Mechanism Of Rare Earths On The Strengthening Of WC-Co Cemented Carbide And Their Application In The Preparation Of Cemented Carbide By Calcium-rich Raw Materials

The cemented carbides with high properties and high stability have always been a hotspot for researchers.The addition of rare earth elements to enhance the properties of cemented carbides is one of the effective methods.In this paper,the method of introducing rare earth intermediates into the process of tungsten metallurgy was discussed.By adding a certain amount elements of Ce and Y in ammonium paratungstate（APT）,the change rule of the microstructure and property of the samples were studied systematically during the process of the calcination of APT containing rare earth,hydrogen reduction of tungsten oxide,carbonization of tungsten and sintering of cemented carbide.Meanwhile,the effects and their mechanisms of rare earth elements（Ce,Y）on the preparation of cemented carbide by calcium-rich raw materials were discussed.It is expected to provide the theoretical basis for the preparation of qualified cemented carbides by tungsten materials with high content of impurities,and the application of rare earths in cemented carbides.The main conclusions of this research are as follows:1.Rare earth Ce refined WO₃ particles and existed in the phase of Ce₄W₉O₃₃3 in the WO₃-Ce composite powder.During the hydrogen reduction process of tungsten oxide,Ce₄W₉O₃₃3 was reduced to Ce₂（WO₄）₃ and Ce₂W₂O₉,and the Ce-rich phases were distributed inside and outside the tungsten particles to affect their nucleation and growth.Fine tungsten powder with the model of Jujube cake was obtained.The refining mechanism was WO₃-Ce genetic properties,and Ce-rich phases increasing the heterogeneous nucleation number of tungsten,inhibiting the volatilization-deposition growth of tungsten particles and preventing the grain boundary migration of them.The phases of Ce₂（WO₄）₃ and Ce₂W₂O₉ further converted to Ce₂WO₆ and the superfine WC-1%Ce composite powder with an average particle size of 0.39μm was obtained in the stage of carbonization.The fine-grained YG6-1%Ce cemented carbide with an average grain of 0.87μm was prepared using the superfine WC-1%Ce composite powder.The relative density,bending strength and fracture toughness of it were 99.4%,2166 MPa,and 11.7 MPa·m^1/2,respectively.Compared with YG6 alloy,the three properties were improved by 0.3%,4.8%,and 4.5%,respectively.The strength improvement of YG6-1%Ce alloy was mainly related to fine grain strengthening and transgranular fracture.2.Rare earth Y refined WO₃ particles and existed in the phase of Y₂WO₆ in the WO₃-Y composite powder.During the hydrogen reduction process of tungsten oxide,the phase of Y₂WO₆ did not change,which formed the"yttrium barrier effect"with the shape of the external barrier similar to the fortress and the inner lanes of the meridian.The phase of Y₂WO₆ distributing around tungsten particles not only inhibited the volatilization-deposition growth of tungsten particles,but also prevented the grain boundary migration of them.More importantly,the yttrium barrier effect not only inhibited the chemical gas phase migration of WO₂（OH）₂ gas,but also restricted the growth of tungsten particles in a separate and narrow limited space,thus significantly refining the tungsten powder.Superadding the structural heredity of WO₃-Y composite powder,Y refined tungsten powder obviously.During carbonize-tion,part of Y₂WO₆ converted to Y₁₅C₁₉9 and small size WC,Y aggravated the agglomeration of fine tungsten carbide powder,and ultrafine WC-1%Y composite powder with an average particle size of 0.34μm was obtained.The fine-grained YG6-1%Y cemented carbide with an average grain of 0.75μm was prepared using the superfine WC-1%Y composite powder.The relative density,microhardness,bending strength and fracture toughness of it were 99.7%,16.7 GPa,2458 MPa,and12.2 MPa·m^1/2,respectively.Compared with YG6 alloy,the four properties were improved by 0.6%,1.2%,19.0%,and 8.9%,respectively.The strength improvement of YG6-1%Y alloy was mainly related to fine grain strengthening,transgranular fracture,interfacial strengthening and Y-rich phase evolution.3.Ca-rich phase distributing inside and outside of tungsten particles affected the nucleation and growth of tungsten.The fine tungsten powder with the model of Jujube cake was prepared.The agglomeration phenomenon of WC-1%Ca became more serious after carbonization.The property of YG6-1%Ca alloy was greatly decreased.The strength reduction was mainly related to grain boundary weakening and interface weakening.Ca+Ce refined WO₃ particles and existed in the phases of CaWO₄、Ce₄W₉O₃₃3 and Ce₂W₂O₉ in the WO₃-Ca+Ce composite powder.During the hydrogen reduction process of tungsten oxide,the"cerium barrier effect"inhibited the harmful effect of Ca-rich phase on tungsten powder effectively,which obtained tungsten powder with normal morphology.Ca-rich phase existed as CaWO₄ and Ce-rich phase existed as Ce₂（WO₄）₃ and Ce₂W₂O₉ in W powder.During carbonization,Ce-rich phase interacted with Ca-rich phase to produce a new phase of Ce_0.9Ca_0.1O_1.9,and part of CaWO₄ further converted into CaC₂.The cemented carbide with an average grain of 0.90μm was prepared using the superfine WC-0.1%Ca+0.9%Ce composite powder.Compared with YG6-0.1%Ca alloy,the microhardness,bending strength and fracture toughness of YG6-0.1%Ca+0.9%Ce were improved by 1.3%,6.6%,and 4.5%,respectively.The strength improvement of YG6-0.1%Ca+0.9%Ce alloy was mainly related to cerium barrier effect,Ce_0.9Ca_0.1O_1.9.9 phase and Ce₂WO₆phase.4.Ca+Y refined WO₃ particles and existed in the phases of CaWO₄ and Y₂WO₆in the WO₃-Ca+Y composite powder.During the hydrogen reduction process of tungsten oxide,the"yttrium barrier effect"inhibited the harmful effect of Ca-rich phase on tungsten powder effectively,which obtained tungsten powder with normal morphology.Ca-rich phase existed as CaWO₄ and Y-rich phase existed as Y₂WO₆ in W powder.During carbonization,Y-rich phase didn’t interact with Ca-rich phase to produce a new phase,part of CaWO₄ further converted into CaC₂,and Ca+Y refined tungsten carbide powder.The cemented carbide with an average grain of 0.87μm was prepared using the superfine WC-0.01%Ca+0.09%Y composite powder.The micro-hardness,bending strength and fracture toughness of YG6-0.01%Ca+0.09%Y alloy were 16.5 GPa、2200 MPa and 11.5 MPa·m^1/2,respectively.Compared with YG6-0.1%Ca alloy,the three properties were improved more by 3.0%,19.6%,and 3.6%,respectively.Compared with YG6 alloy,the latter two properties were improved by6.5%and 2.7%,respectively.The strength improvement of YG6-0.01%Ca+0.09%Y alloy was mainly related to yttrium barrier effect,fine grain strengthening,interfacial strengthening,and the phases of Y₁₅C₁₉9 and Y₂WO₆.Rare earth Ce could improve the stability of the properties of cemented carbide and rare earth Y could improve the mechanical properties of cemented carbide to a greater extent.When Ca content range was less than 0.1%,qualified YG6 cemented carbide could be made by using rare earth Ce or Y.When Ca content was within0.01%,the properties of YG6 alloy with the addition of rare earth Y were better than that of pure YG6 cemented carbides.The application effect of rare earth in the preparation of cemented carbides using Ca-rich raw materials was good,especially laying a foundation for the direct production of WC by scheelite.