Researchs On Preparation And Mechanical Properties Of WC-Co Gradient Structure Cemented Carbide

WC-Co gradient structure cemented carbide, which has a low cobalt content at the surface, a rich cobalt content at the middle layer, and a balanced cobalt content at the core, has both high strength and high ductility widely used as mining tools.In this paper, the formation mechanism of cobalt gradient, microstructure evolution and mechanical properties of WC-Co gradient structure cemented carbide are investigated by means of phase analysis, microstructure image analysis, thermodynamic analysis, and mechanical properties testing. The major results can be summarized as follow:(1) Gradient structure cemented carbide can be prepared by carbon-deficient cemented carbide as-sintered through carburizing process, and the liquid migration driving forces are attributed to the liquid migration pressure caused by the uneven liquid volume and osmotic pressure caused by the solute concentration gradient in the liquid phase. The concentration gradient of solute atoms in the carburizing process and Dissolution-Precipitation Mechanism cause that the WC average grain size in gradient structure cemented carbide gradually decreases along the gradient direction,(2) The rapid cooling can inhibit WC grain growth, maintain the morphology of WC grains at high temperature which can ease the stress concentration caused by closed angle, improve the solid solution elements content in the Co-phase. The macroscopic residual stress and gradually increase the fracture toughness caused by the gradient distribution of Co-phase can effectively improve the resistance to crack propagation and flexural strength.(3) The fatigue cracks initiation at the subsurface area are mainly attributed to the effect of stress concentration and the martensitic phase transformation of Co-phase; the fatigue endurance limit of gradient structure cemented carbide is slightly higher than that of conventional cemented carbides. Due to the Co-phase martensitic transformation under cyclic stresses, the Co-phase becomes brittle, and the Co-phase is the rapid crack propagation channel, so that the fatigue cracks nucleated along the Co gradient.(4) Adding Mg (OH)2to the A12O3filler can effectively improve the the decarburization atmosphere of hydrogen sintering, and achieve uniform decarburization microstructure at the surface. Pre-sinted compact infiltrated with appropriate concentration of ammonium metatungstate solution can prepare WC-Co cemented carbide with only decarburization microstructure at the surface, which decarburization thickness can be controled, and decarburization extent gradiently changes from the surface to the inner.