Production and analysis of alloy composites exhibiting improved bonding using a novel vacuum casting process

Publication Type:
Thesis
Issue Date:
2008
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A new composite manufacturing process has been developed that permits the production of white iron/steel composites. The key differences of the new vacuum casting process compared to other current processes for composite manufacture include: i. Elimination of machining or grinding ii. Removal of brazing alloy iii. Enhanced design flexibility iv. Enhanced control of microstructural features v. Lower cost of production The new vacuum casting process involves the following key steps: • Heating a white cast iron and steel substrate together within a vacuum furnace until the temperature inside the vacuum furnace is typically 50°C above the liquidus of the white cast iron. • Before the white cast iron becomes molten, adding a partial pressure of inert gas (typically nitrogen) into the vacuum furnace to increase the pressure of the chamber above the vapour pressure of the liquid white cast iron. • Holding the temperature above the liquidus of the white cast iron to allow the white iron to partially dissolve the steel substrate. The experimental work outlined in this research has permitted the development of a low melting point white cast iron having the nominal composition of Fe-12Cr-1.6Mn-1.0Ni-0.5Si-4.1C, with a measured liquidus temperature of 1209°C. The microstructure of the low melting point alloy consists of a small volume fraction of primary austenite, with a eutectic of M₇C₃ carbides and austenite. Some of the M₇C₃ carbides have undergone a quasi-peritectic reaction. The austenite has undergone a partial transformation to form ledeburite (ferrite plus M₃C carbide in the form of cementite). The microstructures of the vacuum cast samples show the presence of four zones within the interface region. i. Zone 1 – original steel substrate, consisting of hypoeutectoid steel ii. Zone 2 – heat affected zone steel substrate iii. Zone 3 – “carbide-free” area of low melting point white cast iron adjacent to interface iv. Zone 4 – low melting point white cast iron Manufacturing and field trials have been conducted on a range of composite products to establish the potential benefit of using composite white iron/steel components in mining wear applications. The vacuum casting process has been used successfully to produce a significant volume of trial wear parts, indicating the process is robust enough to be considered for repetitive production, and can also be adapted to manufacture a wide range of products.
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