Solid anti-friction devices – materials therefor – lubricant or se – Lubricants or separants for moving solid surfaces and... – Elemental metal or boron – or alloyed metal
Reexamination Certificate
2002-01-17
2003-01-28
Howard, Jacqueline V. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Lubricants or separants for moving solid surfaces and...
Elemental metal or boron, or alloyed metal
C508S103000, C508S454000, C508S551000, C508S575000, C419S038000, C075S231000, C075S252000
Reexamination Certificate
active
06511945
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to new lubricants for metallurgical powder compositions as well as metal-powder compositions containing these lubricants. Specifically the invention concerns iron-based powder composition including the new lubricants as well as compacts, which are made from these compositions and which are distinguished by a high green strength.
BACKGROUND OF THE INVENTION
Green strength is one of the most important physical properties of green parts. The importance of this property increases as P/M parts increase in size and geometry becomes more complex. Green strength increases with increasing compact density and is influenced by type and amount of lubricant admixed to the powder. The green strength is also influenced by the type of powder used. Another possibility of achieve high green strength is to perform the mixing and/or compaction of the metal powder at elevated temperatures. A high green strength is required in order to prevent compacts from cracking during the ejection from the compacting tool and prevent them from getting damaged during the handling and the transport between the press and the sintering furnace. Presently used compacts having a relatively high green strength are advantageously prepared from sponge iron powders whereas difficulties have been met as regards the preparation of compacts of atomised powders in spite of the fact that an atomised powder is more compressible and hence gives a higher green density.
OBJECTS OF THE INVENTION
An object of the present invention is to provide compacted bodies having high green strength and to ensure durability for handling after compaction and ejection from the tool.
A second object is to provide a new lubricant enabling the manufacture of such compacts from highly compressible iron powders, such as atomised iron powders or highly compressible iron-based powders.
A third object is to provide an iron-based powder composition, which includes iron-based powder and the new lubricant.
A fourth object is to provide a method for the preparation of compacted bodies having high green strength when compacted at ambient temperature.
A fifth object is to provide a method for the preparation of green bodies having high strength despite a comparatively low density.
Other objects of the invention will be apparent from the following text.
SUMMARY OF THE INVENTION
It has now been found that the above objects can be attained by new lubricants comprising a combination of a polyethylene oxide and an oligomer amide and the present invention thus concerns such lubricants.
The invention also concerns an improved metal-lurgical powder composition comprising a major amount of an iron-based powder having a weight average particle size in the range of about 25-350 &mgr;m and a minor amount of this new lubricant. Furthermore, the invention concerns a method for producing green bodies having high green strength while maintaining a low ejection force and low ejection energy. Additionally the method ensures durability for handling after compaction and ejection from the tool as evidenced by low Rattler values.
The method comprises the steps of mixing an iron-based powder and optional additives with the new lubricant and compacting the obtained powder composition.
DETAILED DESCRIPTION OF THE INVENTION
More specifically the new lubricant essentially consists of polyethylene oxide (PEO), which belongs to the family of polyethers, in an amount between about 10 and 60% by weight of the lubricant, the remainder being the oligomer amide. In order to obtain the high green strength in combination with low Rattler values the PEO content of the new lubricant should be at least 20 and most preferably at least 30%. When the amount of PEO is above 60% the green strength is reduced. Considering the green strength the highest values are obtained with lubricants including between 30 and 50% of PEO, the balance being the oligomer amide.
The use of polyethers, or more specifically those having low molecular weight commonly called poly-ethylene glycols, in combination with iron-based powders is disclosed in the U.S. Pat. No. 6,224,823, according to which high green strengths may be obtained when the polyethylene glycols have a molecular weight less than 7000 g/mol and the compacting operation is performed at elevated temperature. According to the present invention which is concerned with the preparation of green bodies by compacting the powders at ambient temperature (normally about 15 to about 35° C.) it has been found that poly ethylene oxides having molecular weights above 7000 g/mol has unexpected advantages if combined with the oligomer amides.
Suitable polyethylene oxides which may be used according to the present invention are disclosed in the U.S. Pat. No. 5,498,276 which is hereby incorporated by reference. These polyethylene oxides are solid, particulate substances having a weight average molecular weight between about 10,000 and about 4,000,000.
According to the present invention the polyethylene oxide should preferably have a weight average molecular weight between about 20,000 and about 400,000 g/mol. Most preferably the oxide should have a weight average molecular weight between 50,000 and 300,000 g/mol. Examples of preferred materials are oxides having a molecular weight of 100,000 g/mol or 200,000 g/mol. If the molecular weight is less than 20,000 green strength will not be sufficiently high and if the molecular weight exceeds 400000 g/mol particles within the desired size range cannot be obtained with conventional methods.
The use of PEO in connection with powder metal compositions is also from the U.S. Pat. Nos. 5,290,336, 6,126,715 and 6,039,784. These patents teaches i.a. that PEO may be as an agent for improving the green strength and reducing the ejection force. It is also disclosed that PEO may be mixed with various lubricants such as stearates and waxes. According to the U.S. Pat. No. 5,498,276 the PEO should preferably be used in amounts of at least 90 of 100% of the lubricant used in the composition.
In contrast to this teaching it has now been found that, in order to achieve the unexpected results according to the present invention, the PEO should be used in amounts less than 90% and that the PEO should be combined with an oligomer amide, whereas combinations of PEO with various types of other commonly used lubricants, such as ethylene bisstearamide as suggested in the above patents, have not been successful.
The oligomer amides, which are used according to the present invention, are known from the U.S. Pat. No. 5,744,433 which is hereby incorporated by reference. According to this patent the oligomers are used as lubricants in metal powder compositions. These oligomers have a weight-average molecular weight M
W
of 30,000 at the most and, preferably, at least 1,000. Additionally these oligomer amides have a melting point peak in the range of 120° to 200° C. Most preferably M
W
varies between 2,000 and 20,000. It is also taught that at least 80% of the lubricant, preferably at least 85% and most preferably 90% by weight of the lubricant, is made up of the oligomer amide.
Furthermore the U.S. Pat. No. 5744433 teaches that these amides are used for warm compaction. When using these amides for cold compaction, i.e. compaction at ambient temperature, the ejection force will be too high for industrial use. This is in contrast to the present invention according to which the oligomer amides in combination with PEO are advantageously used for cold compaction whereas inferior results are obtained when the powder compositions are compacted at elevated temperatures.
As used in the description and the appended claims, the expression “iron-based powder” encompasses powder essentially made up of pure iron; iron powder that has been prealloyed with other substances improving the strength, the hardening properties, the electromagnetic properties or other desirable properties of the end products; and particles of iron mixed with particles of such alloying elements (diffusion annealed mixture or purely mechanical mix
Burns Doane , Swecker, Mathis LLP
Hëganäs AB
Howard Jacqueline V.
LandOfFree
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