Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2002-02-21
2003-11-18
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
Reexamination Certificate
active
06648941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an iron-based mixed powder for powder metallurgy. More particularly, this invention is suitable for obtaining a sintered compact having excellent machinability, but is not limited to this application.
2. Description of the Related Art
Progress in powder metallurgical techniques has enabled the production of parts having complicated shapes and high dimensional precision near to “net shape” (requiring substantially no machining for obtaining a target shape). An iron-based powder metallurgical product is generally produced by mixing an iron-based powder, a powder for an alloy such as a copper powder, a graphite powder, or the like, and a lubricant such as zinc stearate, lithium stearate or the like to form an iron-based mixed powder for powder metallurgy, filling a die(mold) with the resultant mixed powder, pressing the mixture under pressure, sintering the green compact to form a sintered compact and, if required, machining the product. The thus-produced sintered compact has a high content of voids and, thus, has higher cutting force than metallic materials obtained by a solution process. Therefore, to improve the machinability of the sintered compact, any one of various powders of Pb, Se, Te, S, MnS, BaS, CaS, and the like or an alloy thereof is conventionally added to the iron-based mixed powder.
However, Pb has a melting point as low as 330° C. and, thus, has the problem that it is melted in the sintering process and is not dissolved in iron, thereby causing difficulties in homogeneous dispersing Pb in a base matrix. Se and Te embrittle the sintered compact and, thus, have the problem of significantly deteriorating the mechanical properties of the sintered compact.
Japanese Examined Patent Publication No. 46-39564 discloses a free-cutting metallic material produced by a powder metallurgical method in which BaSO
4
or BaS is added to iron or an iron-based alloy singly or in a mixture. This publication also discloses that the method improves machinability by adding BaSO
4
or BaS singly or in a mixture. Japanese Examined Patent Publication No. 52-16684 discloses a method of producing sintered steel with excellent machinability in which a mixed powder obtained by adding calcium sulfide CaS or calcium sulfate CaSO
4
to an iron-based raw material powder is pressed and then sintered.
However, mixing with S or a S-containing compound such as MnS or the like as a machinability improving powder causes the problem of contaminating the refractory of the sintering furnace, the conveyor mesh belt, heating device, etc. with H
2
S produced in sintering, thereby decreasing the lifetime of these parts. In addition, there is also the problem of deteriorating the appearance of the sintered compact. Therefore, mixing a S-containing compound as a machinability improving powder with the iron-based mixed powder is avoided. Furthermore, when BaS, CaS, or the like remains in the sintered compact, the sintered compact has the problem in which corrosion easily occurs due to hygroscopicity of BaS and CaS.
For these problems, for example, Japanese Unexamined Patent Publication No. 57-198201 discloses a steel powder for sintering which contains 0.001 to 0.10% of Ca and 0.05 to 1.0% of O and which provides a sintered compact having good machinability. The sintered compact produced by using the sintering powder disclosed in Japanese Unexamined Patent Publication No. 57-198201 does not have the problem of contaminating the sintering furnace because S is not contained, but the sintered compact has the problem of deteriorating fluidity of the powder and destabilizing pressing because calcium oxide has hygroscopicity.
Japanese Unexamined Patent Publication No. 7-507358 discloses an iron-based powder composition containing 0.1 to 0.6% by weight of calcium fluoride CaF
2
and having improved machinability. However, according to the findings obtained in research conducted by the inventors, the method of simply mixing calcium fluoride in a free state as disclosed in this publication cannot satisfactorily improve machinability. Furthermore, impurities contained in calcium fluoride CaF
2
might affect the dimensional changes and mechanical properties of the sintered compact. Therefore, this method is preferably carried out with caution, such as using high-purity calcium fluoride.
Japanese Unexamined Patent Publication No.9-279204 discloses an iron-based mixed powder for powder metallurgy which contains 0.02 to 0.3% by weight of CaO—Al
2
O
3
—SiO
2
system compound oxide powder mainly comprising an iron powder and having an anorthite phase and/or gehlenite phase and an average particle diameter of 50 &mgr;m or less. However, unless the CaO—Al
2
O
3
—SiO
2
system compound oxide powder containing fewer impurities and having a limited particle size is used, there is the problem of deteriorating the properties of the powder and the sintered compact.
Japanese Unexamined Patent Publication No. 63-137137 discloses a method of producing sintered steel in which a graphite powder comprising an alkali earth fluoride in an amount corresponding to 0.1 to 1.2% by weight of a raw material iron powder, the alkali earth fluoride being partially or entirely adhered to the surfaces of the graphite powder, is added to the raw material iron powder, and the resultant mixture is sintered. This publication also discloses that the sintered steel produced by the method has excellent machinability. However, the technique disclosed in Japanese Unexamined Patent Publication No. 63-137137 must use an alkali earth fluoride ground to a small particle size of about {fraction (1/10)} of the particle size of the graphite powder, thereby causing the problem of adding the step of grinding the alkali earth fluoride as a separate step and significantly increasing the production cost.
It would accordingly be advantageous to solve the above problems of conventional techniques and to provide an iron-based mixed powder capable of improving machinability of a sintered compact without deteriorating the mechanical properties of the sintered compact and a sintering furnace.
SUMMARY OF THE INVENTION
We intensively studied machinability improving powders capable of improving machinability without deteriorating the mechanical properties of a sintered compact. As a result, it was found that a phosphate compound of an alkali earth metal, particularly, calcium phosphate compound is effective. As a result of further experiment and research based on that finding, this invention was achieved.
In accordance with a first aspect of the invention, an iron-based mixed powder for powder metallurgy comprises an iron-based powder, an alloy powder (a powder for an alloy), a machinability improving powder, and a lubricant, wherein the machinability improving powder contains a phosphate compound of an alkali earth metal.
The invention also provides an iron-based sintered compact obtained by pressing the iron-based mixed powder of the invention resulting in a green compact, and then sintering the green compact.
The phosphate compound of an alkali earth metal comprises preferably calcium phosphate compound, and more preferably hydroxyapatite. The machinability improving powder preferably substantially consists of calcium phosphate compound (more preferably, hydroxyapatite) with substantially no additive intentionally added. The hydroxyapatite preferably comprises crystallites of over about 200 Å, preferably over about 600 Å.
In the first aspect of the invention, the machinability improving powder preferably further comprises an alkali earth fluoride, preferably calcium fluoride. In this case, the machinability improving powder preferably comprises calcium phosphate compound and calcium fluoride, and more preferably comprises hydroxyapatite preferably comprising crystallites of over about 200 Å, more preferably over about 600 Å, and calcium fluoride. In this case, the machinability improving powder preferably comprises calcium fluoride and calcium phosphate compound at a ratio (con
Ohta Junichi
Sonobe Akio
Uenosono Satoshi
Kawasaki Steel Corporation
Mai Ngoclan
Piper Rudnick LLP
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