Hardmetals sintering

PcBN compacts are often bonded to a hardmetal or hard steel alloy for manufacturing a tool. This composite can be produced by direct sintering of WC-Co-PcBN mixtures or by brazing. 

Onto the surface of the sintered part (e.g. hardmetal) the mixture of c-BN and/or diamond powder is bonded during high-pressure high-temperature sintering [267]. 

This chapter will follow the different stages of powder metallurgical manufacture. More emphasis will be put on the description of WC powder production methods and qualities, and the preparation of graded powders. Less emphasis will be put on sintering, hardmetal qualities, and applications. In this context, we refer to several excellent books and review articles dealing particularly with hardmetal technology, properties, and applications [9.1, 9.2, 9.4, 9.7-9.9]. 

The desired final carbon content of the WC powder depends on the further production mode of the hardmetal producer (mainly powder milling and sintering conditions), and varies between slightly understoichiometric to stoichiometric (6.13 wt%) to slightly overstoichiometric. 

For most WC powders (APS 0.5-4 pm), there also exists a close relationship between the APS of the WC powder and the average grain size (AGS) of the WC in the sintered hardmetal structure (as long as milling conditions in grade powder preparation and sintering conditions are kept constant). In this case, the APS of the WC powder is used as the main steering parameter for the hardmetal structure. This is shown schematically in Fig. 9.15. 

FIGURE 9.15. Relationship between particle size of W powder, particle/grain (crystallite) size of the derived WC, and WC grain size in the sintered hardmetal (schematic). 

Direct compacting. Direct compacting is applied for near-net shaping of parts. This means that the final hardmetal specimen after sintering already has the desired dimensions and only limited surface treatment is necessary afterward. For that purpose, semiautomatic or automatic mechanical or hydraulic presses are used. The pressure application is only from one direction, resulting in a slightly anisotropic density distribution in the green compact. 

The pressing dies (fabricated of hardmetal) are made to the shape of the desired end product, which means that the die must be greater to allow for shrinkage occurring during sintering. 

FIGURE 9.24. Representative sintering cycle of hardmetal production, indicating the different stages of sintering. 

Pressure sintering. Sometimes, hardmetals exhibit residual porosity (A and B type), which can occur for several reasons. Since the 1970s pressure sintering (HIP-ing) has been applied to remove this porosity. For that purpose, the sintered specimens are loaded in a HIP-ing device and heated again under Ar or He pressure of 50-150 MPa (temperatures and commonly 25-50 °C below the vacuum sintering temperature). Hence any residual porosity can be removed. Exceptions are gas porosities, which either can occur due to incomplete outgassing of the carbon monoxide, or due to the presence of impurities. 

FIGURE 9.25. Three-chamber vacuum sintering furnace for hardmetal production (Ipsen). By courtesy of WIDIA GmbH., Germany. 

FIGURE 9.26. Sinterhip furnace for pressure sintering of hardmetals (max 60 bar). Charges of up to 1000 kg can be sintered within a 24-hour cycle. By courtesy of ALD Vacuum Technologies GmbH., Germany. 

FIGURE 9.27. Cobalt-enriched hardmetal substrate, obtained by gradient sintering with CVD multilayer coating. Outer layer TiCN/TiN middle layen AI2O3 inner layer TiC/TiCN. By courtesy of Kennametal Inc., USA. 

FIGURE 9.35. Dental endmill shown in different stages of manufacturing. Pressed part at the bottom left as-sintered part in the middle brazed onto a steel shaft (right) endmill together with the workpiece in the background. By courtesy of United Hardmetal, Germany. 

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... 

The ternary system W-C-Co is shown in Fig. 24 at 1423°C [102]. WC coexists with the Co phase and forms a pseudobinary lower melting eutectic between WC and Co. This eutectic facilitates full densification during liquid phase sintering. The carbon content must be kept close to the theoretical value because a decrease in total carbon content will lead to the formation of q carbides (W, Co>6C which are brittle and harmful to the performance of these materials. In modern hardmetals, also f.c.c. carbides such as TaC, NbC and TiC are admixed which increase the high-temperature performance. Figure 25 shows a microstructure of a modem WC/Co-based hardmetal containing about lOweight-% of f.c.c. carbides TiC, TaC, and NbC. 

The boundary between hardmetals and cermets is not strict because many of these compacts resemble microstructure features of both type of materials [106] faceted WC crystals together with round-shaped titanium carbonitride-based hard particles. Generally, these titaniiun carbonitride hardmetals are comparable with respect to properties and microstructure to WC-based hardmetals. The powders of these materials are liquid phase sintered with Ni or Ni-Co binder metal alloys. The core-and-rim structure of the hard phase usually exhibit a molybdenum- and carbon-rich (Ti,Mo)C rim and a titanium- and nitrogen-rich Ti(C,N) but can also be inverted (compare Fig. 26). The metallurgy of the phase reactions is (because of the complexity of the multicomponent system) not yet fully understood [69]. 

WC-Co hardmetal is a sintered material consisting of brittle tungsten carbide (WC) crystals bonded by a tough cobalt-based binder. Figure 1 shows the microstructure of a typical commercial WC-Co hardmetal. 

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... 

Belmonte M, et al. Wear resistant CVD diamond tools for turning of sintered hardmet-als. Diamond Relat Mater 2003. 

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. 

Rodinger K, Dreyer K, Gerdes T, Porada MW. Microwave sintering of hardmetals. Int J Refract Metals Hard Mater 1998 16 409-16. 

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