The effect of vanadium and niobium addition on microstructure, wear and hardness of 25% wt% Cr white cast iron for ball mill applications

Mampuru, Lebedike Andrew
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The 25 wt% high chromium white cast iron (HCWCI) is used in the mining industry as a mill liner material for various ores due to its high wear resistance. The material however, suffers from fracture in this high impact application due to its low toughness. This has raised the need to research ways to improve the toughness while retaining good wear resistance and hardness characteristics. A 25 wt% HCWCI (Reference Alloy) were alloyed with vanadium (Alloy 1) and niobium (Alloy 2) in an attempt to improve the toughness. The two alloys (Alloy 1 and Alloy 2) developed in-house showed improved toughness through the addition of vanadium and niobium. The project focused on the microstructure, hardness and wear properties change as a result of the carbide forming elements (niobium and vanadium) added given that the heat treatment was kept the same. The alloys were cast using a 150kg induction furnace, heat treated in muffle furnaces and subjected to chemical composition analysis using the spark emission spectroscopy. The chemical composition results from the spark emission spectroscopy were used to plot the property diagram for all the alloys using the Thermo-CalcTM computer software. Manageable specimens were ground, polished and etched for microstructural study using the optical microscopy, scanning electron microscope (SEM) fitted with energy dispersive X-rays spectroscopy (EDX) and X-ray diffraction (XRD). The wear (erosion and abrasion) tests at 10° and 45° contact angles was done using an in-house developed wear testing rigs. The wear test rigs have been used previously and were particularly effective at ranking materials that experience turbulent flow conditions such as those experienced in pipelines and multiple slurry and dry ores in transportation and handling equipment. The hardness of Alloy 1 and Alloy 2 were similar but lower than the Reference Alloy, this was due to the matrix microstructure difference of the Reference Alloy to the two alloys. The addition of niobium in Alloy 2 did result in the formation of NbC carbides which are not evenly distributed within the microstructure. The presence of these carbides did not add any benefits in terms of wear and hardness properties as the Reference Alloy had superior properties, this was due to softer matrix of the two alloys. No vanadium carbides (VC) were observed in Alloy 1, implying that the V added probably dissolved in the matrix and the M7C3 carbides, which is also supported by the literature.
A research report submitted in partial fulfilment of the requirements for the degree of Master of Science in Engineering (50/50) to the Faculty of Engineering and the Built Environment,School of Chemical and Metallurgical Engineering University of the Witwatersrand, Johannesburg, 2022