Hardfacing is the deposition of thick coatings of hard, wear-resistant materials on a worn or new component surface that is subject to wear in service. Thermal spraying, spray-fuse and welding processes are generally used to apply the hardfacing layer.
Thermal spraying is preferred for applications requiring minimal thermal distortion of the component and good process control. Typical hardfacing materials deposited by thermal spraying include cermets such as WC-Co and alumina-based ceramics. These coatings are applied to a thickness of about 0.3mm. Spray-fuse coatings, also referred to as self-fluxing overlay coatings, are first applied to the component surface using a flame spraying process and then subsequently fused using an oxyacetylene torch or an RF induction coil. The fused coating wets the substrate surface to produce a coating that is metallurgically bonded to the substrate and is free of porosity. There are various alloy types used with the spray-fuse process, the most important are based on the Ni-Cr-B-Si-C alloy system. Depending on composition they melt in the range of 980 to 1200°C.
Weld hard facing is used to deposit very thick (1 to 10mm) dense layers of wear resistant material with high bond strength. Various welding techniques can be used, including metal-inert gas (MIG), tungsten-inert gas (TIG), plasma transferred arc (PTA), submerged arc (SAW) and manual metal arc (MMA). A very broad range of coating materials can be applied. They include cobalt-based alloys (Stellites ® ), martensitic and high-speed steels, nickel alloys and WC-Co cemented carbides. After deposition by any of the above welding processes, it is often necessary to finish the component surface.
Thermal spraying processes - a guide to best practice
Surface engineering at TWI
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