Preparation essentials of coarse grain carbide
The sintering process is also a process of WC grain coarsening and is one of the key steps in obtaining ultra coarse grain carbide. The coarsening of carbide mainly depends on the dissolution of fine grain powder and the growth of coarse grain powder. The grain size and morphology of carbide are jointly determined by raw materials and production processes, with the original WC powder and carbon content being important influencing factors.
WC powder’s quality
Rhombic WC grains have sharp edges and corners, which can easily cause local stress concentration when subjected to loading. Therefore, using circular WC powder as raw material can effectively improve the toughness of carbide and reduce the sensitivity of carbide to cracks. When the cobalt content and carbon content are the same, the performance of carbide depends on the grain size of WC. In addition, after high ball milling, WC powder with smaller original particle size has a higher grain coarsening rate under the same sintering process. Therefore, the particle size distribution of the original powder plays an important role in predicting the coarsening of WC grains.
Carbon content in ultra coarse grain carbide
When the carbon content is low, the fine WC particles produced by ball milling will not undergo recrystallization during sintering. Under the same preparation conditions, compared with low carbon cemented carbide, high carbon cemented carbide can obtain a more uniform grain size distribution and a higher coarsening rate.
The kinetic curves of WC growth at different temperatures and different carbon contents are shown in Figure 1, and it can be clearly seen that low carbon content strongly inhibits the growth of WC. At the same time, the coarsening of WC in medium or high carbon content cemented carbide is highly dependent on temperature, while low carbon content cemented carbide is not sensitive to temperature. The apparent activation energy for WC coarsening in the cemented carbide with the lowest carbon content (5.79%) is 98 kcal/mol, which is close to the activation energy for C self diffusion (88 kcal/mol), so the inhibition mechanism of this type of cemented carbide may be controlled by the self diffusion process of C to WC and liquid phase Co in the WC grains. In addition, the hindering effect of the process of W and C precipitation in WC grains cannot be ruled out in W rich and Co poor cemented carbide.
In carbides 2, 3, and 4, the WC coarse graining process is strongly dependent on the carbon content, and the concentration of tungsten in the binder phase is relatively high. It can be inferred that in this case, the dissolution reaction of C in WC is the slowest. Therefore, the inhibitory effect of reducing carbon content on WC coarse graining may be related to the decrease in carbon concentration in the liquid Co, and in turn, this process is limited by the dissolution rate of C on the surface of WC grains.
The stepped structure on the surface of WC and the irregular shape of WC grains support that nucleation of new atomic layers is the main coarse graining mechanism. Therefore, defects play an important role in the process of grain coarsening by reducing the activation energy and promoting nucleation.
The thickness of the cobalt bonding layer in ultra coarse grade WC Co cemented carbide can reach several micrometers, which plays a significant role in inhibiting the initiation and propagation of thermal cracks and fatigue cracks. However, the hardness and wear resistance of the Co bonding metal are low. When the bonding phase wears quickly, the WC grains are exposed on the surface of the cemented carbide. The WC grains that are not fixed by the bonding phase are easily pulled out and damaged, leading to rapid wear of the entire cemented carbide.
It is worth noting that many attempts to strengthen the bonding agent by adding various chemical elements (Al, Si, Cr, B, etc.) to the bonding metal have failed. Although adding elements can increase the hardness of the cemented carbide, it also significantly reduces the fracture toughness and transverse fracture strength of the cemented carbide.
Trace amounts of VC and CrC can effectively suppress the growth of WC grains in ultrafine and nanocrystalline cemented carbides and enhance the hardness of the cemented carbide. Rare earth elements have the ability to inhibit the growth of WC grains discontinuously and unevenly. By using the characteristics of Cr, V, and RE to change the microstructure of the grains and improve the hardness of the cemented carbide, some scholars have applied them to the field of ultra coarse cemented carbides. Using spherical WC with a Feinman grain size of 14 μm as raw material and adding a small amount of VC, WC 10% Co cemented carbide was produced, and the morphology comparison is shown in Figure 2. It can be seen that the grain size of the cemented carbide is significantly suppressed, and VC also helps to suppress changes in the shape of the WC grains, which still remain circular.
The mechanical properties of ultra coarse grained cemented carbide
Cemented carbides with an average grain size of 5 10 μm are very suitable for use in mining and construction. Toughness increases as the WC grain size increases while maintaining the same hardness. Some scientists have further demonstrated the feasibility of improving the wear resistance and toughness of cemented carbides by increasing the WC grain size while maintaining the hardness of the cemented carbide. The relationship between the hardness and wear resistance of the cemented carbide is shown in Figure 4.
The application of ultra coarse grain carbide is becoming more and more widespread, and the research and development of high quality ultra coarse grain carbide has become a hot topic in research around the world. While learning and drawing on advanced foreign technology, we should also pay attention to exploring the basic theory, establishing a sound theoretical system and standards.
1To explore the preparation process of high quality ultra coarse grain carbide. Strengthen the research on appropriate process parameters and the selection of binders and additives, and seek more simple and efficient methods for promotion and application.
2To explore the strengthening technology of the binder phase, enhance the interface bonding strength between the Co binder phase and the WC hard phase in ultra coarse grained cemented carbide. The wear resistance and service life of the cemented carbide will have a qualitative leap.
3To establish a corresponding relationship system between the grain size, cobalt content, and performance of ultra coarse grain carbide. Establish a sound product evaluation index system and corresponding industry standards, which will help to design and develop new carbide products more scientifically and efficiently, and achieve industrialization faster.