Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Patent
1989-07-21
1990-10-23
Lechert, Jr., Stephen J.
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
75246, 419 11, 419 15, 419 25, 419 29, 419 32, 420 12, C22C 2902
Patent
active
049649086
DESCRIPTION:
BRIEF SUMMARY
It is well known that in order to obtain good mechanical and physical properties in sintered materials, it is important to attain as high a density in the material as possible. Typical sintered densities of ferrous materials may range from 85% to 95% of the theoretical density of the material. As the density of the sintered material approaches 100% of the theoretical density the improvements in both mechanical and physical properties are dramatic. The reduction in the number of pores left in the material after sintering is recognised as being a prime objective if the material is to attain the best properties attainable for any particular material composition.
This development is concerned with the production of ferrous alloys which are useful due to their high mechanical strength, good wear resistance, toughness, and good high temperature properties. These are generally those ferrous alloys with significant elemental addition such as carbon, chromium, molybdenum, tungsten, vanadium and optionally cobalt and nickel, and possibly also other-carbide forming elements such as niobium and titanium and tantalum. Manganese and silicon usually are present as impurities in the starting materials. The ferrous alloys include some of the stainless steels and also cold and hot-working tool steels, including high speed tool steels.
Methods have been developed to attain high densities in materials produced from ferrous powder, some of which are listed below:
(a) Vacuum sintering of compacted metal powders approximately at the solidus temperature of the alloy This technique has the disadvantages that expensive equipment is required, the through-put is relatively small and cycle times are long. As a consequence, the method is only used for high added value products Additionally, the sintering temperatures are very critical, and typically have to be held at .+-.2.degree. C. If the temperature is too low, the material does not attain the high density required, and, if the temperature is too high, problems arise due to the changes that take place in the structure of the material.
(b) Hot isostatic pressing. In this technique, the metal powder is subjected to combined pressure and high temperature in order to promote the sintering together of the metal particles. The method has the major disadvantage that the equipment is very expensive, and, like vacuum sintering, the through-put is comparatively low, resulting in an expensive end product.
(c) Infiltration. In order to ensure that the material has as few remaining pores as is possible, the technique of infiltration can be used. The metal powder is first pressed and sintered, at the required temperature to produce a material which still has interconnected pores. The material is then reheated to a temperature above the melting point of an infiltrant which is placed on, or under the porous, sintered material. On melting the infiltrant passes into the pores by capillary action. It is possible to combine the sintering of the matrix and the infiltration in one heating step.
One disadvantage of infiltration is that it is necessary to press a separate infiltrant mass of the correct weight to exactly fill the pores in the porous, sintered material. Consequently, there are usually two, or more, pressings to be made for each component being fabricated by the method, and this leads to extra costs in manufacture. Additionally, if some of the pores are not interconnected with the surface, they are not filled and the pores remain after the infiltration process has been completed. Although high densities can be obtained, they are typically not as high as 98-99% of the theoretical density of the material and pores still exist. Although the infiltration method is used, it has a limited application.
It is clear that there remains a demand for materials that will attain as close to fill theoretical density as is possible during one sintering operation, and that the sintering method should be a low cost operation. Additionally, the powders from which the material is produced should be capable of b
REFERENCES:
patent: 4032302 (1977-06-01), Nakamura et al.
patent: 4121927 (1978-10-01), Lohman et al.
Bhat Nina
Lechert Jr. Stephen J.
Manganese Bronze Limited
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