The influence of carbon content on formation of carbo-nitride free surface layers in cemented carbides

Abstract

To increase crack propagation resistance in cemented carbide cutting tools, it is sometimes of interest to create tough surface zones in the substrates. A way to do this is to use so-called gradient sintering in the manufacturing of the cutting tool. In this sintering process a nitrogen and titanium containing cemented carbide is sintered in a nitrogen free atmosphere. The difference in nitrogen activity between atmosphere and cutting tool during sintering will create an outward nitrogen diffusion. Due to thermodynamical coupling between nitrogen and titanium, this gives rise to an inward titanium diffusion, which creates a surface zone depleted of hard cubic carbo-nitrides, and enriched in ductile binder phase. By varying the carbon content of the material, the nitrogen activity is affected, and this in turn affects the surface zone formation. In this report, Ti(C,N)–(Ti,W)C–WC–Co, Ti(C,N)–NbC–WC–Co, and Ti(C,N)–TaC–WC–Co cemented carbides were studied. All three materials were produced in series with varied carbon content, in order to study the effect of carbon on gradient surface zone formation.