Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
1999-11-04
2001-05-08
Jenkins, Daniel (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S246000, C419S011000, C419S013000, C419S034000
Reexamination Certificate
active
06228138
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a good machinability Fe-based sintered alloy and a process of manufacture therefor, and more particularly relates to a technique which can improve machinability by sintering a boron compound powder added to a mixed powder of an Fe-based material.
An Fe-based sintered alloy can be produced in near-net shape so that manufacturing cost for processing can be reduced, and moreover, elements may be dispersed therein having specific gravities which differ greatly, and in different alloys in which dissolution is difficult, whereby properties may be obtained such as wear resistance, etc. For this reason, Fe-based sintered alloys are often used in various fields of technology. For example, mechanical parts made of Fe-based sintered alloy can be made without considerable machining processing, even if the parts are of complicated configuration, whereby such parts can be widely employed in valve driving systems, bearings, and the like, in automobiles, motorcycles, etc. However, most mechanical parts made of Fe-based sintered alloys must be machined, therefore poor machinability still present problems.
In order to improve the machinability of Fe-based sintered alloys, many attempts have heretofore been made. In one attempt, an Fe powder containing sulfur is used as a starting material powder. In another attempt, sulfide is added to and mixed with a starting material powder. In still another attempt, a sintered compact is sulfurized in an atmosphere of hydrogen sulfide gas. However, when sulfur as a cutting facilitating component is dispersed in the matrix of a sintered alloy, improvement in machinability is limited. Moreover, sulfur is an element which decreases strength, particularly toughness, in sintered alloys, and also promotes corrosion in sintered alloys; therefore, use of such sintered alloys is limited.
Another technique which fills resin, etc., into pores of a sintered alloy is also available. In such a sintered alloy, the resin in the pore serves as an initiating point for chip breaking, whereby the chip-breaking property is superior. However, in such a technique, using certain types of resin may shorten the service life of a cutting tool such as a cutter. Moreover, a process for removing the resin from the pores after cutting processing may be required, depending on the purpose for which the sintered alloy is to be used.
Therefore, the present applicant proposed an improved method for an Fe-based sintered alloy, in which a boron compound powder is added to a mixed powder of an Fe-based material including carbon, and is sintered, in Japanese Unexamined Patent Application Publication No. 241701/97. According to this proposed technique, diffusion of the carbon into the matrix is suppressed by the boron, whereby machinability can be improved with a decrease in hardness of the Fe-based sintered alloy.
However, further improvement of machinability has been recently demanded to enhance high performance alloys for automobiles.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a good machinability Fe-based sintered alloy which is further improved over the above Fe-based sintered alloy and a process of manufacture therefor. Generally, it is known that such materials harden when the carbon content of an Fe-based sintered part is increased, and that machinability thereof is lowered thereby. However, according to research by the inventors, the following knowledge was obtained. In the case in which the matrix of the Fe-based sintered portion closely resembles pure iron and the hardness thereof is too low, the amount of wear on a cutting tool conversely increases.
FIG. 1
is a chart showing the amount of wear on a cutting tool in cutting processing with respect to 4 kinds of Fe-1.5Cu—C-based sintered parts (A-D) having different hardnesses, which are produced by changing the C content, and an Fe-1.5Cu—C-based sintered part (E), which has improved machinability by a technique disclosed in the above-mentioned Japanese Unexamined Patent Application Publication No. 157706/97.
Hitherto, it was expected that machinability of a part A having the lowest hardness would be most desirable and that the amount of tool wear thereof would be minimal. However, as is apparent from
FIG. 1
, the softest part A in which hardness of a surface thereof ranges from Hv 110 to 120 (load=100 gf) actually has the highest amount of wear, and a part C in which the hardness ranges from Hv 200 to 230 has the least amount of wear. It is apparent that the amount of wear on the cutting tool is drastically reduced in comparison with the amount of wear on the part A, in the case in which hardnesses range from Hv 150 to 250. As a reason for this, it is believed that adhesive wear is generated on an edge of the cutting tool during cutting processing since ferrite, which is a matrix of the Fe-based sintered portion, has high viscosity.
As shown in
FIG. 1
, an Fe-based sintered alloy having improved machinability by a technique shown in the Japanese Unexamined Patent Application Publication No. 157706/97 has the smallest amount of wear, and remarkable improvement in machinability appears. Moreover, it is believed that machinability can be further improved by increasing matrix hardness and suppressing generation of adhesive wear.
Therefore, the inventors found that the amount of wear on a cutting tool is remarkably reduced when hardness is increased by alloying ferrite and is set within a specific range.
In consideration of this situation, a good machinability Fe-based sintered alloy of this invention has an overall composition consisting of, in percent by weight, at least one element selected from the group consisting of P in the amount of 0.1 to 1.0% and Si in the amount of 2.0 to 3.0%, B in the amount of 0.003 to 0.31%, O in the amount of 0.007 to 0.69%, C in the amount of 0.1 to 2.0%, and the balance consisting of Fe and unavoidable impurities, has a matrix hardness ranging from Hv 150 to 250, and has free graphite dispersed therein. Here, the Hv refers to a Vickers hardness at a load of 100 gf.
In this invention, free graphite is dispersed and functions as a solid lubricant, whereby machinability is improved. Boron is contained at 0.003% by weight or more in the Fe-based sintered alloy, whereby the boron prevents graphite from diffusing as C so as to ensure that the graphite remains free and prevents pearite from forming in the matrix. According to the research of the inventors, reasons for the improved machinability due to the boron are as follows.
That is to say, boron compound powder (for example, boron oxide (B
2
O
3
)), added to a powder mixture, dissolves at about 500° C., which is lower than the temperature at which C diffuses into the matrix during heating for sintering, and covers the surfaces of the graphite powder. The C of the graphite powder does not diffuse into the ferrite matrix and cannot form pearite, and remains as free graphite, and the machinability thereof is remarkably improved by functioning as a solid lubricant. In this invention, matrix hardness is particularly set as described above by containing P and Si, whereby further improvement in machinability is achieved.
P: Action of ferrite strengthening is slight when the P content is under 0.1% by weight. As a result, a hard matrix is not obtained, thereby failing to improve machinability. In contrast, when the P content exceeds 1.0% by weight, the generation rate of the Fe—P liquid phase increases in sintering, whereby a green compact easily loses its shape during sintering. Therefore, the P content ranges preferably from 0.1 to 1.0% by weight. Moreover, the P can be added in the form of a simple powder; however, it is preferably added in the form of an Fe—P alloy powder since the simple powder is dangerous.
Si: Si can be added in the form of a simple powder so that it quickly diffuses in the matrix; however, pure Si is expensive, and it is therefore preferably added in the economical form of an Fe—Si alloy powder in consideration of industrial produc
Ishii Kei
Tsutsui Tadayuki
Yamanishi Yuuji
Hitachi Powdered Metals Co. Ltd.
Jenkins Daniel
Scully Scott Murphy & Presser
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