Research On Design, Preparation And Properties Of WC-Co Gradient Cemented Carbides

Conventional homogeneous cemented carbide (CHCC) with homogeneous composition and microstructure in bulk has sharp contradiction between hardness and toughness, which restricts further application areas of it. It is difficult to meet the development of modern social on the double high require, namely high hardness and high toughness. So it seems particularly important to develop a new-type of functionally gradient cemented carbide (FGCC) materials to enable tools with different functions at different positions in bulk. For this reason, supported by the National Natural Science Foundation of China and cooperates with the large-scale state-owned cemented carbide enterprises, with modern material analysis and test methods, such as the finite element analysis software Marc, SEM, XRD, EDS, EPMA and picture analysis software Q550, etc, this paper carried out systematic researches on the gradient structure design base on the aim of thermal residual stress, the microstructures and performances of carbon-deficient cemented carbides (CDCC) and the structural transformation, effects of process parameter on microstructures and performance during preparing of gradient cemented carbides by carburizing treatment, the crack propagation, fracture toughness and high temperature strength of the gradient cemented carbides, getting the following conclusions.1) Putting forward the model suiting for gradient cemented carbides, such as the composition distributing function, the elastic coefficient, the hot expansion coefficient and heat transfer coefficient model, basing on the basic parameter with the mass percent of model. The results of the new established models are very well identical with the test results. The residual stress of YG6 gradient cemented carbides calculated by numerical simulation with the MARC finite element software is basically unanimous with the XRD test values. The best optimization design microstructure of YG6 gradient cemented carbides is the thickness of gradient layer should be of 20% to 30% radius, the value of the gradient distribution exponent p should be between 1.5 to 2.5 and the best cobalt content peak value should be between 12% to 16%.2) In CDCC, carbon content affects the type, amount and distribution ofη phase, and WC shape of alloys, etc. Co₃W₃C phase appears in alloys with relative lower degree of carbon-deficient and whose amount increase with increasing carbon content, whereas Co₆W₆C phase varies reversely. The total amount ofηphase decreases with increasing carbon content. With the increase of degree of carbon-deficient, the homogeneity ofηphase distribution decreases,ηphase tends to form clump shape and WC mostly maintains multangular character.3) The structure and properties of CDCC by Sintering-HIP are better than that of by Vacuum Sintering. The valid controlution of carburization atmosphere during the later stage of Sintering-HIP can directly prepare gradient cemented carbides. The WC-6(xNiyCo) CDCC with binder of xNiyCo alloyed powder, namely some Ni takes place of Co, can get near microstructures and performances of WC-6Co CDCC with same carbon content, which has application prospects in preparing no magnetism or low magnetism gradient cemented carbides for die industry or bearing.4) High performance gradient cemented carbide was successfully be prepared by a simple carburizing treatment on CDCC. The characteristic of the gradient microstructure is thatηphases in surface layer and sublayer have already totally disappeared, where consists of normal WC +γtwo phases. A cobalt-deficient area and a cobalt-rich area was formed on the surface layer and sublayer of the alloy respectively, however, the core area of the alloy has few change includingηphase. The forming of the cobalt gradient is influenced mainly by the carbon diffusion, reaction between carbon andηphase, outward migration of W atoms, pressure difference caused by liquid phase and the capillary force caused by grain grows of WC crystalline, etc.5) The hardness and the fracture toughness take on a continuous gradient change along the gradient direction on the cross section of alloys for the forming of the gradient structure. The gradient cemented carbides have better comprehensive performance, toughened by the gradient distribution of cobalt content, forming compressive stress at surface, inducing crack deflection and crack bridge, etc.6) The high temperature strength of gradient cemented carbides is higher than that of CHCC and CDCC. With the increasing of test temperature, the macroscopical fracture surface tends to smooth and the transcrystalline fracture proportion decreases while the intercrystalline fracture proportion increases. The slow decrease of high temperature strength of the gradient cemented carbides is the combined actions of the forming of cobalt gradient distribution, which weakens the soften and oxidation of metal cobalt at elevated temperature for lower cobalt content on surface layer, the strengthening for more tungsten dissolved in cobalt at surface layer and the relaxation of thermal residual tensile stress with the increasing of test temperature, etc.