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A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. May 2016. / Tungsten carbide cobalt (WC-Co) cemented carbides are widely used for cutting, drilling, machining and as wear resistant materials due to the combination of high hardness and fracture toughness. In this work, we report on as-milled and as-sintered WC-10Co-20Fe samples which were ball milled for 15 hrs and sintered using liquid phase sintering (LPS). These samples were investigated by Vickers hardness test, microstructural analysis, X-ray diffraction (XRD), transmission Mössbauer spectroscopy (TMS) and conversion electron Mössbauer spectroscopy (CEMS) techniques. A mean hardness value of 1160 ± 42 HV was obtained for WC-10Co sample while a value of 776 ± 35 HV was determined for the WC-10Co-20Fe using the Vickers hardness tester. The lower hardness value for WC-10Co-20Fe is attributed to the high volume of the binders (10% Co and 20 %Fe) incorporated in the sample. The microstructural analysis of the as-sintered WC-10Co and WC-10Co-20Fe samples reveals that the light regions represent the WC phases and the dark regions signify the presence of the Co and CoFe phases in the as-sintered WC-10Co and WC-10Co-20Fe samples, respectively. The energy dispersive spectroscopy (EDS) of the as-sintered samples shows the presence of the starting powders used (WC, Co and Fe) and some Cr contamination resulting from either the production process or the starting powders. / GR 2016

This research project is a preliminary investigation of the effect of SiC abrasive breakdown on the wear rate of a WC-12wt%Co mining alloy. Wear tests were carried out on a two body-sliding wear apparatus under (a) “Ideal” (replacing the SiC paper periodically to ensure continual exposure to fresh abrasives), (b) “No debris” (removing the wear debris periodically) and (c) “With debris” (retaining the wear debris for the entire wear test) wear conditions. The WC-12wt%Co specimens and SiC abrasive grits were examined before and after the wear tests using optical, stereo and electron microscopy. As wear progressed, the SiC abrasives blunted thereby increasing the abrasive/specimen contact area, resulting in a reduction in the WC-12wt%Co wear rate. Wear debris clogging the interstices between the abrasive grits caused a further reduction in the WC-12wt%Co wear rate by adding to the abrasive/specimen contact area already created by blunting. Increasing the applied load resulted in an increase in the WC-12wt%Co wear rate under “Ideal” wear conditions. Under the remaining wear conditions, the increased load resulted in a faster deterioration of the SiC grits. The dominant wear mechanisms under all conditions are characterized by hard abrasive wear that caused extensive grooving, Co binder extrusion and cracking and fragmentation of WC grains.

Steel industry prospered in Taiwan greatly from late 70s to early 80s. Automated steel manufacturing machines of high productivity were introduced into all steel factories and billions of NT dollars were invested. In this wave of steel manufacturing, three countries( US, Europe and Japan) dominated the advanced production-line equipment as well as the expensive materials needed for equipment production¡XTungsten carbide roll. This phenomenon put Taiwan steel industry in a less favorable position in the global competition. In view of this downside, PJ Company established a Tungsten carbide factory in Pudong, Shanghai in 1995. After years of hard work, PJ Company successfully developed a Tungsten carbide factory of high quality, breaking the old marketplace monopolized by foreign companies. In the year 2002, PJ took another step in overcoming the technical obstacle of Tungsten carbide alloy and successfully nanonized Tungsten carbide roll, making the company itself a global pioneer of Tungsten carbide manufacturer. The keys to P.J Company¡¦s success are its staged marketing strategy, and subsequent strategy including short, mid, and long-term management. More details are listed below: 1) Establishing the plant in accordance with the availability of raw materials¡Xlow cost strategy 2) Recruiting R&D specialists from China, Japan and Taiwan(especially consultant retired from Mitsubishi heavy industries)¡Xto speed up the establishment and innovation of the technique of powder metallurgy. 3) Brand strategy¡Xusing OEM cost as a counter for brand establishment. 4) Merger strategy¡Xto become a virtual storehouse for European steel factories, to improve service, and to lower time costs and transportation fees for individual product. 5) Strategic alliances¡Xto reduce the fierce competition on relative prices, achieving a bond of peaceful coexistence between companies through cooperation. 6) The nanonization of Tungsten carbide alloy ¡Xto make sure the company is ten years ahead of its rivals in competitiveness, narrowing the chances for other competitors. The success of PJ Company indicates good possibilities for future development and prospective market foresight in those traditional business combined with nanotechnology, the company¡¦s control in cash flow and critical R&D skills, as well as selecting national R&D departments for cooperation with many steel industries in Vietnam to avoid risks. All of these set a good example for Taiwanese businessmen who are or will be establishing factories in Mainland China. China Steel Corporation is currently evaluating the possibility of cooperation between PJ company and Dragon Steel Corporation, a member of CSC in Taichung, hoping to lower DSC¡¦s expenditure on Tungsten carbide roll. PJ Company also began an official collaboration with the factory of Yieh-hsing Company in southern Pintony industry zone on December 1st, 2005. At first the two companies worked together about the machinery process of Tungsten carbide to enhance productivity, to strengthen the techniques needed to complete the steps to producing roll and to achieve the goal of having the technique of nanonization of Tungsten carbide alloy to be replanted and stays in Taiwan. We are therefore extremely proud of this progress and look forward it.

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The influence of mechanical alloying (MA) milling time, temperature, sintering method and microstructure on the mechanical properties of a tungsten carbide-cobalt (WC-Co) hardmetal, based on 10wt% Co, has been established. The effects of high-energy milling for 30, 60, 180 and 300 min and the interrelation between milling time and powder properties, and the resultant effects on the mechanical properties of the consolidated WC-10Co material, has been obtained for a horizontally designed ball mill. Nanostructured WC-10Co powder was synthesised after 60 min cyclic milling at room temperature with an average WC domain size of 21 nm. In direct comparison, a WC-10Co composition MA at -30°C for 60 min produced an average WC domain size of 26 nm with a higher lattice strain. WC domain size showed a slight increase with milling time, measured at 27 nm after 300 min ball milling. Extended ball milling (300 min) reduced the mean particle size from 0.148 μm for 60 min milling to 0.117 μm. Thermal analysis showed that the onset temperature of the WC-Co eutectic was related to particle size with increased milling time reducing the onset temperature from 1344°C after 60 min milling to 1312°C after 300 min milling. Onset temperature was further reduced by the addition of vanadium carbide (VC), reducing the onset temperature to 1283°C after 300 min milling. Powder contamination increased with increased milling time with Fe content measured at ~ 3wt% after 300 min ball milling. Milling at -30°C reduced Fe contamination to an almost undetectable level. Increased ball milling time resulted in decreased levels of green density with the powders milled for 30 and 300 min achieving 62.5% and 59.5% TD, respectively. Relative density increased for the powder milled at -30°C compared to the RT milled powder due to its flattened, slightly rounded morphology. A large difference in VC starting particle size compared to WC and Co led to non-uniform dispersion of the inhibitor during milling. Densification and hardness reached optimum levels for the 60 min milled powder for both pressureless sintering and sinter-HIP. Both properties decreased with increased milling time, regardless of the sintering method. Low temperature milling resulted in a higher hardness value of 1390 HV30 compared to 1326 HV30 for the 60 min, RT milled material after pressureless sintering. Densification levels of the doped materials were restricted to < 90% TD for both sintering methods due to inhomogeneity in the microstructures. Palmqvist fracture toughness (WK) of the RT milled powders increased with increased milling time and increasing WC grain size for both sintering methods. WK reached 11.6 MN.m3/2 with 300 min milling after pressureless sintering but reached 16.1 MN.m32 for the same material after sinter-HIP due to the effect of mean WC grain size and binder phase mean free path. The -30°C milled powder exhibited higher fracture toughness for both sintering methods than the 60 min, RT milled material. Spark plasma sintering (SPS) showed that the onset of densification was dependent upon particle size with the powder from 300 min milling showing an onset temperature of ~ 800°C compared to ~ 1000°C for the 60 min milled powder. The low temperature milled powder showed an onset temperature of ~ 980°C, which suggested that low temperature milling provided enhanced densification kinetics.