Towards carbon control in the manufacture of WC-Co hardmetal

• Main Author: Parker, Simon
• Other Authors: Whiting, Mark ; Yeomans, Julie
• Published: University of Surrey 2017
• Subjects: 669
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.714748

WC-Co hardmetal, found in applications ranging from mining tools to valves in deep-sea gas pipelines, is valued for its hardness and toughness provided by the unique chemistry of the tungsten carbide – cobalt pairing. Properties of WC-Co hardmetal are very sensitive to carbon content, variation of 0.01 wt.% can lead to alteration of hardness yet carbon content is very difficult to control in manufacture of hardmetal, a powder metallurgical process involving high temperatures and interactions with different atmospheres. Accurate measurement of carbon content is difficult in hardmetal as the total carbon content is high compared to the sensitivity. Work has been reported on carbon measurement in hardmetals using x-ray diffraction (XRD) but attempts to replicate this work were unsuccessful due to limitations in accuracy of equipment. Examination of a commercial hardmetal production process sought to identify manufacturing variables that lead to changes in carbon content. The Vickers hardness, Palmqvist toughness, density and magnetic saturation of samples processed under different processed under different conditions found in commercial manufacture of hardmetal were compared. No clear correlation between any of the process variables examined and carbon content of sintered hardmetal could be found, motivating work to actively alter carbon content. An obvious solution to limited carbon control is to alter the balance of carbon and tungsten in powder blends to compensate for anticipated changes and work was undertaken to investigate this approach. In addition to mechanical and magnetic characterisation, the carbon content of samples was measured using the infra-red gas absorption method. Altering the carbon content of samples by adding carbon or tungsten did not appear to offer control of carbon content in sintered hardmetal as the amount of carbon added did not appear to correlate to the carbon content of sintered samples. Heat treatments in carbonaceous atmospheres has also been explored and was demonstrated to have potential as a method of controlling carbon content in the manufacture of WC-Co hardmetal. Pre-sintering, a heat treatment often applied to compacts before sintering, was replicated at laboratory scale using an atmosphere of nitrogen and hydrogen with various additions of methane at different temperatures. Samples were examined using Vickers microhardness and confocal light scanning microscopy (CLSM) to capture spatial variation in sample properties and by electron backscatter diffraction (EBSD) to obtain information on grain size distributions. It was found that carbon content increases with the amount of methane in the heat treatment atmosphere and with heat treatment temperature though the reliability and accurate control required to make the technique commercially viable were not achieved. Results demonstrate that with further refinement heat treatment of pressed compacts in carbonaceous atmosphere could be used to accurately control the carbon content of sintered hardmetal components.