Recommendation on the composition adjustment in double carbide powders

2016-02-23

First, the properties and similarities of tantalum and niobium:

from 1801, British chemist and in 1802 after the discovery of niobium, tantalum Swedish chemist discovered, due to very similar nature, so in a very long period of time, they had been considered to be the same substance. Until 1844, the German chemist Luojie only proven non-identical elements tantalum and niobium. Because of this, they play the same role in cemented carbide: improving red hardness, wear resistance and corrosion resistance, and can effectively inhibit the growth of carbide grains.

Second, the physical and chemical similarity:

tantalum and niobium belong to the fifth sub-group of the periodic table of elements, the same outermost electrons, body-centered cubic crystal structure, and its close lattice constant (Ta: a = 3.296 angstroms, Nb: a = 3.294 Å). Their melting point, boiling point, thermal expansion coefficient, electrical resistivity are also very close. The most important point is that tantalum and niobium at high temperature can make unlimited solid solution. Based on those similar properties, we have many theories to support that niobium can replace tantalum.

Third, the practical application of present, there are a considerable number of domestic manufacturers to adopt 8: 2 series of tantalum and niobium binary solid solution into the YW, YS carbide. This direct addition method, although extremely wrong, it can avoid carbide red hardness and inhibit grain growth. But if we take (W,Ti,Ta,Nb)C solid solution method to replace (Ta,Nb)C,the effect will be more better. Because the direct experience of a solid solution of (Ta,Nb)C was added to bring the following drawbacks: 1, tantalum, niobium gathered. We know that in normal cemented carbide sintering temperature (1500 degrees), WC solubility in TaC, NbC is very bad, WC, TaC, NbC was weakly dissolved in TiC. Thus tantalum and niobium will form aggregates without exception . The TaC, NbC are of poor wettability in cobalt, they must rely on the WC to form a solid solution in order to achieve good wetting effect. Thus,once tantalum and niobium carbide aggregate, it will inevitably cause carbide intercrystalline fracture,eventually cemented carbide will show weak strength, spalling edges or angles, and bad wear resistance; 2, cemented carbide structure is complex, difficult to form a stable alloy microstructure. As a direct added TaC, NbC carbide during sintering can not form a complete and stable solid solution with WC, TiC, and Carbide mixture ingredients is very complex, which will greatly effect the quality of cemented carbide .For those reasons, the (W,Ti,Ta,Nb)C solid solution instead of (Ta,Nb)C will solve all the problems..

Tantalum and niobium proportion in (W,Ti,Ta,Nb)C: although many similarities between tantalum and niobium, we cannot replace tantalum by niobium to an unlimited extent. Here we are talking about one aspect of the problem, mentioned before, because (Ta, Nb) C in Co wettability is poor, they rely on the job and WC solid solution, therefore, tantalum and niobium content can not be increased indefinitely in cemented carbide powder. Ta: atomic weight of 180.95, Nb atomic weight:92.91, tantalum in number of atoms nearly twice more than of niobium, while they Ti, W substitutional solid solution is a solid solution, the solid solution is greatly influenced by the number of atoms. Therefore, the original alloy powder tantalum content can not be replaced with unlimited niobium. So how much will be appropriate?, I think TaC: NbC 8: 2 the most appropriate (specific proportion will depend on cement carbide performance). Because the cemented carbide with such a proportion show the same performance of the original full-containing tantalum carbide. In addition, there is not much difference in carbide density, shrinkage factor compacts, sintering system. If the ratio is smaller, there is no longer the economic value of this transformation.

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