Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Patent
1999-05-24
2000-12-12
Mai, Ngoclan
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
75246, 419 11, 419 38, C22C 3302
Patent
active
061592668
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to sintered powder metal bodies suitable for various kinds of machine parts, and a process for producing the sintered powder metal bodies.
BACKGROUND ART
The process of producing sintered powder metal bodies basically includes the steps of mixing powders of raw materials, compressing the powdery mixture into a green compact, sintering the green compact, and conducting after-treatments such as heat treatment to form a final product. Although the final product may be produced by only the basic process, in many cases the final product are subjected to additional working and/or treatments depending on various applications thereof.
For instance, in order to obtain machine parts with an increased mechanical strength, Japanese Patent Application First Publication No. 1-123005 discloses a process for making sintered powder metal bodies. The process includes the steps of compacting a powder metal mixture into a green compact, sintering the green compact to form a preform, compressing the preform by cold forging, and sintering the compressed preform to form a final product. Specifically, the compressing (cold forging) step includes a first temporary-compressing step and a second regular-compressing step. The preform is coated with a lubricant before being compressed in the first temporary-compression step. Subsequent to being compressed in the first temporary-compressing step, the preform is subjected to a negative pressure so that the lubricant present in fine voids in the porous structure of the preform is evaporated and removed therefrom. Then, the preform is re-compressed in the second regular-compressing step.
In accordance with the proposed process, since the lubricant in the fine voids in the porous structure of the preform is removed, the porous structure is squeezed and the fine voids are eliminated in the second regular-compressing step. As a result, the preform may be compressed into the final product having a relatively high density of approximately 7.4-7.5 g /cm.sup.3 which enhances the mechanical strength of the final product.
Meanwhile, in order to further enhance the mechanical strength of the final product, it will be appreciated to increase a carbon content of the final product, namely, an amount of a graphite powder admixed with a metal powder. However, generally, as the amount of the graphite powder admixed increases, an elongation of a sintered body obtained by sintering a preform which is made from the powdery mixture, decreases and a hardness thereof increases. This causes such a problem that, when the preform is re-compressed, deformability of the preform is reduced whereby the re-compression of the preform cannot be sufficiently achieved.
For example, a pamphlet entitled "The Second Presentation of Developments in Powder Metallurgy" issued on Nov. 15, 1985 by Japan Powder Metallurgy Association, page 90, discloses that a sintered body having a carbon content of 0.05-0.5% has an elongation of 10% and a hardness of Rockwell B Scale (HRB) 83. It is empirically known that, if a sintered body has an elongation of not more than 10% and a hardness of more than 60 HRB, the sintered body cannot be readily re-compressed into the final product. Therefore, there is a demand for obtaining a sintered body having an increased elongation and a reduced hardness and thus excellent deformability.
The present inventors have been devoted themselves to studies to obtain machine parts having a higher mechanical strength which are made from sintered metal. According to the studies by the present inventors, it has been noticed that, in a case where the machine parts are obtained by the steps of temporarily compressing a metal powder to form a preform, sintering the preform to form a sintered body and re-compressing and re-sintering the sintered body, characteristics of the sintered body are the important factors that determine the compressibility in the re-compressing step and mechanical properties of the machine parts to be obtained. Further, the present inventors h
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-Pamphlet --, "The Second Presentation of Development in Powder Metallurgy", Nov. 15, 1985, Japan Powder Metallurgy Association.
Amma Hiroyuki
Fujinaga Masashi
Yoshimura Takashi
Kawasaki Steel Corporation
Mai Ngoclan
Unisia Jecs Corporation
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