Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2000-01-31
2001-06-12
Jenkins, Daniel (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S037000, C419S017000, C419S018000
Reexamination Certificate
active
06245288
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to pressable powders of transition metal carbides, iron group metals or mixtures thereof. In particular, the invention relates to pressable powders of WC mixed with Co.
BACKGROUND OF THE INVENTION
Generally, cemented tungsten carbide parts are made from powders of WC and Co mixed with an organic binder, such as wax, which are subsequently pressed and sintered. The binder is added to facilitate, for example, the flowability and cohesiveness of a part formed from the powders. To ensure a homogeneous mixture, the WC, Co and binder are typically mixed (e.g., ball or attritor milled) in a liquid. The liquid is generally a flammable solvent, such as heptane, to decrease the tendency for the WC to decarburize and for the WC and Co to pick up oxygen, for example, when mixed in water or air. The decarburization of the WC and introduction of excessive oxygen must be avoided because undesirable phases in the cemented carbide tend to occur, generally causing reduced strength.
Unfortunately, the use of a flammable solvent requires significant safety, environment and health precautions, resulting in a significant amount of cost to produce the pressable powder. To avoid some of these problems, WC particles greater than about 1 micrometer in diameter with cobalt and binders have been mixed or milled in water (U.S. Pat. Nos. 4,070,184; 4,397,889 4,478,888; 4,886,638; 4,902,471; 5,007,957 and 5,045,277). Almost all of these methods require the mixing of the WC powders with just the organic binder and, subsequently, heating the mixture until the binder melts and coats all of the WC particles before milling with Co in water.
Smaller WC particles (e.g., less than 0.5 micrometer in diameter) are now being used to increase the strength and hardness of cemented tungsten carbide parts. However, because of the increased specific surface area (m
2
/g) of these WC powders, the avoidance of oxygen pickup has become more difficult. Consequently, the use of these smaller particles has tended to require the milling time to be longer to ensure a uniform mixture of WC with Co, exacerbating the problem of oxygen pick up. Because of these problems, these small powders, generally, are always processed in a solvent, such as heptane.
Thus, it would be desirable to provide a method to form a pressable powder that avoids one or more of the problems of the prior art, such as one or more of those described above.
SUMMARY OF THE INVENTION
A first aspect of the invention is a method to prepare a pressable powder, the method comprises mixing, in essentially deoxygenated water, a first powder selected from the group consisting of a transition metal carbide and transition metal with an additional component selected from the group consisting of (i) a second powder comprised of a transition metal carbide, transition metal or mixture thereof; (ii) an organic binder and (iii) combination thereof and drying the mixed mixture to form the pressable powder, wherein the second powder is chemically different than the first powder. Herein, chemically different is when the first powder has a different chemistry. Illustrative examples include mixes of (1) WC with W, (2) WC with Co, (3) WC with VC, (4) WC with W
2
C, (5) WC with Cr
3
C
2
and (6) Co with Ni.
A second aspect is a pressable powder made by the method of the first aspect. A final aspect is a densified body made from the pressable powder of the second aspect.
Surprisingly, it has been discovered that by mixing in essentially deoxygenated water, a transition metal carbide (e.g., WC), transition metal (e.g., Ni, Co, and Fe) and mixtures thereof may be mixed for long times and still not pick up any more oxygen than when mixing, for example, in heptane. Consequently, the densified shaped part of this invention may have the same properties as those made from powder mixed in heptane without any further processing or manipulations (e.g., addition of carbon in WC-Co systems). This has been evident even when using submicron WC powders, Co or mixtures thereof.
DETAILED DESCRIPTION OF THE INVENTION
The method comprises mixing of a first powder with an additional component in essentially deoxygenated water. In performing the method, it is critical that the water is essentially deoxygenated so as to avoid oxygen pick up during the milling. Herein, essentially deoxygenated water corresponds to an amount of dissolved oxygen in the water of at most about 2.0 mlligrams/liter (mg/L). Preferably the amount of dissolved oxygen is at most about 1 mg/L, more preferably at most about 0.5 mg/L, even more preferably at most about 0.1 mg/L and most preferably at most about 0.05 mg/L. A suitable amount of dissolved oxygen is also when the amount of dissolved oxygen is below the detection limit of Corning Model 312 Dissolved Oxygen Meter (Corning Inc., Scientific Div., Corning, N.Y.).
The water generally is deoxygenated, prior to mixing, by (i) addition of a deoxygenating compound, (ii) bubbling of a gas essentially free of oxygen through the water or (iii) combination thereof. Preferably the water is deoxygenated by bubbling gas essentially free of oxygen through the water so as to minimize any adverse effects the deoxygenating compound may have, for example, on the densification of a shaped part made from the pressable powder. Examples of suitable gases include nitrogen, hydrogen, helium, neon, argon, krypton, xenon, radon or mixtures thereof. More preferably the gas is argon or nitrogen. Most preferably the gas is nitrogen. Examples of useful deoxygenating compounds, when used, include those described in U.S. Pat. Nos. 4,269,717; 5,384,050; 5,512,243 and 5,167,835, each incorporated herein by reference. Preferred deoxygenating compounds include hydrazine and carbohydrazides (available under the Trademark ELIMlN-OX, Nalco Chemical Company, Naperville, Ill.).
The essentially deoxygenated water is preferably formed using distilled and deionized water and more preferably the water is high purity liquid chromatography (HPLC) grade water, available from Fisher Scientific, Pittsburgh, Pa. The pH of the water may be any pH but preferably the pH is basic. More preferably the pH of the water is at least 8 to at most 10. The pH may be changed by addition of an inorganic acid or base, such as nitric acid or ammonia.
The first powder is either a transition metal carbide or transition metal powder. When the first powder is a transition metal carbide it may be any transition metal carbide but preferably the first powder is a carbide of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten or mixtures thereof. Most preferably the first powder is tungsten carbide.
When the first powder is a transition metal it may be any transition metal but preferably is manganese, iron, cobalt, nickel, copper, molybdenum, tantalum, tungsten, rhenium or mixtures thereof. More preferably the first powder is iron, cobalt, nickel or mixtures thereof. Most preferably the first powder is cobalt.
The first powder may be any size useful in making a densified part by powder metallurgical methods. However, the average particle size of the first powder is preferably at most about 25 micrometers, more preferably at most about 10 micrometers, even more preferably at most about 1 micrometer and most preferably at most about 0.5 micrometer to greater than 0.001 micrometer.
The first powder is mixed with an additional component selected from the group consisting of (i) a second powder comprised of a transition metal carbide, transition metal or mixture thereof; (ii) an organic binder and (iii) combination thereof, provided that when the second component is comprised of a second powder the second powder is chemically different, as previously described.
When present, the second powder may be comprised of any transition metal carbide but preferably the transition metal carbide is one of the preferred carbides previously described for the first powder. When present, the second powder may be comprised of any transition metal but preferably the transition metal is one of t
Jenkins Daniel
Kalow & Springut LLP
OMG Americas, Inc.
Santalone, Esq. John J.
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