WC-Co cemented carbides having cobalt metal binder ranging between 12 and 17.5 Vol.% and different carbide grain size (0.7-2.2 μm) were investigated in terms both of microstructure and mechanical behaviour (hardness, flexural strength and fracture toughness). Although high hardness occurs to match the best performance in terms of wear resistance, consequence of a proper choice of percentage of cobalt vs. WC carbide, the absence of cracks nucleating during the life of a generic hardmetal component is a further side improving the reliability of such a family of materials. For such a reason, the evaluation of the toughness behaviour characterizing the different commercial hardmetals is becoming more and more frequently requested along with the traditional mechanical characterization carried out by hardness, wear resistance or flexural strength tests. In the present paper four cemented carbide grades, different in the cobalt binder percentage, or in the average WC carbide size d, or in the mean free path in the binder phase λ, were investigated. In particular the presence of 0.2% of TaC acting as grain-growth inibitor characterises hardmetal grade 3 and 4. That alloying addition strongly increases the flexural strength, but does not adversely affect the toughness. Such a result (different from that expected from the literature [3-4]), was interpreted by means of the influence of testing procedure on the critical intensity factor, KIc determination. In fact, usually for the cemented carbides, the critical fracture mechanics parameter is calculated by means of indentation tests in which radial cracks are induced applying different loads to an indenter apparatus for the Vickers hardness measurements (in the present paper minimum applied load=l5 kg, maximum applied load=150 kg). Considering that indentation fracture behaviour of cemented carbides is extremely sensitive to the surface sample preparation [19-22], and knowing that usually the tested samples are grounded and polished, it is important to define the influence of the surface residual stress data on the fracture toughness behaviour evaluated by means of indentation techniques. Consequently, residual stress data were measured by means of an X-ray diffrattometer both in the hipped condition and after grinding or grinding plus automatic polishing. Tensile residual stresses in the transversal direction and significant compressive residual stress values in the longitudinal direction are the results of the grinding. Applying the fracture mechanics approach instead of the indentation technique allows to limit the effects of surface finishing on data results. On contemporary, researchers are dispensed from choosing one of the empirical equation fitting the experimental results, avoiding a further method variable which also would affect the toughness data. By this way, the result is, therefore, effectively representative of the tested material. Increasing the percentage of the metal binder, evident increasing in the fracture toughness was obtained, but an effect on the fracture toughness at prefixed cobalt percentage must be furthermore related to the microstructural parameters: both the carbide grain size and the mean free path in the binder phase are effective in the variation of mechanical properties. No changing in the fracture mechanism was detected in presence or absence of TaC: ductile fracture characterizes the areas in which metal binder is present, while brittle fracture mechanism (cleavage) is active in correspondence of tungsten carbides. Adding TaC to the chemical composition of cemented carbides, at a fixed percentage of cobalt, improves hardness and flexural strength, but this is not a circumstance, which automatically reduce the toughness behaviour of the sintered hardmetals. Such a result must be carefully considered in all those applications in which, not only wear behaviour are important, but (for instance the case of dies) also the presence of cracks due to an accidental overload can cause the discarding of the tool produced using cemented carbides.
Mechanical behaviour of cemented carbides interpreted by means of microstructureand surface residual stress distribution
La Vecchia G. M.
;D'Errico F.
2003-01-01
Abstract
WC-Co cemented carbides having cobalt metal binder ranging between 12 and 17.5 Vol.% and different carbide grain size (0.7-2.2 μm) were investigated in terms both of microstructure and mechanical behaviour (hardness, flexural strength and fracture toughness). Although high hardness occurs to match the best performance in terms of wear resistance, consequence of a proper choice of percentage of cobalt vs. WC carbide, the absence of cracks nucleating during the life of a generic hardmetal component is a further side improving the reliability of such a family of materials. For such a reason, the evaluation of the toughness behaviour characterizing the different commercial hardmetals is becoming more and more frequently requested along with the traditional mechanical characterization carried out by hardness, wear resistance or flexural strength tests. In the present paper four cemented carbide grades, different in the cobalt binder percentage, or in the average WC carbide size d, or in the mean free path in the binder phase λ, were investigated. In particular the presence of 0.2% of TaC acting as grain-growth inibitor characterises hardmetal grade 3 and 4. That alloying addition strongly increases the flexural strength, but does not adversely affect the toughness. Such a result (different from that expected from the literature [3-4]), was interpreted by means of the influence of testing procedure on the critical intensity factor, KIc determination. In fact, usually for the cemented carbides, the critical fracture mechanics parameter is calculated by means of indentation tests in which radial cracks are induced applying different loads to an indenter apparatus for the Vickers hardness measurements (in the present paper minimum applied load=l5 kg, maximum applied load=150 kg). Considering that indentation fracture behaviour of cemented carbides is extremely sensitive to the surface sample preparation [19-22], and knowing that usually the tested samples are grounded and polished, it is important to define the influence of the surface residual stress data on the fracture toughness behaviour evaluated by means of indentation techniques. Consequently, residual stress data were measured by means of an X-ray diffrattometer both in the hipped condition and after grinding or grinding plus automatic polishing. Tensile residual stresses in the transversal direction and significant compressive residual stress values in the longitudinal direction are the results of the grinding. Applying the fracture mechanics approach instead of the indentation technique allows to limit the effects of surface finishing on data results. On contemporary, researchers are dispensed from choosing one of the empirical equation fitting the experimental results, avoiding a further method variable which also would affect the toughness data. By this way, the result is, therefore, effectively representative of the tested material. Increasing the percentage of the metal binder, evident increasing in the fracture toughness was obtained, but an effect on the fracture toughness at prefixed cobalt percentage must be furthermore related to the microstructural parameters: both the carbide grain size and the mean free path in the binder phase are effective in the variation of mechanical properties. No changing in the fracture mechanism was detected in presence or absence of TaC: ductile fracture characterizes the areas in which metal binder is present, while brittle fracture mechanism (cleavage) is active in correspondence of tungsten carbides. Adding TaC to the chemical composition of cemented carbides, at a fixed percentage of cobalt, improves hardness and flexural strength, but this is not a circumstance, which automatically reduce the toughness behaviour of the sintered hardmetals. Such a result must be carefully considered in all those applications in which, not only wear behaviour are important, but (for instance the case of dies) also the presence of cracks due to an accidental overload can cause the discarding of the tool produced using cemented carbides.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
