Chemistry of inorganic compounds – Carbon or compound thereof – Binary compound
Reexamination Certificate
1996-08-29
2002-12-17
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Binary compound
C423S439000
Reexamination Certificate
active
06495115
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to the production of carbides of the transition metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W and solution carbides of said transition metals.
BACKGROUND OF THE INVENTION
There are generally two forms of tungsten carbide; monotungsten carbide (WC) and ditungsten carbide (W
2
C). It is well-known that WC is useful in the manufacture of commercially worthwhile items such as cutting tools, dies and drilling tools, whereas W
2
C generally is not. In fact, W
2
C degrades the properties such as strength of WC objects even when present only in small quantities.
In producing said WC items, it is common for a tungsten carbide powder to be combined with a metal such as cobalt and subsequently densified into a WC/Co cemented carbide by heating. The heating may take place at a pressure ranging from vacuum to pressures greater than atmospheric pressure.
In a cemented carbide part, the tungsten carbide, grain size and grain size distribution and grain chemistry greatly influence the final part properties. As already stated above, W
2
C should be avoided when making cemented tungsten carbide parts. Generally, smaller grain size in a cemented part results in improved strength. In addition smaller grain sizes often result in higher hardness at a given cobalt addition. Non-uniformity of grain size in a cemented tungsten carbide part adversely affects the strength of and the surface condition of the part after grinding. The non-uniformity of grain size in the cemented WC part is primarily due to exaggerated grain growth during the densification of the part. The grain growth can be controlled by addition of grain growth inhibitors such as VC, Cr
3
C
2
or TaC or starting with a WC powder having as narrow (i.e., uniform) as possible particle size distribution.
WC powder which has an average particle size less than 0.2 to 0.3 micrometer can cause exaggerated grain growth due to the increased reactivity associated with the fine particle size. It has also been reported that standard grain growth inhibitors, as described above, are not effective when sintering a cemented WC part using said fine WC powder. The critical parameter to sinter said fine WC powders was reported to be the WC powder grain size distribution (Suzuki et al,
J. Jap. Soc. Powder and Powder Met.,
Vol. 19, p. 106-112, 1972). Thus, it is desirable to be able to increase the particle size or control the particle size distribution of very fine WC powder (less than 0.2 to 0.3 micrometer) to reduce the possibility of grain growth during the densification of a cemented WC part.
Typically, monotungsten carbide is formed by the carburization of tungsten metal. The basic process steps commonly are:
(a) calcining of ammonium paratungstate or tungstic acid to one of the stable forms of tungsten oxide, such as WO
3
, WO
2.83
, WO
2.65
and WO
2
,
(b) reducing the tungsten oxide to tungsten metal powder,
(c) mixing the tungsten metal powder with a powdered form of carbon,
(d) carburizing the tungsten and carbon mixture at a temperature in excess of 1100° C. in a reducing (hydrogen containing) atmosphere.
The resultant WC particle size is controlled by the size of the W metal powder formed in the above step (b). Tungsten metal particle size, as described by U.S. Pat. No. 3,850,614, is controlled mainly by:
(1) depth of powder bed during reduction,
(2) flow rate of hydrogen,
(3) dew point of the hydrogen gas and
(4) reduction temperature.
Smaller particle size tungsten powder is produced by increasing gas flow, decreasing bed depth, reducing the dew point of the hydrogen gas and decreasing reduction temperature. By reducing the bed depth and reducing the temperature, the amount of tungsten powder that can be carburized to WC in a given period of time is decreased. The mechanism of growth has been attributed to a volatile WOH species directly associated with the water concentration in the gaseous environment (U.S. Pat. No. 3,850,614). Processes requiring the carburization of tungsten metal to form monotungsten carbide are typically limited to producing WC powder having a particle size of about 0.8 micron or larger because of the difficulty in producing W metal much smaller than this size due to, for example, the pyrophoric nature of such a fine tungsten metal powder. Because of the high hardness of WC, it is also difficult to grind WC to this small particle size. Even if WC were easily ground to the fine particle size, the grinding process inherently produces a wide particle size distribution compared to a controlled synthesis process.
Other methods of producing monotungsten carbide include the following methods. Steiger (U.S. Pat. No. 3,848,062) describes reacting a volatile tungsten species such as WCl
5
, WCl
4
, WCl
2
, WO
2
Cl
2
, WOCl
4
, WOF
4
and W(CO)
6
with a vaporous carbon source such as a volatile hydrocarbon or halogenated hydrocarbon. The vaporous carbon source is present in a quantity at least equal to WC stoichiometry during the above vapor phase reaction. The product from this reaction, a mixture of WC, W
2
C and carbon, is then calcined at a temperature of about 1000° C. for about 1 to 2 hours resulting in monotungsten carbide substantially free of ditungsten carbide.
Miyake (U.S. Pat. No. 4,008,090) describes a process having a first step of reacting a tungsten oxide with a carbon powder at a temperature greater than 1000° C., thereby removing the oxygen and a second step of reacting the product of the first step at a temperature higher than the first step in hydrogen to produce monotungsten carbide. Miyake specifies that the temperature must be greater than 1000° C. in the first step to remove the oxygen. The removal of oxygen is necessary to avoid the reaction of hydrogen with oxygen forming water vapor which consequently reacts with carbon forming a volatile carbon-oxygen species, thus effecting the carbon content of the second step product (i.e., desired monotungsten carbide).
Kimmel (U.S. Pat. No. 4,664,899) describes a method to form monotungsten carbide comprising mixing tungsten oxide or ammonium paratungstate with carbon powder to form a resulting mixture, reducing said mixture in a non-reducing atmosphere for a sufficient time at a suitable temperature to produce resulting reduced mixture comprising tungsten, ditungsten carbide and monotungsten carbide, said reducing being carried out in the presence of sufficient carbon to produce a carbon content of less than 6.13 percent by weight in said resulting reduced mixture, determining the carbon content of said resulting reduced mixture, adding sufficient carbon to said resulting reduced mixture to increase the carbon content to at least the stoichiometric amount needed to form monotungsten carbide and carburizing said adjusted reduced mixture to form monotungsten carbide. Kimmel further describes that the product of the reducing of the tungsten oxide is a mixture of W, W
2
C, WC and free carbon and that all of the oxide is reduced.
All of the above described processes to produce monotungsten carbide require the reduction of a tungsten oxide or tungsten compound (e.g., WCl
4
) to either tungsten or a mixture of tungsten metal, carbides of tungsten and free carbon. The tungsten or mixture is substantially free of oxygen (i.e., tungsten oxide) before carburizing to form monotungsten carbide. The oxygen is essentially completely removed to avoid the volatile loss of carbon by oxidation or hydrolysis during the carburization of tungsten metal or said mixture. The removal of carbon during the carburization causes non-uniform carbon contents of the resultant carbide product (i.e., W
2
C in the product). Non-uniform carbon contents are particularly a problem in industrial processes because of the larger volume of carbide that must be processed.
In an industrial process it would be desirable to provide a method to produce a transition metal carbide which is relatively insensitive to the oxygen concentration of the precursor mixture used to make said carbide. In addition, it would be desirable to have a process in which said carbide
Barker Hobart A.
Dunmead Stephen D.
Lasher Gabrielle R.
Nilsen Kevin J.
Repman Joseph F.
Hendrickson Stuart L.
Kalow & Springut LLP
OMG Americas, Inc.
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