HYDROGEN-CONTAINING CHEMICALLY ACTIVE GAS MEDIUM FOR CONTROLLING CARBON CONTENT OF TUNGSTEN-BASE HARD ALLOYS

The interaction between components of tungsten-base hard alloys and hy drogen which is used in sintering as a protective gas medium has been theoretically and experimentally studied. Thermodynamic possibility of reactions responsible for variations in carbon content of alloys has been analyzed. It has been found that when the alloy interacts with hy drogen, the former is decarburized both by hydrogen proper and moistur e and oxygen impurities present in it. Equilibrium constants K-p and e quilibrium compositions for the above reactions have been calculated a t 300-1500 K. It is shown that methane added to hydrogen in amounts ex ceeding its equilibrium concentration with respect to WC but not excee ding its equilibrium concentration with respect to graphite not only s lows down the process of WC decarburization, but recarburizes W and W2 C impurities present in;initial powders to WC with no risk of free car bon liberation. Compositions of methane-hydrogen mixture have been fou nd, which allow a reliable control of the carbon content of sintered h ard alloys. The above gas medium has permitted us to devise a peculiar sintering procedure which allows two-phase alloys uniform with respec t to carbon content, as well as alloys having special-purpose properti es to be fabricated. Copyright (C) 1997 International Association for Hydrogen Energy.