Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2000-02-24
2003-08-12
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S037000, C075S255000
Reexamination Certificate
active
06605251
ABSTRACT:
This invention relates to a lubricant for metallurgical powder compositions, as well as a metal-powder composition 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 compaction. More specifically, the invention concerns lubricants which when pressed result in products having high transverse rupture strength. The lubricant according to the invention further has the advantage that it can be used for both warm and cold compaction.
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. Thus, cold pressing is predominantly used for compacting metal powder (the powder has room temperature). 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 of the compacting tool.
How much the tool is worn 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 compacted and 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 the wear of the compacting tool but may also 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 transverse rupture strength, high green density, as well as sintered products having high sintered density and low ejection force from the lubricant in combination with metal powders. As the compact is subject to considerable stresses when ejected from the compacting tool and as the product must keep together during the handling between compaction and sintering without cracking or being otherwise damaged, it is important with high transverse rupture strength. This is especially important in the case of thin parts.
The lubricant according to the invention contains a polyolefine-based polymer, which has a weight-average molecular weight M
w
of 500-10 000. Polyolefins are a group of thermoplastic polymers with different degrees of crystallinity. Polyolefins are subdivided into simple polyolefins, poly(&agr;-olefins) and copolymers based on olefins and/or &agr;-olefins. The copolymers may also include other types of comonomers such as vinylacetates, acrylates, styrenes, etc. Poly(&agr;-olefins) include polymers such as polypropylene and poly(1-butene). Simple polyolefines, however, include polymers such as branched chain low-density polyethylene and linear chain high-density polyethylene. Linear chain polyethylenes of relatively low molecular weight are termed polyethylene waxes.
The polymer according to the invention is preferably a polyethylene wax. The lubricant according to the invention can be used in both cold and warm compaction, but in warm compaction the weight-average molecular weight M
w
of the lubricant preferably is 1000-10 000.
Preferably, the lubricant of the invention has a polydispersity M
w
/M
n
lower than 2.5, preferably lower than 1.5.
The invention further concerns a metal-powder composition containing a metal powder and the above-mentioned lubricant, as well as methods for making sintered products, both cold and warm compaction. The method for cold compaction according to the invention comprises the steps of
a) mixing a metal powder and a lubricant to a metal-powder composition
b) compacting the metal-powder composition to a compacted body, and
c) sintering the compacted body, use being made of a lubricant according to the invention, which has a weight-average molecular weight M
w
of 500-10 000.
The method for warm compaction according to the invention comprises the steps of
a) mixing a metal powder and a lubricant to a metal-powder composition,
b) preheating the metal-powder composition to a pre-determined temperature,
c) compacting the heated metal-powder composition in a heated tool, and
d) sintering the compacted metal-powder composition, use being made of a lubricant according to the invention, which has a weight-average molecular weight M
w
of 1000-10 000.
The present invention further relates to the use of the lubricant according to the invention in a metallurgical powder composition in cold and warm compaction.
The lubricant can make up 0.1-2.0% by weight of the metal-powder composition according to the invention, preferably 0.2-0.8% by weight, based on the total amount of the metal-powder composition. The possibility of using the lubricant according to the present invention in low amounts is an especially advantageous feature of the invention since it permits that compacts and sintered products having high densities can be achieved cost-effectively.
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 with the remainder 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, boron, phosphorus, or silicon; low melting ternary and quaternary eutectics of carbon and two or three of iron, vanadium, manganese, chromium, and molybdenum; carbides of tungsten or silicon; silic
Burns Doane Swecker & Mathis L.L.P.
Hëganäs AB
Jenkins Daniel J.
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