A wide range of materials are available for hard-surfacing and these are discussed in manufacturer’s literature as well as in several handbooks. Conceptually, any worn part can be rebuilt with an alloy of identical composition by coating techniques. Accordingly, all existing coating alloys can be called hard-facing alloys. These can be divided into five different categories:
Iron-Base Alloys range from low carbon steels to highly alloyed chromium irons. The iron-base alloy system is generally characterized by excellent abrasive wear resistance and impact resistance depending on the composition. In general, iron-base alloys lack the heat and corrosion resistance of their nickel or cobalt-base counterparts.
Cobalt-Base Alloys are characterized by excellent metal-to-metal wear resistance, galling resistance and corrosion and heat resistance.
Nickel-Base Alloys are characterized by extremely high abrasion resistance, combined with corrosion resistance and good metal-to-metal wear resistance.
Copper-Base Alloys are primarily bronzes which resist metal-to-metal wear, corrosion and cavitations-erosion.
Carbide Composites are usually composed of extremely hard carbides of tungsten, titanium or vanadium in a relatively softer matrix of an iron, nickel or cobalt-base alloy.
Hard-facing applications for wear control vary widely. On the one hand, we have very severe abrasive wear service such as rock crushing and pulverizing where a few kilograms of material are quickly worn away. On the other hand, we may need to minimize metal-to-metal wear in control valve, where even a few thousands of a millimeter of wear is intolerable. Hard-facing is used for controlling abrasive wear, such as encountered by mill harmers, digging tools, extrusion screws, cutting shears, parts of earthmoving equipments, ball mills and crusher parts. It is also used to control the wear of un-lubricated or poorly lubricated metal-to-metal sliding contacts such as occur in control valves, undercarriage parts of tractors, shovels and high performance bearings. In addition, hard facing is applied to control combinations of wear and corrosion as encountered by seals, plows in roasting ovens, knives in the food processing industry, valve and pumps handling corrosive liquids or slurries.
In most instances, parts are typically made of either plain carbon steel or stainless steel which by themselves, do not provide desirable wear life. The hard-surfacing alloys are applied in critical wear areas either of original equipment or during the reclaiming of worn parts. Selection of the right hard-facing alloy for a given application is best achieved through careful analysis of the service conditions and matching alloy properties to those conditions. Quite often, a balance between wear properties, environmental resistance and coating ability can be achieved for maximum reliability at minimum cost with technological inheritance technique.
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