Alloyed W–(Co,Ni,Fe)–C phases for reaction sintering of hardmetals

Abstract

It has been shown that W–Co–C phases could dissolve a substantial amount of metals such as V, Cr and Ta, which are known to positively influence the microstructure of hardmetals with respect to uniform grain size distribution and fine grain size. This offers a tool to circumvent the conventional doping of hardmetals with individual carbides. In the present study we used double- and triple-alloyed κ-W9Co3C4 (i.e. κ-(W,V,Cr)9Co3C4 and κ-(W,V,Cr,Ta)9Co3C4) and applied a variety of sintering experiments to obtain WC–Co, WC–(Ti,Ta,Nb)C–Co and WC–(Ti,Ta,Nb)(C,N)–Co hardmetals. We also prepared κ-W9Fe3C4, alloyed κ-W9Ni3C4, and κ-W9(Fe/Ni)3C4, and used the latter for sintering. Sintering was studied in situ by mass spectrometric outgassing experiments (MS-EGA) and dilatometry (DIL). The reactively-sintered hardmetals can be obtained with a very low amount of platelets and show the same properties as hardmetals with platelets. The latter are obviously not responsible for the high KIc values in these hardmetals. The alloy status of the starting alloyed κ-W9Co3C4, especially a certain amount of Ta, plays a role in WC grain growth for avoiding platelets. The good KIc is most probably due to a uniform Co distribution (the binder phase and WC form simultaneously by reaction of κ phase with C. Upon this formation, V, Cr and Ta are directly involved, because they are dissolved in the κ phase). The influence of the starting grain size of alloyed κ-W9Co3C4 on the grain size of reactively sintered hardmetal is crucial. This is probably due to the short diffusion distance of C, which causes the formation of very fine WC particles upon reaction with C. Together with alloyed κ-W9Co3C4, oxides were used to form fcc carbides by in situ carburisation. The microstructure of such prepared WC–(Ti,Ta,Nb)C–Co hardmetals is finer and the interpenetration of the WC and fcc particles is better than by use of fcc carbides in the starting mixture. The same is true if nitrogen atmosphere is used (and C level in the starting formulation is reduced) to form WC–(Ti,Ta,Nb)(C,N)–Co hardmetals. With respect to the fracture toughness/hardness relationship, some of the prepared hardmetal grades show better properties than industrial grades and could thus possibly outperform the latter in cutting applications.