Abstract
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.