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NKG150-125化工离心泵设计(含12张CAD图纸)

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设计外文翻译 设计外文翻译 院(部): 专业班级: 学 号: 学生姓名: 指导教师: 原文原文The requirements for a successful pump installation are performance and life.Performanceis the rating of the pump head, capacity, and efficiency. Life is the total number of hours ofoperation before one or more pump components must be replaced to maintain an acceptableperformance. The initial performance is the responsibility of the pump manufacturer andis inherent in the pump design. Life is primarily a measure of the resistance of the mate-rials of construction to corrosion, erosion, wear, and other factors that can influence thematerials when the pump has been placed in service.The need to maximize reliability andextend the pump life makes the selection of appropriate materials of construction crucial.The selection of materials that are both cost-effective and technically suitable for theapplication requires a knowledge not only of the pump design and manufacturing pro-cesses,but also of the engineering properties of the material,particularly its corrosion andwear resistance properties when subjected to the conditions encountered in the pump.Suf-ficient information is available in the corrosion and metallurgical literature as well asfrom the experience of pump manufacturers to make appropriate material choices for vir-tually any pumping application.It is known that several factors lead to a long pump life.These include Neutral liquids at near-ambient temperatures Appropriate material selections for pumps in aggressive services The absence of abrasive particles Continuous operation at or near the maximum efficiency capacity of the pump An adequate margin of available NPSH over NPSH required as stated on the manu-facturers rating curve A low velocity (developed head/rotative speed)Pumping installations that satisfy all these criteria will have a long life. A typicalexample would be a waterworks pump. Some waterworks pumps with bronze impellersSECTION 5.1METALLIC MATERIALSOF PUMP CONSTRUCTION (AND THEIR DAMAGEMECHANISMS)COLIN O. McCAULRONALD S. MILLER5.35.4CHAPTER FIVEFIGURE 1A small fragment of Ductile Ni-Resist from the lower casing of a vertical pump. The microstructure isalso shown on the right side of this figure, illustrating the depth of the corrosions penetration. This is a classicexample of general corrosion (right photo at 100?).and cast-iron casings have a life of 50 years or more. At the other extreme might be achemical pump handling a hot corrosive liquid with abrasive particles carried in suspen-sion.The life of this pump might be measured in months rather than in years, despite thefact that construction was based on the most resistant materials available.Most pumping applications fall somewhere between these two extremes. The pumpdesigner needs to be familiar with the various types of degradation that can affect thecomponents of the pump and reduce its useful life.These can be grouped into the generalcategories of corrosion, wear, and fatigue, with corrosion and wear being the predominantlife-limiting mechanisms.TYPES OF CORROSION _General CorrosionGeneral corrosion is corrosion that proceeds without an appreciablelocalization of attack. This type of corrosion occurs on metals or alloys that do not developan effective passive film on the surface. Usually, the corrosion mechanism is oxidation withthe formation of metal oxide corrosion products.General corrosion is most often encounteredin pumps with carbon steels and copper base alloys.Cast irons also experience a specializedform of general corrosion,known as graphitic corrosion,which will be considered separately.Carbon steel does not develop a protective oxide film and will corrode at a rate depen-dent upon several characteristics of the water or other fluid, including temperature, oxy-gen content, pH, and fluid chemistry. Several empirical indices based on water chemistryexist and can be used to calculate the relative corrosivity of natural waters to carbon steeland similar ferrous alloys.The Langelier Index is best known.The rate of corrosion is alsovery dependent on velocity and increases with an increasing velocity.In most pump appli-cations,with the notable exception of hydrocarbons,the corrosion rate of carbon steel is toohigh for this material to provide a useful life.However,carbon steel is frequently used,par-ticularly in vertical pumps,with some form of protective coating to prevent corrosion.Coaltar epoxy is a preferred coating for many water services.Copper alloys,including both brasses and bronzes,are also subject to general corrosionin the water applications where they are most commonly used in the pump industry.Thecorrosion rate will be increased by the presence of small amounts of sulfides in the water.Copper alloys gradually develop a protective copper oxide corrosion film in most applica-tions. The corrosion rate gradually decreases over time as this film develops. The rate ofgeneral corrosion varies with the specific type or grade of copper alloy. Among the alloyscommonly used in pumps, nickel aluminum bronzes have the lowest corrosion rate andbest tolerance for higher velocities.The general corrosion of a Ductile Ni-Resist casing from a vertical pump is shown in Fig-ure 1.A metallographic cross section was removed to show the depth of the corrosion attack.5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.5FIGURE 2The interface between the advancing graphitized front and the sound base metal. Graphitic corrosionpropagates along the path of the graphite flakes (50?).DealloyingDealloying is the preferential removal of one phase from a multi-phasealloy, or one element from a material. Several types of dealloying occur in the pumpindustry. One of the most common is the graphitic corrosion of gray cast iron.This mate-rial is low cost, easy to machine, and well suited for a variety of applications, especiallyin the waterworks industry. It is probably the most widely used material in the pumpindustry.Gray cast iron corrodes by a fundamentally different mechanism than carbon steel orductile cast iron.The structure of gray cast iron consists of interconnected graphite flakesin a matrix that is predominantly iron. In the presence of an electrolyte, which is usuallywater, a galvanic cell is established between the iron and graphite.The iron corrodes, andthe corrosion products are largely flushed away with the fluid passing through the pump.The original casting is gradually reduced to a porous graphite structure that may containsome iron oxide corrosion product.This is frequently referred to as graphitization.The sur-face of a gray iron casting that has suffered graphitic corrosion will retain its originalshape and dimensions, but the surface will be largely graphite, which can be cut with aknife.The casting will lose some fraction of its mechanical properties and become increas-ingly susceptible to brittle failure, resulting from modest shock or impact loads. This isalso the corrosion mechanism for Ni-Resist in seawater. Figure 2 shows the interfacebetween the sound base metal and the graphitzed front.It is important to recognize that the rate of graphitic corrosion varies with the waterchemistry,and that this type of corrosion can occur in both fresh and salt waters.The highconductivity of salt water corresponds to a higher corrosion rate. Graphitic corrosion willproceed at a slower pace in waters that have a high mineral content.Minerals tend to plugthe graphitic layer on the surface, sealing off the base metal from exposure to the fluid,thereby reducing the corrosion rate.As the surface of a cast-iron component, such as a pump casing, gradually graphitizes,the galvanic relationships with other components within the pump will be altered. It hasbeen observed that the bronze impeller originally supplied in a cast-iron pump handlingseawater will provide a significantly longer life than bronze impellers that are installedafter the pump has been in service for several years. The reduced life of the replacementimpellers is caused by an altered galvanic relationship with the pump casing.Initially,thecasing was cast iron,which is anodic to a bronze impeller.With time,as the casing graphi-tizes, it gradually becomes cathodic, due to the influence of the graphite. The bronzeimpeller is now the anode and corrodes at a much higher rate.This example highlights theinfluence that graphitic corrosion can have on other components within the pump and theimportance of carefully selecting materials for use in conductive fluids,such as salt water.Several other types of dealloying can also occur in pumps.Brass and bronze alloys con-taining more than about 14 percent zinc are subject to a form of dealloying known as dez-incification.The zinc is preferentially corroded from the matrix of the material, leaving aspongy, copper-rich residue. Dezincification can occur either uniformly in a shallow layer5.6CHAPTER FIVEFIGURE 3The dealloying of a vertical turbine pump impeller. Note the change in color across the cross section.The unaffected bronze (light color) material is surrounded by a dezincified layer (1.3?).over the surface of the casting or as a distinct plug confined to a small area.Plug-type dez-incification is a more serious problem because the plug is weak and will cause leakage ifit penetrates a pressure boundary, but it should be emphasized that copper alloys con-taining less than 14 percent zinc are not susceptible to this form of corrosion. Conse-quently,the requirement often imposed upon pump manufacturers for zinc-free bronzes toavoid dezincification is without technical justification.Figure 3 shows the dealloying of animpeller.The final type of dealloying that occasionally occurs in pumps is dealuminification inaluminum bronzes. These are metallurgically complex materials. Some compositions canform an aluminum-rich phase that can be preferentially corroded in aggressive fluids,especially seawater. The detrimental phase can be mitigated by a special heat treatmentknown as temper annealing. This heat treatment must be specified by the designer forsusceptible compositions, because it is not a mandatory requirement of national materialspecifications. The chemistry of some aluminum bronze alloys from Europe has beenadjusted to preclude the formation of the detrimental aluminum-rich phase without theneed for the temper annealing heat treatment. The temper anneal can serve as a stressrelief operation for fabricated aluminum bronze structures, which is a secondary benefitfor products in this category.Galvanic CorrosionGalvanic corrosion refers to the corrosion that occurs when onealloy is electrically coupled to another and exposed in a conductive liquid.Usually,the cor-rosion rate of the more noble alloy will be less than if it were exposed uncoupled.The cor-rosion rate of the less noble material will be greater than if it were exposed uncoupled.Several factors influence the rate of galvanic corrosion of both metals.This corrosion isgreatly influenced by the conductivity of the fluid.In a fluid such as fresh water,which hasa low conductivity, galvanic corrosion will be less severe and generally confined to theimmediate location where the metals contact one another.However,in a highly conductivefluid, such as seawater, galvanic corrosion will be more severe and will occur over a widerarea. The pump designer needs to consider the possibility of such corrosion when usingdissimilar metals in a conductive fluid.Galvanic corrosion problems in seawater and other conductive fluids can be avoided bythe careful use of materials. Galvanic corrosion is related to the area ratios of the coupledmetals.It is always desirable to have the area of the anode,or less noble metal,equal to orgreater than that of the more noble metal. In this way, the additional corrosion experi-enced by the less noble metal will be spread over a relatively large area and will not beexcessive because of being coupled. An example of the effective use of this galvanic rela-tionship involves centrifugal pumps having a Ni-Resist casing and austenitic stainlesssteel internals.This combination is often specified for seawater services.The Ni-Resist is5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.7anodic to the stainless steel and will protect it from localized corrosion when the pump isshut down and contains stagnant water.The area of Ni-Resist is considerably larger thanthat of stainless steel. The increased galvanic corrosion of the Ni-Resist is spread over alarge area and is negligible.The amount of corrosion that will occur in a galvanic couple also depends on the freelycorroding potentials of the coupled metals. Less corrosion-resistant metals, such as zinc,cast iron, and steel will usually have more negative potentials when measured against astandard reference electrode.More corrosion-resistant metals,such as stainless steels,willhave less negative potentials.The corrosion potentials for many commonly used engineering alloys in slowly movingseawater are shown in Table 1.The alloys are listed in the order of the potential that theyexhibit in flowing seawater. Certain alloys (indicated by solid colored boxes preceding thename of the alloy) in low-velocity or poorly aerated water and at shielded areas maybecome active and exhibit a potential near ?0.5 volts.The extent of galvanic corrosion thatwill occur when two metals are electrically coupled will depend on the potential differencebetween the metals.The corrosion rate of zinc coupled to stainless steel will increase dra-matically because of the large potential difference between these two metals.A nickel alu-minum bronze coupled to austenitic stainless steel will experience little galvanic corrosionbecause the potentials of these two metals are close to one another. The pump designerneeds to be aware of the corrosion potentials of dissimilar metals used in conductive flu-ids in order to avoid unanticipated galvanic corrosion problems.The use of coatings can decisively alter the galvanic relationships in a pump. If themore anodic component,such as a steel casing,is coated,one can expect a high rate of cor-rosion at those locations where the coating eventually begins to fail. This will be causedby a very unfavorable area ratio, with a small area of exposed carbon steel coupled to alarge area of some more noble metal, such as stainless steel or bronze. For this reason,coatings should be employed with caution in pumps handling conductive fluids that areconstructed of dissimilar metals.It is generally advisable in these applications not to coatthe anodic component. Figure 4 documents the galvanic corrosion on the interior diame-ter of a carbon steel flange connected to a stainless steel shroud. The accelerated corro-sion is due to the unfavorable ratio of stainless steel to carbon steel in this component.Stress Corrosion CrackingStress corrosion cracking (SCC) is a particularly danger-ous form of corrosion because it is not easily detected before it has progressed to such anextent that it can cause sudden catastrophic damage. Although relatively uncommon inpumps, it can occur in several classes of materials. The pump designer should be awareof the potential combinations of material and environment that can cause SCC.Stress corrosion requires that several factors be present. These include tensile stress,which can be either residual or applied,a susceptible material,an environment capable ofcausing stress corrosion, and time.The materials used in the pump industry that may experience SCC include austeniticand martensitic stainless steels, some copper base alloys, and, occasionally, Ni-Resist.Theaustenitic stainless steels are susceptible to stress corrosion in aqueous chlorides at tem-peratures above about 140F (60C). Cast alloys, which contain some fraction of ferrite inthe microstructure,are significantly more resistant to stress corrosion than their wroughtcounterparts.The possibility of cracking is increased in situations where chlorides are con-centrated, as by evaporation. High residual stress, often present in as-welded structures,also enhances the possibility of cracking. Increasing nickel content in austenitic stainlessalloys enhances the resistance to SCC.The high nickel grade,commonly known as Alloy 20,is often used in chemical applications where the optimum resistance to stress corrosion isnecessary.The SCC of austenitic stainless steels in pumps is relatively uncommon.Martensitic stainless steels are susceptible to cracking in the presence of hydrogen sul-fide and is often referred to as sulfide stress corrosion cracking (SSC). These steels, par-ticularly CA-15 and CA-6NM, are commonly used in pumping applications in oilproduction and refining where hydrogen sulfide can be present. SCC can be avoided bygiving these materials a special heat treatment intended to reduce hardness below a cer-tain threshold level, below which cracking will not occur.This has also been correlated tothe yield strength of a material. It is often seen in literature that ferrous materials used5.8CHAPTER FIVEVolts: Saturated Calomel Half-Cell Reference Electrode+0.3+0.2+0.10-0.1-0.2-0.3-0.4-0.5-0.6-0.7-0.8-0.9-1.0-1.1-1.2-1.3-1.4-1.5-1.6-1.7MagnesiumZincBerylliumAluminum AlloysCadmiumMild Steel, Cast IronLow Alloy SteelAustenitic Nickel Cast IronAluminum BronzeNavel Brass, Yellow Brass, Red BrassTinCopperPb-Sn Solder (50/50)Admiralty Brass, Aluminum BrassManganese BronzeSilicon BronzeTin Bronzes (G & M)Stainless Steel - Types 410, 416Nickel Silver90-10 Copper-Nickel90-10 Copper-NickelStainless Steel - Type 430Lead70-30 Copper-NickelNickel-Aluminum BronzeNickel-Chromium alloy 600Silver Braze AlloysNickel 200SilverStainless Steel - Types 302, 304, 321, 347Nickel-Copper alloys 400, K-500Stainless Steel - Types 316, 317Alloy “20” Stainless Steels, cast and wroughtNickel-Iron-Chromium alloy 825Ni-Cr-Mo-Cu-Si alloy BTitaniumNi-Cr-Mo alloy CPlatinumGraphiteTABLE 1Corrosion potentials in flowing seawater (813 ft/s, 5080F/2.44.0 m/s,1026C)in these services should have a hardness no greater than 22 Rcor a yield strength nohigher than 90,000 lb/in2(620MPa). Technical standards, including API 610 and NACEMR-01-75, can be used to specify appropriate requirements for martensitic steels, whichwill be used in environments containing hydrogen sulfide.5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.9FIGURE 4Galvanic corrosion is evident on this pump section. Note the high corrosion rate on the interiordiameter of the carbon steel flange that is attached to the stainless steel shroud.Copper alloys are susceptible to SCC in the presence of ammonia, although consid-erable variations take place in the susceptibility of the various types of bronzes, withaluminum bronzes being the most resistant. Polluted natural waters can containammonia, and for this reason, bronze pumps are usually not a good choice for theseapplications.High-strength manganese bronzes are susceptible to cracking in natural waters. Castimpellers in these alloys have been known to suffer severe cracking.Residual stress in thecasting may also be sufficient to induce cracking.These alloys should not be used in pumpsbecause of their susceptibility to such problems.Ni-Resist is an austenitic cast iron that contains 15 to 20% nickel.This material is com-monly used in large, seawater vertical pumps. Experience has shown that it is subject toSCC, especially in the diffuser section of these pumps, unless the castings are furnacestress-relieved. This must be specified by the purchaser, as it is not a requirement ofnational material specifications.Hydrogen EmbrittlementHydrogen damage is a form of environmentally assisted fail-ure that results from the combined action of hydrogen and residual or applied tensilestress. Hydrogen damage to specific alloys or groups of alloys manifests itself in manyways, such as cracking, blistering, hydriding, or as a loss of tensile ductility. Collectively,these various forms of damage are often referred to as hydrogen embrittlement.Damage caused by hydrogen is occasionally encountered in pumps. Some platingprocesses, such as chrome plating, which is often used to rebuild pump shafts, generatehydrogen.This hydrogen can enter the surface of the metal.Microscopic cracks can occurin higher strength steels (greater than a 90,000-lb/in2or 620-MPa yield strength). Abu-sive grinding can work-harden the surface of lower strength steels and increase theprobability that hydrogen will cause cracking. Microscopic cracks resulting from hydro-gen damage act as stress risers and can propagate failure catastrophically by mechani-cal fatigue. This problem can be avoided by utilizing proper grinding practices beforeplating.Higher strength steels should be baked,to drive off hydrogen,immediately afterplating.Hydrogen can also be introduced into metals during welding. In order to avoid thehydrogen damage associated with welding, ferritic and martensitic steels should bewelded with low hydrogen electrodes. Coated electrodes should be baked, in accordance5.10CHAPTER FIVEwith manufacturers instructions,prior to usage in order to drive off moisture,which is themajor source of hydrogen contamination of welds.Microbiologically Induced CorrosionLiving organisms can promote corrosion inmany different environments.A variety of biological organisms thrive in both aerobic andanaerobic environments.Corrosion attributable to microbiological activity occurs most fre-quently in stagnant water,which remains in a pump when it is shut down for an extendedlength of time.Sulfate-reducing bacteria are found in many waters. They will form slimy, reddishhemispherical shaped mounds or colonies on cast iron or carbon steel.These are known astubercles. If scraped off, there will invariably be a saucer-shaped pit beneath the tubercle.The inside of the pit will contain a wet,black deposit.The pitting is caused by traces of sul-furic acid excreted by the bacteria.This type of corrosion will usually not result in prema-ture failure.Several more serious types of microbiologically induced corrosion afflict stainlesssteels.A certain class of metal ion concentrating/oxidizing microbes appears to concentrateferric and manganic chlorides,both of which are potent pitting agents.These bacteria formcolonies preferentially at welds in austenitic stainless steels and are capable of causingsevere pitting corrosion in a relatively short time. This problem has been encountered ina variety of equipment in both salt and fresh water. It is often discovered only when thewelds begin leaking. Pumps employing welded stainless steel fabrications can be afflictedby this problem if permitted to sit idle with stagnant water, either fresh or salt, for anextended period. Biocides can be used to mitigate this problem in some instances.Finally, the decay of biological organisms can generate hydrogen sulfide, whichadversely affects the protective oxide film on copper base alloys. The enhanced biologicalactivity in warmer tropical waters, especially under stagnant conditions, can impair thecorrosion resistance of bronzes and reduce the threshold velocity at which accelerated cor-rosion will occur. Bronzes should be used with caution in applications where macrobiolog-ical activity is anticipated and the possibility of extended shutdowns is possible.Intergranular CorrosionThis infrequent type of corrosion preferentially attacks amaterial at the grain boundaries.This is caused by local chemical differences such as thechrome-depleted regions of an austenitic stainless steel. Bronze alloys susceptible to thistype of corrosion include aluminum brasses, silicon bronzes, Muntz metal, and admiraltymetal.Two things are necessary:a sensitized material and a corrosive media,such as sea-water. Sensitization can occur during heat treatment or more commonly during weldrepair. This type of corrosion often leads to corrosion-assisted fatigue cracks when cyclicloading is present.The improper heat treatment of 300 series austenitic stainless steels can result in sen-sitization to intergranular corrosion. Sensitization occurs when stainless steels that con-tain more than .03% carbon are held at temperatures between 800 and 1550F (between425 and 850C). At these temperatures, chrome carbides precipitate along the grainboundaries,resulting in chrome depletion in the adjacent areas.These adjacent areas havereduced corrosion resistance.Austenitic stainless steels contain approximately 16 to 18%chrome. The chromium content in the areas surrounding a chrome carbide particle candrop below the 12% necessary to maintain a passive state.A galvanic cell is set up with alarge cathode (grains) and a small anode (grain boundaries). In this undesirable scenario,corrosion occurs along the anodic grain boundaries. The extent of the corrosion damagedepends on the length of time held within the sensitization temperature range.The degreeof sensitization is a function of the carbon content; the higher the carbon content, theshorter the period of time the material can be held within this range without sensitizationoccurring.A graph of the temperature versus time for various carbon contents illustratesthis point in Figure 5. Intergranular corrosion of an improperly heat-treated stuffing boxcover is shown in Figure 6.Austenitic stainless steels can also be sensitized during normal welding procedures.Care must be taken to avoid the sensitization range during welding followed by properpost-weld heat treatment when necessary.Sensitization can be avoided or corrected by several methods: Heat the material to a temperature high enough to dissolve the chrome carbides,typically 1900 to 2100F (1040 to 1150C), followed by rapid cooling through thesensitization range. Localized heat treatment of welded areas will not desensitize amaterial. Use a stainless steel that is stabilized by the addition of niobium or titanium.These twoelements will tie up the carbon, thus preventing chrome carbides. Reduce the carbon content to a low level (less than .03 percent). The lower the carboncontent, the longer it takes chrome carbide precipitation to occur.When austenitic stainless steels are necessary in the pump industry, materials com-monly used in services where intergranular attack is anticipated include 316L,304L,CF-3, and CF-3M. Intergranular corrosion is not a concern in alloys containing 25% or morechromium.Cavitation ErosionCavitation erosion is primarily a mechanical process, although itacts synergistically with corrosion and is often considered with other forms of corrosion.Cavitation erosion can be defined as metal removal from the surface caused by high5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.11FIGURE 5Time-temperature sensitization curves as determined by the Strauss Test for 18-8 stainless steel.Note that a low carbon grade of stainless (0.03% C) requires five to 10 hours exposure, while a standard grade(0.08%) need only minutes of exposure time.FIGURE 6The surface of a stuffing box cover that experienced intergranular corrosion due to sensitization. Thegrains are clearly evident on the interior of the bore as well.5.12CHAPTER FIVEFIGURE 7Cavitation erosion of an impeller, indicated by the porous appearance of cavitated regions on thesurfacestresses associated with the collapse of vapor bubbles in the fluid. Cavitation occurs in apump when the local pressure of the fluid is reduced to the vapor pressure. In a multi-stage pump, vapor bubbles form in the low-pressure areas at the impeller inlet and areswept by the flow into regions of higher pressure where they collapse.A great many bub-bles may form and collapse in a small area, producing many microjets of high kineticenergy. The energy released by the bubble collapse is expended as impact loading on themetal surface. This situation is aggravated if protective oxide films are present becausethese are damaged, exposing fresh metal to the corrosive action of the fluid. This cyclicloading eventually causes the formation of microscopic fatigue cracks.These cracks prop-agate and intersect, resulting in the removal of metal from the surface and the charac-teristic spongy or porous appearance of cavitation damage. An example of a cavitatedimpeller is shown in Figure 7.Although every effort should be made in the design and application of centrifugalpumps to prevent cavitation, it is not always possible to do so at capacities less than therated maximum efficiency capacity of the pump. It must be recognized that at a low flowoperation, the stated NPSH required curve is not usually sufficient to suppress all cavi-tation damage. The stated NPSH required is that needed to produce the head, capacity,and efficiency shown on the rating curve.At low flows, some cavitation damage should beexpected. It may be impractical to supply an NPSH that would suppress all cavitation atthese low flows, as it could be many times that it is required at the best efficiency point.Therefore, the possibility of cavitation damage frequently becomes a consideration whenselecting material for impellers.Open-type mixed flow impellers that produce heads in excess of 35 ft (10.7 m) are par-ticularly susceptible to cavitation erosion in the clearance space between the rotatingvanes and the stationary housing. This is usually referred to as vane tip erosion and iscaused by a cavitating vortex in the clearance space between the vane and the housing. Itis also impractical in this instance to provide sufficient NPSH to eliminate the cavitation.Any evaluation of the impeller and housing for a pump of this type should include the pos-sibility of vane tip erosion.It was conventional wisdom in the pump industry until recent years that the cavitationresistance of a material was directly related to its hardness.A more sophisticated under-5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.13standing has been developed in recent years that has led to the development of a new classof nonstandard stainless steels with exceptional cavitation resistance.The relationship between cavitation resistance and hardness was first critically inves-tigated in the 1970s when it was observed that cobalt base alloys of a modest hardnessdeveloped a very high resistance to cavitation damage. Cavitation resistance was relatedto the capability of the material to transform at the surface when subject to cavitationloading into a harder, more resistant metallurgical phase. This work was extended toaustenitic stainless steels, whose chemical composition was adjusted to promote the for-mation of a stress-induced martensite under cavitation loading. New alloys were devel-oped initially as weld filler metals to repair cavitation damage and later as impellercastings for pumps. These alloys have relatively low hardness in the solution-annealedcondition, comparable to standard austenitic grades, but transform to a much hardermartensite at the surface upon exposure to cavitation loading. The hard surface layerresists the initiation of fatigue cracks. If these cracks eventually develop after extendedexposure to cavitation bubbles, propagation into the soft ductile base metal is difficult.Cavitation-resistant austenitic stainless steel castings, alloyed with chrome and man-ganese, develop cavitation resistance similar to that of cobalt base alloys.Extensive laboratory tests of the resistance of a wide range of materials to cavitationerosion have produced data for all the materials commonly used in centrifugal pump con-struction. It is possible to make a good correlation between the laboratory data and fieldexperience to develop the following tabulation of the cavitation-resistance properties ofpump materials, listed in order of decreasing cavitation resistance: Stellite Chrome-manganese austenitic stainless Carburized 12% chrome stainless casting Titanium 6AL-4V Cast nickel-aluminum bronze Cast duplex stainless steel Cast precipitation hardening stainless steel Ductile NiResist Cast CF-8M Cast CA6-NM Cast CA-15 Monel Manganese bronze Carbon steel (cast) Leaded bronze Cast ironSelecting materials with adequate cavitation resistance will afford the pump designermuch greater leeway in the range of conditions under which the pump can be operated. Italso permits the design of smaller, lighter pumps that can be operated at higher speeds.The judicious use of materials significantly extends the time between outages caused bycavitation damage and can dramatically reduce maintenance costs.TYPES OF WEAR_Rotating equipment, including pumps, can suffer from damage as a result of mechanismsunrelated to corrosion. The relative motion between parts that are in close proximity toeach other can produce wear when these components come into contact with one another.5.14CHAPTER FIVECatastrophic damage may occur if the parts make contact under high loading conditionsor when foreign bodies are entrapped between the rotating and stationary components.Anaccelerated material loss or catastrophic seizure of these components can result in costlyrepairs or replacements. Erosion, due to the presence of solid particles in the liquid beingpumped, can also limit the life of internal pump components.Wear mechanisms have been categorized into more than 20 individual processes.1However, only a few mechanisms are frequently recognized as damaging to a pump: Adhesive wear: material-to-material contact Abrasive wear: solids interacting with internal components Erosion: solid particle impingement Fretting: small amplitude motion of parts causing oxidation damageIdentifying the wear mechanism is somewhat difficult at times as wear, or the loss ofmaterial, within a pump can result from more than one mechanism at a time.The study of friction and wear as a science,known as tribology,had its beginning in thelate 1930s. These early studies fostered an increased awareness of wear damage mecha-nisms that, in addition to corrosion and material fatigue, account for the life-limiting fac-tors of pumps.Additional information on the study of wear can be found in current tradejournals and texts.Adhesive WearOne of the primary causes of material loss on rotating components ina pump handling clear liquids (with no solids entrained in the fluid stream) is adhesivewear. This material loss is due to material-to-material contact producing surface disrup-tions, material grooving, a transfer of material, and possibly galling.Two important char-acteristics to consider for a pair of materials that may come into contact are their adhesivewear traits and their galling threshold. Galling of a material is considered a severe caseof adhesive wear.The wear of two surfaces in relative motion is complex. Some alternative theories ofsliding wear have been proposed in addition to the adhesive wear model. They are thedelamination theory, the oxidation theory, the surface delamination theory, a fatiguemodel,and combinations of several of the theories mentioned.However,only the adhesivewear theory offers a general wear equation to quantitatively predict wear, thus providinga means to rank materials with respect to their wear characteristics.A multitude of adhesive wear tests exist, including ring and block, pin and vee block,4-ball,and pin on disk.Wear tests are performed in order to screen material combinationsfor potential usage.Therefore, wear tests are designed to simulate, as closely as possible,the actual service conditions and parameters.The wear testing of materials under adhesive wear conditions has resulted in severalgeneralities that are safeguards to the successful use of materials that may experiencecontact during service.Studies supported by EPRI,U.S.Naval research,and private indus-tries result in lists of materials that are considered acceptable with regard to wear com-patibility when contact does occur. From this testing, the materials hardness isdetermined to be the critical parameter for successful running combinations. The follow-ing guidelines should be used when selecting materials for services where adhesive wearis expected:1. Like materials are not expected to run well under adhesive wear conditions (except formaterials designed for antigalling resistance such as Nitronic 60 and Waukesha 88).2. Combinations with hardness values less than 45 Rcrequire a hardness differential ofat least 10 Rc.3. Combinations with hardness values greater than 45 Rccan have the same hardness.Based upon extensive empirical testing and field experiences, several sound rules ofthumb have been developed through the years when selecting pump wear ring materials.Three factors are used to select materials for wear surfaces in clear liquid environments:5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.15EnvironmentMaterialsHardnessNon-corrosiveCast iron/leaded bronzeUnimportantMildly Martensitic stainless steels (locally orLess than 45 corrosivethrough hardened)Rc,10-point differentialGreater than 45 Rc, same hardness acceptableCorrosiveCorrosion-resistant, non-gallingNot applicableaustenitic stainless steel (Nitronic 50/Nitronic 60 or Waukesha 88/Nitronic 50)Severely Highly alloyed austenitic stainless corrosiveseel with hard-faced materials such as Stellite or ColmonoyNot applicable Corrosiveness of the fluid Amount of wear allowed Galling stressCorrosion determines the class of material to be used.These classes generally fall intothree groupings: non-corrosive, mildly corrosive, and corrosive. Of course, additional con-straints occur when selecting an appropriate material within the corrosive materialgrouping that will need to be addressed by application experience.Other material characteristics, such as additives, can significantly affect performancewith regard to adhesive wear and galling. For example, copper alloys with lead additionsare considered to be bearing alloys because of the capability of the lead to provide lubric-ity between contacting surfaces.Alternatives are being evaluated today to replace leadedbronze alloys to avoid the health considerations of lead usage.This is also true of tin andbismuth additions to nickel-based alloys.A general guide for materials in several environments is as follows:Using these industry-wide accepted rules of thumb will help avoid catastrophic dam-age normally resulting in costly repairs.Some special applications have produced unique material applications for given envi-ronments.These include low specific gravity applications where the use of mechanical car-bon materials is desirable because of the non-lubricating nature of these fluids. Commonpractice is to make the stationary component metal-filled graphite if the specific gravityis 0.5 or less. Stationary mechanical carbon components are also used in liquid CO2ser-vices and other potential dry start applications, such as the upper bearing in verticalpumps.Currently,non-metallic wear components,such as advanced polymers and ceram-ics,are being looked at to solve nagging problems encountered in a variety of applications.Usually, these are glass-filled polymers or ceramic composites with various additives toenhance their wear resistance.FrettingFretting can be considered a special case of adhesive wear. It occurs when twoparts in contact experience a repeated, small amplitude relative motion between close-fitting surfaces such as a loose impeller on a shaft. Researchers have described frettingdamage as a four-stage event:21. Adhesive wear of the asperities on the mating materials2. Abrasive wear caused by the wear debris produced in step one3. Abraded particles filling the asperity valleys4. Elastic contact producing cold working of the surface and micro-pitting5.16CHAPTER FIVEFIGURE 8The fretting damage of a shaft beneath an impeller that experienced small amplitude motion. Themottled appearance is typical of the damage caused by fretting (2.2?).In a pump, there is the potential for small amplitude motion at loose fitting impellers,beneath loose bearings,and between impeller wear rings and the impeller hub.The designengineer does not intentionally create a circumstance that will generate this type ofmotion, but when it occurs, fretting damage can lead to other problems.Fretting can be identified by a red powdery oxide that forms along the fretted surface.In a pump, the red-colored debris is often washed away, but a distinct damaged surfaceappearance will develop on the fretted surfaces.This damage is often described as havinga mottled appearance and is best depicted as a flat, eroded surface with no directionalityto the damage.Although the oxide may be washed from the surface, some staining of theadjacent component can be observed after disassembly of the pump. This has led to themisinterpretation that fretting is a corrosion mechanism, but it is actually a special wearphenomenon.Figure 8 shows the fretting damage of a pump shaft along the impeller-fitted areawhere a loose fit enables the oscillation of the impeller during operation.Since the motionnecessary to cause fretting can be of a small amplitude, large vibrations in the pump maynot be present.This makes the detection of fretting during any operation impossible.Theimpeller in this example would have similar damage along its bore.Fretting damage can be avoided with a few relatively simple guidelines. You shouldeliminate or prevent the possibility of motion between the two components by eithertighter clearances, or shrink fitting the assembly, which increases the clamping force. Iffretting is unavoidable in a particular design, methods of mitigation can be used. Theseinclude various coatings or providing the contact zone with an appropriate lubricant.Coat-ings that may be used include flame-sprayed high-nickel alloys, silver plating, or possiblyadding a thin, dense chrome plating to one or both of the faces in contact.Abrasive WearAbrasive wear is often categorized into two main classifications: two-body and three-body wear.The name indicates the mechanism of wear. For the most part,three-body abrasive wear is the primary mechanism of damage in centrifugal pumps.Thiscan occur when hard solid particles entrained in the fluid enter between ring fit areas orimpeller keyway faces. In fluids with high concentrations of solids, another form of three-5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.17FIGURE 9Three possible conditions between wear surface clearances and solid particle size. Condition “A” isconducive to maximum three-body abrasive wear.body wear is produced. Solids carried in the fluid stream can strike the internal pumpsurfaces. This is more commonly referred to as erosion. This type of damage is observedin the impeller and cutwaters of the casing. The degree of material damage, due to thismechanism, depends upon the bulk hardness of the material, the carbon content, and thecharacteristics of the solids present.Important particle characteristics include size,shape,hardness, and mass.To minimize three-body abrasive wear, a couple of variables must be taken into con-sideration.The wear ring clearance influences damage.The relationship between the sizeof the particles in the fluid stream and the gap into which they can enter is important.This is graphically illustrated in Figure 9,which shows three types of particle-to-gap rela-tionships.Condition A is logically the most damaging three-body abrasive case.A high rateof damage will result as these particles are entrapped between the two components. Incondition B, large particles relative to the ring clearance will not enter and produce dam-age. This condition enables the particles to flow with the fluid stream through the eye ofthe impeller and exit the pump. In condition C, very fine or relatively small particles willnot be entrapped and ground between the rings and will not result in collateral damage ofthe components.For the most part, particles in a fluid service will be in a range of sizes, so all the con-ditions will exist.Typically, a particle size and distribution analysis is performed to char-acterize the amount of particles that will cause condition A to exist. This is relativelysimple to accomplish by extracting the solids from a fluid sample and performing a sieveanalysis. The percentage of solids present in the fluid stream is extremely important fordetermining the appropriate material and design considerations. This will be addressedlater with guidelines given for appropriate material selections.Wear particle hardness is also extremely important. If particles are soft and friable,such as talc,little damage would be expected to occur on metal pump components becauseof three-body abrasive wear.The amount of damage is expected to be greater if the parti-cles are extremely hard. These particles include welding scale or silicon dioxide (SiO2),which is sand. The particle geometry also contributes to the amount of damage that canresult in three-body abrasive wear. Often, particles of SiO2are found in a rounded condi-tion. Pumps used to handle river water or seawater on ships frequently encounter theseconfigurations. Hard, round particles are less damaging than particles of equal hardnesswith sharp, angular configurations. Fly ash, a very hard, sharp, angular particle, is one ofthe most abrasive services encountered in the pump industry.5.18CHAPTER FIVEA materials resistance to abrasive wear can be characterized by a standard ASTM testprocedure. Each testing procedure attempts to simulate the mechanism that most appro-priately addresses the class of abrasive wear. In general, materials that are resistant totwo-body abrasive wear are resistant to three-body abrasive wear also.Test results show that the primary property responsible for increasing resistance toabrasive wear is the hardness of a metal alloy. Zum Gahr has provided test results tographically illustrate this fact.3Small microstructural differences, alloying, and surface-condition differences within alloy groups also can influence the abrasion resistance of amaterial. Some of these conclusions include the following: Abrasion resistance is increased with increasing bulk material hardness. At the same bulk hardness, steels with higher carbon content have higher abrasionresistance. Cold working, which increases a materials surface hardness, does not significantlyincrease the abrasion resistance of the alloy. Precipitation hardening increases the bulk material hardness and abrasion resistanceof an alloy. Gray cast irons show a decreasing abrasion resistance at higher hardnesses. Softer, austenitic, white cast irons exhibit improved abrasion resistance over marten-sitic, white cast irons. Carbides are important for the wear resistance of steels and chromium-alloyed, whitecast irons. A carbide volume fraction of 30% maximizes the abrasive wear resistance for materialswith a soft matrix.An example ofthree-body abrasive wear is shown in Figure 10.It shows a laser-hardenedshaft sleeve after approximately one year of service in a mine dewatering operation whereabrasive wear caused a significant wear of other material combinations.The abrasive wearwas caused by fine tailings in this gold mine application.To increase the life of rings in ser-vices like this, the use of hardened wear rings is a good start.This is the reason why pumpproducers use coated rings in applications where significant abrasive wear is anticipated.However, depending upon the severity of the service, a choice of a ring material containingcarbides may be necessary.For mildly abrasive services, the following materials should be considered: Ni-ResistIts resistance is due to chromium carbides in the matrix. It has goodadhesive wear resistance also. Selectively hardening the surface of AISI 420 (laser hardened 50-55 Rc). Surface hard-ening is not susceptible to hydrogen embrittlement or SCC. Carburized and hardened 12% chromium stainless steel.For more abrasive services, the following is often considered: Hardened AISI-440C (5055 Rc) Stellite or colmonoy-coated (hard-faced) austenitic stainless steel Solid stellite Tungsten carbide Silicon carbide Partially stabilized zirconia (PSZ)Recent advances involving the use of ceramics, metal-matrix composite materials,laser-surface alloying, and laser-surface modifications to a substrate that normally couldnot survive in an abrasive service are examples of ongoing material developments.5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.19FIGURE 10The three-body abrasive wear of a laser-hardened shaft sleeve in an abrasive service. Note the fineconcentric scoring of the hardened surface. The helix pattern is the laser-beam overlapped zone produced by thelaser process.ErosionMost fluids handled by pumps are considered clear liquids, meaning they donot have significant amounts of solid particulates present. The corrosive nature of thesefluids dictates the required pump materials. Guidelines for many of these services areembodied in “Corrosion in Pumps,” a tutorial published in the Ninth International PumpUsers Symposium.4However,many fluid-handling applications requiring pumps are far from clear liquids.Solid particulates can be removed with costly filtration systems that must work flawlesslyat all times.Fabricated piping systems may introduce suspended solids from weld slag andpipe burn. Naturally occurring suspended solids are those found in water sources such asriver water or seawater, as mentioned previously in the abrasive wear section.The following factors should be considered during the material and pump selectionphase of the procurement process: The hardness of the particles The quantity of particles Size distribution Nature (geometry) The velocity of the pumpage The angle of fluid impingementThe first four items listed deal with the suspended solids. These variables can varyfrom application to application.The hardness of the particles is important to understandin determining the materials necessary to yield an acceptable pump lifespan. Hardnesscan range from relatively soft substances, such as cellulose fiber in pulp and paper appli-cations,to very hard abrasive particles such as silicon or rock in mining pumps.The Miller5.20CHAPTER FIVEnumber index, as described in ASTM G75,5is used to characterize the abrasivity of hardparticles.The Miller number was developed to determine the relative abrasivity and attrition ofsolid particles making up a slurry. In a closed loop test, the abrasivity of the particlesbecomes less damaging with time due to the fracturing and rounding (or friability) of theparticles as they strike each other and/or impinge on a pump or casing wall.The Miller number is therefore reported with two numbers. The first number char-acterizes the abrasivity of the particles and the second is the loss of abrasivity (attri-tion) of the particles during the slurry test.The abrasivity portion of the Miller numberis useful in practical applications because this more closely characterizes a slurrysdamaging potential.The attrition number has found little use other than characterizinga test loops influence on a slurry.A slurry with a Miller number less than 50 is not con-sidered abrasive in a reciprocating pump. Examples of slurries with a Miller numberbelow 50 are limestone, sulfur, and detergent. It has been determined that a slurry con-sisting of finer particles is less abrasive then one containing larger particles. Test datashows that Corundum at 220 mesh is about four times as abrasive as the same mater-ial at 400 mesh.5Particle velocity plays a major role in the degree of damage that occurs in a pump han-dling slurries.In this case,the potential energy is converted into kinetic energy,producinga material loss by the transfer of energy from the particle to the component.The amountof material damage on an individual particle scale depends specifically upon particle veloc-ity,v,and mass,m (kinetic energy,defined as mv2).This is demonstrated by Finnies equa-tion6for hard materials:Of course, the pump part that absorbs the kinetic energy resulting from the particleimpact has a role to play also. The material hardness and/or resilience of the pump com-ponent in absorbing the particles impact energy will also determine the amount of mate-rial loss.Chen and Hu7have performed laboratory tests on materials while changing the parti-cle variables previously described.Their test results show the following: An increased particle hardness increases the material loss up to 1700-kg/mm2micro-hardness (greater than 75 Rc). Beyond this hardness, a decrease in wear occurs.Thisis most likely the result of the hard, brittle particle fracturing, which absorbs some ofthe kinetic energy. Sharp, angular particles increase the erosion rate over round particles. Erosion increases with increasing concentrations of abrasive particles. An increased fluid (and particle) velocity increases the erosion rate. Minimal erosion occurs at an impingement angle of 0 (tangent to the target surface)and increases to a maximum amount of wear at a 65 angle.A review of the literature shows that several authors have plotted the solid particleimpingement angle versus the amount of erosion.8,9These plots show that for ductilematerials, erosion increases with the increasing impingement angle to a maximum mate-rial loss at an angle of 25. Then the erosion damage decreases to the 65 impingementangle previously mentioned. Brittle materials, such as glass, are quite different. As theimpingement angle increases from 0 to 90, the volume of material loss continuouslyincreases.The characteristic features of erosion damage due to solid particle impingement areusually recognizable.However,when an aggressive fluid is present,the effects of solid par-ticle impingement may not be easily identified. These effects can appear very much likecorrosion-erosion,which is a fluid velocity-controlled damage mechanism where entrainedsolids are not present. If this damage is misdiagnosed, an improper material substitutioncan be made that may not solve the real problem. Conversely, a more likely situation isWear rate ? 1# of impinging particles2 ? 1average particle mass2? 1impingement velocity22? 1angle of impingement2FIGURE 13An austenitic stainless steel impeller inan abrasive fly ash service that shows severe erosion.Increased erosion occurs with an increasing fluidvelocity near the periphery of the impeller.FIGURE 14Erosion damage of an AISI-type, 440Cstainless steel ball valve in a coal slurry service5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.21FIGURE 11The severe erosion of a carbon steelcasing in a 17% bauxite and sand service. Note thegouging due to the local turbulence of the slurry.FIGURE 12Erosion at the exit vane tips of a duplex,stainless steel CD4MCu impeller in a bauxite service.that the observed damage resulting from the erosion-corrosion is misinterpreted as solidparticle erosion.A full understanding of the pumpage including the fluid velocity,fluid cor-rosiveness, content, and nature of the solid particles present is necessary for the appro-priate action to be taken in improving the life of a damaged pump.An example of solid particle erosion in a pump is shown in Figures 11 and 12. Thesevere erosion damage of a casing is illustrated by the gouging of surfaces that weredirectly impinged or scoured by glancing blows of the solid particles in the fluid stream.This pump handles a bauxite slurry where the percentage and velocity of alumina (Al2O3)and sand are too high for the carbon steel casing and CD4MCu impeller.Figure 13 shows a CF3M impeller in a fly ash service. This shows that the greatestdamage to the impeller is at the outer periphery, which corresponds to the highest veloc-ity of the slurry.The least amount of damage is near the impeller inlet eye. Figure 13 alsoshows that the lower velocity region of the impeller inlet eye has the least damage. Thisconfirms the laboratory data that shows an increased erosion with an increased slurryvelocity. Note that the damage increases near the outside of the inlet eye and is almostnonexistent at the impeller hub where the fluid velocities are lower.Erosion damage can also be encountered in reciprocating pumps. Figure 14 showsextensive erosion of an AISI-type, 440C stainless steel ball from a ball valve after it5.22CHAPTER FIVEbecame stuck and unable to rotate in a coal slurry application. This caused a slurryimpingement on a concentrated region of the ball.The particle velocity and impingement angle are design factors that can be used to mit-igate erosion in pumps.The challenge in the coal liquefaction program investigated by theDepartment of Energy in the 1970s was to develop a high-speed pump for handling coal-oil slurries.8This was attempted because traditional slurry pumps are usually large,slow-moving machines that increased the capital and operating costs of pilot plants built duringthat era.Most of the slurry pump industry utilizes large,slow-moving,single-stage pumpsto address the solid particle erosion problem. Many of these pumps are rubber-lined toabsorb the particles impingement energy.Erosion damage, once identified, has a limited number of solutions to prolong thelongevity of pump materials. This can be accomplished by the selection of hard, wear-resistant replaceable liners,elastomeric liners,or,in cases where liners cannot be utilized,hard materials. Such metallic materials include white cast iron (such as Ni-Hard), highchromium (13 to 28 percent) alloy steels, cobalt-based super alloys (such as Stellite), andnickel-based alloys.FATIGUE_Centrifugal and reciprocating pumps are subjected to cyclical loading, which, if not con-sidered during design, will result in a limited life due to material fatigue. In combinationwith a corrosive environment, material fatigue can be accelerated due to what is com-monly referred to as environmentally assisted fatigue.The one essential parameter in component fatigue is the presence of an alternating orcyclic load.In general,pumps are machines that have either fluid or mechanically inducedcyclic loading on their components. Although centrifugal pumps are for the most partsteady-state rotational equipment, pulsations or fluctuating applied stresses are encoun-tered. The source of these cyclic stresses can be from fluid interaction between impellerexit vanes and diffuser vanes or, in a volute pump, the impeller vanes and the casing cut-water.Mechanically induced forces are due to bending moments acting on the pump shaftor possibly a component imbalance in the rotor assembly.Reciprocating pumps experiencea cyclic loading of the internal and external components from the action of the machinery.In fact, these pumps can be thought of as large fatigue-testing machines due to the pul-sating action of the pumping process.When cyclic forces are applied to materials in a pump over a period of time,a crack mayinitiate at the components surface. After initiation, the crack will grow with continuedcyclic loading until the part finally fractures.Fractures can occur,even though the loadingproduces stresses that are far less than the tensile strength of the material. Engineershave been aware of this potential mode of component fracture for many years and havedeveloped design criteria that take this anomaly into account.The study of cyclic loadingand material behavior based upon cyclic stress history and flaw size is beyond the scopeof this text.It should be noted,however,that the field of fracture mechanics offers an engi-neering design tool that can predict the life of an engineered component.Fatigue is a three-stage process consisting of (1) crack initiation,sometimes associatedwith preexisting defects, (2) crack propagation, and (3) the final fracture, associated withcrack instability,as suggested by Wohler.10The applied stress level,sample geometry,flawsize, and mechanical properties determine the existence and extent of these stages.Fatigue was first studied by August Wohler in 1852.11Wohlers work included the con-cept of alternating applied stress,S,and the number of cycles,N,applied to a sample untila fracture occurs. This work is the basis for todays S/N curves used by design engineers.A laboratory-generated S/N curve is shown in Figure 15. This curve was generated bysmooth, rotating-beam test specimens. These specimens are machined carefully to avoidmetallurgical notches on their surfaces that would lower the applied stresses required toproduce a failure during testing.When a corrosive media is introduced, many crack initiation sites are produced. Thelower curve shows the resulting drop in the endurance limit.Since corrosion over time can5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.23FIGURE 15A laboratory-generated S/N curve for a smooth bar rotating beam test specimenTABLE 2Corrosion fatigue strength of alloys in sea water*AlloyUTSCFSTi-6Al-4V15488Inconel 71818960Inconel 62514950Hastelloy C10832Monel alloy K-50017626Ni Al bronze (cast)11515304 Stainless7915316 Stainless8514304L Stainless7514316L Stainless791317-4PH - cast1070-30 Cu-Ni (cast)839Ni Mn Bronze829Mn Bronze738D-2 Ni-Resist7.5Mild steel2*Test parameters: ambient temperature, 1750 rpm, 23 ft/s (0.60.9 m/s). Corrosion fatiguestrength (CFS) given at 100,000,000 cycles. All values are in ksi; 1 ksi = 6.894759 mPa. (UTS is ulti-mate tensile strength of material in air.)increasingly damage the material,the endurance limit will correspondingly decrease withthe exposure time. No true fatigue limit exists for materials in a corrosive environment.For this reason, the corrosion-assisted fatigue life of a material is usually published withcautionary statements. Given enough time, corrosion can penetrate completely through afatigue test specimen, resulting in a data point of zero load and zero cycles. For this rea-son, corrosion-influenced fatigue test results usually specify the corrosive media, the testtemperature, the details of the sample pre-exposure to the corrosive media, and the testfrequency with respect to the applied cyclic loading.Published data varies because laboratories that use a low frequency of applied stressesincrease the influence that corrosion has upon the test specimens. This is in comparisonto laboratories that conduct these tests at a high frequency that minimizes the influenceof the corrosive media. Published values for the ultimate tensile strength and corrosionfatigue strength of various alloys are shown in Table 2.12Wohlers investigation shows that a mechanical notch can reduce a materials fatigue.10This is shown in Figure 16 as a shift in the S/N curve below the curve produced by asmooth bar specimen.The severity of the notch determines the amount of divergence fromthe smooth bar curve. As shown, the surface degradation mechanisms lower the stress5.24CHAPTER FIVEFIGURE 17SEM photo of a fracture smooth fatiguetest specimen with a single origin. The arrow indicatesthe fatigue crack origin.FIGURE 16A shift in the S/N curve below thecurve produced by a smooth bar specimen. Theseverity of the notch determines the amount ofdivergence from the smooth bar curve.needed to produce the specimen failure after a certain number of cycles. This in turnreflects a lowering of the materials endurance limit.The three stages of fatigue cracking can be observed on the fracture face, renderingthem easily identifiable. This is especially true if no other secondary damage masks thecharacteristic appearance. Especially in fractures that occur over long periods, lines arevisible on the fracture surface.These bands are sometimes referred to as clamshell mark-ings, crack arrest lines, or beach marks and reflect different periods of crack growth.Ratchet lines,which represent the joining of two different crack fronts on different planesinto one, are observed in multiple origin fatigue cracks. Multiple origin fatigue fracturesare often associated with rotating components.Figure 17 shows a high magnified view of a smooth bar fatigue specimen after a frac-ture.The arrow shows a single origin of this specimen.This fatigue crack propagated acrossthe entire specimen diameter until the final fracture occurred,shown as a small circle.Thefinal fracture is sometimes referred to as the ductile overload zone or the fast fracture zone.An example of a fatigue fracture on a pump shaft is shown in Figure 18.The arrows inthis figure indicate the location of many crack origins.The flat,smooth surface appearanceFIGURE 18Multiple origin fatigue fracture of a pump shaft. The arrows show the locations of the many fatiguecrack origins. A and B correspond to the final fracture zone of each fracture face.5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.25FIGURE 19Overall view of a CF-3M impeller thathas two corrosion-assisted fatigue fractures in the frontshroud wallFIGURE 20A higher magnification of one of thefatigue fractures that originated at a corrosion pit atthe exit vane tip and shroud intersection. Additionalcorrosion pitting can be seen on the impeller in thisfigure.of this fracture face is characteristic of most fatigue fractures. This flat fracture appear-ance is sometimes mistaken for a brittle fracture because no evidence of plastic deforma-tion is observed on or near the break.Shown almost directly in the center of the fracturedshaft section is a small area of ductile overload,marked A and B,which corresponds to thefinal fracture area. The relatively small size of this area indicates the crack propagatedunder low alternating loads. In other words, the only material holding the two halves ofthe shaft together was the last area to fracture. This is a mere fraction of the total cross-sectional area of this shaft. The multiple arrows at the OD of the shaft shows the manycrack origins. Fatigue ratchet marks are at each of these locations. This type of fatiguefracture is referred to as multiple origin, high cycle fatigue.As mentioned before, an investigator uses the relative size of each fatigue crack stageto determine the magnitude of the loads acting on the component.The identification of thecrack origin is also of prime concern in conducting a failure analysis. The crack origin isimportant to determine if the fatigue crack initiated from a flaw in the material or a notchproduced in service or during manufacturing.Corrosion, often the primary cause of pump material damage, can increase the likeli-hood of fatigue cracking. Corrosion-assisted fatigue is the name given to this special typeof cracking. Corrosion damage can change the surface texture and significantly increasethe local stresses acting on the pump component.If the corrosion damage is severe enoughto produce a sharp notch in a region of high cyclic loading, then fatigue cracking of thecomponent is inevitable.In some cases,the propagation phase is also influenced by oxida-tion, which can mask the telltale features of the fatigue mechanism. Corrosion oxides,which form along the crack face, can produce a wedging effect, which mechanicallyincreases the local tensile forces acting on the crack tip. This increases the crack propa-gation rate.An example of corrosion-assisted fatigue at two locations in the front shroud wall of animpeller is shown in Figures 19 and 20. The evidence of corrosion pitting on the surfaceindicates a strong possibility that corrosion influenced the fracture mode. Further inves-tigation shows that both shroud wall fatigue fractures were initiated at corrosion pitslocated in highly stressed areas of the impeller.The fluid pulsations acting on the exit vanetip result in alternating loading.Corrosion is not the only mechanism of surface degradation that can promote this formof cracking.Surface disruptions through fretting or wear can also provide sites for fatiguecrack initiation. Sharp radii and defects at the material surface such as porosity and poormachining act as stress concentrations.Once the mechanism of fatigue cracking has been identified,suitable corrective actionscan be implemented.These include the following:5.26CHAPTER FIVE Higher strength materialsA good approximation for the endurance limit of a metalis 50% of the materials tensile strength. This is for high-cycle fatigue where no macroplastic loading is experienced. A graph published in Deformation & Fracture ofEngineering Materials10shows this rule of thumb. Design modificationThe stress acting upon a component can be reduced with anincreased section size. Reducing the stress on a component will increase its life. Thedesign criteria for mean stress in an alternating loading environment can be deter-mined using several analytical models. Since components are subjected to a range ofloading (not a constant amplitude),a fluctuating mean stress is encountered.The antic-ipated load history can aid in the design process to avoid fatigue fractures.The predic-tion of potential component life can be based upon a fluctuating mean stress designcriterion, referred to as the Pamgren-Miner cumulative damage law.10 Surface treatmentsThe introduction of compressive stresses to the surface of a partincreases the fatigue life of a component. This is usually performed at crack-sensitiveregions such as sharp corners or notches.If compressive stresses are introduced into thesurface of a material,cyclic tensile stresses in excess of the compressive stress value areneeded to cancel their effect before fatigue damage can occur. Therefore, any form ofcompressive stress will benefit a component with respect to fatigue cracking. Compres-sive stresses can be introduced by (1) cold working, (2) shot peening, or (3) a local heattreatment that introduces beneficial,compressive residual stresses (such as laser hard-ening or induction hardening). Increased corrosion-resistant materialsThe use of more highly corrosion-resistant materials is beneficial in cases where corrosion has decreased a componentslife by degradation of its surface condition.MATERIALS OF CONSTRUCTION _ImpellersThe pump designer needs to consider several criteria when selecting thematerial for the impeller: Corrosion resistance Abrasive wear resistance Cavitation resistance Casting and machining properties Weldability (for repair) CostFor many water and other noncorrosive services,bronze satisfies these criteria and,asa result, is the most widely used impeller material for these services. Bronze impellersshould not be used for pumping temperatures in excess of 250F (120C).This is a limita-tion imposed primarily because of the differential rate of expansion between the bronzeimpeller and the steel shaft. Above 250F (120C), the differential rate of expansionbetween bronze and steel will produce an unacceptable clearance between the impellerand the shaft.The result will be a loose impeller on the shaft.Leaded bronzes have been used extensively in the past as impellers, especially in lessdemanding applications.The lead addition to bronze enhances its castability and machin-ability. In recent years, environmental concerns associated with lead have caused manynonferrous foundries to stop producing these alloys and pump manufacturers are increas-ing their use of nonleaded bronzes for impeller applications.It should be noted that bronzes have velocity limitations above which they will sufferaccelerated erosion corrosion. The maximum velocity, which will correspond with theperiphery of the impeller, is higher in fresh water than in salt water. The most resistantbronzes, able to tolerate the highest velocities, are the nickel aluminum bronzes. These译文译文第五章第五章 结构的材料结构的材料 5.1 节节 泵结构的金属材料泵结构的金属材料(以及他们的损伤机制以及他们的损伤机制) 一台成功的泵装置需要的是它的性能和寿命。性能就是泵头的额定功率,容量和效率。 寿命就是在需要更换一个或者多个泵部件以保持有效的性能之前总的运行小时数。 最初的泵性能是泵制造者的责任,并且包含在泵设计当中。 寿命是当泵在运行时,结构材料对腐蚀、冲蚀、磨损等影响材料因素的抗性的主要衡量方式。 最大限度地提高泵的可靠性和延长泵的使用寿命的需求,使得选择合适的结构材料至关重要。 选择既符合成本效益,在技术上又宜应用的材料需要的知识不仅在于泵的设计和制造工艺,而且包括材料的工程性质,特别是当材料处于泵运行环境中时其耐腐蚀性和耐磨损性。 在腐蚀和冶金文献中以及从泵制造商的经验里已经有足够的信息使我们能够为几乎所有应用环境中的泵选择适当的材料。 众所周知,有很多因素能够保证泵的使用寿命。这些因素包括: 在接近环境温度下的中性液体。在恶劣的运行环境中选择合适的泵材料。去除会造成磨蚀的小颗粒。在接近最大效率点处持续运行。根据制造商提供的工作曲线要求的汽蚀余量预留充分且有效的余量低速率(扬程/转速)。符合这些条件的泵装置将会有一个长的寿命。供水泵就是一个典型的例子。一些用铜叶轮和铸铁壳体的供水泵有 50 年的寿命或者更长。另一个极端就是处理热腐蚀性并带有悬浮颗粒液体的化工泵。 这种泵的寿命就应该以月来衡量而不是以年。尽管事实上这种泵的结构采用了最耐受的材料。 大多数的泵应用处于上述这两个极端之间。 泵的设计者需要熟悉各种影响泵部件以及它的使用寿命的剥蚀类型。这些剥蚀一般可以分为腐蚀,磨损和疲劳,其中腐蚀和磨损是影响寿命的主要机理。 腐蚀的类别 腐蚀的类别 普通腐蚀 普通腐蚀 普通腐蚀在发生时没有明确可预知的地点。 这种类型的腐蚀发生于表面没有涂有效钝化膜的金属或者合金上。通常,腐蚀机理是伴随着金属氧化物腐蚀产物形成的氧化作用。普通腐蚀大多数时候经常发生在用碳钢和铜合金的泵中。铸铁也会有一种需要被单独列出的普通腐蚀形式,称为石墨化腐蚀。 碳钢不会产生保护性氧化膜, 而是会根据水或其他液体的一些特性,包括温度、含氧量、pH 值以及流体的化学性质以一定的速率腐蚀。有一些基于水的化学成份的经验指标可以用来计算自然水体对碳钢以及类似的铁合金的相对腐蚀度。 朗格利尔指数(Langelier Index)是最有名的。腐蚀率也很大程度上取决于速度并且随速度的增加而增加。在大多数的泵应用中,显然碳氢化合物除外,碳钢的腐蚀率太高以至于不能够提供一个有效的寿命。然而,碳钢却被频繁应用,特别是在立式泵中,用一些形式的保护涂层来避免腐蚀。煤焦油环氧树脂是许多水运行环境下的理想涂层。 铜合金,包括黄铜和青铜,也在最常使用他们的泵工业中遭受着腐蚀。水中少量硫化物的存在会增加腐蚀速率。 铜合金在大多数情况中逐渐形成一种保护性的氧化铜腐蚀膜。 随着这个薄膜的不断形成, 腐蚀速率逐渐减小。 不同的合金种类,不同的等级,他们的腐蚀速率也各不相同。在泵常用的合金中,镍铝青铜腐蚀率最低,对高速度的耐受最好。 图 1 中展示了一个立式泵中镍铜铬耐蚀铸铁壳体的普通腐蚀。 金相截面局部图显示了腐蚀的深度 图 1 某立式泵的下壳体镍铜铬耐蚀铸铁的碎片。右图中展示的微观结构显示了腐蚀渗入的深度。此为普通腐蚀的典型的例子(右图放大倍率为 100 倍) 脱合金成分腐蚀脱合金成分腐蚀 脱合金成分腐蚀会优先腐蚀掉多相合金中的某相,或者金属中的一种元素。在泵工业中,脱合金成分腐蚀的发生有很多形式。其中最常见的是灰铸铁的石墨化腐蚀。这种材料价格低,易于加工,而且非常适于各种应用情况,尤其是在供水系统工业。可以说是泵工业中应用最为广泛的一种材料。 与碳钢或球墨铸铁相比, 灰铸铁腐蚀有着本质上不同的机理。 灰铸铁的结构是在一个主要是铁的晶阵中由石墨薄片互相连接而成。在电解液的存在下,通常是水, 在铁和石墨之间建立了一个原电池。铁腐蚀,腐蚀产物绝大部分随着介质液体流过泵被冲刷带走。 最初的铸造体会逐渐减少变成包含一些氧化铁的腐蚀产物的多孔石墨结构。 这通常被称为石墨化。灰铸铁表面的石墨腐蚀不会改变其原来的形状和尺寸,但表面大部分将会变成可以用小刀切割的石墨。铸件将丧失其部分机械性能, 并逐渐变得容易受到轻微的冲击或冲击载荷而造成脆性破坏的影响。 这也是在海水中镍铜铬耐蚀铸铁的腐蚀机制。图 2 展示了金属中完好基本金属和它石墨化的前端的交界面。 石墨的腐蚀率会随水化学性质的不同而变化,而且这种类型的腐蚀在淡水和盐水中均会发生,这一点至关重要。盐水的高电导性越好,腐蚀率越高。在高矿物质含量的水中,石墨化腐蚀会以缓慢的速度进行。矿物会堵塞在石墨层的表面上,封闭基本金属使其不暴露在流体中,从而降低腐蚀速率。 随着铸铁件,如泵壳的表面逐渐石墨化,与泵内其他部件的电偶关系将会被改变。相比于被安装在已经工作很多年的泵中的青铜叶轮,一开始就被安装在用于处理海水的铸铁泵中的青铜叶轮会有更长的寿命。 更换叶轮后其寿命的减少是因为改变与泵壳的电偶关系。 最初,壳体是铸铁,相对于青铜叶轮它是阳极。随着时间的推移,当壳体逐渐石墨化,由于石墨的影响而渐渐变成阴极。 青铜叶轮现在就成为了阳极并且以一个高的速率腐蚀。 这个例子突显出石墨腐蚀对泵内其他部件的影响以及谨慎选择用于导电流体如盐水中的材料的重要性。 其他一些类型的脱合金成分腐蚀也会在泵中发生。含有超过 14%锌的黄铜和青铜合金遭受的脱合金成分腐蚀称为脱锌。锌被优先从材料的基体上腐蚀掉,留下一个有弹性的富铜渣。 脱锌能够均匀地发生在一个铸件表面的浅层或作为一个局限在一个小区域独立的塞头上发生。塞头处脱锌是更严重的问题,因为塞头很薄弱,而且如果穿过边界压力则会造成泄漏,但应该强调的是,锌含量小于 14%的铜合金不容易受到这种形式的腐蚀。因此,在无锌青铜上的需求通常强加于泵制造商来避免脱锌,这是没有技术理由的。图 3 展示了一个叶轮的脱合金成分腐蚀。 图 2 石墨化前和可靠的基底金属之间的交界面。石墨化腐蚀沿着石墨片的路径发生和传播(50) 图 3 一个立式泵叶轮的脱合金成分腐蚀。注意横截面颜色的变化。未受影响的铜(浅色)材料被脱锌层包裹着(1.3) 最后一种偶尔会发生在泵中的脱合金成分腐蚀是铝铜合金中脱铝。 这些都是冶金复合材料。 某些组成成分可以形成一个富铝相并且会在有侵蚀性的液体中优先腐蚀,特别是海水。 一种特殊的热处理可以减轻有害相。 这种热处理被称为回火退火。这种热处理必须由设计者指定易感成分,因为它不是国家材料规格的强制性要求。 欧洲的一些铝青铜合金的化学成分已经被调整以去除富铝相的形成而不需要回火退火热处理。 回火退火可以作为焊接的铝青铜结构的一种应力消除操作,这是一个产品在这一类的二次受益。 电化学腐蚀电化学腐蚀 电化学腐蚀这种腐蚀是发生在当一种合金被电偶合到另一种合金上而且暴露在一种导电液体中时。 通常,相比于未形成电偶暴露情况下,越是贵金属合金的腐蚀率会越低,而非贵金属的腐蚀率越高。 这两种金属的电化学腐蚀率受多种因素的影响。 这种腐蚀很大程度上受液体导电性的影响。例如在导电性较低的清水中,电化学腐蚀将不那么严重,一般只限于金属互相接触的位置。 然而,在导电性很高的流体中,如海水,电化学腐蚀将更严重并且发生在更广泛的区域中。 泵的设计者需要考虑当在一种导电性液体中使用不同的金属时这种腐蚀发生的可能性。 谨慎选用材料可以避免在海水或者其他导电液体中的电化学腐蚀问题。 电化学腐蚀与偶和金属的接触区域比率相关。它总是发生在阳极区域,或者低贵金属区域,不常发生于贵金属所在区域。因此,非贵金属的额外腐蚀将被传播在一个相对比较大的区域,因为被耦合,所以并不会过度。 一个有效利用这种电解关系的例子就是在离心泵中采用镍铜铬耐蚀铸铁壳体而内部部件采用奥氏体不锈钢。 具体来讲这种组合经常被应用于海水工况下。 当泵停机并含有积水时, 镍铜铬耐蚀铸铁相对于不锈钢就是阳极,从而保护它不受局部腐蚀。 镍铜铬耐蚀铸铁的面积明显比不锈钢大。 镍铜铬耐蚀铸铁电化学腐蚀的增加会覆盖一个大的区域,而且可以忽略不计。 在电偶中发生的腐蚀程度也取决于耦合金属的自由腐蚀电位。低耐腐蚀金属,如锌、 铸铁和钢当相对于参考电极测量时通常会有更多的负电位。 高耐腐蚀金属,如不锈钢,则负电位较少。 常用工程合金在缓慢移动海水中的腐蚀电位见表 1。这些合金以它们在流动的海水中展现出来的电位的顺序列出。某些合金(固体颜色框标出的合金名称)在低速或低碳酸水以及在屏蔽的区域可能会变得活跃,并表现出一个近-0.5 伏的负电位。 当两种金属电耦合时电化学腐蚀发生的程度将取决于两种金属的电位差。由于锌与不锈钢这两者之间巨大的电位差, 锌与不锈钢耦合的腐蚀率会大大增加。镍-铝青铜耦合奥氏体不锈钢时电化学腐蚀程度很小,因为这两者的电位是彼此接近的。泵设计者需要注意用于导电液体中的不同金属的腐蚀电位,以避免预料不到的电化学腐蚀问题。 涂料的使用可以决定性的改变泵中的电位关系。 如果像钢壳体这样的阳极部件被涂上涂层,预计在这些位置会有一个高的腐蚀率, 而这些位置的涂层最终将会脱落。这将由一个非常不利的面积比造成,一个小面积暴露的碳钢与一个大面积的一些贵金属耦合,如不锈钢或者青铜。 因此,当泵处理具有不同金属的导电性流体时,应谨慎使用涂料。在这些应用中通常不建议在阳极部件上涂涂层。图 4 统计了连接在不锈钢罩上的碳钢法兰内径的电化学腐蚀。 加速腐蚀是由在这个部件的不锈钢相对于碳钢的不当比例造成的。 应力腐蚀开裂应力腐蚀开裂 应力腐蚀开裂(SCC)是一个特别危险的腐蚀形式,因为在其发展到可以引起突然严重破坏之前不易被察觉。虽然应力腐蚀开裂在泵中相对比较少见,但它可能在几类材料中发生。 泵的设计者应该警惕可能引起应力腐蚀开裂的材料和环境的组合。 应力腐蚀要求几个因素的存在。这些因素包括拉伸应力,它既可以是残余的拉引力也可以加载的,敏感材料,足够导致应力腐蚀的环境和时间。 在水泵行业有应用且可能导致应力腐蚀开裂的材料包括奥氏体和马氏体不锈钢, 某些铜基合金, 偶尔会有镍铜铬耐蚀铸铁。 奥氏体不锈钢在水溶性氯化物中,当温度超过 140F(60C)时易于发生应力腐蚀。铸造合金在微观组织中包含部分铁素体,相对于锻造件来说,明显的更耐应力腐蚀。 开裂的可能性会在氯化物通过蒸发而浓缩的情况下增加。高残余应力,往往存在于焊接结构,也提高了开裂的可能性。 奥氏体不锈钢中镍含量的增加提高了其抵抗 SCC 的性能。 高品位镍,一般称为 20 合金,通常在最需要抵抗应力腐蚀的化学应用中使用。奥氏体不锈钢在泵体中的应力腐蚀开裂相对较少见。 马氏体不锈钢容易在硫化氢存在下开裂,通常被称为硫化物应力腐蚀开裂(SSC)。这些钢,特别是 CA-15 和 CA-6NM,通常应用在有硫化氢存在下的石油和炼油产业的泵中。 通过给这些材料一种特殊的热处理,使其可以降低硬度低于某一临界值的水平,从而避免腐蚀开裂。这也与材料的屈服强度有关。通常在文献中可以看到,应用在这些工况下的含铁材料应该硬度不大于 22 的钢筋混凝土或者屈服强度不高于 90000 磅/平方英寸(620MPa)。技术标准,包括 API 610 和NACE MR-01-75,在含硫化氢环境中,可用于指定为马氏体钢的相应要求。 图 4 在该泵剖面上的电流腐蚀是明显的。注意在不锈钢罩上连接的碳钢法兰的内径的高腐蚀速率。 表 1 流动海水中的腐蚀电位(813 ft/s, 5080F/2。44。0 m/s, 1026C)。伏:饱和甘汞电极作为参考电极 铜合金在氨的存在下易发生应力腐蚀开裂, 不同种类的青铜易受应力腐蚀开裂的耐受程度不同且相差特别大,而铝青铜是耐性是最强的。受污染的自然水域可能含有氨,因此,这些场合下青铜材质的泵通常不是一个很好的选择。 高强度锰青铜很容易在自然水域中开裂。 这种合金铸造叶轮已经被发现会遭受严重的侵蚀。铸件残余应力也足以引起开裂。这些合金由于在这些问题中的敏感性而不应该用于泵中。 镍铜铬耐蚀铸铁是含 15%至 20%镍的奥氏体铸铁。在大型的海水立式泵中通常应用这种材料。 经验表明, 它是倾向于应力腐蚀, 特别是在这些泵的扩散管部分,除非铸件进行了炉内的应力消除。这必须由买方指定,因为这不是国家材料标准的要求。 氢脆化氢脆化 氢损伤是一种环境辅助失效的形式, 这是氢和残余或外加拉伸应力的综合作用造成的。 特定的合金或组成合金的氢损伤表现在许多方面, 如开裂、 起泡、 氢化,或拉伸塑性的失效。总的来说,这些各种形式的损坏通常被称为作为氢脆化。 在泵中偶尔遇到由氢造成的损坏。一些电镀工艺,如镀铬,通常用于再次加工泵轴,会产生氢气。这种情况下氢可以进入金属表面。微观裂纹会发生于高强度钢中(屈服强度大于 90000 磅/平方英寸或 620 MPa)。过度研磨可以使低强度钢表面硬化,但会增加氢导致裂纹的几率。氢损伤产生的微观裂纹会形成应力集中,通过机械疲劳不断延伸失效从而产生灾难性的故障。这个问题可以在电镀前通过使用适当的研磨工艺加以避免。高强度钢在电镀后应该立即烘烤以去除氢。 在焊接过程中,也可以将氢引入金属中。为了避免与焊接相关的氢损伤,铁素体和马氏体钢应该用低氢电极焊接。涂层电极应根据制造商的指示,在使用前进行烘烤,以使其脱落水分,因为这是焊接中氢污染的主要来源。 微生物腐蚀微生物腐蚀 生物体可以在很多不同的环境中促进腐蚀。 在好氧和厌氧环境中都有多种生物有机体的生长。 由微生物活动引起的腐蚀最常发生在因泵较长时间停机而滞留在其中的水中。 在许多水域会发现硫酸盐还原菌。 它们将会形成粘稠的红色半球聚集物附着在铸铁或碳钢上。 这被称为结节。 如果把结节刮下来, 在结节下总会有一个碟形坑。坑内将包含一个湿的,黑色的沉淀。点蚀是由细菌分泌的硫酸的痕迹引起的。这种类型的腐蚀通常不会导致过早失效。 不锈钢遭受着许多更严重类型的微生物的腐蚀。一类金属离子浓缩/氧化微生物出现在精矿铁和锰的氯化物,这两者都是有效的腐蚀剂。这些细菌优先在奥氏体不锈钢焊缝形成附着,并能够在一个相对短的时间内造成严重的点蚀。这个问题已经在应用于盐水和淡水的各种设备中遇到了。 只有当焊缝开始泄漏时才发现这个问题。如果对留滞的水不管是淡水还是盐水长时间放任不管,采用不锈钢的焊接的泵就会受到这个问题的困扰。 在特定情况下杀菌剂可以用来减轻这个问题。 最后,生物有机体的衰变会产生硫化氢,对铜基合金的保护性氧化膜产生不利的影响。温暖的热带水域会强化生物活性,尤其是水停滞的条件下,可以影响青铜的腐蚀抗性以及降低会发生加速腐蚀地方的阈值速度。 在有可能有微生物活跃的地方以及有长时间停机可能的情况下, 青铜应该谨慎使用。 晶间腐蚀晶间腐蚀 这种不常见的腐蚀类型优先攻击的是材料的晶界。 这是由局部化学性质差异造成的, 如奥氏体不锈钢的铬缺乏区。 青铜合金易受这种类型的腐蚀,包括铝黄铜、硅青铜、熟铜、海事金属。晶间腐蚀所需要的两点是:一种致敏材料和腐蚀性介质,如海水。在焊接修复过程中,热处理过程中可能会出现增敏作用。当循环荷载时,这种类型的腐蚀往往会导致腐蚀辅助疲劳裂纹 300 系列奥氏体不锈钢的热处理不当会导致对晶间腐蚀的敏感。当不锈钢含有超过 0.03%的碳,并且温度在 800和 1550C(425 至 850之间)时,会发生致敏作用。 在这种温度下, 铬的碳化物沿晶界析出, 导致在相邻区域中的铬消耗。这些相邻区域的耐腐蚀性已经降低。奥氏体不锈钢含有约 16%至 18%铬。碳化铬周围区域的铬含量可以降到低于 12%个的范围内以保持一个钝化的状态。一个大的阴极(颗粒)和一个小的阳极(晶界)建立了一个原电池。在这种不希望的情况下, 腐蚀沿阳极晶界发生。腐蚀损伤程度取决于处在敏化温度范围内的时间长度。敏化的程度是碳含量的一种功能。碳含量越高,材料在这个范围内不发生致敏存在的时间越短。图 5 中的温度随时间变化的曲线图,说明了这一点。图 6中展示了一个不恰当热处理的填料箱盖的晶间腐蚀。 奥氏体不锈钢也可以在正常焊接过程中致敏化。 在焊接过错中必须非常小心以避开致敏范围,如果有需要,随后可以进行适当的焊后热处理。 通过一些方法可以避免或改正致敏: 将材料加热到足够高的温度,以溶解铬的碳化物,通常为 1900 至 2100F(1040 至 1150C),随后通过感光范围随后快速冷却。焊接区局部热处理不会使材料脱敏。 使用添加铌或钛的不锈钢。这两个元素将与碳捆绑在一起,从而防止铬的碳化物。 将碳含量降低到一个较低的水平(小于 0.03%)。碳含量越低,碳化铬产生所需要的时间越长。 当奥氏体不锈钢在泵行业中必须使用时, 材料通常应该用在可以预料到晶间腐蚀的地方,包括 304L,316L,CF-3,和 CF-3M。含 25%或更多铬的合金的晶间腐蚀是不用担心的。 汽蚀汽蚀 汽蚀本质上是一个机械的过程,尽管它与腐蚀协同作用而且通常被认为是其他形式的腐蚀。 汽蚀可以被定义为流体中由于蒸汽气泡的溃裂而引起的高压而造成的表层金属的脱落。当流体的局部压力降为蒸汽压力时,将会发生空化现象。在多级泵中,在叶轮入口的低压区中形成气泡,并被流到更高压力的地区,在那里他们溃破。 许多气泡可能会产生并在一个区域破裂, 产生许多高动能的微射流。由气泡溃破释放的能量以冲击载荷的形式被消耗在金属表面上。 如果保护氧化层缺失,这种情况是具有破坏性的,因为保护氧化层被破坏从而将金属直接暴露在腐蚀液中。 这种循环加载最终导致微观疲劳裂纹的形成。 这些裂缝的传播和交叉,导致从表面上金属的脱落和独特的海绵状或多孔外观的气蚀损坏。 图 7 展示了一个受汽蚀破坏的叶轮的例子。 图 5 18-8 不锈钢的由斯特劳斯试验确定的时间温度敏感曲线。 注意一个低碳等级的不锈钢 (0.03%)需要 5 至 10 小时的曝光,而一个标准级(0.08%)只需要几分钟的曝光时间。 图 6 一种填料箱盖的表面,由于敏化作用,有晶间腐蚀。颗粒在孔的内部也很明显 图 7 一个叶轮的汽蚀破坏,通过表面空化区域的多孔状显示出来。 尽管在设计和应用中应该尽各种努力去避免离心泵汽蚀的发生,但在容量低于最大额定效率容量的泵中实现时这个并不总是可行的。必须注意到的是,在低流量运行时,所要求的 NPSH 曲线通常是不足以抑制所有的空蚀破坏的。规定的汽蚀余量是满足流量特性曲线中的扬程、容量和效率所需要的汽蚀余量。在低流量时,应该考虑会有一定的汽蚀破坏。可以在低流量时抑制空化是不切实际的,因为最佳效率点的需求,它可能发生多次。因此,空蚀破坏成为选择叶轮材料时一个频繁被考虑的因素。 扬程超过 35 英尺(10.7m)的开放式混流叶轮在旋转叶片和静止的壳体的间隙之间特别容易产生汽蚀。这通常被称为叶尖侵蚀,是由于叶片和壳体之间间隙的空化涡流引起的。 在这种情况下提供足够的汽蚀余量去消除空化也是不切实际的。这种类型的泵的叶轮和壳体的任何评估都应包括对叶片尖端侵蚀可能性的考察。 近几年来泵行业一致认为材料的抗汽蚀性与它的硬度直接有关。 近几年又有了一个更深的理解, 并且已经促成了一类具有优异抗汽蚀性的非标准不锈钢的发展。 在二十世纪七十年代, 当发现有一定硬度的钴基合金能够形成一个很高的抗汽蚀破坏能力时,抗汽蚀性与硬度之间的关系在此时第一次被精密计算。抗汽蚀能力与当遭受汽蚀时,材料表面向更硬、更具有抗性的晶相阶段的转变能力有关。这个过程和奥氏体不锈钢有关, 奥氏体不锈钢的化学成分在空化作用下的应力会诱发马氏体的形成。 最初的新合金被开发出来作为焊接填充金属修复气蚀损坏而使用, 后来作为泵的叶轮铸件的材料。这些合金在退火情况下具有比标准的奥氏体等级相对低的硬度, 但当其暴露在空化载荷下在表面会转变成一个更硬的马氏体。 硬表层会抵抗疲劳裂纹的产生。如果材料持续暴露于空化气泡中最终形成了裂纹, 裂纹想传播到软韧性基底金属也是困难的。抗蚀性奥氏体不锈钢铸件与铬和锰的合金,有与钴基合金相似的抗蚀性机理。 大量材料空化腐蚀抗性的实验测试已经为所以常用于离心泵部件中的材料提供了一套数据。 可以在实验室数据和现场经验之间做出一个好的相关性联系以制定出泵材料抗空蚀性能特性列表,表格中的材料按抗空化性能逐渐降低排列: 钨铬钴合金 铬锰奥氏体不锈钢 渗碳 12%铬不锈钢铸件 钛合金 6Al - 4V 铸造镍铝青铜 铸造双相不锈钢 铸造沉淀硬化不锈钢 球墨铸铁耐蚀镍合金 铸造 cf-8m 铸造 CA6-NM 铸造 CA-15 蒙乃尔合金 锰青铜 碳钢(铸) 铅青铜 铸铁 选择合适的抗空蚀材料,将使泵设计人员在泵可以操作的条件下,在工况范围内留有更大的操作余地。它还允许可以在更高的速度运行得更小、更轻的泵的设计。 明智地使用的材料能显著的延长汽蚀破坏造成的停机之间的时间,可以大大降低维护成本。 磨损类型磨损类型 旋转设备,包括泵,会遭受与腐蚀无关的机械损伤。当这些组件接触到另一个组件时,它们之间彼此接近的相对运动会产生磨损。如果在高载荷条件下或当异物被包裹在旋转和固定部件之间,可能会出现灾难性的损伤。这些组件材料的加速损失或部件严重性的脱落,可以导致昂贵的维修或更换费用。侵蚀,由于在液体中的固体颗粒的存在,也会限制泵内部组件的寿命。 磨损机理已被分为 20 多个单独的过程。然而,只有几个机理经常被认定为可以损伤一个泵: 粘着磨损:材料与材料接触 磨料磨损:与内部元件相互作用的固体 侵蚀:固体颗粒撞击 微动:引起氧化损伤的小幅度运动 确定磨损机制有时是困难的,因为泵内的磨损或材料的脱落,可能同时由一个以上的机制所导致。 摩擦和磨损的研究作为一门学科, 被称为摩擦学, 在二十世纪三十年代末开始。这些早期的研究加强了对磨损损伤机理的认识, 这种磨损损伤机理是除了腐蚀和材料的疲劳之外对泵寿命有影响的因素。在目前的行业杂志和文章中,可以找到关于磨损研究的许多资料。 粘着磨损粘着磨损 在一个介质为清水(没有夹带固体的液流)的泵中,旋转部件材料损失的一个主要的原因就是粘着磨损。这种材料的损失是由于材料表面接触产生破坏,材料出现缺口,材料转移,以及可能的磨损。考虑一对可能接触到的材料的两个重要特点是粘着磨损特性及其磨损阈值。 材料的磨损被认为是一个严重的粘着磨损的后果。 相对运动的两个表面的磨损是复杂的。除了粘着磨损模型之外,一些其他的滑动磨损理论还被提了出来。它们是分层理论,氧化理论,表面分层理论,疲劳模型,以及前面提到的几个理论的组合。然而,只有粘着磨损理论提供了一个一般的磨损方程来定量的预测磨损, 从而提供了一种以它们磨损特点为手段的材料分类方法。 存在很多的粘着磨损试验,包括环块、销钉和 V 形块,4-球,和钉在圆盘上。进行磨损试验,以筛选潜在用途的材料组合。因此,磨损试验的目的是模拟,尽可能接近实际的工作条件和参数。 粘着磨损条件下,材料的磨损试验得出了几个概论,以便在那些可能在运行中经历接触的情况下保障材料的正确选择。由 EPRI 支持,美国海军研究和私人企业的研发出了一系列被认为当接触发生时它们磨损的兼容性是可以接受的材料。根据测试,材料的硬度被认为是成功的运行组合的关键参数。当预计会有粘着磨损发生的工况下,下面的指导应该被用来选择材料: 1、 如材料在粘着磨损条件下预计不会良好运行(除专为 antigalling 电阻设 计的材料,如氮 60 和肖 88 材料)。 2、组合硬度低于 45 的钢筋混凝土需要硬度差别至少为 10 的钢筋混凝土。 3、组合硬度高于 45 的钢筋混凝土可以有相同的硬度。 在广泛的经验测试和现场经验的基础上,多年来,在选用泵口耐磨环材料时,已经开发出了一些经验丰富的经验法则。 有三个因素用来选择在干净液体环境中的磨损表面的材料: 流体的腐蚀性 允许磨损量 磨损应力 腐蚀决定了所使用的材料的类别。这些类一般分为三个分组:非腐蚀性,轻度腐蚀性和腐蚀性。当然,在腐蚀性材料组中选择一个正确材料时的附加约束需要通过实际经验来处理。 其他的材料特性,如添加剂,可以显著影响与粘着磨损和擦伤相关的性能。例如,添加铅的铜合金被认为是耐磨损合金因其为接触面之间铅能提供润滑性能。目前正在评估以取代含铅青铜合金的替代品,以避免使用铅而产生的健康问题。锡铋和镍基合金的添加也同样如此。 几种环境中材料的大致使用指南如下: 环境 材料 硬度 无腐蚀、轻度腐蚀 铸铁/加铅青铜的马氏体不锈钢(局部或者彻底变硬) 低于 45 钢筋混凝土时不重要,高于 45 钢筋混凝土时应有 10 个点的差别,相同硬度是接受的 腐蚀 抗腐蚀,无磨损的奥氏体不锈钢(氮 50/氮 60 或者 88/氮 50) 不适用 严重腐蚀 高合金奥氏体不锈钢和硬面材料,例如钨铬钴合金或者铬化硼系化合物 不适用 使用这些全行业公认的经验法则将有助于避免通常导致昂贵的维修的灾难性的损害
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