Research On Preparation Of WC Base Cemented Carbides With Plate-like WC Grains And Surface Carburizing Treatment Technology

WC based cemented carbides with plate-like WC grains were prepared by vacuum sintering.The formation and growth mechanism of plate-like WC grains during sintering and the effects of chemical composition on the microstructure and mechanical properties of cemented carbides were investigated by differential scanning calorimetry?DSC?,X-ray diffraction?XRD?,scanning electron microscopy?SEM?and energy dispersive spectrdmeter?EDS?.Based on this,graded cemented carbides were prepared through gas carburication of WC-based cemented carbides with plate-like WC grains.In the first part of the thesis,the phase and microstructure transformation of plate-like WC grains during sintering were studied.It was found that:During solid sintering?700?¹200??,plate-like WC grains were formed through direct reaction between C and most of the flattened W powders.In the same time,small amount of?phase were formed through reaction between C,Co and remaining W powders.Subsequently,these?phase further reacted with C to form WC and Co,which could promote the formation of plate-like WC grains.Small equiaxial hard phase grains formed in solid sintering stage dissolved into the binder phase and then precipitated on the surface of coarse plate-like WC grains during the subsequent liquid phase sintering.Then,the heating rate between 1200?and 1400?on the microstructures and properties of cemented carbides with plate-like WC grains were studied.As the heating rate increased,the average grain size of hard phase decreased,the length-thickness ratio of the hard phase firstly increased and then decreased,the hardness and transverse rapture strength firstly increased and then decreased,and the fracture toughness decreased.When the heating rate between 1200?and 1400?was 3?/min,cemented carbides showed superior comprehensive mechanical properties.Effects of C,NbC and TaC content on the microstructures and properties of cemented carbides with plate-like WC grains were studied.The results showed that:As the content of C increased,the?phase disappeared,the average grain size of hard phase firstly decreased and then increased,and the length-thickness ratio of the hard phase firstly increased and then decreased.When the C content was 5.44wt.%,the cemented carbides showed the best mechanical properties with the TRS of 2951MPa,hardness of 91.1HRA,K_ICC of 15.2 MPa·m^1/2.As the addition of NbC increased,the average grain size of hard phase firstly decreased and then increased and decreased again,the length-thickness ratio of the hard phase decreased.When the addition of NbC was1wt.%,the cemented carbides showed mechanical properties with the TRS of 2604MPa,hardness of 91.4HRA,K_ICC of 14.3 MPa·m^1/2.As the addition of TaC increased,the average grain size of hard phase firstly decreased and then increased,the length-thickness ratio of the hard phase didn't change obviously at first and then decreased.When the content of TaC was 0.5wt.%,the cemented carbides showed mechanical properties with the TRS of 2617MPa,hardness of 91.5HRA,K_ICC of15.4 MPa·m^1/2.Finally,graded cemented carbides with plate-like WC grains were prepared through carburication of as-sintered cemented carbides,and the effects of main process parameters of carburizing treatment on the binder-deficient layer's thickness of cemented carbides were studied.The results showed that:After carburization,binder-deficient layer was formed in the surface zone,which enhanced the surface hardness of cemented carbides.The phenomenon of element migration during carburization is caused by the difference in affinity between carbon and other alloying elements.When treated at the same carbonizing process,the thickness of binder-deficient layer of cemented carbides increased with the increase of C content in as-sintered materials.However,when the C content of as-sintered material reached 5.44wt.%,graphite phase appeared after carburization,due to the exceeded C content.At the same carbonizing pressure,the thickness of the binder-deficient layer treated at 1300?was thicker than those treated at 1250?and1350?.At the same carbonizing temperature,the thickness of the binder-deficient layer treated at0.3atm was thicker than those treated at 0.2atm and 0.4atm.In addition,a binder-deficient layer about 75?m was formed when treated at 1300?and 0.3atm,the hardness of its surface is 1749HV,250HV higher its core section.