Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2001-07-03
2003-03-18
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
Reexamination Certificate
active
06533836
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention is directed to iron-based mixed powders for use in metallurgy.
2. Description of Related Art
Iron-based mixed powders for use in powder metallurgy hereinafter also referred to simply as “iron-based mixed powder”) are manufactured, generally, by adding: (1) an iron powder for an iron-based powder as a substrate material (which can be a mixture of one or more kinds of iron powder), (2) alloying powder(s) (one or more kinds of alloying powder such as a copper powder, graphite powder and iron phosphide powder), optionally, (3) a lubricant such as zinc stearate (which can be a mixture of one or more kinds of lubricant) and, optionally, (4) machinability improving powder(s) (one or more kinds of machinability improving powder).
However, the iron-based mixed powders described above have a problem that the starting powder, particularly, the alloying powder(s) tends to cause segregation. This is because the iron-based mixed powder contains plural kinds of powder of different sizes, shape and density. Specifically, the distribution of starting powders in the iron-based mixed powder is not uniform during transportation after mixing, charging to a hopper, discharging from the hopper, or upon charging to the mold or during pressing.
For example, it is well-known for the mixed powder of the iron powder and the graphite powder that the iron powder and the graphite powder move and displace independently of each other in a transportation container during track transportation and, as a result, the graphite powder of lower specific gravity floats to the surface and causes segregation. Further, because the mixed powder of the iron powder and the graphite powder charged in the hopper segregates due to movement in the hopper, it is also well-known that the concentration of the graphite powder is different, for example, between each of the initial stage, the middle stage and the final stage of discharging from the hopper.
When the segregated iron-based mixed powder is charged in a mold and pressed into a molding product and the molding product is finally sintered into a sintered body as a final product, the composition fluctuates for every product (sintered product). As a result of the fluctuation of the composition, the size and the strength of products vary greatly to cause failed products.
Further, because each of the alloying powders to be mixed, such as copper powder, graphite powder and iron phosphide powder, is finer than the iron-based powder, the specific surface area of the iron-based mixed powder increases by the mixing of the alloying powder(s) to lower the fluidity of the iron-based mixed powder. Lowering the fluidity of the iron-based mixed powder lowers the charging rate of the iron-based mixed powder into the mold and, therefore, lowers the production speed of the molding product (also referred to as compact powder or green compact).
As a countermeasure for such problems in iron-based mixed powders, as a technique of preventing segregation, Japanese Patent Laid-Open No. 219101/1989, for example, proposes an iron powder for use in powder metallurgy, comprising from 0.3 to 1.3% of a lubricant, from 0.1 to 10% of an alloying element powder and the balance of an iron powder, in which the alloying element powder is adhered on the surface of the iron powder. According to this publication, the iron powder causes no segregation of the ingredients during handling and enables to obtain homogeneous sintered products.
Further, Japanese Patent Laid-Open 162502/1991 discloses a method of manufacturing an iron-based mixed powder for use in powder metallurgy with less segregation of additives and less aging change of the fluidity. The method described in Japanese Patent Laid-Open No. 162502/1991 comprises conducting primary mixing by adding a fatty acid to an iron-based powder, then conducting secondary mixing by adding a metal soap to the alloying powder(s), elevating the temperature during or after the secondary mixing, and then applying cooling during tertiary mixing, thereby adhering the alloying powder(s) to the surface of the iron-based powder by a binding effect of a co-molten product of the fatty acid and the metal soap.
Japanese Patent Publication No. 3004800 discloses an iron-based mixed powder using a binder not containing a metal compound as a binder for the alloying powder(s) to the surface of the iron-based powder. It is described that contamination to a sintering furnace can be reduced by the use of the binder material not containing the metal compound.
However, the iron-based mixed powder applied with the segregation-preventive treatment by each of the publications described above has a problem in the die filling property to a mold and, particularly, has a property that the amount of charge to a narrow width portion of the mold (thin-walled cavity) tends to be decreased.
In view of the above, the present inventors have experimentally confirmed the die filling property of the iron-based mixed powder applied with the segregation-preventive treatment disclosed by the publications described above. First, the result of this experiment is explained as follows.
To an atomized iron powder as the iron-based powder, 2 mass % of a copper powder and 0.8 mass % of a graphite powder as the alloying powder(s), and 0.4 parts by weight of zinc stearate and 0.2 parts by weight of machine oil (spindle oil) as the binder based on 100 parts by weight of the total sum of the iron power and the alloying power, were mixed and heated to adhere the alloying powder(s) to the surface of the iron powder (example of a binder treatment). Then, 0.3 parts by weight of zinc stearate was mixed with these components as a free lubricant. An iron-based mixed powder including a mixture of an iron powder and a free lubricant, in which alloying powder(s) is adhered on the surface of the iron powder (known product), was obtained by this treatment. 150 g of the iron-based mixed powder was charged in a shoe box sized 20 mm×60 mm×100 mm, as shown in FIG.
1
.
The shoe box was moved in a direction to a mold at a speed of 200 mm/s, stood stationary just above the mold for 1 second, and then retracted to the original position in the arrangement, as shown in FIG.
1
. The iron-based mixed powder was charged into the mold by the operation. The mold used has a cavity with a thickness of T mm, length, L, of 60 mm and depth, D, of 60 mm. The thickness T mm was varied as 1, 2 and 5 mm.
After charging, the iron-based mixed powder charged in the cavity was molded at a pressure of 488 MPa and the weight of the obtained molding product was measured. Then, the charged density (=the molding product weight/mold volume) was calculated to evaluate the die filling property of the iron-based mixed powder to the mold. The result for the iron-based mixed powder (known product) is shown in FIG.
2
. It can be seen from
FIG. 2
that the charged density decreases as the cavity thickness T of the mold decreases in the known product. For example, when the cavity thickness T of the mold is 1 mm, the existent iron-based mixed powder is charged by less than one-half for the apparent density. As described above, when the cavity thickness of the mold is thin, die filling property of the iron-based mixed powder treated for segregation by the known techniques is deteriorated.
In the known product of the reduced die filling property as described above, when it is charged into a mold, for example, of a gear shape, the charged density is lower at a narrow width portion of the tooth tip as compared with other portions of the gear. Then, when it is pressurized as it is into the molding product and further sintered, because the amount of shrinkage differs depending on the portions, the dimensional accuracy of a part is deteriorated. Generally, when the charged density is different and the green density is different for different portions, the rate of dimensional change upon sintering also differs and, further, the sintering density is also different. Accordingly, in the portion at t
Ozaki Yukiko
Uenosono Satoshi
Kawasaki Steel Corporation
Mai Ngoclan
Oliff & Berridg,e PLC
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