Method of producing powder-metallurgical objects, specifically elongate objects such as rods, sections, tubes or the like
BACKGROUND OF THE PRESENT INVENTION 
     The present invention relates to a method of producing powder-metallurgical objects, specifically elongate objects such as rods, sections, tubes or the like, wherein a powder of metal/and or metal alloys of great hardness, specifically tool or high-speed steel powder, is charged into a thin-walled capsule, said capsule is then sealed so as to be airtight, is heated and subjected to isostatic pressure so as to produce a blank which will then be hot-worked, particularly extruded, to form the finished product. 
     Such a method is known, for instance, from the German Patent DE-C-35 30 741. Tests have shown that the known method cannot be employed for the use of tool steel or high-speed steel powder to produce a flawless, specifically crack-free finished product. The reason is to be found in the low plasticity and malleability of the metal powder used which provides for a maximum powder density of 75% of the theoretical value under the common conditions of cold isostatic compression. Due to the hardness of the metal powder here used, the isostatic compression does not allow for a tight bonding between the material of the capsule and the adjoining metal powder, which results in the formation of folds or &#34;wrinkles&#34; in the comparatively soft material of the capsule in the extrusion step. This phenomenon leads to surface cracks and fissures in the finished product which will become apparent after removal of the capsule material. 
     Knowing the problems described above, experts have proposed, in the Swedish Patent SE-A-442 486, to heat a high-speed steel powder initially to a temperature between 850° C. and 900° C., in a non-oxidizing environment. Then the thus obtained &#34;powder cake&#34; is pulverized again by mechanical means, i.e. by grinding. Then the powder is reheated to a temperature of 850° C. to 900° C. A shaping tool is then used to press the thus pretreated powder into a blank which will be sintered subsequently at a temperature between 1,150° C. and 1,250° C. until the so-called &#34;communicating porosity&#34; in the blank will have been eliminated, which means that a closed-pore blank is produced. This step is followed by a warm isostatic final compression to a density of practically 100% of the theoretical value. It is obvious that this known method of producing powder-metallurgical objects involves a comparatively expensive process which is not suitable anyhow for the production of elongate objects. 
     The Australian Patent AT-A-377 718 describes a method of manufacturing objects made of tool steel powder, wherein the powder is charged into a capsule, this capsule is sealed so as to be airtight, is then heated, and wherein the airtight and sealed capsule is heated to a temperature between 700° and 1,000° C., the introduction of air into the capsule is permitted again, and the capsule is then heated to a temperature between 1,050° C. and 1,200° C. This known method obviously still requires that the extruded rods be annealed for several hours whereupon they are continuously cooled, initially in steps to a lower temperature and subsequently in ambient air without temperature control. This method, too, is rather expensive and troublesome, while it appears to be problematic to open the capsule again after heating the same to a temperature between 700° C. and 1,000° C. for heating it to an even higher temperature. This method involves an increased risk of oxidation so that the suitability of this method for practical realization well appears doubtful. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is now based on the problem of providing a method of the type defined by way of introduction, which will allow for an unproblematic and easy production of flawless or crack-less objects, specifically elongate objects, using a powder of metal and/or metal alloys of great hardness, specifically tool steel or high-speed steel powder. 
     This problem is solved by processing the sealed capsule blank containing a hard powder of metal and/or a metal alloy and heating the sealed blank to a temperature greater than 1,000° C. and preferably in the range of 1,100° C. to 1,200° C. The heating is carried out for a period to through-heat the capsule blank. The heated capsule blank is held at an elevated temperature above 1000° C. for a period greater than the heating period. The capsule blank is subsequently cold worked to obtain the final product. 
     The inventive measures contribute to the achievement of a powder density better than 75% of the theoretical density, prior to the hot working, specifically extrusion, of the capsule. In spite of this low powder density, the remaining inventive features contribute to a crack-free finished product. The comparatively hard powder is actually rendered &#34;obedient&#34; for hot working, due to the inventive measures. It was a surprise to detect that the inventive method is suitable for the production of acceptable finished products even if the employed powder consists of metal and/or metal alloys whose carbon percentage is as high as up to 2.5% by weight. 
     In the one method, a cooling time is provided which is noticeably longer than the through-heating time and the time for which the capsule is maintained at an elevated temperature level. The cooling time of the capsule corresponds preferably to some 3 to 5 hours. The capsule may be cooled down to the ambient temperature in the furnace in which it had been heated and maintained at an elevated temperature. This contributes to an extremely &#34;gentle&#34; cooling, i.e. a comparatively low cooling rate without any quenching effect. 
     The cold isostatic compression of the capsule is expediently realized at a pressure in the range of some 4,500 to 5,500 bar. The metal powder, which has been rendered comparatively easy to manage by the thermal pretreatment, undergoes a compaction in this compression step which is sufficiently strong so as to furnish flawless products in the subsequent hot-working step. 
     Successful tests have been made with a powder of metal and metal alloys, which present(s) a carbon percentage of at least 0.5% to 2.5%. The mean particle size was roughly 125μ, with a maximum particle size ranging between 600 and 800μ. The length/diameter ratio of the capsule charged with powder was some (4-5):1 in the tests carried through, with capsules being used that had a length between 600 and 1,100 mm while their diameters ranged from 120 to 236 mm. 
     In an alternative method, which also solves the problem underlying the present invention, it is not always necessary to cool the pressed blank prior to hot-working. Good results can be achieved also when the hot-working step will have commenced immediately after maintaining the pressed blank at an elevated temperature higher than 1,000° C. 
     In the alternative method the pressed blank should be maintained at an elevated temperature for a prolonged period, preferably for a time as long as 4 to 5 hours. 
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     In the following, the method according to the present invention will be described in more detail with reference to some comparative examples: 
     1st Alternative 
     Example 1 
     An elongate section was produced for tool steel powder presenting the following composition (expressed in % by weight): 
     C=0.85; W=6.0; Mo=5.0; Cr=4.0; V=2.0; Fe=balance, 
     in accordance with the present invention: 
     The tool steel powder, presenting a mean particle size of 125μ and a maximum particle size of roughly 600μ, was charged into a thin-walled capsule of low-carbon steel, having a diameter of 120 mm and a length or height, respectively, of 600 mm. Then the capsule was sealed and exposed to cold-isostatic compression, at a pressure of roughly 5,000 bar to form a pressed blank. This furnished an appropriate powder density of 75% of the theoretical value. 
     Then the pressed blank was heated to the hot-working temperature. Subsequently the blank was extruded. The finished product presented unacceptable cracks and fissures and had to be rejected. Even the application of a higher pressure in the cold-isostatic compression step failed to lead to a better result. 
     Example 2 
     The tool steel powder from Example 1 was charged, like in Example 1, into a capsule. Then the capsule was sealed so as to be airtight. Subsequently, the capsule was heated to 1,150° C. until the capsule had reached this temperature level all through. Then the capsule was maintained at this level for roughly 1 hour, and was thereupon slowly cooled in the furnace. The cooling down to the ambient temperature lasted for 4 hours. Then the capsule was subjected to cold isostatic compression and extrusion. Due to this cold isostatic compression, a powder density of roughly 80% of the theoretical value was achieved. A higher density was not required, in spite of tool steel powder being used which presents a comparatively high resistance to deformation not only at low but also at high temperatures. In spite of this property of the metal powder employed, the theoretical density was achieved in the finished product. Moreover, the finished product did not show any flaws, i.e. cracks or fissures. 
     It should be mentioned here that in all of the tests the powder particles presented an approximately spherical shape, or else the metal powder would not have been suited for processing in the aforedescribed manner. 
     2nd Alternative 
     Example 3 
     The capsule charged with tool steel powder according to Example 1, was sealed so as to be airtight and then subjected to cold isostatic compression, achieving a powder density of roughly 75% of the theoretical value. This intermediate blank was then heated to 1,150° C. Following through-heating, the pressed blank was maintained at this temperature for roughly 1 hour and then slowly cooled. The cooling time in the furnace corresponded to 3 hours, approximately. After this treatment, the powder density was roughly 80% of the theoretical density. Then the compressed blank was extruded. The finished product presented the theoretical density and did not show any flaws, specifically cracks or fissures. 
     In both alternatives of the method, it is thus not necessary to demand a powder density higher than 80% of the theoretical value prior to extrusion of any other hot-working step, in the attempt to produce flawless powder-metallurgical finished products with a density of practically 100% of the theoretical value. 
     Finally, it should be emphasized that results will be achieved which are the better, the finer or smaller the powder particles are dimensioned. Moreover, it should be noted that with the application of the aforedescribed method the carbon percentage in the metal powder is not particularly critical, which means that even high-carbon metal powders may be processed in such a powder-metallurgical method to furnish acceptable and flawless finished products.