RU2028273C1 - Method of preparing of tungsten carbide dispersed powder - Google Patents

Abstract

FIELD: solid alloys. SUBSTANCE: metallic tungsten powder with specific surface 2 m²/g (not less) is put into molybdenum combustion boat, heated up to 840-1100 C and kept in the flow of methane-hydrogen gaseous mixture containing 0.4-3.2 vol. % methane. At low temperature of carbidizing methane content in the mixture is high, and at high temperature - low. Specific surface of tungsten carbide is 2,0-8,0 m²/g, powder particle size is 0.05-0.20 mcm, content of free carbon is 0.1 wt.-%, not more. EFFECT: improved method of tungsten carbide powder preparing. 2 cl, 1 tbl

Description

 The invention relates to powder metallurgy, in particular to a technology for producing tungsten carbide, the main component of sintered hard alloys.

Currently, the most prevalent in the industry is a method of producing WC, on which the charge containing tungsten and carbon black karbidiziruyut at a temperature of 1400-1600 ^{° C} [1].

 The disadvantage of this method is the need for the process at high temperature, which leads to grain growth of the obtained carbide due to collective recrystallization.

It is known to obtain dispersed tungsten carbide with a bulk density of 2.0-2.1 g / cm ³ and a total and free carbon content that meets the technical specifications by carbide solidification of a mixture containing tungsten α and β modification with an oxygen content of not more than 3 wt.% at 1000-1200 ^о С [2].

Closest to the claimed is a method of producing tungsten carbide and a tungsten-cobalt carbide mixture by carbidization of tungsten with a specific surface of up to 1 m ² / g, or a mixture of intermetallic Co ₇ W ₆ + W in a methane-hydrogen gas mixture at 900-1000 ^about With the methane content in the mixture in the range of 2.5-10% CH ₄ [3].

The optimal concentration of methane is 10%. In this case, tungsten carbidization takes place in 5 hours, and the mixture Co ₇ W ₆ + W for 40 minutes. With increasing temperature from 900 to 1000 ^about With the degree of carbidization increases from 83 to 99.9%, and the content of free carbon from 0.44 to 0.79 wt.%.

The disadvantage of this method is to obtain a sufficiently large powder of tungsten carbide with a particle size of 0.5-1.7 microns. Another disadvantage of this method is the carbidization in a nonequilibrium methane-hydrogen mixture, as a result of which stable production of tungsten carbide powder with a stoichiometric content of bound carbon and a free carbon content of not more than 0.1 wt.% Is not ensured. To ensure the required free carbon content of the aforementioned work, it carries out an additional processing operation of the obtained carbide and tungsten-cobalt carbide mixtures in hydrogen or in a mixture of H ₂ + 1% CH ₄ . This operation ensures the removal of free carbon in the gas phase in the form of methane.

The invention solves the problem of obtaining highly dispersed tungsten carbide with a specific surface area of 2.0-8.0 m ² / g particle size 0.05-0.20 microns and providing a stoichiometric composition of the material with a free carbon content of not more than 0.1 wt.% With a minimum possible time costs. In this case, the carbidization process is carried out in one stage and the subsequent operation of removing excess free carbon is excluded.

This is achieved by the fact that in the known method for producing tungsten carbide by carbidization of tungsten powder in a methane-hydrogen gas medium according to the invention, tungsten is used with a specific surface area of at least 2.0 m ² / g, and carbidization is carried out in a methane-hydrogen gas medium with a methane content of 0 , 4-3.2 vol.% At 850-1110 ^о С for a time sufficient to ensure the stoichiometric composition of tungsten carbide. At a low carbidization temperature, the methane content in the mixture is high, and at a high temperature it is low.

С+2Н₂ (1) и поэтому исключается возможность осаждения избыточного свободного углерода. В то же время это содержание метана превышает равновесное над WC и поэтому обеспечивается стехиометрический состав карбида вольфрама, образующегося по реакции:
W + CH₄ = WC + 2H₂ (2)
Высокодисперсный порошок вольфрама с удельной поверхностью не менее 2,0 м²/г получают плазмохимическим способом, либо низкотемпературным восстановлением водородом триоксида вольфрама.The selected composition of the methane-hydrogen gas mixture does not reach equilibrium over carbon for the reaction of CH ₄

C + 2H ₂ (1) and therefore the possibility of deposition of excess free carbon is excluded. At the same time, this methane content exceeds the equilibrium value over WC and therefore the stoichiometric composition of tungsten carbide formed by the reaction is ensured:
W + CH ₄ = WC + 2H ₂ (2)
Highly dispersed tungsten powder with a specific surface area of at least 2.0 m ² / g is obtained by the plasma-chemical method, or by low-temperature hydrogen reduction of tungsten trioxide.

 Plasma-chemical tungsten has a high dispersion and activity and usually contains a significant amount of oxygen, which is a harmful impurity in the production of hard alloys.

The use of a methane-hydrogen medium with a methane content of 2.5-20.0% (prototype) when using finely dispersed tungsten powder with a specific surface area of at least 2.0 m ² / g allows to obtain finely dispersed tungsten carbide powder with the required specific surface area and particle size, however, it does not provide a stoichiometric carbide powder with a free carbon content of less than 0.1%. The regulation of carbon content under these conditions is even more complicated due to the high activity of dispersed powders and the increased oxygen content in them.

The invention also makes it possible to obtain highly dispersed tungsten carbide with a stoichiometric carbon content by carbidization of a W + WO ₂ mixture with an oxygen content of up to 11 wt.%. A higher oxygen content leads to coarsening of the obtained product, therefore, the use of a powder W with an O ₂ content _of more than 11 wt.% In the method according to the invention is undesirable.

Plasma-chemical tungsten of various dispersion with an oxygen content of 2.5% is used as the initial powder; fine tungsten powder with a specific surface area of 11.0 m ² obtained by multistage low-temperature reduction in acutely dried hydrogen with an oxygen content of 3.2%, as well as a W + WO ₂ mixture with a specific surface area of 2.6 m ² / g and an oxygen content of 11 wt. .%.

Carbidization is carried out in a VT 400/40 type furnace, into which a methane-hydrogen mixture of a given composition is fed. The methane concentration in the gas mixture is monitored and maintained with an accuracy of +0.05 vol% CH ₄ using a GIAM-5M gas analyzer.

 The thickness of the tungsten powder backfill layer is 6-8 mm.

PRI me R 1. The powder of plasma-chemical tungsten with a specific surface area of 2.6 m ² / g is poured into a molybdenum boat with a layer thickness of 8 mm Boat into the furnace BT 400/40, was heated to 900 ^C and kept at this temperature under a methane-hydrogen gas mixture flow rate of 2 liters / min and a concentration of 1.8% methane. After exposure (2.5 hours), the oven was turned off and the powder was cooled to room temperature.

 In the resulting product, the specific surface area is determined by the BET method and the content of total and free carbon by standard methods. According to the specific surface area, the average particle size of the powder is calculated. The results are shown in the table (example 1).

 The method was also implemented with boundary and beyond the boundary values of the dispersion of the initial tungsten powder (examples 1-4) and carbidization conditions in a methane-hydrogen medium at an optimal concentration of methane for each temperature (examples 5-8).

 The table presents data on the specific surface, particle size and carbon content in the tungsten carbide powder obtained according to the invention, depending on the specific surface of the starting powder and the carbidization mode.

All tungsten carbide powders obtained by carbidization by the present method have a specific surface area of 2.0-8.0 m ² / g, a particle size of 0.05-0.20 μm, a stoichiometric content of bound carbon and a free carbon content of not more than 0.1 %

The table also shows that when using the initial tungsten powder with a specific surface of less than 2 m ² / g or during carbidization at a higher temperature, a tungsten carbide powder with a reduced specific surface is obtained (examples 2 and 8, respectively).

The use of dispersed tungsten powder with a specific surface area of at least 2 m ² / g with an acceptable high oxygen content (up to 11 wt.%) And a methane-hydrogen gas mixture of the specified composition allows to obtain highly dispersed tungsten carbide of stoichiometric composition with a specific surface area of 2.0-8, 0 m ² / g, with a particle size of 0.05-0.20 microns, which can significantly reduce the granularity of both tungsten carbide and hard alloys based on it, significantly increase the wear resistance when machining cast iron of high hardness.

 The invention also avoids the release of excess free carbon and thereby reduces the decarburization annealing operation.

Claims (2)

1. METHOD FOR PRODUCING DISPERSED POWDER OF TUNGSTEN CARBIDE, including carbidization of tungsten metal powder in a methane-hydrogen gas mixture at elevated temperature, characterized in that they take tungsten powder with a specific surface area of at least 2 m ² / g, the gas mixture is used with a methane content of 0.4- 3.2 vol.% And carbidization is carried out at 840-1110 ^o C.  2. The method according to claim 1, characterized in that at a low carbidization temperature, a methane-hydrogen mixture with a high methane content is used, and at a high temperature - with a low methane content.

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