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AMAZON multi-meters discounts AMAZON oscilloscope discounts Abrasion Resistance Wear rates for chromium are substantially lower than for cast iron, as the data in Tables 7 and 8 show. The data were developed in a test of two engines run for nine hours under load, with one a cast iron cylinder and the other a porous-chrome surfaced cylinder. Allis-Chalmers Type B abrasive dust (85 percent below five microns, 100 percent below 15 microns) was added to the initial charge of crankcase oil, with a proportion of abrasive dust of 8.6 percent by weight of the lube oil, to accelerate wear. FIG. 10. Cylinder wear on chrome and cast-iron cylinders. The contrast in wear ratios between the cast iron and chromium in this test is substantial, reaching as much as four to one in the cylinder and three to one for the pistons and rings. FIG. 10 shows a plot of these data for the cylinder wear comparison. Often, the boring out of worn cylinders requires the deposition of extra thick layers of chrome to bring the surface back to standard size. This is not advisable, because on the next resalvaging, it may not be possible to bore down further into the basis metal and still retain enough structural strength in the liner wall to justify salvaging. With another proprietary process, 99.9 percent pure iron is electro deposited on the basis metal with a special bond, to build up the basis metal, to a thickness where normal chrome layer thicknesses are practical. Chromium in Turbocharged Engines The operation of turbocharged engines involves the exaggeration of all the wear factors described in this section because the temperatures are higher, fuel and lube oil consumption are higher, the engine runs faster, and corrosive agents seem to be more active and destructive. Turbo charging, however, increases the horsepower of an engine from 10 to 25 percent. During the last decade, many stationary engines were retrofitted for turbocharging, and engines with liners not surfaced with chrome have had the chance to be upgraded. Just as an example, the high heat of turbocharged engines creates a lubrication problem with cast iron liner surfaces. Even microporosity in iron casting won’t retain oil under such high temperatures. The corresponding increase in wear factor effects will accelerate liner and piston ring wear and increase downtime. Special chromium, with variable porosity tailored to the operating characteristics of the engine, can make the difference between a productive engine installation and a liability. FIG. 11. Graphs of cylinder liner wear. Curves A and C refer to opposed piston engines and curves B, D, and E are for poppet valve engines. Curves D and E show results using chromium plated liners. Operating Verification In a detailed study assessing the conditions and circumstances influencing machinery maintenance on motor ships, Vacca10 plots the operating performance of several marine engine liners and arrives at a documented conclusion that chrome-plated liners show a wear rate that is less than half that of nonchrome-plated liners. The indirect result is considerable improvement in fuel economy and ship speed. FIG. 11 shows these data plots. Another application study emphasized the benefits of chrome-plating engine liners and was seen to have a direct effect on labor requirements and the workloading of engine room staffs. For more documented low wear rates, a study on engine liner performance by Dansk-Franske Dampskibsselskab of Copenhagen on one of their ships, the "Holland," produced some interesting statistics. All cylinder liners were preventive plated with chromium before they were installed. The results well repaid the effort, in less overhaul, reduced ring wear, and extremely low cylinder wear. The highest wear rates on the six cylinder liners were 0.20mm/10,000 hours, as shown on the chart in FIG. 12. This negligible wear led to the conclusion that the liners: "...will still have a life of more than 10,000 hours...In fact, it means that this ship will never need any liner replacement."11 Even though these studies represented only a fraction of the operating and test data that supports this contention, they indicate the considerable benefits in terms of cost-savings and long-lived performance that the use of chrome-plating can provide. The fact that the studies cited were performed on motor ships, in salt-water environments, where corrosion agents are more active than in stationary facilities, adds further emphasis to this position. The question of chrome plating economy has been raised and can be answered by an example. Chrome-plating offers a twofold economy. First, in the cost of restandardizing with chrome over the cost of a new liner, and second, the extended length of operating life of the plated liner, whether new or reconditioned. FIG. 12. Cylinder liner wear-with chrome plating. As stated in the Diesel Engineering Handbook: "A (chrome) plating... will cost 65 to 75 percent of the price of a new unplated cast iron liner, or 50 to 60 percent of the price of a new chrome-plated liner. It must be remembered that the plated liner will have three to five times the life of a new unplated liner."12 The significance of the last sentence in the quote is often overlooked. Even if the chrome-plating restandardsizing of the worn liner were 100 percent of the cost of a new unplated liner, a cost savings will be achieved because the replated liner will still last three to five times as long. At 100 percent, the replated liner is thus still only about 30 percent of the cost of all the new replacement liners that would be required to match its normal operating life. Conclusion Our principal conclusions can be summarized as follows. Directly or indirectly, all of the effects of the wear factors described in this section can be mitigated or eliminated completely with the use of special chromium-plating on cylinder liners, crankshafts, and piston rods. Whether the method of liner salvage is restandardsizing or oversize boring with oversize piston rings, or even with new liners and parts to be conditioned for long wear before going into service, proprietary chromium plating processes can add years of useful operating life in a continuing, cost effective solution to the problems of wear. On-Site Electroplating Techniques. Where parts cannot be moved to a plating work station, deposition of metal by the brush electroplating technique may be considered. This process serves the same varied functions that bath electroplating serves. Brush electroplating of machinery components is used for corrosion protection, wear resistance, improved solderability or brazing characteristics and the salvaging of worn or mis matched parts. Housed in a clean room, the equipment needed for the process is: 1. The power pack. 2. A lathe. 3. Plating tools. 4. Masking equipment and plating solutions. 5. Drip retrieval tray. 6. Pump to return solutions through a filter to the storage bath. 7. Trained operator. 8. Supply of clean water for rinsing parts between plating operations. Brush electroplating thickness in excess of 0.070 in. is generally more economic if done in a plating bath. Electrochemical metalizing, another form of electroplating, is a hybrid between electric arc welding and bath electroplating. It’s a portable system for adding metal to metal. As a special type of metalizing, the process is claimed to offer better adhesion, less porosity, and more precise thickness control than conventional flame spray or plasma types of metalizing. Unlike conventional metalizing or bulk welding, the base metal is not heated to high temperatures, thus avoiding thermal stresses. In the rebuilding of main bearing saddle caps-a typical application--one flexible lead is connected to a working tool or "stylus" of appropriate size and shape. The stylus serves as an anode, and is wrapped in an absorbent material. The absorbent is a vehicle for the aqueous metallic plating solution. Metal deposits rapidly onto the cathodic-negative charged-workpiece surface. Deposit rates of 0.002 in. per minute are typical. One repair shop uses multistep processes in which the prepared metal surface initially is built to approximate dimensions with a heavy build alkaline copper alloy solution. Then a hardened outer surface is created by depositing a tungsten alloy from a second solution. Not only engine saddle caps, but cylinder heads, crankcases, manifolds, engine blocks, crankshafts, and other machinery castings have been successfully repaired using the electrochemical metalizing process. The process has replaced conventional oxyacetylene high-heat bronze welding that was used to build new metal onto worn saddle caps. The high-heat welding associated with oxyacetylene spraying had disadvantages in terms of excessive machining time, metal waste, lost time in cool-down, and high temperature distortion of the workpiece. In field use, the hardness and durability of electrochemically metallized material appears to equal the original casting. In contrast to other metal rebuilding methods, flaking or cracking of parts rebuilt with the process has not been experienced. The following equipment is required for an electrochemical plating process: 1. The power pack and flexible leads. 2. Turning heads and assorted stylus tools. The turning head is a low speed reversible, variable speed rotational device for use in electro-chemically plating cylindrical components. It enables rotation of shafts, bearings and housings, so that either inside or outside diameters can be uniformly plated. 3. Handles and selected anodes. 4. Accessories such as cotton batting, wrapping material, stylus holders, evaporating dishes, solution pump, and tubing. 5. Selection of plating solutions from some 100 different primary metals or alloy solutions. 6. A trained operator. Hardening of Machinery Components. In trying to achieve improved wear resistance it would be well not to neglect proven traditional steel hardening methods. In surface hardening of alloy steels the core of a machinery part may be treated to produce a desired structure for machinability or a strength level of service, whereas the surface may be subsequently hardened for high strength and wear resistance. Flame hardening involves very rapid surface heating with a direct high temperature flame, followed by cooling at a suitable rate for hardening. The process utilizes a fuel gas plus air or oxygen for heating. Steels commonly flame hardened are of the medium, 0.30 to 0.60 percent carbon range with alloy suitable for the application. The quenching medium may be caustic, brine, water, oil, or air, as required. Normally quenchants are sprayed, but immersion quenching is used in some instances. To maintain uniformity of hardening, it’s necessary to use mechanical equipment to locate and time the application of heat, and to control the quench. As with conventional hardening, residual stresses may cause cracking if they are not immediately relieved by tempering. In some instances residual heat after quenching may be sufficient to satisfactorily relieve hardening stresses. As size dictates, either conventional furnace tempering or flame tempering may be used. With flame tempering, the heat is applied in a manner similar to that used for hardening but utilizing smaller flame heads with less heat output. Carburizing is one of the oldest heat treating processes. Evidence exists that in ancient times sword blades and primitive tools were made by carburization of low carbon wrought irons. Today, the process is a science whereby carbon is added to steel within desired limitations to a controlled amount and depth. Carburizing is usually, but not necessarily, performed on steels initially low in carbon. If selective or local case hardening of a part is desired, it may be done in one of three ways: 1. Carburize only the areas to have a hardened case. 2. Remove the case from the areas desired to be soft, either before or after hardening. 3. Case carburize the entire surface, but harden only the desired areas. The first method is the most popular and can be applied to the greatest variety of work. Restricting the carburizing action to selective areas is usually done by means of a coating that the carburizing gas or liquid won’t penetrate. A copper plate deposited electrolytically, or certain commercial pastes generally prove satisfactory. The several methods employed in adding carbon come under the general classification of park carburizing, gas carburizing, and liquid carburizing. TBL. 9 Composition of Various Nitriding Steels Nitriding is a process for the case hardening of alloy steel in an atmosphere of ammonia gas and dissociated ammonia mixed in suitable pro portions. The steel used is of special composition, as seen in TBL. 9. The process is carried out at a temperature below the transformation range for steel and no quenching operation is involved unless optimum core properties are desired. Nitrided parts evidence desirable dimensional stability and are, therefore, adaptable to some types of close tolerance elevated temperature applications. The parts to be nitrided are placed in an airtight container and the nitriding atmosphere is supplied continuously while the temperature is raised and held at 900° to 1,150°F. A temperature range of 900° to 1,000°F is generally considered optimum to produce the best combination of hardness and penetration. The hardening reaction takes place when nitrogen from the ammonia diffuses into the steel and reacts with the nitride formers (aluminum, chromium, molybdenum, vanadium, and tungsten) to produce precipitates of alloy nitrides. Nitrogen is absorbed by the steel only in the atomic state, and there fore, it’s necessary to keep fresh ammonia surrounding the steel surfaces. This is accomplished by adequate flow rates and circulating the gases effectively within the container. The nitriding cycle is quite long depending upon the depth of case required. A 50 hour cycle will give approximately 0.021 in. case of which 0.005 to 0.007 in. exceeds 900 Vickers Diamond Pyramid hardness. The handling of nitrided steels in general is similar to that of any other alloy steel. However, due to their high aluminum content, these steels don’t flow as readily in forging as other alloy steels and, therefore, require some what greater pressures. Where large sections are encountered, normalizing prior to nitriding is recommended. To develop optimum core properties, nitriding steels must be quenched and tempered before nitriding. If the part is not properly heat treated and all traces of decarburization removed from the surface, nitrogen will penetrate along the ferrite grain boundaries and thereby produce a brittle case that has a tendency to fail by spalling. In tempering, the temperature must exceed the nitriding tempera ture; otherwise, significant distortion may result during the nitriding cycle. If a large amount of machining is to be done, it’s sometimes advisable to anneal, rough machine, heat treat, and finish machine. In very large parts, it’s advisable to stress relieve before final machining if the parts were rough machined in the heat treated condition. In all instances where machining is done after heat treatment, it’s important that sufficient surface be removed to ensure freedom from decarburization. Nitrided surfaces can be ground, but whenever possible this should be avoided. In nitriding, some growth does occur due to the increase in volume of the case. However, this is constant and predictable for a given part and cycle. Therefore, in most instances, parts are machined very close to final dimensions before nitriding. When necessary, lapping or honing is preferred to grinding because the extremely hard surface is shallow. If threads and fillets are to be protected or areas are to be machined after nitriding, an effective means of doing so is to tin plate those locations which are to remain soft. A 1 : 1 mixture of tin and lead is commonly used when electroplating is not possible. Since the nitriding temperatures exceed the melting point of the tin and tin alloys, it’s essential that an extremely thin coat be applied to prevent the coating from flowing onto surfaces other than those to be protected. Nitrided parts have a combination of properties that are desirable in many engineering applications. These properties include: 1. An exceptionally high surface hardness which is retained after heating to as high as 1,100°F. 2. Very superior wear resistance particularly for applications involving metal-to-metal wear. 3. Low tendency to gall and seize. 4. Minimum warpage or distortion and reduced finishing costs. 5. High resistance to fatigue. 6. Improved corrosion resistance. Here is a list of typical machinery applications: Bushings Piston Rods Cams Plungers Camshafts Pump Sleeves Connecting Rods Pump Shafts Crankshafts Push Rods Cylinder Barrels Racks and Pinions Cylinder Liners Ratchets Diesel Engine Fuel Retaining Rings Injector Pump Parts Seats and Valves Gears Shafts Guides Splines King Pins Sprockets Knuckle Pins Studs Needle Valves and Seats Thrust Washers Nozzles Timing Gears Pinions Thrust Washers Pinion Shafts Water Pump Shafts Piston Pins Wear Plates Pistons Wrenches FIG. 13. Principles of the diffusion alloy process. Diffusion Alloys.* Since carburizing dealt with earlier is, by definition, a diffusion alloying system, the primal history of diffusion alloys is quite lost in antiquity. But, we can state that the modern systems began during World War II in Germany when precious chromium was diffused into steel parts to form a stainless surface. Until recently, almost the sole beneficiaries of this work were gas turbine and rocket engine manufacturers. These engines make use of diffusion alloys resistant to high temperature oxidation and sulfidation. Now we are able to produce diffusion alloys tailored to specific industrial needs: Hardness, corrosion resistance, erosion resistance, and oxidation resistance, including combinations of these properties. Diffusion alloys can be produced on a wide spectrum of alloys, allowing interesting combinations of substrate properties and alloys optimized for cost, strength, or other considerations. Diffusion alloys are alloys and/or intermetallic compounds formed by the high temperature reaction of atoms at the surface of the part to be alloyed with atoms brought to that surface by a suitable process such as chemical vapor deposition (CVD). This is illustrated schematically in FIG. 13. Since diffusion alloy deposition is conducted at fairly high temperatures there is significant atom mobility for both alloy and substrate elements, i.e., diffusion of all atom species will occur. Properties of diffusion alloys are quite different from metals in many respects. In general they are single phase, but if multiple phases should exist, these are not intermingled but occur in layers. There are no grain boundaries, and grain boundaries that exist in the substrate disappear in the alloying. Although the ductility of the alloys is limited, they are not glass-brittle and will allow some plastic deformation of the substrate without cracking. Unlike overlay coatings such as plasma spray, there is no weak interface between the alloy and the substrate to sometimes fail under thermal shock or differential thermal expansion; dif fusion alloys are an integral part of the system. In reality then, they are not a "coating," but a conversion of the surface. How can a thin diffusion alloy prevent erosion? Nothing totally pre vents erosion, but erosion can be slowed by a diffusion alloy. As pointed out previously, this is a single phase system. In a hardened metal, the hard precipitate is slowly eroded, but the soft matrix in which it’s held erodes very quickly. As soon as the support for the hard particle is worn away, the particle simply drops off. By producing a hard, single phase system on the surface, there is no soft matrix to erode, and a much slower erosion rate results. This rate is low enough so that increases in life of 3 to 30 times are common. Tungsten carbide and diffusion alloying. There are a number of advantages to diffusion alloying tungsten carbide. Like hardened metals, tungsten carbide is a two-phase system and the matrix is readily eroded. Technology has developed a system that not only hardens the matrix, but reacts with the tungsten carbide particles to form an even harder material. Another advantage is realized by using carbides with higher binder content. The more erosion-resistant grades of tungsten carbide contain very little binder. This results, however, in an extremely brittle material, having low resistance to both thermal and mechanical shock. By utilizing a diffusion alloy with the higher binder carbides, the properties of the alloy are not impaired and a better structural part is produced. How does a diffusion alloy prevent wear? As described before, wear can be divided into two basic types, adhesive wear and rubbing wear. Adhesive wear usually occurs when two metals rub against each other under either very heavy pressure or extremely de-oxidizing conditions. In both cases metal migrates across the interface of the parts, resulting in an actual weld. Further movement tears a piece of the material from one or the other of the two parts. This is usually called seizing or galling. Again, the high bond strength between the atoms of an intermetallic compound prevents their migration across the interface with the mating part. When there is no migration there is no welding. How corrosion resistant are diffusion alloys? Different combinations of metals in the part and elements introduced by the process give differing results in corrosion. Generally, diffusion alloys are acid resistant, and various combinations will yield resistance to hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid. Oxidation resistance can be imparted to over 2,000°F. Most of the diffusion alloys are resistant to hydrogen sulfide and mercaptans. Diffusion alloys can be tailored for specific properties. An intermetallic compound behaves, chemically, very differently from those elements of which it’s composed. === TBL. 10 Hardness of Diffusion Alloys Surface Treatment Hardness (Vickers) Nitriding 600-950 Carbonitriding 700-820 Carburizing 700-820 Hard Chrome Plating 950-1,100 TMT-5 Steel 1,600-2,000 TMT-5 WC 2,200-2,350 TMT-5 Molybdenum 2,900-3,100=== Application. Diffusion alloys build with remarkable uniformity, following each asperity of the original surface. Total alloy thickness variation on a part is normally 0.0001 to 0.0002 in. If the original surface is eight microinches rms (root mean square), then the surface of the diffusion alloyed part will also be eight rms. Finer finishes require slight lapping. Hardnesses of diffusion alloys are shown in TBL. 10. How brittle is such a hard material? Although the hard diffusion alloys cannot stand extensive elongation, they are sufficiently ductile, For example, to allow straightening of shafts which have been heat treated following diffusion alloying. As long as the plastic deformation is below about five percent, the alloy won’t crack. Equally, thermal and mechanical shock don’t have any effect. Unlike the "stuck on" coatings, thermal differentials don’t load an interface in shear. There is no true interface to load. What metals can be diffusion alloyed? Almost any alloy of iron, nickel, or cobalt can be diffusion alloyed. Naturally, some alloys are preferred for specific systems, but the general rule holds. Aluminum, copper, zinc, and cadmium cannot be diffusion alloyed. Tungsten molybdenum, niobium, and titanium can be diffusion alloyed. Is a high strength alloy affected by the high temperature process? In general, when a part is diffusion alloyed it’s in the annealed state. If high strength is required, the part is heat treated following alloying. With some simple precautions, the heat treating can be carried out in a normal manner. Can a diffusion alloy be formed in any shape? There are virtually no configuration restraints. Internal passages and blind holes pose no problems. The elements added are transported in a gaseous phase. Spray patterns or "line of sight" are not a part of the system. The following specific process machinery applications of diffusion alloys have been successfully implemented: 1. Pump impellers and casings in fluid catalytic cracking units suffering from erosion by catalytic fines. 2. Pump impellers and casings used in coking service. 3. Pump impellers in lime slurry service. 4. Steam turbine nozzles and blades. 5. Expander turbines in contaminated gas streams. TBL. 11 represents a more comprehensive overview of diffusion alloy applications. TBL. 11 Characteristics and Applications of Diffusion Alloys System Substrates Characteristics Applications Electro-Spark Deposition Coatings. Electro-spark deposition involves the transfer of minute molten droplets of the desired coating material from a contacting electrode to the surface of the part. At the completion of the spark-induced transfer, the droplet welds to the part. By careful control, usually by computer, these microwelds will overlap, yielding a complete new surface. Heat input to the part is extremely low, and the maximum temperature rise is just a few degrees above ambient. Some of the remarkable things that are being done by this process include carbide coatings on aluminum, carbide coatings on titanium, nickel or gold on aluminum, and nickel aluminide on steels, as well as the seemingly simple coating of stainless steel with stainless steel. Bond strengths equal those of the base components, rather than being limited to 10,000 or 12,000 psi. Thus, the ESD coatings will withstand bend tests, thermal shock, and mechanical shock that no other coating system can match. This process is now in use in critical nuclear reactor components and may well be the answer to some of the most difficult wear and corrosion problems facing us today. FIG. 14 illustrates Triboloy 700 applied by electro-spark deposition to seal ring surfaces to prevent fretting. FIG. 14. Triboloy 700 applied by Electro-Spark Deposition to seal ring surfaces to prevent fretting. High-Velocity Thermal Spray Coatings. These coatings are available for steam-turbine blading and other components. In a number of applications, high-velocity thermal spray systems have produced coatings that are equal to or better than D-Gun and high-energy plasma spray deposits when evaluated for bond strength, density, and oxide content. More specifically, the bond strength of tungsten carbide/cobalt coatings produced with the HV system has been measured at more than 12,000 psi on a grit-blasted surface. High-velocity thermal spray systems use high-velocity combustion exhaust gases to heat and propel metallic powder onto a workpiece, thereby producing a coating. The exhaust is produced by internal combustion of oxygen and a fuel gas. Propylene, MAPP, and hydrogen have all been used as fuels with propylene being the recommended fuel. The combustion temperature is approximately 5,000°F, with exhaust velocities of 4,500 ft/sec, more than four times the speed of sound. The powder particles are introduced axially into the center of the exhaust jet. When the powder particles (hot and possessing high kinetic energy) hit a solid workpiece, they are deformed and quenched. The resulting coatings exhibit high bond strength and density and are exceedingly smooth. TBL. 12 highlights the characteristics and principal applications for high-velocity thermal sprays. TBL. 12 Characteristics and Applications of High-Velocity Thermal Sprays Description Characteristics Application Other Coatings for Machinery Components. There are many proprietary coating processes that can be applied to machinery components either in a restorative or preventive manner. These coatings may be used for ser vices in moderate wear and corrosion environments but also in applications where metal to metal contact is made and the danger of galling of the two surfaces exists. The iron-manganese-phosphate bath process is a typical example. The use of this process is especially indicated for cams, rollers, and gears. The iron-manganese-phosphate process adds from 0.0002 to 0.0003 in. to the surface of the workpiece. The process specification calls for the cleaning of the workpiece to be coated, preheating in a water bath to 200°F and immersion into the iron-manganese-phosphate bath until all reaction stops. The piece is then rinsed and immersed in a hot solution of soluble oil and colloidal graphite. It’s finally wiped and dried thoroughly. Hard coating treatment of aluminum alloys (anodizing) is a process that increases surface hardness and abrasion and corrosion resistance of aluminum and aluminum alloys. This is accomplished by formation of a dense aluminum oxide in a suitable electrolyte. Coating thicknesses range from 0.0015 to 0.0025 in. Typical applications are the coating of reciprocating compressor pistons and centrifugal compressor labyrinths. Application of thin films of Teflon® to metals. These films often have the advantage over other dry lubricants by producing very low coefficients of friction. The operable temperature range of thin Teflon lubricating coatings is from -80°F to 550°F. These coatings have also been successfully applied for corrosion protection of machinery parts. Teflon® coatings are formed by baking an air-dried coating deposited from an aqueous dispersion. Several aqueous suspensions are available. Application methods are well defined. Film thickness ranges between 0.0002 and 0.0003 in. for most lubrication applications. For corrosion protection, multiple coatings are applied for a final thickness of 0.0015 to 0.003 in. Fluoropolymer (Teflon) Infusion Process. This process entails the infusion of a friction-reducing fluoropolymer into the surface of machinery parts. The process does not result in a surface coating, although an initial coating of 0.005 in. is provided after treatment. Since the infusion process is not a rebuilding process, the workpiece must be serviceable before treatment. The process has been applied to steam turbine trip valve stems, pump plungers, compressor sliding parts, shafts, and bushings. The treatment adds built-in lubrication and corrosion resistance but does not harden the original surface. TBL. 13 Elastomer Preference by Application DYNAMIC -- STATIC NUTRILE (B, C, OR D) NITRILE (B, C, OR D) ETHYLENE-PROPYLENE (E) ETHYLENE-PROPYLENE (E) SBR (G) NEOPRENE (N) FLUOROCARBON (V) SBR (G) NEOPRENE (N) SILICONE (S) PHOSPHONITRILIC FLUOROCARBON (V) FLUOROELASTOMER (Q) POLYURETHANE (U) POLYACRVLATE (L) POLYACRYLATE (L) FLUOROSILICONE (F) BUTYL (J) POLYURETHANE (U) EPICHLOROHYDRIN (Z) BUTYL (J) PHOSPHONITRILIC FLUORDELASTOMER (Q) EPICHLOROHYDRIN (Z) POLYSULFIDE (K) CHLOROSULFONATED POLYETHYLENE (H) . Concluding Comments on Coatings and Procedures TBL. 14 Elastomer Temperature Ranges Recall that the coatings and compositions given in this text are representative of typical industry practices and availabilities. There are hundreds of variations and proprietary formulations. Users are encouraged to seek out experienced vendors, i.e., vendors that use proven materials and application processes. Refer to Sub-section 10-A For examples. Selection and Application of O-Rings‡ In hydrocarbon processing plants, mechanical seals for pumps and compressors, tube fittings and pipe flanges often use O-rings to prevent fluid flow or leakage. According to application, O-rings can be categorized as static (seal between flange facings) and dynamic (subjected to movement or wobble). TBL. 13 lists the commonly available O-ring materials in decreasing order of preference based on an overall desirability for O-ring sealing service, with cost and availability considered secondary. When following the design steps results in several candidate elastomers for a specific application, this table may be used for final selection. (Letter suffixes identify elastomers compound designations.) Next, the user has to consider temperature limitations of the elastomers. Here TBL. 14 will be helpful. Chemical compatibility of O-rings with a process fluid and temperature limits will define the method of O-ring production, using full-circle molding, ambient adhesive bonding and hot bonding or vulcanizing. Having no joint and hence no weak point, full-circle molded O-rings are the most common for reliability in operation. Available in a wide range of stock sizes and materials, O-rings of this type also can be custom molded. Ambient adhesive-bonded O-rings of any diameter can be quickly and easily made, using cord stock of most materials except silicone rubber. A simple jig used for cutting square ends and aligning them for bonding gives a smooth joint, which can sometimes be made in place without machine disassembly. Vulcanizing is considered to be an intermediate method in terms of non-stock O-ring delivery, chemical, and temperature resistance. Thermal and chemical resistance of the hot bonded O-rings is superior to the adhesive-bonded, but inferior to the molded ones. O-ring failure analysis can be instructive to prevent machine failure. There are several common causes of O-ring failure. Deterioration in Storage. Some synthetic rubbers such as neoprene and Buna N are sensitive to ultraviolet radiation, others to heat and ozone. O rings should therefore not be exposed to temperatures above 120°F (49°C) or air, light, ozone, and radiation generating electrical devices. Generally, storing O-rings in polyethylene bags inside larger cardboard boxes under normal warehouse conditions will ensure maximum storage life. Temperature. Exceeding an allowable temperature is a common cause of O-ring failure. Many O-rings fail from overheating because they are deprived of lubricant and/or coolant. Others are unable to recover from compression set at a high temperature, and they remain "flattened out," so to speak. Yet others fail because of overheating or chemical attack. TBL. 14 lists temperature limits for common O-ring materials. These limits can often be exceeded for short periods. Process fluid temperature may not be equal to O-ring temperature. A cool flush may reduce O-ring temperature, as may heat dissipation through a barrier structure. On the other hand, localized frictional heating may increase the O-ring temperature. If high or low temperatures are suspected of causing failure, it may be practical to change the environmental temperature or the O-ring material. In extreme cases, an all-metal bellows seal may permit elimination of O-rings. Mechanical Damage. Tearing, pinching, foreign matter embedment, dry rubbing and various other mechanical damage can occur during installation, operation, and removal. A sharp steel tool used for O-ring removal can scratch the groove or sealing surface and cause leakage. Brass, wood or plastic tools can be used without risk of scratching. Removal and installation instructions are shown in FIG. 15. Chemical Attack. Tables 10-15 and 10-16 are useful in selecting O-ring materials compatible with various fluids. Experimental verification is sometimes worthwhile. Complications may occur because of different properties of supposedly identical O-rings produced by various manufacturers, Also, a loss of identification in storage and handling is possible. To minimize unpleasant surprises for critical services, consider discarding even new O-rings which come with new and rebuilt seal assemblies, and replace them with O-rings of known material from a single manufacturer whom you consider reliable. To check whether an unknown, used O-ring is Viton or something else, you can immerse it in carbon tetrachloride. If it sinks, it’s probably Viton; if it floats, it’s not. Kalrez® perfluoroelastomer would also sink, but this material is uncommon and usually identified by tagging or other means. FIG. 15. O-ring removal and installation instructions. TBL. 16 Elastomer Capabilities Guide What Makes an O-ring O-rings are manufactured from a variety of elastomers which are blended to form compounds. These compounds exhibit unique properties such as resistance to certain fluids, temperature extremes, and life. The following section describes the most prominent elastomers and their inherent properties. Nitrite, Buna N, or NBR. Nitrile is the most widely used elastomer in the seal industry. The popularity of nitrile is due to its excellent resistance to petroleum products and its ability to be compounded for service over a temperature range of -67° to 257°F (-55°C to 125°C). Nitrile is a copolymer of butadiene and acrylonitrile. Variation in pro portions of these polymers is possible to accommodate specific requirements. An increase in acrylonitrile content increases resistance to heat plus petroleum base oils and fuels but decreases low temperature flexibility. Military AN and MS O-ring specifications require nitrile compounds with low acrylonitrile content to ensure low temperature performance. Nitrile provides excellent compression set, tear, and abrasion resistance. The major limiting properties of nitrile are its poor ozone and weather resistance and moderate heat resistance. Advantages: • Good balance of desirable properties • Excellent oil and fuel resistance • Good water resistance Disadvantages: • Poor weather resistance • Moderate heat resistance Ethylene-Propylene, EP, EPT, or EPDM. Ethylene-propylene compounds are used frequently to seal phosphate ester fire resistant hydraulic fluids such as Skydrol. They are also effective in brake systems, and for sealing hot water and steam. Ethylene-propylene compounds have good resistance to mild acids, alkalis, silicone oils and greases, ketones, and alcohols. They are not recommended for petroleum oils or diester lubricants. Ethylene-propylene has a temperature range of -67°F to 302°F (-55°C to 150°C). It’s compatible with polar fluids that adversely affect other elastomers. Advantages: • Excellent weather resistance • Good low temperature flexibility • Excellent chemical resistance • Good heat resistance Disadvantage: • Poor petroleum oil and solvent resistance Chloroprene, Neoprene, or CR. Neoprene is a polymer of chlorobutadiene and is unusual in that it’s moderately resistant to both petroleum oils and weather (ozone, sunlight, oxygen). This qualifies neoprene for O-ring service where many other elastomers would not be satisfactory. It’s also used extensively for sealing refrigeration fluids. Neoprene has good compression set characteristics and a temperature range of -57°F to 284°F (-55°C to 140°C). Advantages: • Moderate weather resistance • Moderate oil resistance • Versatile Disadvantage: • Moderate solvent and water resistance Fluorocarbon, Viton, Fluorel, or FKM. Fluorocarbon combines more resistance to a broader range of chemicals than any of the other elastomers. It constitutes the closest available approach to the universal O-ring elastomer. Although most fluorocarbon compounds become quite hard at temperatures below -4°F (-20°C), they don’t easily fracture, and are thus serviceable at much lower temperatures. Fluorocarbon com pounds provide a continuous 437°F (225°C) high temperature capability. Advantages: • Excellent chemical resistance • Excellent heat resistance • Good mechanical properties • Good compression set resistance Disadvantage: • Fair low temperature resistance Silicone or PVMQ. Silicone is a semi-organic elastomer with outstanding resistance to extremes of temperature. Specially compounded, it can provide reliable service at temperatures as low as -175°F (-115°C) to as high as 482°F (250°C) continuously. Silicone also has good resistance to compression set. Low physical strength and abrasion resistance combined with high friction limit silicone to static seals. Silicone is used primarily for dry heat static seals. Although it swells considerably in petroleum lubricants, this is not detrimental in most static sealing applications. Advantages: • Excellent at temperature extremes • Excellent compression set resistance Disadvantages: • Poor physical strength Fluorosilicone or FVMQ. Fluorosilicones combine most of the attributes of silicone with resistance to petroleum oils and hydrocarbon fuels. Low physical strength and abrasion resistance combined with high friction limit fluorosilicone to static seals. Fluorosilicones are used primarily in aircraft fuel systems over a temperature range of -85°F to 347°F (-65°C to 175°C). Advantages: • Excellent at temperature extremes • Good resistance to petroleum oils and fuels • Good compression set resistance Disadvantage: • Poor physical strength Styrene-Butadiene or SBR. Styrene-butadiene compounds have properties similar to those of natural rubber and are primarily used in the manufacture of tires. Their use in O-rings has been mostly in automobile brake systems and plumbing. Ethylene-propylene, a more recent development, is gradually replacing styrene-butadiene in brake service. Temperature range is -67°F to 212°F (-55°C to 100°C). Advantages: • Good resistance to brake fluids • Good resistance to water Disadvantages: • Poor weather resistance • Poor petroleum oil and solvent resistance Polyacrylate or ACM. Polyacrylate compounds retain their properties when sealing petroleum oils at continuous temperatures as high as 347°F (175°C). Polyacrylate O-rings are used extensively in automotive trans missions and other automotive applications. They provide some of the attributes of fluorocarbon O-rings. A recent variation, ethylene-acrylate, provides improved low temperature characteristics with some sacrifice in hot oil resistance. Advantages: • Excellent resistance to petroleum oils • Excellent weather resistance Disadvantages: • Fair low temperature properties • Fair to poor water resistance • Fair compression set resistance Polyurethane, AU, or EU. Polyurethane compounds exhibit outstanding tensile strength and abrasion resistance in comparison with other elastomers. Fluid compatibility is similar to that of nitrile at temperatures up to 158°F (70°C). At higher temperatures, polyurethane has a tendency to soften and lose both strength and fluid resistance advantages over other elastomers. Some types are readily damaged by water, even high humidity. Polyurethane seals offer outstanding performance in high pressure hydraulic systems with abrasive contamination, high shock loads, and related adverse conditions provided temperature is below l58°F (70°C). Advantages: • Excellent strength and abrasion resistance • Good resistance to petroleum oils • Good weather resistance Disadvantages: • Poor resistance to water • Poor high temperature capabilities Butyl or IIR. Butyl is a copolymer of isobutylene and isoprene. It has largely been replaced by ethylene-propylene for O-ring usage. Butyl is resistant to the same fluid types as ethylene-propylene and, except for resistance to gas permeation, it’s somewhat inferior to ethylene propylene for O-ring service. Temperature range is -67°F to 212°F (-55°C to 100°C). Advantages: • Excellent weather resistance • Excellent gas permeation resistance Disadvantage: • Poor petroleum oil and fuel resistance Polysulfide, Thiokol, or T. Polysulfide was one of the first commercial synthetic elastomers. Although polysulfide compounds have limited O-ring usage, they are essential for applications involving combinations of ethers, ketones, and petroleum solvents used by the paint and insecticide industries. Temperature range is -67°F to 212°F (-55°C to 100°C). Disadvantages: • Poor high temperature capabilities • Poor mechanical strength • Poor resistance to compression set Chlorosulfonated Polyethylene, Hypalon, or CSM. Chlorosulfonated poly ethylene compounds demonstrate excellent resistance to oxygen, ozone, heat, and weathering. But their mechanical properties and compression set are inferior to most other elastomers, and they are seldom used to advantage as O-rings. Temperature range is -65°F to 257°F (-55°C to 125°C). Advantages: • Excellent resistance to weather • Good resistance to heat Disadvantages: • Poor tear and abrasion resistance • Poor resistance to compression set Epichlorohydrin, Hydrin, or ECO. Epichlorohydrin is a relatively recent development. Compounds of this elastomer provide excellent resistance to fuels and oils plus a broader temperature range, -65°F to 275°F (-55°C to 135°C), than nitrile. Initial usage has been in military aircraft where the particular advantages of epichlorohydrin over nitrile are of immediate benefit. Advantages: • Excellent oil and fuel resistance • Excellent weather resistance • Good low temperature resistance Disadvantage: • Fair resistance to compression set Phosphonitrilic Fluoroelastomer, Polyphosphazene, PNF, or PZ. This is another new elastomer family. O-rings of phosphonitrilic fluoroelastomer are rapidly accommodating aircraft sealing requirements where the physical strength of fluorosilicone is inadequate. In other regards, the functional characteristics of phosphonitrilic fluoroelastomer and fluorosilicone are similar. Temperature range is -85°F to 347°F (-65°C to 175°C). Advantages: • Excellent oil and fuel resistance • Wide temperature range • Good compression set resistance Disadvantage: • Poor water resistance TBL. 17 UTEX HTCR® Fluororubber. Typical of many recent elastomeric com pounds, this copolymer of tetrafluoroethylene and propylene is too new to be on most charts. In application range, it fits somewhere between fluorocarbon (Viton) and Kalrez®. HTRC is thermally stable for continuous use in temperatures of 450°F, and depending on the specific application, has serviceability in environments up to 550°F. The US manufacturer, UTEX, claims excellent resistance to a wide variety of chemical environments. TBL. 17 provides an indication of its chemical resistance. Since temperature, concentration, mixtures and elastomer compound selection can affect performance, this chart provides guidelines only. Perfluoroelastomer (Kalrez®). Kalrez® O-rings have mechanical properties similar to other fluorinated elastomers but exhibit greater heat resistance and chemical inertness. They have thermal, chemical resistance, and electrical properties similar to Teflon® fluorocarbon resins but, made from a true elastomer, possess excellent resistance to creep and set. Generally, Kalrez O-rings are capable of providing continuous service at temperatures of 500°-550°F (260°-288°C) and can operate at 600°F (316°C) for shorter periods as long as they are in static service. For long term dynamic sealing duties, an operating temperature of 450°F (232°C) would be a reasonable limit. The chemical resistance of Kalrez® O-rings is outstanding. When using specially formulated compositions, little or no measurable effect is found in almost all chemicals, excepting fluorinated solvents which induce moderate swelling. The parts have excellent resistance to permeation by most chemicals. Resistance to attack is especially advantageous in hot, corrosive environments such as: • Polar solvents (ketones, esters, ethers) • Strong commercial solvents (tetrahydrofuran, dimethyl formamide, benzene) • Inorganic and organic acids (hydrochloric, nitric, sulfuric, trichloroacetic) and bases (hot caustic soda) • Strong oxidizing agents (dinitrogen tetroxide, fuming nitric acid) • Metal halogen compounds (titanium tetrachloride, diethylaluminum chloride) • Hot mercury/caustic soda • Chlorine, wet and dry • Inorganic salt solutions • Fuels (aviation gas, kerosene, JP-5, Jet Fuel, ASTM Reference Fuel C) • Hydraulic fluids, synthetics and transmission fluids • Heat transfer fluids • Oil well sour gas (methane/hydrogen sulfide/carbon dioxide/steam) • Steam TBL. 18 Gland Design Guide INCHES MILLIMETERS FIG. 16. Back-up rings used with O-rings. Back-Up Rings. Back-up rings, as shown in FIG. 16, are often used to prevent extrusion in high pressure applications, or to correct problems such as spiral failure or nibbling. They are sometimes used in normal pressure range applications to provide an added measure of protection or to prolong O-ring life. These devices also permit the use of a wider clearance gap when close tolerances are impossible to maintain. A back-up ring is simply a ring made from a material harder than the O-ring, designed to fit in the downstream side of the groove and close to the clearance gap to provide support for the O-ring. Quite often, O-rings are used as back-up rings, even though back-up rings don’t perform any sealing function. O-Ring, Back-Up Ring, and Gland Dimensions. O-ring sizes have been standardized and range in size from an inside diameter of 0.029 in. and a cross section of 0.040 in. to O-rings with an inside diameter of 16 or more in. and a cross section of 0.210 or more in. Installation dimensions vary with duty and application and the user may find it easy to consult manufacturers' catalogs, which are typically con figured as shown in FIG. 17. Note the small differences in gland dimensions. They depend on whether the O-ring will be axially squeezed, radially squeezed, or will perform dynamic piston and rod sealing duty. To calculate your own gland design, refer to TBL. 18, "Gland Design Guide." FIG. 17. A sampling of O-ring and gland dimensions. ++++ ++++ Sub-section 10-A Part Documentation Record TBL. A-1 Typical Part Documentation Record Sheet THE INTENT OF THIS FORM IS TO RECORD SPECIFIC PART CHARACTERISTICS THAT WILL BE USED FOR FUTURE EVALUATION. PART IDENTIFICATION _________________ UNIQUE ID ____________ CUSTOMER __________________ CUSTOMER P.O. _____________ INSPECTOR ______________________________ DATE ________________________ PST JOB NO. ______________________ DRAWING NO. _______________________ HARDNESS __________________ COATING TYPE _____________ OAL ________________ MAJOR DIAMETER _________________________________ LENGTH OF COATING FROM THE CROSSHEAD END OF THE SHAFT __________ LENGTH OF COATING _________ MAGNAFLUX-ACCEPTED/REJECTED COMMENTS __________________________ TBL. A-2 Coating Designations and Physical Properties of Materials Typically Used by Praxair Surface Technologies, Houston, Texas Physical Properties Average Metallographic Evaluation Typical Coating Designation TBL. A-4 Fusion High Velocity Oxygen Fuel (HVOF) Coating Procedure for Repair of Industrial Crankshafts Scope Listed on this document are approved standards to be used in the repair of a crankshaft when a customer specification, or no other specification, exists. A customer drawing or specification will always be used in place of these standards. The HVOF repair method will provide a much harder surface than original hardness of substrate, while offering the bond strength and optimum density of compressive coatings. This will allow resistance to wear and corrosion. It’s not intended to restore tensile or torsional strength. Diameter Tolerance: Rod journal diameter: +0/-0.001= Main journal diameter: +0/-0.001= Gear fits: +0/-0.001= Seal areas: +0/-0.001= Circular Runout Limits: Main journals: 0.002= total indicator reading (T.I.R.); crankshaft supported at each end. TBL. A-5 Documentation (Typical Only) Identifying Procedure Changes from a Previous Revision Fusion High Velocity Oxygen Fuel (HVOF) Coating Procedure for Repair of Industrial Crankshafts: Changes from Revision 5 to Revision 6 TBL. A-6 Repair Procedure for Piston Rods and Plungers Prev. | Next |