Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Patent
1989-01-23
1990-10-23
Hunt, Brooks H.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
419 48, 419 58, 419 68, 419 67, B22F 700
Patent
active
049650432
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method of powder-metallurgical production of objects, specifically tubes, rods, or the like, in accordance with introduction of a gaseous mixture of nitrogen and hydrogen with a metal powder into a sealed capsule for cold-isotatic pressure prior to hot working.
The objects to be manufactured should consist particularly of stainless steel or high-alloyed nickel steel with good high-temperature characteristics, such as steels containing some 80% of nickel and 20% of chromium. The objects to be produced may, however, also consist of high-speed steels or any other metals and/or metal alloys. The capsules for use in production of these objects are made of a thin metal sheet having a gauge in the range of 1 to 2 mm. For a leakage test of the capsules it has been proposed to introduce helium into the capsule (see German Patent 31 15 095). When this test proved a sufficient sealinig the capsule was charged with a powder of metal and/or metal alloys, then closed and exposed to a cold-isostatic pressure for subsequent hot-working. This type of leakage test, however, has shown that one part of the introduced helium remains in the capsule, in spite of previous flushing with nitrogen or a similar gas, specifically when the capsules are comparatively long and tubular and have a narrow annular gap at one end. Moreover, the diffusion of the enclosed helium gas out of the closed capsule requires a very long time. With a major helium volume inside the capsule the manufactured product tends to become brittle, which is obviously due to occluded helium. Apart from that, the aforementioned method does not allow for leakage testing of the charged and closed capsule, which results in the consequence that the closed capsule may well still present a certain degree of porosity, in particular in the area of the closed powder-charging opening. Thus there is the risk of water penetrating into the capsule during the cold-isostatic pressing step, which leads to actual explosion of the capsule in the subsequent heating step.
The present invention is based on the problem of eliminating the safety risk mentioned last, and, in particular, of obtaining top quality products (with increased homogeneity, improved strength and uniform surface quality, as well as lack of brittleness).
SUMMARY OF THE INVENTION
The present invention solves the problem discussed above.
The present invention is directed to a method of powder-metallurgical production of objects such as tubes, rods or the like which includes charging a thin-walled capsule with a metal powder selected from a metal and/or a metal alloy. The capsule is closed and exposed to a cold-isostatic pressure so as to form a pressing for subsequent hot-working such as hot extrusion.
In accordance with the present invention, a nitrogen and hydrogen gas mixture is introduced into the capsule together with the metal powder and simultaneously pre-compacts the powder to about a density of 60 to 80% of the theoretical density and preferably to about 70%. The gas mixture generally includes 70 to 95% of nitrogen and 30 to 5% hydrogen, and in one preferred process about 80% of nitrogen and 20% of hydrogen and in another about 90% of nitrogen and 10% of hydrogen. The pre-compacted capsule is subjected to the cold-isostatic pressure for compaction up to about 95% of the theoretical density. The capsule is then tested for leakage in a vacuum chamber prior to the heating and hot-working.
In a particular application, the metal powder is selected from the group of stainless steel, high-alloyed nickel steels and the like which have good high-temperature characteristics. The metal powder preferably contains about 80% of nickel and about 20% of chromium or high-speed steels. The capsule may consist of carbon steel with a carbon percentage less than 0.004%.
By contrast to the known proposal, the present invention provides for a porosity or leakage test performed on the finished capsule after the cold-isostatic pressing step or on the pressing itself so
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Avesta Nyby Powder AB
Hunt Brooks H.
Mai Ngoulant
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