Wear resistant materials ABRASIVE
Abrasive wear is the loss of material from a surface that results from the motion of a hard material across this surface. There are several types of abrasive wear. Since the properties required of a wear-resistant material will depend on the type of wear the material has to withstand, a brief mention of these types of wear may be useful. There are three main types of wear generally.considered: gouging abrasion (impact), Figure 18.1; high-stress abrasion crushing),Figure 18.2; and low-stressabrasion (sliding),Figure 18.3. This classificationis made more on the basis of operating 3rressesthan on the actual abrading action.
Gouging abrasion This is wear that occurs when coarse material tears off sizeable particles from wearing surfaces. This normally im-olves high imposed stresses and is most often encountered when handling large lumps.
This is encountered when two working surfaces rub together to crush granular abrasive materials. Gross loads may be low, while localised stresses are high. Moderate metal toughness is required; medium abrasion resistance is attainable. Rubber now competes with metals as rod and ball mill linings with some success. Main advantages claimed are longer lifer at a given cost, with no reduction III throughput, lower noise level, reduced driving power consumption, less load on mill bearings and more uniform wear on rods.
Low-stress abrasion This occurs mainly where an abrasive material slides freely over a surface, such as in chutes, bunkers, hoppers, skip cars, or in erosive conditions. Toughness requirements are low, and the attainable abrasion resistance is high.
Figure 18.2 Types of high-stress abrasion: (a) rod and ball mills; (b) roll crushing
DEFORMATION AND SUBSEQUENT REMOVAL OF PROTRUSIONS
Figure 18.1 Types of gouging abrasion
Figure 18.3 Low-stress abrasion
Wear resistant materials
MATERIAL SELECTION Abrasiveness of these particles. Type of wear to which wear surface is subjected (i.e. gouging, sliding, impact, etc.). Velocity of flow of material in contact with wear surface. Moisture content or level of corrosive conditions. General conditions (e.g. design of equipment, headroom available, accessibility, acceptable periods of non-availability of equipment).
Very generally speaking the property required of a wear-resistant material is the right combination of hardness and toughness. Since these are often conflicting requirements, the selection of the best material will always be a compromise. Apart from the two properties mentioned above, there are few general properties. Usually the right material for a given wear-resistant application can only be selected after taking into consideration other factors that determine the rate of wear. Of these the most important are:
Tables 18.1 and 18.2 give some general guidance on material selection and methods of attaching replaceable components. Table 18.3 gives examples of actual wear rates of various materials when handling abrasive materials. The subsequent tables give more detailed information on the various wear resistant materials.
Ambient temperature, or temperature of material in contact with the wear surface. Size distribution of particles flowing over the wear surface.
Table 18.1 Suggested materials for various operating conditions Operating
Austenitic manganese steel, rubber of adequate thickness
High stress, impact
Great toughness; properties
Low stress, sliding
I, Great hardness; 2, toughness less important; 3, quick replacement
Hardened and/or heat-treated metals, hardfacing, ceramics
I, Cheapness of basic material; 2, replacing time less important
Ceramics, quarry tiles, concretes
wear resistance; cost is
immaterial Gouging wear
Usually metals, i.e. irons and steels, hard facing
Wet and corrosive conditions
Stainless metals, ceramics, rubbers, plastics
Low stress; contact of fine particles; low abrasiveness
Low coefficient offriction
Polyurethane, PTFE, smooth metal surfaces
Resistance to cracking, spallillg, thermal shocks; general resistance to elevated temperawres
Chromium-containing alloys of iron and steel; some ceramics
Minimum periods of shut-down of plant
Ease of replacement
Any material that can be bolted in position and/or does not require cunng
Curved, non-uniform irregular surface and shapes
Anyone or a combination ofthe above properties
Hardfacing weld metal; most trowellable materials
Arduous and hot conditions
Hardfacing weld metal
Wear resistant materials Table 18.2 Methods of attachment of replaceable wear-resistant components Method oj fixing
Suitable jar: Metals, ceramics, rubbers, plastics
Bolting, nuts, or nuts and bolts
Sticking, adhesives or cement mortar
WEAR - RESISTANT
metal trays, provided with studs, then fixed as I above
Ceramics, concretes, plastics, rubbers
Cast-in bolts or studs
Concretes, pastes, poured plastics
Cast irons, ceramics, concretes
J-CERAMIC ~••.• __-_-_-_-_-_-_-_-_-_-_-~_-_-_-~-_-_-_-_-_-~_-_-_-_-_~_ METAL
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Metal plates previously plasticoated or coated with weld or spray metal
Mainly for steel or steelbased components