WC grain size distribution during sintering of WC-Co cemented carbides

Sammanfattning: The grain growth of WC in WC-7%Co cemented carbides with Cr-addition was investigated for cemented carbides synthesized from four different raw materials varying in grain size. Cr additions were 4%of the Co binder phase. To investigate the effect of Cr on grain growth two of the raw materials were also synthesized without Cr-additions. To investigate the effect of cobalt content on grain growth cemented carbide with 13%Co with Cr addition was synthesized. All samples were sintered in four different cycles: 1300˚C for 10min, 1380˚C for 30min, 1430˚C for 1h and 1430˚C for 2h. Three of the samples were also sintered in a fifth sintering cycle at 1200˚C for 30min to investigate a sintering cycle with negligible grain growth for comparison with laser diffraction (LD) measurements of the WC powders, as milled. The evolution of the grain size distribution (GSD) for these sintering conditions were evaluated with electron backscatter diffraction (EBSD) measurements in a scanning electron microscope (SEM). The grain growth observed during these sintering conditions is divided into three stages: A primary stage of rapid grain growth during solid state sintering with a prominent grain growth for the fine fraction, resulting in a narrowing of the GSD; A secondary stage of slower grain growth during liquid phase sintering; A tertiary stage where the large grains grow out of proportion to the rest of the population. The tertiary stage of grain growth was inhibited and the secondary was slowed down by Cr-additions, also the onset of densification in the initial stage was delayed with Cr-additions. Most likely, the delay of the initial stage is related to the presence of Cr- surface oxides which delay the spreading of Co at the beginning of densification. However, the delay of densification does not seem to have any significant effect on grain growth as sintering proceeds since Cr-additions lowers the melting temperature of Co. The inhibition at the secondary and tertiary stages is likely related to an inhibited precipitation of W and C atoms dissolved in the Co binder onto existing WC grains. Variation in the critical misorientation angle for the determination of grains post EBSD-analysis is consistent with grain growth by coalescence in the early stage of grain growth. Also the dependences of hardness, fracture toughness and coercivity on grain size were evaluated. From these, correlations between coercivity and hardness/ fracture toughness were made which provides reasonable estimates of hardness and fracture toughness from coercivity for WC-7%Co with similar carbon content and Gaussian GSD.