The Effect of Cooling Rate on Sintered Cemented Carbides

Abstract: Magnetic measurements are useful tools for quality control of cemented carbides. Previous work at Sandvik Mining and Rock Technology has shown that the coercivity increases with increased cooling rate during sintering for a specific grade.  This study aims to investigate why the coercivity changes with the cooling rate and if this is true for other cemented carbide grades as well. Three different cemented carbide grades were sintered with different cooling rates and evaluated with coercivity, Cobalt-magnetic saturation and hardness measurements, and with microscopy and Electron Backscatter Diffraction analysis. It was found that the coercivity increased with increasing cooling rates for the previously studied grade, but not for the two other grades. It was expected that the increased coercivity would indicate a decrease in WC grain size, but the results showed that the WC grain size of the fastest and slowest cooling rate were the same. However, a change in size of the Co areas between the WC grains was found. The fast cooled sample showed smaller Co areas than the slow cooled sample. These Co/WC grain boundaries increase the coercivity. An increased fraction of hcp-Co/fcc-Co was also found for the fast cooled material which also increases the coercivity. No relationship between the hardness and the coercivity or the cooling rate was found.   The contiguity for the different grades was also calculated.  No significant difference in contiguity between the different cooling rates of each material was found but the contiguity values between the different materials differed.  This is probably mainly dependent on the different binder contents of the materials.