Method for producing hard metal mixtures

Details Method for producing hard metal mixtures Method for producing hard metal mixtures

2001-07-13

2003-09-30

Mai, Ngoclan (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Consolidated metal powder compositions

C419S032000, C428S404000

Reexamination Certificate

active

06626975

ABSTRACT:

Hard metals are materials comprising hard materials and binder metals. They are important as wear-resistant materials and are used in shaping operations with and without metal cutting.
Hard materials are carbides, nitrides or carbonitrides of refractory metals of Subgroups IV, V and VI of the periodic table of the elements, the most important of which being titanium carbide (TiC), titanium carbonitride (Ti(C,N)) and particularly tungsten carbide (WC).
Cobalt is used in particular as a binder metal. However, mixed metal powders or alloy powders comprising cobalt, nickel and iron and optionally other constituents are used in minor amounts.
In order to produce hard metals, hard materials and binder metal, each in powder form, are intimately mixed, pressed and subsequently sintered, whereupon the binder metal results in the formation of a melt during sintering and thus facilitates very extensive densification and the formation of a multi-phase microstructure which exhibits an advantageous bending strength and fracture toughness. The optimum effect of the binder metal is achieved if complete wetting of the hard material phase is effected. The solubility of the hard material in the binder, which depends on the sintering temperature, results in partial redissolution and rearrangement of the hard material, so that a microstructure is obtained which is very resistant to crack propagation. The result of sintering can be represented in the form of the residual porosity. A necessary prerequisite for the achievement of a satisfactory fracture toughness is that the residual porosity must not be less than a defined value.
Hard materials with average particle sizes according to ASTM B 330 of 3 to 20&mgr;, preferably 3 to 10&mgr;, are normally used. Very finely divided contents of hard materials should be avoided, since they tend to recrystallise during liquid phase sintering (OstwaId ripening). Crystallites which have grown in this manner contain multi-dimensional point defects, which are disadvantageous with regard to certain service properties of hard metals, particularly when they are used for the machining of steel, in mining and for impact tools. For example, tungsten carbide can be plastically deformed to a certain degree if multi-dimensional point defects are removed at high temperatures above 1900° C. The carburisation temperature at which the tungsten carbide was produced therefore has a significant effect on the service properties of hard metals. The fraction of the tungsten carbide phase which remains undissolved in the hard metal at the sintering temperature, which is typically between 1360 and 1450° C., is qualitatively inferior to the non-redissolved fraction as regards these service properties. Further embrittlement can occur due to the incorporation of binder metals in the lattice by WC fractions which have grown by redissolution.
The binder metal which is used is generally of smaller particle size, and is typically about 1 to 2&mgr; according to ASTM B 330.
The binder metal is used in an amount such that that it corresponds to about 3 to 25% by weight of the hard metal.
Up to 50% of ground, recycled sinterable hard metal powders can advantageously be used in conjunction.
Apart from the choice of hard material which is suitable in each case (particle size, particle size distribution, crystal structure) and of a suitable binder metal (composition, amount, fraction of hard metal), and of the sintering conditions, the production of suitable hard metal mixtures, i.e. the mixing of hard material and binder before sintering, plays a decisive part with respect to the subsequent properties of the hard metal.
On account of the electrostatic forces of repulsion between fine particles of powder (which always result in finer powders having lower bulk densities), different particle sizes and densities, and the unfavourable quantitative relationship between the two components, dry mixing has not been successful according to the prior art. The electrostatic forces of repulsion between the particles can in fact be overcome by employing dry grinding, but this would result in comminution of the particles, particularly those of the hard metal, since very many fine fractions would be produced. Moreover, unavoidable wear on the grinding tools is a problem which has not been solved hitherto.
Accordingly, wet grinding in an attrition mill or in a ball mill using an organic grinding liquid and grinding balls has been widely used as the method which is employed industrially for the production of hard metal mixtures. Moreover, by using a grinding liquid, the electrostatic forces of repulsion are effectively suppressed. In fact, by employing wet mixing in an attrition mill it is possible to keep the comminution of the grains of the hard material within acceptable limits, but mixed grinding is a very costly procedure, firstly due to its high space requirement on account of the requisite ratio by volume of grinding agents to the material being ground of about 6:1, and secondly due to the grinding times of 4 to 48 hours which are required. Added to this, there is the requirement of separating the grinding balls from the hard metal mixture by sieving after mixed grinding, and of separating the organic grinding liquid by evaporation. A certain degree of wear on the mill and a certain degree of comminution of the grains also have to be accepted when wet mixed grinding is employed. Those WC powders which are carburised at at least 1900° C., which have a narrow particle size distribution without a proportion of fines, and which therefore have to be converted into very high grade hard metals without redissolution processes, are particularly affected by the above disadvantages.
According to one very old proposal (GB Patent 346 473), the problems of mixing hard materials and binder metal are claimed to be solved by electrolytically coating the hard material with the binder metal. However, this method has not achieved widespread use. According to more recent proposals (U.S. Pat. Nos. 5,505,902 and 5,529,804), the binder metal, particularly cobalt, is chemically deposited on the particles of hard material. However, this necessitates the use of organic liquid phases which are not without effect on the carbon content of the hard metal. The object of the present invention is to provide a method of producing a hard metal mixture which avoids the disadvantages of the prior art, which in particular is less costly on an industrial scale, and which furthermore, due to the homogeneity of the mixture and due to the avoidance of comminution of the particles of the hard material after sintering, results in hard metals which have outstanding service properties due to the minimization of the redissolved fraction of the WC phase.
It has been found that this object can be achieved by effecting short-range mixing of the mix material with the generation of a high shearing impact velocity of the powder particles, and by effecting long-range mixing by recirculation of the mix material.
In this manner, dry mixing of the hard material and binder metal powders can be achieved substantially without particle comminution, without the use of grinding agents, grinding aids or liquid suspending agents.
The expression “short-range mixing” is to be understood according to the invention as the mixing of a partial amount of the mix material, whereas long-range mixing denotes the mixing of the bulk of the mixture batch, i.e. of the partial amounts thereof with each other.
The method according to the invention therefore firstly consists of mixing the powder particles with each other using short-range mixing with a high input of mixing energy (with respect to the amount of powder acted upon by the mixing element) in order to overcome the electrostatic forces of repulsion between the powder particles, and secondly consists of effecting long-range mixing with a reduced energy input in order to homogenise the powder mixture.
According to the invention, different mixing units are preferred for short-range and for long-range mixing.
The bulk of the mix ma

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