| According to the grading standard of Sandvik, a leading enterprise in cemented carbide industry, the WC grain size of cemented carbide within3.5～4.9μm,5.0～7.9μm,8.0～14μm could be defined as coarse grain, extra coarse and super extra coarse cemented carbide, respectively. Compared with conventional cemented carbide, under the premise with the same Co content, coarse cemented carbide with better resistance to thermal fatigue, thermal shock resistance, and thus the coarse carbide is widely used in mining alloy area. As the upgrading of the coarse carbide, extra coarse and super extra coarse carbide with growing proportion in mining alloy area, this type of alloy has a good market prospect. But at present, the research of extra coarse and super extra coarse carbide is still at the beginning in our country.In this paper, using WC powder (Fischer sub-sieve size32.1μm) and Co powder as raw materials, Cr3C2, VC and RE(mixture of rare earth) as additives, five alloys with different composition were prepared by sintering-HIP:WC-8.4Co, WC-8.4Co-0.4Cr3C2, WC-8.4Co-0.4Cr3C2-0.05RE, WC-8.4Co-0.4VC and WC-8.4Co-0.4VC-0.05RE, respectively. The influence of Cr3C2, VC and RE (mixture of rare earth) on the microstructure and mechanical properties of the alloys was investigated, and the microstructure and nano-indentation mechanical properties of Co binder phase was studied too. The main results were as following:1. The average WC grain size of five alloys reached7μm. The porosity of five alloys reached the level of≤A02B00C00, just mean that volume of the pores were smaller than10μm and less than0.2%of the total volume and no pore was bigger than25μm. Cr3C2, VC also had the ability to inhibit WC grain growth in extra coarse, super extra coarse carbide system, and the ability of VC was better than Cr3C2. The similar characteristic was found in ultrafine grained carbide system. The coercive force of alloys decreased slightly with the increasing of the average WC grain size of the alloys and trace RE (mixture of rare earth) had no significant effect on the coercivity of the alloys. The alloy with Cr3C2additive was characterized with regular WC shape and homogeneous microstructure. However, the addition of V resulted in the polarization of the WC grain size and the irregular of the WC shape.2. Doping VC deteriorated the Palmqvist toughness of extra coarse carbide, nevertheless doping Cr3C2and RE (mixture of rare earth) had no significant negative impact to extra coarse carbide. Cr3C2could significantly improve the nano-indentation hardness of the Co binder phase in ultra coarse carbide; VC had the same effect, but the impact was less than Cr; RE (mixture of rare earth) could reduce the nano-indentation hardness of Co binder phase slightly. The elastic modulus of Co binder phase was not sensitive to the changes of the composition of alloys.3. The trace RE (mixture of rare earth) had no significantly effect on the microstructure and mechanical properties (including hardness and toughness) and physical properties (coercive force) of the alloys.4. Under the condition of addition with0.4wt%, the solid solubility of Cr in the Co binder phase of the alloys was three to four times that of V; More than90%of Cr dissolved in the Co binder phase, only27%of V dissolved in the Co binder phase. Additive with Cr, the lattice parameter of Co binder phase increase a little; Additive with V, the lattice parameter of Co binder phase decrease slightly. The Co binder phase of alloys WC-8.4Co, WC-8.4Co-0.4Cr3C2and WC-8.4Co-0.4Cr3C2-0.05RE had flat pore walls, but there were lots of steps nearly0.5μm on the pore walls of Co binder phase of alloys WC-8.4Co-0.4VC and WC-8.4Co-0.4VC-0.05RE, which maybe explain why VC reduced the mechanical properties of extra coarse carbide. |
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