Production of gradient ultrafine-grained hard alloys from powders obtained from scrap VK15 alloy dispersed in water and scrap VK8 alloy dispersed in oil

Abstract

An experiment has been carried out aimed at producing a low-cost tool material which would have an extra hard surface and a crack resistant base phase. A double-layer gradient alloy was produced out of powders with excessive and insufficient carbon obtained from alloys VK8 and VK15 by electrospark dispersion. To obtain the initial powder, plates of the VK8 alloy scrap material initially containing 5.6% of carbon were dispersed in oil. This caused excessive carbon (3.1%), which was then partially reduced to 0.6% by CO2 heat treatment. The VK15 alloy with 5.3% of initial carbon was dispersed in water to achieve an insufficient carbon concentration (2.8 %) in it. It was then heat treated in the CO atmosphere to partially lower the carbon concentration to 0.7%. The dispersed powder was found to consist of spherical microparticles, as well as agglomerated nanoparticles obtained through crystallization of liquid and vapour phases. Mixing of tungsten and cobalt phases resulted in much smaller diameters of WC grains in the newly sintered alloy. This combination of deviations in the carbon concentration between the layers helped maintain the unequal concentration of cobalt (8 and 15%) and thus ensure an extra hard surface (1820HV) and a crack resistant base (14.2 MPa√m). Analysis was carried out to estimate the power costs, as well as the performance of the laboratory unit used for electrospark dispersion of hard alloys.