US2568251A - Process for refining refractory carbides - Google Patents

Description

Patented Sept. 18, 1951 PROCESS FOR REFINING REFRACTORY CARBIDES Marcel Oswald, Paris, France, assignor to Societe Le Carbone-Lorraine, Paris, France No Drawing. Application October 20, 1947, SerialNo. 780,998. In France August 4, 1941 Claims. l 1

This invention relates to refining process for deoxidizing and/or denitrifying carbides or mixtures of carbides of refractory metals of the fourth and fifth periodic groups.

Refractory carbides of transition metals are generally prepared by reducing the corresponding oxides in the presence of a carburising substance and in a neutral or reducing atmosphere or in vacuum. When the oxides are very difficultly reducible, solid solutions of the protoxides in the carbides, for instance of the type Ti (C. O.) in the case of titanium carbide, are frequently obtained. If, furthermore, the nitrifying action of the nitrogen has not been avoided, ternary solid solutions of carbide, nitride and oxide, particularly Ti (C. N. 0.), are obtained. Experience has shown that solid solutions of oxides and/or of nitrides in the carbides derived from the metals of the periodic groups IV and V are difficult to sinter with a more fusible auxiliary metal or alloy, because these solutions are not weLted by the melted auxiliary constituent. Instead of obtaining a hard tough alloy withstanding wear, a badly agglomerated substance full of blow-holes, fragile and hardly usable is produced.

Frequently, carbides of the group VI, particularly tungsten carbide, are associated with the carbides of the groups IV and V. The tungsten carbide enters into solid solution in the carbides of the other groups, but its diffusion is very much delayed by the presence of nitrogen and/or oxygen. Attempts have, therefore, been made to heat-treat the alloys at a very high temperature in order to facilitate diffusion, but the final product is found to contain blow-holes which areall the larger and the more numerous as the carbide of the groups IV and V include a larger percentage of foreign elements, such as oxygen and nitrogen, and the operation has been carried out at a higher temperature. In conclusion, it has not been possible to prepare good heat-treated alloys by starting from carbides comprising oxygen and/or nitrogen.

The process which forms the subject matter of the invention makes it possible to refine impure carbides and to incorporate them in sintered alloys, provided the latter also contain tungsten carbide and/or molybdenum carbide. To that end, the diffusion of tungsten carbide or molybdenum carbide, or of these two carbides, is set up in the carbide, or the mixture of carbides, to be refined. This preliminary operation, carried out at a high temperature, produces a solid solution of a carbide of the Vlth group in the carbide or carbides subjected to refining. Accord- Section 1, Public Law 690, August 8, 1946 Patent expires August 4, 1961 ing to a variant, the refining is carried out in the presence of a more fusible metal, which serves as a flux. It is important to incorporate with the pulverulent mixture intended for refining, the

quantity of free carbon calculated according to the composition to be refined, and according to the proportion of carbon which it is desired to leave in the refined product.

The two examples below, referred to each of the main modes of operation, explain the invention. The proportions given are by weight.

Example 1 For refining titanium carbide containing 1% of oxygen, tungsten carbide is used as a reducing agent. Reduction by tungsten carbide follows the reaction 2WC+O CO+WzC and requires 24.5 parts of tungsten carbide for combination with the oxygen in parts of titanium carbide containing 1% oxygen. But it is much better to use an excess of tungsten carbide relatively to the above-indicated stoichiometrically calculated quantity, as, for instance, about 50 to about parts of tungsten carbide per unit of oxygen in the carbide to be refined. According to the invention, the proportion preferred in this case comprises the quantity of tungsten carbide,

which makes it possible to obtain the composition of the double carbide Ti3WC4. In these conditions, the refining mixture is as follows:

Parts Titanium carbide with 1% of oxygen 100 Tungsten carbide 108 Carbon (lamp-black) 1.5

The constituent powders are intimately mixed. The mixture is heated in a gastight' vessel and a vacuum is kept up during the whole operation and during the subsequent cooling. The temperature is raised to 1600 C. to 2000 C., and it is kept at the higher temperature for from one to three hours. After cooling, 3, solid solution is .produced which has substantially the composipending upon the temperature above 1300 C. to

which the solid solution has been brought.

According to the proportion, the nature of the primary substances and the treatment chosen, it is possible to obtain a solid solution or compound such as Ti3WC4 or ZI3WC4 sufiiciently refined for use.

Example II In the presence of a flux, the refining is more rapid. The most convenient way is to use as flux the auxiliary metal which is to serve as a binder during the sintering or else one of the metals incorporated with the auxiliary alloy. The presence of the flux makes it possible to shorten the refining time at constant temperature or to lower the refining temperature with constant time, A metal of the iron family is preferably chosen in the proportion of less than of the weight of the carbides to be treated, usually comprising between about 0.5 and 4%. The correct dispersion of the flux requires some care. It is convenient to prepare beforehand an intimate mixture of the flux and the tungsten or molybdenum carbide by liberating the auxiliary metal and the refractory metal simultaneously in the nascent state by the action of a reducing and carburising gas such as town gas on an intimate mixture of reducible compounds of the auxiliary and refractory metals, so as simultaneously to produce powdered carbon which is distributed between the metallic grains and prevents them from growing. The further carbon necessary to carburise the refractory metal as required is then added and the mixturecarburised to-produce an intimatemixture of the auxiliary metal and the refractor metal carbide.

For instance, with the composition shown in Example 1, may be incorporated 3 parts of cobalt to 100 parts of mixture of carbides, the mixture being as follows:

Parts Titanium carbide with 1% of oxygen 100 Tungsten carbide 108 Cobalt 6.24 Carbon 1.5

The refining is carried out at a temperature of 14001700 C. for from one to four hours, in a maintained vacuum. A part of the cobalt distils towards the cold parts of the oven. It is, therefore, necessary to analyse the refined product in order to calculate the supplemental quantity of auxiliary metal to be incorporated with the mixture for the purpose of sintering.

B refining with manganese, or an alloy rich in manganese, it is possible more easily to distil this very volatile metal under chosen operative conditions. Whatever the flux is, its amount must remain insufficient to cause the binding of. the mixture. In the particular case chosen, the operation may be concluded by the heat treatment described in the aforesaid application Serial No. 764,900 and the double carbide TiaWC-i, correctly refined, may be obtained.

The processes described in Examples I and II apply, without any notable modifications, to the refining of several carbides, either mixed or already in solid solutions, under the action of tungsten or molybdenum carbide. It is possible, for example, to refine by this process mixtures of titanium carbide and tantalum carbide, or of solid solutions of both these carbides, using tungsten carbide for the refining. Finally, it is possible to refine a carbide, a mixture of carbides, a solid solution of carbides, or a mixture of solid solutions of carbides, by the simultaneous action of tungsten and molybdenum carbides, or by the gotion of a solid solution of these carbides with or without a flux.

The only limit to the application of the process is as follows: the carbides to be refined, or their solid solutions, must allow the diffusion of one at least of the tungsten or molybdenum carbides in solid solution; it is, therefore, necessary that the products to be refined should retain a solvent power adequate for the molybdenum and tungsten carbides.

Apart from the above-described methods of execution, the invention comprises different variants. Particularly, the carbide of tungsten and the carbide of molybdenum can be made to act in their nascent state, for instance by mixing pulverulent tungsten or molybdenum with carbon in a calculated quantity, and with the carbide to be refined, or else it is possible to associate with the reaction mixture carbide of tungsten or of molybdenum, metallic tungsten or molybdenum, and carbon, with the eventual addition of a flux. The nascent carbide then becomes more rapidly diffused, and often the refining action is improved.

I claim:

1. Method of refining impure carbide of metal selected from the group consisting of titanium, zirconium, tantalum and columbium, said carbide being preformed and contaminated with oxygen as an impurity, the method comprising forming an intimate powder mixture of said impure carbide with carbide of metal selected from the group consisting of tungsten and molybdenum and with added free carbon, and heating said mixture to substantially eliminate said impurity and simultaneously produce a solid solution of carbides of the selected metals.

2, Method of refining impure carbide of metal selected from the group consisting of titanium, zirconium, tantalum and columbium, said carbide being preformed and having a non-metallic contamination including oxygen chemically com bined with said metal, the method comprising eliminating said contamination and substituting carbon therefor by heating said impure carbide in the presence of carbide of metal selected from the group consisting of tungsten and molybdenum and free carbon in an amount of stoichiometrically equivalent to that of said non-metallic contamination in said impure carbide, to substantially eliminate said contamination and substitute said carbon therefor and simultaneously produce a solid solution of carbides of the selected metals.

3. Method of refining impure carbide of metal selected from the group consisting of titanium, zirconium, tantalum and columbium, comprising determining the impurity content of oxygen and nitrogen in said carbide, forming an intimate powder mixture of said carbide with carbide of metal selected from the group consisting of tungsten and molybdenum, and with added free carbon in an amount stoichiometrically equivalent to that of said impurity content, heating said mixture to a temperature ranging from about 1600 C. to about 2000C. in vacuo to substantially eliminate said impurity content and substitute said carbon therefor in said impure carbide and simultaneously produce a solid solution of said carbides.

4. Method of refining impure carbide of metal selected from the group consisting of titanium, zirconium, tantalum and columbium, comprising determining the impurity content of oxygen and nitrogen in said carbide, forming an intimate powder mixture of said carbide with carbide of metal selected from the group consisting of tung- ,n f-i-s sten and molybdenum}: and with added free carbon in an amount stoichiometrically equivalent to that of saidirnpurity content, heating said mixture to ateinperature ranging from about 1600 C. to about 2000C. in vacuo for about one to three hours, cooling the resultant product while still under vacuo, and fsubjecting said product to a heat treatment comprising bringing said prodnot to a temperature at least about 1300 C. and thereafter slowly cooling to about 1000 C. in about two to four hours.

5. Method of refining impure titanium carbide containing about 1% of oxygen impurity chemically combined with titanium, comprising heating an intimate powder mixture of about 100 parts of titanium carbide with about 108 parts of tungsten carbide and about 1.5 parts of carbon for about one to three hours at about 1600 to 6 2000 C. in vacuo, cooling the product thereby formed while still under vacuum, and subjecting said product to a heat treatment comprising bringing said product to a temperature at least about 1300 C. and thereafter slowly cooling to about 1000 C. in about two to four hours.

MARCEL OSWALD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 2,119,489 Beer May 31, 1938 2,289,104 Dawihl et a1. 'July '7, 1942 $2,300,558 Driggs Nov. 3, 1942 2,356,009 Schwartzkopf Aug. 15, 1944

Claims (1)

1. METHOD OF REFINING IMPURE CARBIDE OF METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIM, TANTALUM AND COLUMBIUM, SAID CARBIDE BEING PREFORMED AND CONTAMINATED WITH OXYGEN AS AN IMPURITY, THE METHOD COMPRISING FORMING AN INTIMATE POWDER MIXTURE OF SAID IMPURE CARBIDE WITH CARBIDE OF METAL SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN AND MOLYBDENUM AND WITH ADDED FREE CARBON, AND HEATING SAID MIXTURE TO SUBSTANTIALLY ELIMINATE SAID IMPURITY AND SIMULTANEOUSLY PRODUCE A SOLID SOLUTION OF CARBIDES OF THE SELECTED METALS.