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.com
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 · 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 casehardened 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
Materials with better inherent adhesive wear resistance can be selected Martensitic and precipitation-hardened(Ph) stainless steels have generally poor galling resistance Austenitic stainless steels, with high work-hardening rate, are better For carbon and alloy steels, higher hardness is beneficial Stainless steel S21800 and nickel casting alloy CY5SnBiM are very resistant to adhesive wear Surface finish and coatings are important for minimizing adhesion Relatively smooth surfaces do not have projections (asperities) that penetrate the lubricant film, which, in turn should have as high of a viscosity as possible for the application A surface that is too smooth(<10 uin )will not carry the lubricant into the contact zone and will not tolerate debris Hard coatings, plating, or overlays can be beneficial Diffusion coatings, such as nitriding, tuftriding, and sulfidization, can be good Use mutually insoluble metals with low shear strength to prevent adhesion.( Gold or silver plating is sometimes used on one of two mating high-speed gears, or other contacting parts. Fretting, sometimes referred to as static adhesive wear, is wear that occurs between two closely contacting surfaces having oscillatory relative motion of extremely small magnitude. The term fretting is derived from the French word "frottement, which means rubbing. Fretting wear has also been called fretting corrosion, friction oxidation, chafing fatigue, and wear oxidation. Fretting is the cause of discoloration and damage in close-fitting joints or between any two closely fitted parts that are then subjected to vibration during service. Some examples are Press fits on shafts at coupling hubs, gears, wheels, and bearing inner races Bolts, rivets, and pins under the heads and in their holes Rolling-element bearings while stationary during shipping Wire rope between the strands Turbine and compressor blade roots Heat exchanger tubes in tube sheets and baffles Instead of having a dynamic sliding motion as discussed previously, fretting involves adhesive wear in a joint that is essentially stationary except for the small vibratory elastic motion that inevitably occurs during machine operation. In other words, the joint is supposed to be stationary, but actually it is not, due to a minute metal-to-metal rubbing motion at the interface. Again, microwelding occurs, tearing tiny fragments from one or both surfaces. This wear debris remains between the surfaces and oxidizes to form a reddish-brown cocoa"(mostly aFe2O3 iron oxide)on iron and steel, or a black powder(aluminum oxide)on aluminum. It is the wear debris that oxidizes when exposed to air that causes fretting to also be called fretting corrosion, although the"corrosion"seems to be incidental to the wear resulting from the adhesion One of the major problems with fretting is not the destruction of the joint itself, but the fact that it frequently leads to fatigue cracks and fracture originating in the interface. Fretting can cause the endurance limit of a steel to be reduced from, say, 515 to about 375 MPa(75 to about 55 ksi), often enough to cause a service failure. In many cases, the site where the fretting fatigue originates is different from the expected location of fracture, particularly if the expected failure location has been strengthened by heat treating or mechanical prestressing of the surface The major variables in fretting are the tightness of the interface and the magnitude of the vibration that is causing relative motion. The probability of the fretting leading to fatigue failure depends on the magnitude of the cyclic stresses in fretted part and the reduction in the endurance or fatigue limit. Fretting is not easy to prevent or eliminate, although it be minimized by one or more of the following procedures Increase the interference of press-fit components, so as to increase the clamping force and prevent relative motion. Recommended values are given in design handbooks Use plastic or rubber pads, gaskets, or bushings to absorb some or all of the vibration Increase the hardness of one or both surfaces to increase the resistance to microfracture Roughen the surfaces in an attempt to"lock " them together, although this probably will not work unless the pressure or interference fit is also increased between the two surfaces Chromium plate one surface to reduce the coefficient of friction and try to prevent microwelding to the other surface Heat treat with a salt bath or gas atmosphere diffusion treatment to form an efen compound in the surface that resists fretting of ferrous metals
Materials with better inherent adhesive wear resistance can be selected: · Martensitic and precipitation-hardened (PH) stainless steels have generally poor galling resistance. · Austenitic stainless steels, with high work-hardening rate, are better. · For carbon and alloy steels, higher hardness is beneficial. · Stainless steel S21800 and nickel casting alloy CY5SnBiM are very resistant to adhesive wear. Surface finish and coatings are important for minimizing adhesion: · Relatively smooth surfaces do not have projections (asperities) that penetrate the lubricant film, which, in turn, should have as high of a viscosity as possible for the application. · A surface that is too smooth (<10 μin.) will not carry the lubricant into the contact zone and will not tolerate debris. · Hard coatings, plating, or overlays can be beneficial. · Diffusion coatings, such as nitriding, tuftriding, and sulfidization, can be good. · Use mutually insoluble metals with low shear strength to prevent adhesion. (Gold or silver plating is sometimes used on one of two mating high-speed gears, or other contacting parts.) Fretting, sometimes referred to as static adhesive wear, is wear that occurs between two closely contacting surfaces having oscillatory relative motion of extremely small magnitude. The term fretting is derived from the French word “frottement,” which means rubbing. Fretting wear has also been called fretting corrosion, friction oxidation, chafing fatigue, and wear oxidation. Fretting is the cause of discoloration and damage in close-fitting joints or between any two closely fitted parts that are then subjected to vibration during service. Some examples are: · Press fits on shafts at coupling hubs, gears, wheels, and bearing inner races · Bolts, rivets, and pins under the heads and in their holes · Rolling-element bearings while stationary during shipping · Wire rope between the strands · Turbine and compressor blade roots · Heat exchanger tubes in tube sheets and baffles Instead of having a dynamic sliding motion as discussed previously, fretting involves adhesive wear in a joint that is essentially stationary except for the small vibratory elastic motion that inevitably occurs during machine operation. In other words, the joint is supposed to be stationary, but actually it is not, due to a minute metal-to-metal rubbing motion at the interface. Again, microwelding occurs, tearing tiny fragments from one or both surfaces. This wear debris remains between the surfaces and oxidizes to form a reddish-brown “cocoa” (mostly αFe2O3 iron oxide) on iron and steel, or a black powder (aluminum oxide) on aluminum. It is the wear debris that oxidizes when exposed to air that causes fretting to also be called fretting corrosion, although the “corrosion” seems to be incidental to the wear resulting from the adhesion. One of the major problems with fretting is not the destruction of the joint itself, but the fact that it frequently leads to fatigue cracks and fracture originating in the interface. Fretting can cause the endurance limit of a steel to be reduced from, say, 515 to about 375 MPa (75 to about 55 ksi), often enough to cause a service failure. In many cases, the site where the fretting fatigue originates is different from the expected location of fracture, particularly if the expected failure location has been strengthened by heat treating or mechanical prestressing of the surface. The major variables in fretting are the tightness of the interface and the magnitude of the vibration that is causing relative motion. The probability of the fretting leading to fatigue failure depends on the magnitude of the cyclic stresses in the fretted part and the reduction in the endurance or fatigue limit. Fretting is not easy to prevent or eliminate, although it can be minimized by one or more of the following procedures: · Increase the interference of press-fit components, so as to increase the clamping force and prevent relative motion. Recommended values are given in design handbooks. · Use plastic or rubber pads, gaskets, or bushings to absorb some or all of the vibration. · Increase the hardness of one or both surfaces to increase the resistance to microfracture. · Roughen the surfaces in an attempt to “lock” them together, although this probably will not work unless the pressure or interference fit is also increased between the two surfaces. · Chromium plate one surface to reduce the coefficient of friction and try to prevent microwelding to the other surface. · Heat treat with a salt bath or gas atmosphere diffusion treatment to form an εFeN compound in the surface that resists fretting of ferrous metals
Mechanically prestress the surface, such as by shot peening, to prevent the fretting from causing fatigue cracking and fracture Elimination of the vibration causing the fretting is often impossible, because it is a normal aspect of the operation of the equipment or because it is so small as to be beyond one's ability to correct(e.g, by better balancing of rotating components) Cavitation is the removal of material from a surface by the formation and rapid collapse of bubbles of gas or vapor in an only Cnt liquid. Sometimes called cavitation corrosion or cavitation erosion, this type of wear is somewhat unique in that one metal surface is involved. The other material is a liquid, frequently water or water/antifreeze. The basic mechanism is true pitting fatigue, resulting in pits in the surfaces of parts such as diesel engine wet cylinder sleeves, hydraulic turbines, nozzles, impellers, and propellers, or any other part that moves rapidly through liquid. It can also occur in pressure let-down valves in rapidly flowing liquid piping systems. High-speed flow of liquid in these devices causes local hydrodynamic pressures to vary widely and rapidly. The resulting pits can be quite small or very large and deep, depending on the particular circumstances Cavitation pitting is caused by rapid, repetitive, relative movement between the metal and the liquid. When the local pressure is reduced, as when the metal and liquid momentarily move in opposite directions, small cavities are formed in the liquid at the interface with the metal in the low-pressure regions. These gas or vapor cavities form when the local pressure drops below the vapor pressure of the liquid or dissolved gas. Then, when the metal and the liquid move toward each other with reversal of movement, the cavities at the interface implode, or collapse, on the metal surface, causi high local-contact stresses. If vibration or other relative movement is continued for a large number of load applications at these local points on the metal, the implosion forces cause fatigue fracture or pits to form in the metal surface. Continued service of this type can cause complete penetration of the metal and/or large pits in the metal. One of the characteristics of cavitation pitting is that the pitting occurs primarily in a pattern on the surface, with most of the pits concentrated in specific areas, depending on the local vibration or flow characteristics. Another characteristic of cavitation damage is that the pitting is very rough and jagged For example, if cavitation is occurring in a centrifugal pump, the damage will generally be on the back side of each vane of the impeller at the same location near the inlet and will be jagged. Erosion damage, on the other hand, will be on the front or pressure side of the vanes and will generally be grooved Cavitation pitting of diesel engine wet cylinder sleeves can result as the sleeve wall moves in and out due to the internal pressure cycles. It can cause complete destruction of an engine if the coolant gets into the combustion chamber or can cause loss of efficiency of an impeller or propeller. Any corrosion is incidental to the pitting fatigue that causes the pits, although corrosion can easily occur on fresh, unprotected metal fracture surfaces Some of the variables involved in cavitation are Liquid composition, density, surface tension, and viscosity Dissolved gas content Pressure relative to the vapor pressure of the liquid at the system temperature Temperature relative to the boiling point at the local pressure Vibration levels Flow velocity--there is a power-law relationship with an exponent between 3 and 7 Material Cavitation pitting is not easily prevented, because the vibration and liquid flow characteristics are not easily controlled Some possible ways to avoid it are to streamline fluid flow by design changes to avoid sudden pressure drops, stiffen the part to change vibration characteristics, increase the vapor pressure of the liquid or the total pressure in closed systems and reduce surface roughness. Pumps should be operated at or very near their design flow rate and head pressure; cavitation occurs in off-design operation. Material changes, such as using a metal with a higher fatigue strength, can slow but not eliminate damage. For example, a Stellite overlay can increase resistance by 10 times or more over bare type 304 stainless steel, but will not stop the cavitation. If none of these possible solutions work, it may be necessary to simply replace the parts during scheduled maintenance Liquid-droplet impingement is, in some ways, the inverse of cavitation, but it produces damage that is very similar to cavitation damage in appearance. Liquid-droplet impingement is metal loss from the repeated impact of liquid droplets that are being carried along in a gas or vapor stream. The damage is generated by the initial high-pressure pulse in the area of impact causing cracks and radially displaced metal, followed by breaking off of the asperities thus formed by subsequent rapid radial flow from subsequent droplet impacts. The droplets usually form by condensation from a vapor stream, as the temperature, the pressure, or both are reduced. The most common places for this damage to occur are the blades in the late stages of steam turbines and downstream of elbows in piping systems. Similar to cavitation, the area where metal has been removed by liquid-droplet impingement will be very rough and jagged The significant variables in liquid-droplet impingement damage include Thefileisdownloadedfromwww.bzfxw.com
· Mechanically prestress the surface, such as by shot peening, to prevent the fretting from causing fatigue cracking and fracture. Elimination of the vibration causing the fretting is often impossible, because it is a normal aspect of the operation of the equipment or because it is so small as to be beyond one's ability to correct (e.g., by better balancing of rotating components). Cavitation is the removal of material from a surface by the formation and rapid collapse of bubbles of gas or vapor in an adjacent liquid. Sometimes called cavitation corrosion or cavitation erosion, this type of wear is somewhat unique in that only one metal surface is involved. The other material is a liquid, frequently water or water/antifreeze. The basic mechanism is true pitting fatigue, resulting in pits in the surfaces of parts such as diesel engine wet cylinder sleeves, hydraulic turbines, nozzles, impellers, and propellers, or any other part that moves rapidly through liquid. It can also occur in pressure let-down valves in rapidly flowing liquid piping systems. High-speed flow of liquid in these devices causes local hydrodynamic pressures to vary widely and rapidly. The resulting pits can be quite small or very large and deep, depending on the particular circumstances. Cavitation pitting is caused by rapid, repetitive, relative movement between the metal and the liquid. When the local pressure is reduced, as when the metal and liquid momentarily move in opposite directions, small cavities are formed in the liquid at the interface with the metal in the low-pressure regions. These gas or vapor cavities form when the local pressure drops below the vapor pressure of the liquid or dissolved gas. Then, when the metal and the liquid move toward each other with reversal of movement, the cavities at the interface implode, or collapse, on the metal surface, causing very high local-contact stresses. If vibration or other relative movement is continued for a large number of load applications at these local points on the metal, the implosion forces cause fatigue fracture or pits to form in the metal surface. Continued service of this type can cause complete penetration of the metal and/or large pits in the metal. One of the characteristics of cavitation pitting is that the pitting occurs primarily in a pattern on the surface, with most of the pits concentrated in specific areas, depending on the local vibration or flow characteristics. Another characteristic of cavitation damage is that the pitting is very rough and jagged. For example, if cavitation is occurring in a centrifugal pump, the damage will generally be on the back side of each vane of the impeller at the same location near the inlet and will be jagged. Erosion damage, on the other hand, will be on the front or pressure side of the vanes and will generally be grooved. Cavitation pitting of diesel engine wet cylinder sleeves can result as the sleeve wall moves in and out due to the internal pressure cycles. It can cause complete destruction of an engine if the coolant gets into the combustion chamber or can cause loss of efficiency of an impeller or propeller. Any corrosion is incidental to the pitting fatigue that causes the pits, although corrosion can easily occur on fresh, unprotected metal fracture surfaces. Some of the variables involved in cavitation are: · Liquid composition, density, surface tension, and viscosity · Dissolved gas content · Pressure relative to the vapor pressure of the liquid at the system temperature · Temperature relative to the boiling point at the local pressure · Vibration levels · Flow velocity—there is a power-law relationship with an exponent between 3 and 7 · Material Cavitation pitting is not easily prevented, because the vibration and liquid flow characteristics are not easily controlled. Some possible ways to avoid it are to streamline fluid flow by design changes to avoid sudden pressure drops, stiffen the part to change vibration characteristics, increase the vapor pressure of the liquid or the total pressure in closed systems, and reduce surface roughness. Pumps should be operated at or very near their design flow rate and head pressure; cavitation occurs in off-design operation. Material changes, such as using a metal with a higher fatigue strength, can slow but not eliminate damage. For example, a Stellite overlay can increase resistance by 10 times or more over bare type 304 stainless steel, but will not stop the cavitation. If none of these possible solutions work, it may be necessary to simply replace the parts during scheduled maintenance. Liquid-droplet impingement is, in some ways, the inverse of cavitation, but it produces damage that is very similar to cavitation damage in appearance. Liquid-droplet impingement is metal loss from the repeated impact of liquid droplets that are being carried along in a gas or vapor stream. The damage is generated by the initial high-pressure pulse in the area of impact causing cracks and radially displaced metal, followed by breaking off of the asperities thus formed by subsequent rapid radial flow from subsequent droplet impacts. The droplets usually form by condensation from a vapor stream, as the temperature, the pressure, or both are reduced. The most common places for this damage to occur are the blades in the late stages of steam turbines and downstream of elbows in piping systems. Similar to cavitation, the area where metal has been removed by liquid-droplet impingement will be very rough and jagged. The significant variables in liquid-droplet impingement damage include: The file is downloaded from www.bzfxw.com
Material resistance Flow velocit Pressure and temperature drops Droplet quantity and iquid specific gravity The primary direction to take in combating this form of wear damage is to modify the flowing gas properties. Design or process changes can be used to eliminate, or at least reduce, the size and quantity of the droplets. Material changes to minimize damage could include ncreasing the local hardness in the damage zone, such as by flame or induction hardening Using weld overlay, laser clad, or brazed-in inserts of a material such as Stellite Applying very hard coatings, such as HVOF tungsten carbide Rolling-contact fatigue is surface damage that results by fatigue from repeated rolling, or rolling and sliding contact between curved metal surfaces. The key words descriptive of this type of wear are"pitting fatigue "with origins on or near contacting surfaces. Although the cause of the fatigue and the locations of the origins are somewhat different in each subtype, these are true fatigue fractures that are caused by shear-stress origins due to contact stresses between two metal surfaces. Three mechanisms have been identified and are described: subsurface-origin pitting, surface-origin pitting, and subcase-origin spalling. This form of wear damage is most commonly seen in rolling-element(ball and roller) bearings, gears, and rolling-mill rolls The common thread linking the three types of rolling-contact fatigue is that each represents the limit of load-carrying capacity; improvements are made only with great difficulty, if at all, assuming that the materials are proper and the geometries are accurate. Where metal-to-metal contact is concerned, such as in gears or rolling-contact bearings, the materials are usually hardened steels, with hardness values near 60 HRC. More details on rolling-contact fatigue are given in the article"Fatigue Failures in this volume Corrosive wear really is not a separate type of wear, but is simply a combination of corrosion and wear occurring either alternatively or simultaneously Corrosive wear can result in more rapid degradation of material than would be observed by either the corrosion or the wear acting individually. Two conditions can occur. In one case, corrosion deposits that form are more easily removed by one of the wear mechanisms, such as erosion. In the other case, the wear process can remove protective deposits or passive layers, exposing bare metal to the corrosive environment Corrosive wear is a major reason for gasoline engine wear, particularly if the engine is stopped before it has had a chance to heat up to operating temperature. The moisture that was formed as a product of combustion cannot evaporate from the cold engine, but remains inside and condenses on various internal parts. Corrosion may occur if the lubricant does not only to reform again because the moisture cannot evaporate from the cold engie roducts will be worn off by operation, prevent it. Then, when the engine is again operated for a short time, the corrosion Wear of parts operating in a corrosive environment is particularly difficult to prevent because of the difficulties in specifying and testing alternate metals. Frequently, various types of surface coatings are of great help in solving the problems of corrosive wear. In some cases, the use of lubricants containing corrosion inhibitors can be beneficial Lubricants Almost any surface film acts as a lubricant, preventing cold welding of asperities on opposing surfaces or allowing opposing surfaces to slide across one another at a lower frictional force than would prevail if the film was not present Lubricants may be either gas, liquid, or solid. The main function of a lubricant is to maintain separation of the surfaces by pressure within the film, which opposes the applied contact force. Another function of a lubricant is to carry away generated by two surfaces sliding under contact pressure. Liquid lubricants dissipate heat better than solid or sem lubricants, but the shear properties of all types of lubricant are critical to performance. A third function of lubricants is to flush dirt and wear debris out of the contact zone, so they can be removed subsequently by filtration Lubricating oils are relatively free-flowing organic substances that lower the coefficient of friction in mechanical devices They are available in a broad range of viscosities, and many are blended or contain additives to make them suitable for specific uses. In general, lubricating substances that are fluid at 20C(68F)are termed oils, lubricating substances that are solid or semifluid at 20C(68F)are termed greases Oils are derived from petroleum(mineral oils), plants, or animals(fixed oils ). Commercial mineral, oil-base products consist mainly of saturated hydrocarbons(even though naphthene-base crudes are predominantly unsaturated) in the form of chain or ring molecules that are chemically inactive and do not have polar heads. These commercial products may or may not contain waxes, volatile compounds, fixed"oils, and special-purpose additives Silicone-based oils have been developed that can be used for somewhat higher temperatures, where mineral oils or fixed oils would break down and degrade
· Material resistance · Flow velocity · Pressure and temperature drops · Droplet quantity and size · Liquid specific gravity The primary direction to take in combating this form of wear damage is to modify the flowing gas properties. Design or process changes can be used to eliminate, or at least reduce, the size and quantity of the droplets. Material changes to minimize damage could include: · Increasing the local hardness in the damage zone, such as by flame or induction hardening · Using weld overlay, laser clad, or brazed-in inserts of a material such as Stellite · Applying very hard coatings, such as HVOF tungsten carbide Rolling-contact fatigue is surface damage that results by fatigue from repeated rolling, or rolling and sliding contact between curved metal surfaces. The key words descriptive of this type of wear are “pitting fatigue” with origins on or near contacting surfaces. Although the cause of the fatigue and the locations of the origins are somewhat different in each subtype, these are true fatigue fractures that are caused by shear-stress origins due to contact stresses between two metal surfaces. Three mechanisms have been identified and are described: subsurface-origin pitting, surface-origin pitting, and subcase-origin spalling. This form of wear damage is most commonly seen in rolling-element (ball and roller) bearings, gears, and rolling-mill rolls. The common thread linking the three types of rolling-contact fatigue is that each represents the limit of load-carrying capacity; improvements are made only with great difficulty, if at all, assuming that the materials are proper and the geometries are accurate. Where metal-to-metal contact is concerned, such as in gears or rolling-contact bearings, the materials are usually hardened steels, with hardness values near 60 HRC. More details on rolling-contact fatigue are given in the article “Fatigue Failures” in this Volume. Corrosive wear really is not a separate type of wear, but is simply a combination of corrosion and wear occurring either alternatively or simultaneously. Corrosive wear can result in more rapid degradation of material than would be observed by either the corrosion or the wear acting individually. Two conditions can occur. In one case, corrosion deposits that form are more easily removed by one of the wear mechanisms, such as erosion. In the other case, the wear process can remove protective deposits or passive layers, exposing bare metal to the corrosive environment. Corrosive wear is a major reason for gasoline engine wear, particularly if the engine is stopped before it has had a chance to heat up to operating temperature. The moisture that was formed as a product of combustion cannot evaporate from the cold engine, but remains inside and condenses on various internal parts. Corrosion may occur if the lubricant does not prevent it. Then, when the engine is again operated for a short time, the corrosion products will be worn off by operation, only to reform again because the moisture cannot evaporate from the cold engine. Wear of parts operating in a corrosive environment is particularly difficult to prevent because of the difficulties in specifying and testing alternate metals. Frequently, various types of surface coatings are of great help in solving the problems of corrosive wear. In some cases, the use of lubricants containing corrosion inhibitors can be beneficial. Lubricants Almost any surface film acts as a lubricant, preventing cold welding of asperities on opposing surfaces or allowing opposing surfaces to slide across one another at a lower frictional force than would prevail if the film was not present. Lubricants may be either gas, liquid, or solid. The main function of a lubricant is to maintain separation of the surfaces by pressure within the film, which opposes the applied contact force. Another function of a lubricant is to carry away heat generated by two surfaces sliding under contact pressure. Liquid lubricants dissipate heat better than solid or semifluid lubricants, but the shear properties of all types of lubricant are critical to performance. A third function of liquid lubricants is to flush dirt and wear debris out of the contact zone, so they can be removed subsequently by filtration. Lubricating oils are relatively free-flowing organic substances that lower the coefficient of friction in mechanical devices. They are available in a broad range of viscosities, and many are blended or contain additives to make them suitable for specific uses. In general, lubricating substances that are fluid at 20 °C (68 °F) are termed oils; lubricating substances that are solid or semifluid at 20 °C (68 °F) are termed greases. Oils are derived from petroleum (mineral oils), plants, or animals (fixed oils). Commercial mineral, oil-base products consist mainly of saturated hydrocarbons (even though naphthene-base crudes are predominantly unsaturated) in the form of chain or ring molecules that are chemically inactive and do not have polar heads. These commercial products may or may not contain waxes, volatile compounds, “fixed” oils, and special-purpose additives. Silicone-based oils have been developed that can be used for somewhat higher temperatures, where mineral oils or fixed oils would break down and degrade
Fixed oils are usually considered to have greater"oiliness"than mineral oils. Oiliness is a term that describes the relative ability of any lubricant to act as a boundary lubricant. Electron-diffraction experiments have shown that molecules of an effective lubricating agent-a long-chain fatty acid of high molecular weight, such as stearic acid or oleic acid-are attached to a metallic surface by polar bonding. They stand up much like individual strands in a pile carpet. The result is a surface layer with high adhesion and high resistance to contact stress and low resistance to lateral shear along the surface Lubricating grease is a semifluid product consisting of a dispersion of a thickening agent in a liquid lubricant. In more practical terms, most greases are stabilized mixtures of mineral oil and metallic soap. The soap is usually a calcium, sodium, or lithium compound and is present in the form of fibers whose size and configuration are characteristic of the metallic radical in the soap compound Some greases may contain additives to make them more suitable for specific applications, such as metalworking or threads on fasteners. Some of these materials include graphite, molybdenum disulfide, polytetrafluoroethylene(PTFE or Teflon), copper, nickel, and zinc powders Solid lubricants provide certain advantages in high-vacuum, aerospace, or cryogenic applications, where liquids would evaporate or congeal, and at somewhat higher temperatures, where liquids could decompose. Numerous solid inorganic and organic compounds as well as certain composite materials may be classified as solid lubricants. Notable examples are raphite, molybdenum disulfide, and PTFE. Dozens of proprietary compounds may also be classified as solid lubricant Although solid lubricants may be applied to achieve design simplification or weight reduction, they usually are adopted because of their good stability, retaining their lubricity at elevated temperatures, in chemically active environments, and under exposure to nuclear radiation. However, there has been some evidence in the literature that molybdenum disulfide can break down in hot, moist environments to release hydrogen sulfide, which can cause hydrogen embrittlement of some steels Lubrication Modes. Several basic lubrication modes should be understood in order to analyze and correct wear failures. In all modes, contact surfaces are separated by a lubricating medium, which may be a solid, a semisolid, or a pressurized liquid or gaseous film. In hydrodynamic lubrication, the shape and relative motion of the sliding surfaces cause the formation of a fluid film having sufficient pressure to separate the surfaces. In hydrostatic lubrication, the lubricant is applied under sufficient external pressure to separate the opposing surfaces by a fluid film. Elastohydrodynamic lubrication is a system in which the friction and film thickness between the two bodies in relative motion are determined by the elastic properties of the bodies, in combination with the viscous properties of the lubricant at the prevailing pressure, temperature, and shear rate. In dry-film (solid-film) lubrication, a coating of solid lubricant separates the opposing surfaces and the lubricant itself wears away. Boundary lubrication and thin-film lubrication are two modes in which friction and wear are affected by properties of the lubricant. In boundary lubrication, each surface is covered by a chemically bonded fluid or semisolid film, which may or may not serve to separate opposing surfaces, and viscosity of the lubricant is not a factor affecting friction and wear. In thin-film lubrication, the lubricant usually is not bonded to the surfaces, it separates opposing surfaces, and lubricant viscosity affects friction and wear Lubricant Failures. Devices depending on lubricants to minimize friction and wear can fail when the lubricant fails. Most lubricant failures occur by chemical decomposition, contamination, changes in properties from exposure to elevated temperature, or loss of film. Lubricating oils and greases can fail by any one of the aforementioned processes alone; however, in most situations, chemical decomposition, contamination, and temperature are all involved and are interrelated. In addition, pressure-lubricating systems are susceptible to certain types of failure that result in the inability of the system to provide the required flow rate where it is most needed Thus, failure by lubricant starvation often results Decomposition. When oil is heated in the presence of air, oxidation occurs. Oxidation increases the viscosity and organic acid concentration of mineral oils, causing varnish and lacquer deposits to form on hot metal surfaces. Under severe conditions, the deposits may be converted to hard, carbonaceous substances. Fixed oils absorb oxygen more readily than do mineral oils, and some may dry, thicken, and form elastic solids. Certain fixed oils(castor, olive, sperm whale, and lard oils) oxidize more slowly than others. These fixed oils are more widely used in blended oils because of their nondrying characteristics. Temperature also affects oxidation rates. In mineral and blended oils, for example, the rate doubles with each 10C(18F)rise in temperature. Oxidation rates also are higher when the oil is agitated or when catalysts such as copper or acids are present In general, solid-film lubricants fail by mechanical removal of microscopically thin layers. Wear debris, which consists primarily of lubricant particles, is generated by the sliding action of a sharp edge against the bonded film on a contact urface. The sharp edge shears a layer of the film(and sometimes the entire film) from the substrate Contact between a rolling element and a sharp ridge can chip the film, often initiating more extensive failure. This process eventually causes a lack of dynamic stability(as would result from excessive clearance in a bearing)or adhesive wear of metallic contact Many bonded, solid lubricants derive their adhesion from binders, which are incorporated into the film in quantities up to about 20% by volume. Metals, oxides, silicates, or other ceramics are the most common binders. When wear debris contains binder particles, it abrades the remaining film more rapidly than when it consists solely of particles of the lubricating substance Thefileisdownloadedfromwww.bzfxw.com
Fixed oils are usually considered to have greater “oiliness” than mineral oils. Oiliness is a term that describes the relative ability of any lubricant to act as a boundary lubricant. Electron-diffraction experiments have shown that molecules of an effective lubricating agent—a long-chain fatty acid of high molecular weight, such as stearic acid or oleic acid—are attached to a metallic surface by polar bonding. They stand up much like individual strands in a pile carpet. The result is a surface layer with high adhesion and high resistance to contact stress and low resistance to lateral shear along the surface. Lubricating grease is a semifluid product consisting of a dispersion of a thickening agent in a liquid lubricant. In more practical terms, most greases are stabilized mixtures of mineral oil and metallic soap. The soap is usually a calcium, sodium, or lithium compound and is present in the form of fibers whose size and configuration are characteristic of the metallic radical in the soap compound. Some greases may contain additives to make them more suitable for specific applications, such as metalworking or threads on fasteners. Some of these materials include graphite, molybdenum disulfide, polytetrafluoroethylene (PTFE or Teflon), copper, nickel, and zinc powders. Solid lubricants provide certain advantages in high-vacuum, aerospace, or cryogenic applications, where liquids would evaporate or congeal, and at somewhat higher temperatures, where liquids could decompose. Numerous solid inorganic and organic compounds as well as certain composite materials may be classified as solid lubricants. Notable examples are graphite, molybdenum disulfide, and PTFE. Dozens of proprietary compounds may also be classified as solid lubricants. Although solid lubricants may be applied to achieve design simplification or weight reduction, they usually are adopted because of their good stability, retaining their lubricity at elevated temperatures, in chemically active environments, and under exposure to nuclear radiation. However, there has been some evidence in the literature that molybdenum disulfide can break down in hot, moist environments to release hydrogen sulfide, which can cause hydrogen embrittlement of some steels. Lubrication Modes. Several basic lubrication modes should be understood in order to analyze and correct wear failures. In all modes, contact surfaces are separated by a lubricating medium, which may be a solid, a semisolid, or a pressurized liquid or gaseous film. In hydrodynamic lubrication, the shape and relative motion of the sliding surfaces cause the formation of a fluid film having sufficient pressure to separate the surfaces. In hydrostatic lubrication, the lubricant is supplied under sufficient external pressure to separate the opposing surfaces by a fluid film. Elastohydrodynamic lubrication is a system in which the friction and film thickness between the two bodies in relative motion are determined by the elastic properties of the bodies, in combination with the viscous properties of the lubricant at the prevailing pressure, temperature, and shear rate. In dry-film (solid-film) lubrication, a coating of solid lubricant separates the opposing surfaces and the lubricant itself wears away. Boundary lubrication and thin-film lubrication are two modes in which friction and wear are affected by properties of the lubricant. In boundary lubrication, each surface is covered by a chemically bonded fluid or semisolid film, which may or may not serve to separate opposing surfaces, and viscosity of the lubricant is not a factor affecting friction and wear. In thin-film lubrication, the lubricant usually is not bonded to the surfaces, it separates opposing surfaces, and lubricant viscosity affects friction and wear. Lubricant Failures. Devices depending on lubricants to minimize friction and wear can fail when the lubricant fails. Most lubricant failures occur by chemical decomposition, contamination, changes in properties from exposure to elevated temperature, or loss of film. Lubricating oils and greases can fail by any one of the aforementioned processes alone; however, in most situations, chemical decomposition, contamination, and temperature are all involved and are interrelated. In addition, pressure-lubricating systems are susceptible to certain types of failure that result in the inability of the system to provide the required flow rate where it is most needed. Thus, failure by lubricant starvation often results. Decomposition. When oil is heated in the presence of air, oxidation occurs. Oxidation increases the viscosity and organicacid concentration of mineral oils, causing varnish and lacquer deposits to form on hot metal surfaces. Under severe conditions, the deposits may be converted to hard, carbonaceous substances. Fixed oils absorb oxygen more readily than do mineral oils, and some may dry, thicken, and form elastic solids. Certain fixed oils (castor, olive, sperm whale, and lard oils) oxidize more slowly than others. These fixed oils are more widely used in blended oils because of their nondrying characteristics. Temperature also affects oxidation rates. In mineral and blended oils, for example, the rate doubles with each 10 °C (18 °F) rise in temperature. Oxidation rates also are higher when the oil is agitated or when catalysts such as copper or acids are present. In general, solid-film lubricants fail by mechanical removal of microscopically thin layers. Wear debris, which consists primarily of lubricant particles, is generated by the sliding action of a sharp edge against the bonded film on a contact surface. The sharp edge shears a layer of the film (and sometimes the entire film) from the substrate. Contact between a rolling element and a sharp ridge can chip the film, often initiating more extensive failure. This process eventually causes a lack of dynamic stability (as would result from excessive clearance in a bearing) or adhesive wear of metallic contact surfaces. Many bonded, solid lubricants derive their adhesion from binders, which are incorporated into the film in quantities up to about 20% by volume. Metals, oxides, silicates, or other ceramics are the most common binders. When wear debris contains binder particles, it abrades the remaining film more rapidly than when it consists solely of particles of the lubricating substance. The file is downloaded from www.bzfxw.com