The Preparation Of Ultra-coarse WC Particles And Behavior Research On Carbonization

Tungsten carbide (WC) is one major part of cemented carbide. WC grain size has an important effect on properties of cemented carbide. Ultra-coarse WC with high quality has a series of advantages that fine grain WC does not have, such as few structural defects, high micro-hardness, low micro-strain and good plasticity. The application of ultra-coarse WC in cemented carbide can significantly improve the anti-fracture properties of cemented carbide and then improve its service life. It is widely used in production of mining tools, boring drills and so on.In this paper, different sizes of single-crystalline W particles (denoted as W-1, W-2, W-3, W-4, W-5, W-6, respectively) have been used to prepare ultra-coarse WC particles under high temperature. The following properties such as carbonization effect, carbon content, grain size and micro-defect are analyzed with SEM, XRD, chemical analysis and granulometry etc. The change of crystal structure and phase during the carbonization, the reason why the particles explode and the generation mechanism of grain boundary are further investigated. The conclusions are as follows:(1) A carbonization process has been applied in this research with the carbonization temperature at2200℃and holding time of6-7hours. The total time of this process is30hours. Sample W-1and W-2can still not be completely carbonized after multiply repeating this procedure. Sample W-3, W-4, W-5and W-6are carbonized only once. All other three samples are completely carbonized except sample W-3. The fully-carbonized WC particles have a silvery surface color with a uniform size. Most of them are rubbly while a small amount is flaky. Those samples have a combined carbon content of6.13%±0.03%and free carbon content lower than0.03%. WC particles with a large grain size have been prepared.(2) With an additional treatment of ball milling, single-crystalline W particles changed into poly-crystalline WC particles with large amount of grain boundaries after complete carbonization. The reasons are as follows:strain hardening will exist in W particles after ball milling and the strain energy will be stored in the particles with the form of structural defect. During the high temperature carbonization, recovery and recrystallization occur. The nucleation of recrystallization originates from the movement of sub-boundary, and the co-grown-up of multiple nuclei leads to the formation of multiple grains in the particles, resulting in the poly-crystallization of single-crystalline W particles after carbonization.(3) W has a Bcc structure. During the high temperature carbonization, Carbon atom diffuses along the most close-packed direction in the lattice of W particles, entering the tetrahedral interstice of W crystal lattice and then changing the interatomic bonding force balance of surrounding atoms. Thus the surrounding atoms relocated, leading to the generation of close-packed hexagonal W. During the carbonization, Carbon atoms first form into W2C or WC(1-x) in the surface of W particles, and then WC. For the existence of concentration gradient, Carbon atoms gradually diffuse into the inner parts of W particles. Once the particles are too large, due to the concentration gradient gradually reducing from the outer parts to the inner parts, the inner part cannot be carbonized completely. Throughout the carbonization process, there exists a phase change of Wâ†'W2C (or WC (1-x))â†'WC from the surface to the core.(4) In the high-temperature carbonization, there happens a particle-explosion phenomenon. The reason is that during the heating-up and cooling process, thermal stress in the W particles and generating transformation stress because of the solid-state phase change during the carbonization, which results in the explosion of some W particles after carbonization. Particle size and carbonization process counts have a big impact on the particle-explosion process.(5) There exist many defects in the carbonized particles. During the ball milling and carbonizing of W particles, as a result of cold deformation and thermal stress, cracks will generate in the inner part of W particles. During the crystallization, microporosity will easily forms at the intersection of the grains.