surface. The very large number of very small particles tends to cause channeling, forming grooves and removing soft surface layers(e.g, paint). Erosive wear also tends to round sharp edges of various types of parts, reducing efficiency and eventually leading to complete destruction In general, erosive wear resistance is increased with the hardness of the surface. Frequently, a relatively thick hardfacing or a relatively thin, hard, wear-resistant coating is used, because it may not be desirable to increase the general hardness of the metal. The increased hardness may lead to brittle fracture, which could be a worse problem than the erosive wear Some specific steps that can be taken are to change flow conditions by Reducing fluid velocity, but particle dropout must be avoided Eliminating turbulence at misalignments, diameter changes, gaps at joints, and so forth A voiding sharp bends less than about 3.5 pipe diameters and change material to Harder material, but must be harder than particles, for example: high-chromium white cast iron or nitride case hardened steel Hard coatings, for example: cement-lined pipe; tungsten carbide, hard chrome, electroless nickel; or cobalt-base alloy flame, arc, or HVOF deposits Elastomer or rubber lining in slurry piping Adhesive wear occurs by dynamic metal-to-metal contact between two surfaces sliding relative to one another, when there is poor or nonexistent lubrication. The principal mechanism of adhesive wear is described by the key word microwelding, which is similar to friction welding. At low stress, bonding is on a microscale at high points, called asperities, at least initially. At later stages and higher stresses, bonding is more extensive, and severe damage occurs Adhesive wear, then, involves microwelding between two metals that are mutually soluble; that is, they are inherently capable of being welded together. Since adhesive wear is often basically a lubrication problem, a lubricant is frequently involved and must be considered as part of the system. Discussion of lubricants and lubrication is contained in a later section of this article hile the term dynamic adhesive wear is preferred, other terms are frequently used to describe varying degrees of damage. Some of these terms, in order of increasing severity, are Scuffing: superficial scratches on the mating faces Scoring: grooves cut into the surface of one of the components Galling: severe tearing and deep grooving of one face and buildup on the mating surface White layer: in steels, the formation of a very hard (800 HV) white etching phase by frictional heating Seizure: friction welding " of the mating parts so they can no longer move These terms are less accurate, and it is preferred to use the general term of adhesive wear. The dynamic"aspect refers to the movement of one surface sliding past another surface, such as a shaft rotating in a sleeve bearing, making and breaking of threaded connections, or two gear teeth contacting under load. Adhesion is favored by chemically clea surfaces, nonoxidizing conditions, and by chemical and structural similarities between the sliding couple If a poor lubricant, or no lubricant, is present in the interface, the adhesion between the two surfaces rapidly escalates and very large wear scars may occur, accompanied by gross overheating. The heat comes from the friction between the two urfaces(see the information on friction in a later section of this article). In some cases, complete destruction of surfaces may occur. In order to have destruction of the surface, frictional heating must bring the local temperature at the interface into the 870 to 1090C (1600 to 2000F)range, or higher, for steel. At this temperature, great changes occur in the microstructure of hardened steels, which are temperature-sensitive. Rehardening will occur, with the formation of a very hard, brittle, untempered martensite, white layer, at the surface, surrounded by softer, highly tempered martensite below the surface. The structure has a pattern that is quite similar to that of grinding burn To prevent or minimize adhesive wear, all facets of the situation must be considered; a system approach must be taken The result may be that several courses of action will be taken. The considerations must include the nature of the wearing metals, the surfaces of these mating components, and the lubrication or other environment that may be present. Adhesive wear is the one wear mechanism that is most degraded by poor lubrication, or lack thereof, and the most benefited by good lubi Lubrication to separate and cool the surfaces and remove wear debris can be used to minimize adhesive wear Keep bulk lubricant cool to prevent overheating at the contact surfaces from frictional heat. Use lubricating oil with EP or other additives that form chemical films to prevent metal-to-metal contact. At higher temperatures use greases or solid lubricants Thefileisdownloadedfromwww.bzfxw.comsurface. The very large number of very small particles tends to cause channeling, forming grooves and removing soft surface layers (e.g., paint). Erosive wear also tends to round sharp edges of various types of parts, reducing efficiency and eventually leading to complete destruction. In general, erosive wear resistance is increased with the hardness of the surface. Frequently, a relatively thick hardfacing or a relatively thin, hard, wear-resistant coating is used, because it may not be desirable to increase the general hardness of the metal. The increased hardness may lead to brittle fracture, which could be a worse problem than the erosive wear. Some specific steps that can be taken are to change flow conditions by: · Reducing fluid velocity, but particle dropout must be avoided · Eliminating turbulence at misalignments, diameter changes, gaps at joints, and so forth · Avoiding sharp bends less than about 3.5 pipe diameters and change material to: · Harder material, but must be harder than particles, for example: high-chromium white cast iron or nitride case￾hardened steel · Hard coatings, for example: cement-lined pipe; tungsten carbide, hard chrome, electroless nickel; or cobalt-base alloy flame, arc, or HVOF deposits · Elastomer or rubber lining in slurry piping Adhesive wear occurs by dynamic metal-to-metal contact between two surfaces sliding relative to one another, when there is poor or nonexistent lubrication. The principal mechanism of adhesive wear is described by the key word “microwelding,” which is similar to friction welding. At low stress, bonding is on a microscale at high points, called asperities, at least initially. At later stages and higher stresses, bonding is more extensive, and severe damage occurs. Adhesive wear, then, involves microwelding between two metals that are mutually soluble; that is, they are inherently capable of being welded together. Since adhesive wear is often basically a lubrication problem, a lubricant is frequently involved and must be considered as part of the system. Discussion of lubricants and lubrication is contained in a later section of this article. While the term dynamic adhesive wear is preferred, other terms are frequently used to describe varying degrees of damage. Some of these terms, in order of increasing severity, are: · Scuffing: superficial scratches on the mating faces · Scoring: grooves cut into the surface of one of the components · Galling: severe tearing and deep grooving of one face and buildup on the mating surface · White layer: in steels, the formation of a very hard (>800 HV) white etching phase by frictional heating · Seizure: “friction welding” of the mating parts so they can no longer move These terms are less accurate, and it is preferred to use the general term of adhesive wear. The “dynamic” aspect refers to the movement of one surface sliding past another surface, such as a shaft rotating in a sleeve bearing, making and breaking of threaded connections, or two gear teeth contacting under load. Adhesion is favored by chemically clean surfaces, nonoxidizing conditions, and by chemical and structural similarities between the sliding couple. If a poor lubricant, or no lubricant, is present in the interface, the adhesion between the two surfaces rapidly escalates and very large wear scars may occur, accompanied by gross overheating. The heat comes from the friction between the two surfaces (see the information on friction in a later section of this article). In some cases, complete destruction of surfaces may occur. In order to have destruction of the surface, frictional heating must bring the local temperature at the interface into the 870 to 1090 °C (1600 to 2000 °F) range, or higher, for steel. At this temperature, great changes occur in the microstructure of hardened steels, which are temperature-sensitive. Rehardening will occur, with the formation of a very hard, brittle, untempered martensite, “white layer,” at the surface, surrounded by softer, highly tempered martensite below the surface. The structure has a pattern that is quite similar to that of grinding burn. To prevent or minimize adhesive wear, all facets of the situation must be considered; a system approach must be taken. The result may be that several courses of action will be taken. The considerations must include the nature of the wearing metals, the surfaces of these mating components, and the lubrication or other environment that may be present. Adhesive wear is the one wear mechanism that is most degraded by poor lubrication, or lack thereof, and the most benefited by good lubrication. Lubrication to separate and cool the surfaces and remove wear debris can be used to minimize adhesive wear: · Keep bulk lubricant cool to prevent overheating at the contact surfaces from frictional heat. · Use lubricating oil with EP or other additives that form chemical films to prevent metal-to-metal contact. · At higher temperatures, use greases or solid lubricants. The file is downloaded from www.bzfxw.com