The blank made for comparison in a mechanical press was heated and extruded in the same manner. After removing the capsule material, the result-ing tube was useless. The folds and creases produced on pressing had given rise to cracks and other material flaws which rendered the tube useless.

In another case a compound tube was made in the following manner;
Ic a sheet metal capsule corresponding to example 1 having a continuous inner central tube~ a thin-walled tube was placed half way between the external and inner wall of the capsule. In the outer intermediate space, whilst simultaneously vibrating~ a spherical powder of a 2S% chromium steel was placed which had high contents of silicon and aluminium. The grain size was less than 600 /u. It is emphasized that a blank of this quality is exceedingly difficult to make with conventional methods~ i.e. smelting met-allurgy~ The material is particularly suitable for powder metallurgical .

~)S0~7 production. It is known that products of this quality are of very great industrial significance.
Spherical stainless powder of a chromium-nickel steel (18% Cr and 8% Ni) having a grain size of less than 600 /u was placed in the inner inter-mediate space with simultaneous vibration. After removing the intermediate wall and evacuating and sealing the capsule the latter was exposed to a cold-isostatlc pressure of 5000 bar (about 72~500 psi). Thereafter, the blank was heated and extruded to form a seamless tube as described in example 1.
The capsule material was also removed in a nitric acid bath. A structural investigation of the compound tube showed that the structure was completely dense and completely uniform. There was a total bond in the junction region of the two materials, i.e. without flaws. It is emphasized that the faultless production of a compound tube is practically impossible with hitherto known methods.

The same powder and capsule material as in example 1 was not subjected to an isostatic compression but heated directly to 1200 and extruded to a finished tube. The tube had pronounced surface flaws due to wrinkling of the capsule, itself due to the low starting density of the powder body. This test thus shows that a compacting of the blank prior to the extrusion is necessary to avoid the known phenomenon of creasing of the capsule and thus to avoid the occurrence of surface flaws.

The same powder and the same capsule material as in example 1 was subjected to an isostatic pressure of 2000 bar (about 29000 psi); the capsule shrank without wrinkling. The density of the powder was increased to 82% of the theoretical density.
The blank was heated and extruded in the manner described above.
The tube obtained was faultless and did not exhibit any creasing or wrinkling.

The test proves that a cold~isostatic compacting of up to 80% is sufficient to give a flawless product.

Of 8 capsules, 4 were filled with stainless steel powder of irreg-ular form (powder atomized in water) and 4 with regular spherical grain form (powder atomized in argon or another inert gas). The capsules were subjected to a cold-isostatic pressure of 2000 (about 29000 psi), 4000 (about 58000 psi), 6000 (about 87000psi) and 8000 (about 116000 psi) bar which led to densities as illustrated in Figure 1.
The 4 capsules which had been filled with powder of irregular form exhibited pronounced wrinkling and creasing at the surface. The capsule with spherical powder, in contrast, did not exhibit any flaws. The tests show that it is essential to use spherical powder~ which also gives a high apparent density, if wrinkling (creasing) and other flaws are to be avoided when using cold-isostatic pressure for achieving densities above 80%.
The diagram illustrates the ratio between the cold-isostatic pressure and the densities achieved on compressing inert-atomized powder (full line) and water-atomized powder (dot-dash line) and the fact that densities above 80% were achieved with considerably less pressure with inert-ætomized powder.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A structure from which tubes, bars or other elongate metal objects can be produced by extrusion, which structure consists of a thin ductile sheet metal capsule whose wall thickness is less than 3 % of the external diameter of the capsule and which is sealed after having been filled with powder of one or more metals or metal alloys or mixtures thereof or of a mixture of metal and/or alloy powder with cermaic powder, said powder consisting at least predominantly of spherical grains and having a density of about 60 to 70 % of the theoretical density.

2. A capsule according to Claim 1, in which the container is filled with stainless powder.

3. A capsule according to Claim 2, in which the stainless powder is argon-atomized powder.

4. A capsule according to Claim 1, 2 or 3, in which said powder consists of spherical grains having a size less than 600µ.

5. A capsule according to Claim 1, 2 or 3 wherein said sheet metal is carbon steel.

6. A capsule according to Claim 1, 2 or 3 wherein said sheet metal is nicke

7. A capsule according to Claim 1, 2 or 3 in which the wall thick-ness is less than 1 % of said external diameter.

8. A capsule according to Claim 1, 2 or 3 in which the wall thick-ness is between 0.2 mm and 3 mm.

9. A capsule according to Claim 1, in which the container is separated by one or more partitions into two or more regions each of which contains a different powder.

10. A capsule according to claim 9 wherein the wall thickness is between 0.2 and 3 mm.

11. A structure from which tubes, bars or other elongate metal objects can be produced by extrusion, which structure consists of a thin ductile sheet metal capsule whose wall thickness is less than 3% of the external diameter of the capsule and which is sealed after having been filled with powder of one or more metals or metal alloys or mixtures thereof or of a mixture of metal and/or alloy powder with ceramic powder, said powder con-sisting at least predominately of spherical grains and having a density of about 60 to 70% of the theoretical density.

12. A capsule according to claim 9, in which the wall thickness is less than 1% of said external diameter.

13. A capsule according to claim 9 in which the wall thickness is between 0.2 mm and 3 mm.