At present, there are mainly three types of compound carbides used in the domestic production of titanium-containing cemented carbides: CK24, CK32, and CK40. Among them, CK32 is the largest. But from the aspects of alloy structure, solubility and solid solubility of compound carbides, and thermodynamics of substance diffusion, CK40 should be the best. It is also possible to use CK24 when making gravity cutting inserts.
First, let's analyze the double carbide of CK24. The main feature of the solid solution is: at the alloy sintering temperature, the solubility of WC in TiC is saturated, so the alloy structure is relatively stable and the process is easy to control. However, the production of different grades of alloys should be approximately 70/30 instead of the traditional WC/TiC=71/29 (the book has another comment on this point).
About CK40 duplex carbide. We know that the compound carbide is a severely unsaturated solid solution, and TiC can still dissolve a large amount of WC during the alloy sintering process to form a "ring structure". The formation of the "ring structure" is mainly due to the fact that the solid solution is of a severely unsaturated type, and the WC added in the alloy will continuously dissolve into the solid solution during the alloy sintering process. The alloy sintering temperature and time can not satisfy the complete dissolution of WC in the solid solution. Therefore, from the inside to the outside, TiC gradually decreases, while WC gradually increases, so that the outermost layer is complete WC (in theory). Therefore, the alloy metallography forms different levels of color changes, which look like a "ring".
So is the appearance of the "ring" good or bad for the alloy? We think that good control will definitely be very beneficial, and bad control will have bad effects. First of all, it is certain that the appearance of "ring" can enhance the wear resistance of the alloy, but at the same time increase the brittleness of the alloy. But as long as it is well controlled, it is still disorderly in the short-range and orderly in the long-range. However, since the outer layer of the "ring" is WC, this actually enhances the wettability of cobalt to the hard phase. Therefore, the brittleness of the alloy due to the "ring" is cushioned and offset.
That being the case, why is CK32 not the preferred solid solution? This is mainly determined by the structure of the alloy gold image made and other physical inspection indicators and the actual use effect. There is not much theoretical demonstration yet.
If more ingredients such as Ta and Nb need to be added to the alloy, the choice of compound carbide is definitely CK40. This is because cobalt has poor wettability to Ta and Nb, that is to say, Ta and Nb must be dissolved in W and Ti before they can be wetted. Therefore, only the solid solution of CK40 can dissolve Ta to the maximum in the alloy sintering process. , Nb. The CK24 is the worst.
