Binder Migration in Double Pressed Drill Bit Inserts

Sammanfattning: It is possible to produce functionally graded cemented carbides on a large scale by double pressing. Being able to predict the binder migration in WC-Co cemented carbides during the sintering is essential to the design of such cemented carbides. The cobalt content is one of the main factors determining the hardness gradient. The cobalt binder migrates from large to small grains, from high cobalt concentrations to low and from high carbon concentrations to low. This has been shown qualitatively and quantitatively. However, the models and theory is not developed enough to be able to accurately predict and design hardness gradients in cemented carbides. This research begins by investigating past experiments and models. Then proceeds by producing and investigating double pressed drill bit inserts for top hammers. Lastly, the research made new and further develop earlier migration equilibrium models and diffusion simulations. This research shows that the migration pressure models are reasonably accurate for certain grain size ranges. A modified version of a surface/interface energy minimization model without any fitting was also used. Which can predict the migration to a similar extent and it can be applied to a larger range of grain sizes. It also provides a maximum gradient in cobalt due to a grain size gradient. This paper also shows that diffusion simulations alone cannot adequately explain the migration, and that some type of bulk flow is likely to occur during sintering. Some double pressed drill bit inserts with hardness gradients can be designed using these models of binder migration. More theoretical development and experiments are suggested to test and develop our understanding of binder migration in WC-Co cemented carbides.