The Effect of Nb,Cr And Ti AS Matrix elements in WC-6Co Hard Metal

         The present work is a comparative study of hard metals obtained from a commercial WC-6% Co composite by adding 1wt.% of the elements Nb, Ti, Cr and Mo, respectively. The powder metallurgical production process of these compositions was studied monitoring the process parameters green density, sintered density, shrinkage and hardness. The sintering temperature of 1450°C was adequate for WC-6Co, WC-6Co-Nb and WC-6Co-Mo. Potentiodynamic tests in H2SO4 showed that WC-6Co-Ti has the best corrosion resistance.



  A hard metal is a composite material which consists of a hard phase embedded in a ductile binder phase. The most common composition is tungsten carbide (WC) in a cobalt or nickel matrix.Hard metals are widely used as tools for metal cutting operations . For other industrial applications, such as seal rings and valves, sufficient corrosion resistance of the hard metal is essential.



  The mechanical properties of WC/Co hard metal depend critically on its composition, microstructure, and the chemical purity of the carbide. In general, decreasing WC particle size improves mechanical properties such as hardness, wear resistance, and even transverse rupture strength of the composite.The addition of small quantities of other carbide forming elements, such as Ti, Ta, V, Nb, Cr has the function to inhibit WC grain growth during sintering.WC dissolves in the Co matrix during sintering and precipitates again at other carbide particles during cooling. In this way particle coarsening occurs since small carbides dissolve preferentially and reprecipitate at greater particles . Grain growth is also believed to occur by grain boundary migration.



  Increasing the volume fraction of Co increases the fracture toughness at the expense of hardness and wear resistance. Co possesses a very good wettability of tungsten carbide in the liquid phase. Commercial hard metals have a Co content between 3 and 25 wt%. For applications in aggressive environments modifications of the binder phase are made in order to improve the corrosion resistance. In contact with the nobler carbide phase the binder is the part of the hard metal which dissolves preferentially. To improve the binder resistance, Ni has been used as binder, as well as the addition to Co of elements like Cr, Nb, Mo, Ti, i.e., elements which are known to improve corrosion resistance of ferrous alloys, which form carbides, leaving less binder phase which can be attacked or which are very resistant as pure elements and promote the formation of protective oxides.



  Among the elemental powders added to WC-6Co, Mo showed the best wettability. As a consequence a lower hardness of WC-6Co-Mo was observed due to accelerated grain growth at the sintering temperature.Potentiodynamic test in sulfuric acid revealed WC-6Co-Ti as the hard metal with the best corrosion resistance. For all hard metals with addition of elementary metal powder the corrosion current densities obtained by Tafel extrapolation were lower than that of the commercial WC-6Co hard metal.