The manufacture and wear of a cast iron matrix / WC-Co composite material

M.Tech (Metallurgy)Abrasive wear is a major factor in the production costs of the mining industry in South Africa (as is the case in the rest of the world). These costs arise from the need to replace consumables such as digger teeth, mill liners, screens and chutes. Some materials used in these areas have been used for years with little or no change to their properties such as Hadfields manganese steel; others have been significantly modified to improve their performance, as is the case with high chromium white cast iron. Some areas in the mining industry have made a complete chanlfe of material in order to reduce wear rates; the use of 9% Cr stainless steel ( ) is an example of this. In some applications metals have been successfully replaced by ceramics for example the use of alumina in combination with cemented tungsten carbides ash conditioner blades (2) at ESKOM power stations. Significant improvements in profitability (in the form of reduced consumable costs and increased productivity) can be realised by the development of new abrasion resistant materials; hence many organisations are constantly involved in such work. The fundamental property required to resist abrasive wear is hardness, however a degree of toughness is always required depending on the application. Composite materials have the best possibility of combining these properties, for example high chrome white iron can be regarded as a composite material on a microscopic scale with very hard carbides supported by a tough martensitic matrix. This material performs extremely well in many highly erosive environments; it is also relatively cheap as the "composite microstructure" forms directlJ: from casting with a heat treatment process providing a hardened matrix 3). Another example of a composite material is cemented tungsten carbide; this is manufactured as a true composite, i.e. discrete particles of tungsten carbide are sintered with cobalt particles to form a relatively tough extremely hard material. This combines the hard brittle WC with the relatively soft tough cobalt binder acting as a matrix. This material has the best resistance to abrasive wear of all metallic materials. Ceramics have higher hardness but are significantly more brittle hence their application is limited to erosive rather than abrasive environments. Cemented tungsten carbide inserts are brazed or shrunk into steel holders and used as drill tips for rock drilling; this can be regarded as a composite product, using the strong and tough steel to hold the hard tungsten carbide inserts in place