There are mainly three kinds of compound carbides (CK materials) for YT hard metals: 3/7(4K24)、4/6(4K32)、5/5(4K40),among of them, 4/6(4K32) is the most popular. Yet considering alloy structure, solubility and solid solubility of compound carbide, and the thermodynamics of Substance diffusion, 4/6(4K32) is the one to be suspended. Without the futher study on the solid alloy structure and solid solution, 4/6(4K32) is the simply average between 3/7 and 5/5 by the original designer.
Firstly, we will analyse the 3/7 solid solution. Its main characteristic is: WC dissolves into TiC under the sintering temperature, which results in the stable alloy structure and easy process. As mentioned in Technical Research and The Communication with Alloy Factories, different alloys requires the approximate value 3:7, rather than the traditional TiC/WC=29/71. 3/7 solid solution is not suitable for the factories with high level and advaced controlling methods, and those facotires require 5/5 solid solution, even ultra-fine solution and other alloy raw materials.
As we all know, 5/5 solution is totally non-saturated, thus, it can easily form a “ring structure”, while the additional WC dissolve into the TiC solution when sintered.WC cannot fully dissolve due to the limited temperature and time of sintering.So, from the inside to outside,TiC content descend while the WC ascend, theorily, the outmost ring is total WC, which results in the color changing of the alloy metallography, so it looks like a ring.
So the “ring” is good or bad for alloy? My opinion is if under good control, it is good for alloy and vice versa. Why? In brief, the “ring” means gradient dissolve of WC into TiC, which leads to the partial unbalance. But it is still short range disorder and long rang order. The existence of “ring” enhaces the wear-resistance and fragility of alloy. While the outmost ring is WC, which improves the wettability of Cobalt to hard phase,hence, all of this neutralizes the fragility of alloy.
If Ta and Nb are added in the mixing powder, 5/5 solid solution is adopoted without any consideration, because of the inferior wettability of WC by Cobalt. That is to say, Ta and Nb cannot be wetted without the solid dissolution with W and Ti. Therefore, Ta and Nb can be dissolved to the fullest extent during the sintering process.
In a word, with the development of technology and the enhance of strength, the fine grain and extra fine grain 5/5 solid solution and other alloy raw materials will be selected for solid solution.(3/7 refer to WC:TIC=30:70 4/6 refer to:WC:TIC=40:60 5/5 refer to : WC:TIC=50:50 tungsten titanium series, with YT as its code name)