Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2000-03-23
2002-04-23
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C419S031000, C419S032000, C419S037000, C419S038000
Reexamination Certificate
active
06375709
ABSTRACT:
This invention relates to a lubricant for iron-based metallurgical powder compositions, as well as metal power compositions containing the lubricant. The invention further concerns a method for making sintered products by using the lubricant, as well as use of the lubricant in a metal powder composition in warm compaction. By using the lubricant according to the invention, high green strength may be obtained.
In industry, the use of metal products manufactured by compacting and sintering metal powder compositions is becoming increasingly widespread. A number of different products of varying shape and thickness are being produced, and the quality requirements placed on these products are that the manufactured metal products have high density as well as high strength.
In metal compaction, different standard temperature ranges are used. Both cold pressing and warm pressing require the use of a lubricant.
Compaction at temperatures above room temperature has evident advantages, yielding a product of higher density and higher strength than compaction performed at lower temperatures.
Most of the lubricants used in cold compaction cannot be used in high-temperature compaction since they seem to be effective within a limited temperature range only. An ineffective lubricant considerably increases the wear on the compacting tool.
The degree of wear on the tool is influenced by various factors, such as the hardness of the material of the tool, the pressure applied, and the friction between the compact and the wall of the tool when the compact is ejected. The latter factor is strongly linked to the lubricant used.
The ejection force is the force required for ejecting the compact from the tool. Since a high ejection force not only increases wear on the compacting tool but also may damage the compact, this force should preferably be reduced.
However, the use of a lubricant may create problems in compaction, and it is therefore important that the lubricant is well suited to the type of compaction carried out.
In order to perform satisfactorily, the lubricant should be forced out of the pore structure of the powder composition in the compacting operation, and into the interspace between the compact and the tool, thereby lubricating the walls of the compaction tool. By such lubrication of the walls of the compaction tool, the ejection force is reduced.
Another reason why the lubricant has to emerge from the compact is that it would otherwise create pores in the compact after sintering. It is well-known that large pores have an adverse effect on the dynamic strength properties of the product.
An object of the new lubricant according to the present invention is to make it possible to manufacture compacted products having high green strength, high green density as well as sintered products having high sintered density and low ejecting force from the lubricant in combination with metal powders. As the compact is subject to considerable stress when ejected from the compacting tool and as the product must maintain its integrity during the handling between compaction and sintering without cracking or being otherwise damaged, it is important to have high green strength. This is especially important in the case of thin parts.
The lubricant according to the invention contains a polyester, which is a polymer formed by e.g. the esterification condensation of di-functional alcohols and acids. Polyesters are available as resins and thermoplastics, and are subdivided into aliphatic and aromatic polyesters, mainly depending on the type of acid monomer used. Aromatic polyesters are usually non-hygroscopic, aliphatic polyesters are, however, known to be more sensitive to moisture. Polyesters can be further classified into saturated and unsaturated polyesters, depending on whether double bonds are present in the polymer backbone. While saturated polyesters are relatively unreactive, unsaturated polyesters are suitable as resins by copolymerisation with other monomers, such as styrenes, diallyl phthalates, etc.
The polyester according to the invention is a saturated polyester, aromatic or partly aromatic, which has a number-average molecular weight Mn of 5000-50000, and 50-100% by weight, preferably 60-100% by weight and most preferred 70-100% by weight of the lubricant is made up of this polyester. Apart from the polyester, the lubricant according to the invention, may contain other PM-lubricants, such as zinc stearate, lithium stearate and/or lubricants of amide wax type, such as ethylene bis-stearamid. A preferred lubricant according to the invention contains 0-30% by weight of zinc stearate, 0-30% by weight of lithium stearate, and/or 0-30% by weight of a lubricant of amide wax type, the balance being polyester.
The polyester is preferably a polymer or a copolymer of alkylene phthalate, wherein alkylene phthalate is a C
2
-C
8
-alkylene phthalate, whereby the polyester preferably has a melting point peak above 100° C.
Most preferred, the polyester is a poly(alkylene terephthalate) or a poly(alkylene isophthalate).
The invention further concerns a metal powder composition containing a metal powder and a lubricant according to the invention. This metal powder composition can be used for warm compaction.
The metal powder composition according to the invention comprises 0.1 to 2% by weight of the lubricant according to the invention, 0.005-3% by weight of binding agent, 0-0.5% by weight of plastiziser, 0.01-3% by weight of graphite, 0-2% by weight of thermoplastics, 0-15% by weight, preferably 0-7% by weight of alloying elements, 0 to 2% by weight of processing aids, and 0 to 2% by weight of hard phases, the balance being iron powder selected from the group consisting of essentially pure iron powders, partially prealloyed iron powders and prealloyed iron powders.
The lubricant preferably makes up 0.2-0.8% by weight of the metal powder composition according to the invention, based on the total amount of the metal powder composition. The possibility of using the lubricant according to the present invention in small amounts is an especially advantageous feature of the invention since it permits compacts and sintered products having high densities to be achieved cost-effectively.
As used in the description and the appended claims, the expression “partly aromatic” encompasses a polyester in which some of the aromatic dicarboxylic acids have been replaced by aliphatic dicarboxylic acids in order to modify the temperature dependence/melt behaviour (rheology) of the resulting polyester.
As used in the description and the appended claims, the expression “metal powder” encompasses iron-based powders essentially made up of iron powders containing not more than about 1.0% by weight, preferably not more than about 0.5% by weight, of normal impurities. Examples of such highly compressible, metallurgical-grade iron powders are the ANCORSTEEL 1000 series of pure iron powders, e.g. 1000, 1000B and 1000C, available from Hoeganaes Corporation, Riverton, N.J. and similar powders available from Höganäs AB, Sweden. For example, ANCORSTEEL 1000 iron powder, has a typical screen profile of about 22% by weight of the particles below a No. 325 sieve (U.S. series) and about 10% by weight of the particles larger than a No. 100 sieve, the remainder being between these two sizes (trace amounts larger than No. 60 sieve). The ANCORSTEEL 1000 powder has an apparent density of about 2.85-3.00 g/cm
3
, typically 2.94 g/cm
3
. Other iron powders that can be used in the invention are typical sponge iron powders, such a Hoeganaes' ANCOR MH-100 powder.
The iron-based powders can also include iron, preferably substantially pure iron, that has been prealloyed, diffusion bonded, or admixed with one or more alloying elements. Examples of alloying elements that can be combined with the iron particles include, but are not limited to, molybdenum; manganese; magnesium; chromium; silicon; copper; nickel; gold; vanadium; columbium (niobium); graphite; phosphorus; aluminium; binary alloys of copper and tin or phosphorus; Ferro-alloys of manganese, chromium
Storström Helge
Vidarsson Hilmar
Burns Doane Swecker & Mathis L.L.P.
Hëganäs AB
Mai Ngoclan
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