Hardmetal properties

Relationships between Hardness and Other Hardmetal Properties... 

Co hardmetals, it is possible to improve both properties at the same time (Cherradi et al., 1994) (Fig. 22.13). This can be achieved by a gradation in the binder content from the centre to the surface of the tool. The improved mechanical properties of the FGM-cemented carbide are due to compressive stresses near the surface, which enable a reduction of the Co-phase content from 6 to 3 wt% without any loss in apparent fracture toughness. 

In the systems Co-C and Ni-C and in the other transition metal-carbon systems not mentioned so far, no stable carbide phases are observed. The carbon solnbilities in the metals are of importance for the fabrication and properties of hardmetals (see Section 9.1.1). The phase diagrams are of the entectic type. Metastable carbide phases have been reported in rapidly qnenched Co-C and Ni-C alloys. 

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 extremely high modulus of elasticity of WC (only exceeded by diamond and W2B5), and the high electrical and thermal conductivity are further important criteria for its use in hardmetals. The latter two properties also reflect the strong metallic component of the mixed covalent (W5d-C2p) metallic bonds in the carbide. Fermi surface properties of WC and electronic band structure calculations can be found elsewhere [4.23, 4.24]. 

Manufacture ofhardmetals [9.4]. The manufacture of hardmetals is based on powder metallurgical techniques, which include several steps. Each step must be carefully controlled to achieve a final product with the desired properties. These steps are ... 

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]. 

Although the above characterization in regard to physical and chemical properties seems to be quite rigid and complete, in some cases imexpected and unsatisfactory results may occur diuing the further hardmetal production. The reason for this behavior is based on still insufficient specification, because the carbon black quality, carburization temperature, as well as type and diuation of milling after carburization are not clearly defined, and can be changed by the supplier or may differ between diverse suppliers. 

In order to minimize testing operations for the hardmetal producer, it has become common in recent years to prepare homogeneously blended lots of 5 t or even 10 t. In blending, it is strictly forbidden to use smaller lots of strongly diverging properties to meet the specified values. 

As mentioned earlier in the introduction to this chapter, tfie properties of WC-Co based hardmetals can be varied widely and consequently their applicability is extremely widespread. The properties are intimately connected with their microstructure (including micro- and macroporosity) and surface conditions (grinding cracks and excessive roughness). These can be influenced by several raw material properties and processing conditions ... 

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]. 

As a result, by selecting appropriate combinations of cobalt contents and carbide grain sizes, the hardness of WC-Co can be varied from below 800 HV to more than 2000 HV. The most appropriate combination is determined by the properties which, besides hardness, are required for a specific application. The recent introduction of grain sizes of the order of 10 nm has extended the range of possible hardness well above 2000 HV [16]. However, nano-grade hardmetals are not included in the present review because they always contain grain refiners such as VC or Cr2C3, while this review is limited to two-phase V/C- Co alloys. 

In most hardmetal applications it would be desirable to use the hardest possible grade. However, properties often have reciprocal relationships, i.e. an improvement in one leads to a deterioration in another. Therefore, in order to select the most appropriate grades for a specific application, it is desirable to know quantitatively the relationships between the various properties. So far, extensive work has been... 

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