Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Metal and nonmetal in final product
Reexamination Certificate
2002-10-25
2004-02-24
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Metal and nonmetal in final product
C419S026000, C419S029000, C419S036000
Reexamination Certificate
active
06696014
ABSTRACT:
RELATED APPLICATION
This application is a divisional of application Ser. No. 09/934,428, filed Aug. 21, 2001 now U.S. Pat. No. 6,514,307, which claims priority from Japanese Patent Appln. No. 2000-015655, filed Jan. 24, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an iron-based sintered component formed of an iron-based metal powder as a raw material and suitable to machinery parts, or an iron-based powder metal body as an intermediate material suitable to manufacture of the sintered iron-based component.
2. Description of the Related Art
Powder metallurgical technology can produce a component having a complicated shape as a “near net shape” with high dimensional accuracy and can markedly reduce the cost of cutting and/or finishing. In such a near net shape, almost no mechanical processing is required to obtain or form a target shape. Powder metallurgical products are, therefore, used in a variety of applications in automobiles and other various fields. For reduction in size and weight of the components, demands have recently been made on such powder metallurgical products to have higher strength. Specifically, strong demands have been made on iron-based powder products (sintered iron-based components) to have higher strength.
A basic process for producing a sintered iron-based component (sometimes hereinafter referred to as “sintered iron-based compact” or simply as “sintered compact”) includes the following sequential three steps (1) to (3):
(1) a step of mixing sub-material powders such as a graphite powder and/or copper powder and a lubricant such as zinc stearate or lithium stearate to an iron-based metal powder to yield an iron-based powder mixture;
(2) a step of charging the iron-based powder mixture into a die and pressing the mixed powder to yield a green compact; and
(3) a step of sintering the green compact to yield a sintered compact.
The resulting sintered compact is subjected to a sizing or cutting process according to necessity to thereby yield a product such as a machine component. When a higher strength is required for the sintered compact, it is subjected to heat treatment for carburization or bright quenching and tempering.
The resulting green compact obtained through the steps (1) to (2) has a density of at greatest from about 6.6 to about 7.1 Mg/m
3
and, accordingly, a sintered compact obtained from the green compact has similar density.
In order to further increase the strength of such iron-based powder products (sintered iron-based components), it is effective to increase the density of the green compact to thereby increase the density of the resulting sintered compact obtained by subsequent sintering. The component has fewer voids and better mechanical properties such as tensile strength, impact resistance and fatigue strength when the sintered compact has a higher density.
A hot pressing technique, in which a metal powder is pressed while heating, is disclosed in, for example, Japanese Published Unexamined Patent Application No. 2-156002, Japanese Published Unexamined Patent Application No. 7-103404 and U.S. Pat. No. 5,368,630 as a pressing process for increasing the density of a green compact. For example, 0.5% by mass of a graphite powder and 0.6% by mass of a lubricant are added to a partially alloyed iron powder in which 4 mass % Ni, 0.5 mass % Mo and 1.5 mass % Cu are contained, to yield an iron-based powder mixture. The iron-based powder mixture is subjected to the hot pressing technique at a temperature of 150° C. under a pressure of 686 MPa to thereby yield a green compact having a density of about 7.30 Mg/m
3
. However, application of the hot pressing technique requires heating facilities for heating the powder to a predetermined temperature which increases production cost and decreases dimensional accuracy of the component due to thermal deformation of the die.
Further, Japanese Published Unexamined Patent Applications No. 1-123005, for example, discloses sintering cold forging process as a combination of the powder metallurgical technology and cold forging that can produce a product having a substantially true density.
The sintering cold forging process is a molding/working method for obtaining a final product of high density composition by compacting a metal powder such as an iron-based powder mixture into a preform, preliminarily sintering the preform, cold forging and then re-sintering the same instead of the steps (2) and (3) described above. In this invention, the preliminarily sintered body is particularly referred to as a (iron-based) sintered powder metal body. Further, when it is referred to simply as a sintered body or sintered component, it means a sintered body obtained by re-sintering and/or heat treatment. The technique described in Japanese Published Unexamined Patent Application No. 1-123005 is a method of coating a liquid lubricant on the surface of a preform for cold forging and sintering, provisionally compacting the preform in a die, then applying a negative pressure to the preform to thereby suck and remove the liquid lubricant and then re-compact and re-sinter. According to this method, since the liquid lubricant coated and impregnated to the inside before the provisional compaction is sucked before the re-compaction, minute voids in the inside are collapsed and eliminated during re-compaction to obtain a final product with high density. However, the density of the final sintered product obtained by this method is about 7.5 Mg/m
3
at the greatest and the strength has a limit.
For further improving the strength of the product (sintered body), it is effective to increase the concentration of carbon in the product. It is general in the powder metallurgy to mix a graphite powder as a carbon source with other metal powder materials, and it may be considered a method of obtaining a high strength sintered body by compacting and then preliminarily sintering a metal powder mixed with a graphite powder to form a sintered preform, further re-compacting and re-sintering (application of sintering/cold forging method). However, when preliminary sintering is applied in the existent method, about all of the mixed carbon diffuses into the matrix of the preform upon the preliminary sintering to increase the hardness of the sintered powder metal body. Therefore, when the sintered powder metal body is re-compacted, the re-compacting load increases remarkably and the deformability of the sintered powder metal body is lowered, so that it can not be fabricated into a desired shape. Accordingly, high strength and high density product can not be obtained.
For the problem described above, U.S. Pat. No. 4,393,563, for example, discloses a method of manufacturing a bearing component without pressing at high temperature. The method comprises the steps of mixing an iron powder, an iron alloying powder, a graphite powder and a lubricant, compacting the powder mixture into a preform, preliminarily sintering and then subjecting the same to cold forging with at least 50% plastic working, then re-sintering and annealing and roll forming the compact into a final product (sintered component). For the technique described in U.S. Pat. No. 4,393,563, it is described that when preliminary sintering is applied under the condition of suppressing diffusion of graphite, the preliminarily sintered component (preliminarily sintered body) has high deformability and can lower the compacting load in the subsequent cold forging. U.S. Pat. No. 4,393,563 recommends preliminary sintering conditions of 1100° C.×15-20 min. However, it has been found by the experiment of the present inventors that, under the conditions described above, graphite is completely diffused into the preform to remarkably increase the hardness of the material for sintered preform to make the subsequent cold forging difficult.
For the problem described above, Japanese Published Unexamined Patent Application No. 11-117002 proposes, for example, a sintered powder metal body by compacting a metal powder formed by mixing 0.3% having a structure where graph
Anma Hiroyuki
Fujinaga Masashi
Hatai Yasuo
Iijima Mitsumasa
Koizumi Shin
JFE Steel Corporation
Mai Ngoclan
Piper Rudnick LLP
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