Coarse-grained Wc-co Cemented Carbide Preparation Process

Compared with medium/fine cemented carbides, coarse-grained cemented carbide has a higher thermal conductivity, better thermal fatigue resistance and thermal shock resistance under the condition of the same Co content, mainly applied in mining tools used in continuous excavation in extreme working conditions, punching dies, cold punching dies, rollers and so on. This kind of cemented carbides has very broad market prospects. At present, no production of cemented carbide has been reported with WC grain size above 4.5μm in China. The purpose of this paper is to explore the preparation technique of coarse-grained WC-Co cemented carbide with WC grain size above 4.5μm and high-performance.This study includes three parts. Firstly, the limit of size of WC grain in cemented carbide prepared by traditional wet mixing was studied, according to the effects of raw materials and process parameters on the WC grain size of cemented carbide; Secondly, method of increasing WC grain size in WC-Co cemented carbide was explored through the study of dry mixing; Finally, on the basis of the investigations mentioned above, a novel technology called "chemical-powder-wrapping" is developed, for the preparation of coarse-grained cemented with a WC grain size above 4.5μm. The morphology of the powders and the WC grain in alloys was observed by scanning electron microscopy (SEM); phase composition of the powders was analyzed by X-ray diffraction (XRD), and microstructure of alloys was observed by metallographic microscope. The results are as follows:(1) Although traditional wet mixing ensures Co distributes in WC uniformly, primary particles and secondary particles of WC are so easily crashed that WC grain size is reduced, as WC is brittle material. Thus, reducing wet mixing time to only 24 hours can not prepare coarse-grained cemented carbide with WC grain size above 4.5μm, even if WC powder which has a particle of FSSS 28.3μm is ued.(2) Compared with the traditional wet mixing, dry mixing has no fragmentation effect on grain size of WC in slurry, thus ensures the grain size of WC in alloys. Coarse-grained cemented carbide with WC grain size above 5μm is prepared by dry mixing using WC powder which has a particle size of 6.26μm (FSSS) as starting material; nevertheless, because of bigness, roughness, irregular shape and hard aggregate of WC powder in the slurry prepared by dry mixing, defects in the green compact, e.g. big voids and uncompacted will be present and remain in the alloy, decreasing the mechanical properties of cemented carbide.(3) The "chemical-powder-wrapping" technology does not only overcome the defect of wet mixing reducing WC grain size remarkably, but also improves the distribution of Co in WC and changes the manner of combination of WC and Co, Co is wrapping WC by means of nano-phased self-assembly form structure. WC-Co cemented carbide is prepared by "chemical-powder-wrapping" technology using WC powder which has a particle size of 6.26μm (FSSS) as starting material successfully. Before that, the WC powder has a special pretreatment. The results show that the alloy has a homogeneous microstructure with crystalline perfection of WC whose rims are rounded. The cemented carbide has a average grain size of WC above 4.8μm. WC-20%Co cemented carbide was prepared with this method with transverse rupture strength 2750 MPa and Rockwell hardness 83.4, reaching the level of similar products produced by Sandvik Corporation.