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

• Published in: International journal of refractory metals & hard materials 2010-09, Vol.28 (5), p.638-645
• Main Authors: Reichel, B., Wagner, K., Janisch, D.S., Lengauer, W.
• Format: Article
• Language: English
• Subjects: Alloy plating; Alloying; Cemented carbides; Chromium; Face centered cubic lattice; Grain size; Kappa phase; Phases; Platelets; Reactive sintering; Sintering; Tantalum; Ternary tungsten carbide phases; W 9Co 3C 4; W 9Fe 3C 4; W 9Ni 3C 4
• ISSN: 0263-4368; 2213-3917
• DOI: 10.1016/j.ijrmhm.2010.06.003

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-W 9Co 3C 4 (i.e. κ-(W,V,Cr) 9Co 3C 4 and κ-(W,V,Cr,Ta) 9Co 3C 4) 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-W 9Fe 3C 4, alloyed κ-W 9Ni 3C 4, and κ-W 9(Fe/Ni) 3C 4, 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 K Ic values in these hardmetals. The alloy status of the starting alloyed κ-W 9Co 3C 4, especially a certain amount of Ta, plays a role in WC grain growth for avoiding platelets. The good K Ic 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-W 9Co 3C 4 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-W 9Co 3C 4, 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.