Research On Production Principle Of Graded Cemented Carbide And Formation Mechanism Of Gradient In Cemented Carbide

To enhance the performance of cemented carbide for wear resistance, cutting and impact applications, in which high hardness and high toughness are favored, the composition, producing and the relationship between properties and microstructure of graded cemented carbide were studied by using SEM, TEM, XRD and EDAX. Principles to develop graded cemented carbide and gradient formation mechanism were also discussed in detail in this work, following conclusions are obtained:(1) Dual-structure cemented carbide has been produced by powder lay-up technique to realize gradual variation of contents, microstructure and properties at the interface of two layers, starting from two types of powder mixture in which WC grain size was different. It is found that the formation of the graded structure is due to the migration of Co binder in liquid state from the coarse grain size layer to the fine grain size one driven by capillary force, re-shape of WC framework and dissolution-precipitation of WC, of which migration driven by capillary force takes place at the initial sintering stage.(2) A tri-layer graded cemented carbide has been fabricated by optimization of carbon content, carburizing temperature, carburizing time and carburizing mode. It is found that both excessive high carburizing temperature and exassive low carbon content in the samples are unfavourable for the formation of graded structure and extension of carburizing time results in thickness increase of graded layer.(3) An equation has been set up for calculation of diffusion coefficient of atoms in liquid metal, which better matchs the experimental results compared with former researches.(4) A dynamic equation has been established for calculation of thickness increase of graded layer when cemented carbide with carbon content below stoichiometric one is carburized. The relationship between growth of graded layer and carburizing time calculated by the equation is close to experimental results.(5) It is found that cemented carbide with Co binder gradient can be fabricated by solid state carburizing. The mechanism for this process is the migration of W and Co atoms driven by diffusion of carbon atoms. On the other hand, Co binder gradient formed by liquid state carburizing is due to migration of W atoms driven by diffusion of carbon atoms, leading to coarser and closer WC grains in the surface and inward diffusion of Co atoms.(6) The influence of alloy contents, sintering method and process parameters on the formation of surface layer deficient in cubic carbide has been studied. A coated blade with gradedstructure and high bending strength has been fabricated from cemented carbide with nitride by controlling the difference of nitrogen activity outside and inside the sintered body in a de-nitrogen atmosphere in a batch furnace. It has been found that the outward diffusion of nitrogen and inward diffusion of Ti in the sintered body of cemented carbide with nitride lead to a tougher surface deficient in cubic carbide.(7) Practical tests show that the performance of cemented carbide anvils, coated blade and drill bits with graded structure developed in this dissertation are superior to those with regular structure.