Tungsten in Hardmetals

Cemented carbides represent a group of hard and wear-resistant refiactory composites in which hard carbide particles are bound together or are cemented by a ductile and tough binder matrix. Although the term cemented carbide is still widely used, mainly in English-speaking countries, and well describes the nature of the composite, they are even better known internationally as hardmetals. The latter term will be used in the following. 

Besides WC-based hardmetals, Ti(C,N)-based grades are also used, particularly in Japan. These so-called cermets contain no, or only minor additions of, WC. They will not be discussed in this book. 

As a result of the strong adhesion between the WC grains and the Co binder, straight WC-Co grades exhibit the best hardness-to-toughness ratio of all hard materials produced today. This explains their outstanding role in the manufacturing industry. 

The machining industry is the best example. The dominant tool material today is the coated hardmetal which, in combination with indexable insert technology, has had an enormous impact on the machining industry since its introduction in the late 1960s [9.3]. 

However, despite these changes in the tool industry, hardmetals have not lost their importance. On the contrary, larger amounts are produced than ever before and, in their various areas of application, their proportion is steadily increasing. The reason for this situation is manifold  

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W-Cu, V-ThO2). Electrical discharge machining allows close dimensional tolerances, accuracy of form, and a high-quality surface finish to be achieved. Both hardened steels and hardmetals can be machined (Fig. 7.12). Tungsten in wire eroding is not only used as a guide and contact for the wire electrodes, but also as erosion wire. 

The binder in WC-Co hardmetal is not pure cobalt but a solid solution of carbon and tungsten in cobalt. When cooling the material from the sintering temperature... 

Since 1930, cemented carbides (also called hardmetals) steadily attained a greater share in tungsten consumption. It is of interest to ask why the demand for cemented carbide grew so rapidly. Table 2.9 shows a chronological table indicating the most important events in cemented carbide research and development, a process which is still under way today. We recognize that what we call cemented carbides or hardmetals are in reality a very wide palette of materials with different properties. Cemented carbide properties can be adjusted by several variations and combinations of the components, as shown in Table 2.10. Hence cemented carbides could be applied widely. Figure 2.10 presents a breakdown of the fields of application of cemented carbides. 

The binary tungsten-carbon system (Fig. 4.1) [4.13-4.15] is of high technical importance. It contains three intermediate compounds W2C(P), WCi ((y), and WC(5), the latter being the main constituent in most of the commercial cemented carbides (hardmetals see Chapter 9). Besides, ternary and even more complex compoimds exist which are of interest in alloyed steels and cemented carbides. Tungsten hexacarbonyl is an important precursor for organic synthesis. 

Besides tungsten metal, only those alloys which were described in Chapter 6 are treated in this chapter. Alloys produced by melting metallurgy as well as the cemented carbides (hardmetals) and their application are treated in Chapters 8 and 9, respectively. 

Economic importance. About 23,000 tons/year of hardmetal are currently used in the western world, which correspond to more than 60% of tungsten demand [9.1]. The world market volume for hardmetal cutting tools in 1995 was estimated to be roughly 9 billion DM [9.10]. The price ofhardmetals, on average, equals that of silver. For special tools, like coated indexable inserts, the price is about double that of silver [9.11]. 

Nonmachining. Thirteen percent of tungsten consumed by the hardmetal industry, corresponding to 6% of world tungsten demand, are used in underground mining, road... 

There do not exist any special values for permissible concentrations of hardmetal dust on the work place. Therefore, the MAK values for insoluble W and Co are valid (5mgWC/m and 0.5mgCo/m ). The ITIA (see Section 13.6) is encouraging inducing enhanced activity in regard to acute toxicity tests to be conducted on tungsten compounds including hardmetals (ITIA Newsletter, Jime 1997). 

The most widely used transition metal carbide is tungsten carbide, hexagonal WC, which is employed as the hard constituent in WC-Co hardmetals. Such hardmetals are sintered composite materials with 80-90% of hard particles such as WC embedded in a ductile binder phase such as Co. For these apphcations WC combines a number of... 

A nozzle opening of a sandblaster may suffer severe wear during operation. This can be countered by using hot-pressed boron carbide (B4C), which is expensive but is very hard and has a long lifetime in this application. Sintered alumina or hardmetal (a cermet of tungsten carbide sintered with some cobalt or nickel metal) show more wear but are suitable as well and cheaper to make. 

Tungsten carbide, or hardmetal, was developed in the 1920s for wear-resistant dies to draw incandescent-lamp filament wire. Earlier efforts to manufacture the WC-W C eutectic alloy was unsuccessful because of its inherent brittleness therefore researchers diverted their attention to powder metallurgy techniques. At present, these powder metallurgy techniques are... 

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