Machinery Components Maintenance And Repair Episode 2 Part 11 doc

cylinder, and under the constant sliding of adjacent metal parts, like piston rings. This phenomenon of oil retention is termed “wettability” and describes the dispersive characteristics of oil on a microscopically uneven metal surface. The oil collects in the recesses of the metal surface and disperses outward in an enveloping movement. In contrast, the surface tension of oil will cause an oil drop on a smooth plane surface to exhibit a tendency to reach a state of equilibrium where it will neither spread nor recede, and thus does not provide a lubricating coating for that surface. Such a surface inside a cylinder liner will not be adequately covered with an oil film and will require greater volumes of lube oil to achieve adequate protection from friction, temperatures, corrosion, and abrasion. This proprietary porous chrome surface prevents the action of the oil’s molecular cohesion in trying to achieve a perfect sphere, and form- ing into a drop. The configuration of the chrome surface disrupts this tendency. This porous chrome presents such a varied surface that a portion of the area 1 / 4 in. in diameter may contain from 50 to several hundred pores or crevices depending on the porosity pattern applied to the chrome surface. Even one drop of oil, encountering such a surface, tends to disperse itself indefinitely over the flats, downslopes of pores, and the upsloping side. The importance of lubrication has been discussed in numerous books, and a direct correlation between a successfully maintained oil film and wear on piston rings and liner surfaces can be shown. Thus, the ability of porous chrome surfaces to provide an unbroken oil film indicates its desir- ability in preventing many types of liner wear, including gas erosion, which is due to a leaky piston ring seal; friction and frictional oxidation, by protecting the surface from oxygen in the combustion area; preventing metal stresses resulting in abrasion from excessive loading, which does not break the oil film maintained by the chrome; and by protecting the surface from corrosive agents produced by lube oil breakdown and com- bustion products. The load capacity of porous chrome involves a condition known as boundary-layer lubrication. This term refers to an oil film thickness that is so thin it approaches the characteristics of dry lubrication. It has lost its mobility as a fluid, but reduces the mutual attraction of adjacent, sliding metallic surfaces, and thereby the friction. Fluid lubrication, e.g., thicker layers of lube oil, are not desired under the high-temperature conditions of the combustion area of the cylinder, because of the sus- ceptibility of lube oil to flash point combustion, breakdown into deposits, unnecessarily high lube oil consumption, and the production of air pollutants. Protecting Machinery Parts Against Loss of Surface 565 566 Machinery Component Maintenance and Repair Thermal Conductivity Thermal conductivity in chromium is higher than for cast iron and com- monly used steels, by approximately 40 percent, as shown in Table 10-6. Maximum metal surface temperatures in the cylinder are at the liner surface, especially in the combustion zone, and any improvement in heat transfer provides a lower wall temperature and will improve piston and ring lubrication. The heat reflection qualities of chromium add to the com- bustion and exhaust temperatures, helping to reduce incomplete combus- tion and its products. While the coefficient of expansion, also shown in Table 10-6, of chromium is lower than that of cast iron or steel, there is a decided advan- tage in the difference. The surface of the cylinder liner has a much higher temperature than the underlying basis metal, because of that sharp tem- perature gradient through the wall. The effect of this gradient on a homogeneous metal, e.g., distortion, is eliminated with chromium plating, because it is desirable to have a variable coefficient of expansion ranging from a lower value at the inner wall surface to a higher value in the outer wall, where the coolants are operating. Tests run on air-cooled airplane engines for 700 to 1,000 hours showed no tendency of the chromium surfaces to loosen due to differential expansion between the chrome and the basis metal. This is significant, considering that these engines normally run at higher cylinder wall temperatures than engines in stationary installations. Table 10-6 Expansion Coefficients and Thermal Conductivity Thermal Linear thermal conductivity expansion cal/cm 2 /cm/ in./in. at 68°F deg C/sec at Metal (¥10 -6 ) 20°C Melting point, F° Chromium, electrolytic 4.5 0.165 3,325+ Cast iron 6.6 0.12 2,500 Steels 6.2–6.6 0.11 2,700–2,800 Aluminum 12–13 0.52 1,216 Copper 9.1 0.92 1,981 Protecting Machinery Parts Against Loss of Surface 567 Abrasion Resistance Wear rates for chromium are substantially lower than for cast iron, as the data in Tables 10-7 and 10-8 show. The data were developed in a test of two engines run for nine hours under load, with one a cast iron cylin- der 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 propor- tion of abrasive dust of 8.6 percent by weight of the lube oil, to acceler- ate wear. Table 10-7 Cylinder Wear Diametral wear, in. Wear ratio, Cast-iron Porous-chrome cast-iron cyl./ Location cylinder cylinder porous-chrome cyl. 1 / 8 in. below head end 0.0077 0.0018 4.3–1 1 / 2 in. below head end 0.0071 0.0021 3.4–1 2 in. below head end 0.0083 0 0022 3.8–1 Table 10-8 Piston and Ring Wear In porous-chrome In cast iron cylinder cylinder Wear ratio, Percent of Percent of cast-iron cyl./ original original porous-chrome Grams weight Grams weight cylinder Aluminum piston loss in weight 0.589 0.90 0.316 0.45 1.9 : 1 Top ring lost in weight 2.506 56.0 0.849 18.9 3.0 : 1 Second ring loss in weight 2.586 57.8 0.972 21.8 2.6 : 1 Oil ring loss in weight 1.260 46.5 0.865 31.9 1.5 : 1 568 Machinery Component Maintenance and Repair The constrast 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. Figure 10-10 shows a plot of these data for the cylinder wear comparison 9 . 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 Figure 10-10. Cylinder wear on chrome and cast-iron cylinders. Protecting Machinery Parts Against Loss of Surface 569 iron casting will not retain oil under such high temperatures. The corre- sponding increase in wear factor effects will accelerate liner and piston ring wear and increase downtime. Special chromium, with variable porosity tailored to the operating char- acteristics of the engine, can make the difference between a productive engine installation and a liability. Operating Verification In a detailed study assessing the conditions and circumstances influencing machinery maintenance on motor ships, Vacca 10 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. Figure 10-11 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 perfor- mance by Dansk-Franske Dampskibsselskab of Copenhagen on one of their ships, the “Holland,” produced some interesting statistics. All Figure 10-11. 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. 570 Machinery Component Maintenance and Repair 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 0.20 mm/10,000 hours, as shown on the chart in Figure 10-12. 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. Figure 10-12. Cylinder liner wear—with chrome plating. Protecting Machinery Parts Against Loss of Surface 571 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 func- tions 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. * Dalic Plating Process. 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 metallizing, another form of electroplating, is a hybrid between electric arc welding and bath electroplating. It is a portable system for adding metal to metal. As a special type of metallizing, the process is claimed to offer better adhesion, less porosity, and more precise thickness control than conventional flame spray or plasma types of met- allizing. Unlike conventional metallizing 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 appropri- ate 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 suc- cessfully repaired using the electrochemical metallizing 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 13 . The following equipment is required for an electrochemical plating process 14 : 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- 572 Machinery Component Maintenance and Repair 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 machin- ability or a strength level of service, whereas the surface may be subse- quently 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 quench- ing 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 is 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 resid- ual heat after quenching may be sufficient to satisfactorily relieve hard- ening 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 15 . Carburizing is one of the oldest heat treating processes. Evidence exists that in ancient times sword blades and primitive tools were made by car- burization 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: Protecting Machinery Parts Against Loss of Surface 573 574 Machinery Component Maintenance and Repair 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 will not penetrate. A copper plate deposited electrolytically, or certain commercial pastes gen- erally prove satisfactory. The several methods employed in adding carbon come under the general classification of park carburizing, gas carburiz- ing, and liquid carburizing 16 . Nitriding is a process for the case hardening of alloy steel in an atmos- phere of ammonia gas and dissociated ammonia mixed in suitable pro- portions. The steel used is of special composition, as seen in Table 10-9. 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 17 . The parts to be nitrided are placed in an airtight container and the nitrid- ing 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 hard- ness and penetration. The hardening reaction takes place when nitrogen from the ammonia diffuses into the steel and reacts with the nitride Table 10-9 Composition of Various Nitriding Steels 17 AISI 7140 AMS 6470E AMS 6425 135 Type G N EZ Carbon 0.38–0.43 0.21–0.26 0.30–0.40 0.20–0.27 0.30–0.40 Manganese 0.50–0.70 0.50–0.70 0.40–0.70 0.40–0.70 0.50–1.10 Silicon 0.20–0.40 0.20–0.40 0.20–0.40 0.20–0.40 0.20–0.40 Chromium 1.40–1.80 1.00–1.25 0.90–1.40 1.00–1.30 1.00–1.50 Aluminum 0.95–1.30 1.10–1.40 0.85–1.20 0.85–1.20 0.85–1.20 Molybdenum 0.30–0.40 0.20–0.30 0.15–0.30 0.20–0.30 0.15–0.25 Nickel — 3.25–3.75 — — — Selenium — — — — 0.15–0.25 [...]... 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 Table 10- 12 highlights the characteristics and principal applications for high-velocity thermal sprays Protecting Machinery Parts Against Loss of Surface 583 Table 10- 12 Characteristics and Applications... Oilfield machinery and chemical processing equipment Gas turbine components (Table continued on next page) 584 Machinery Component Maintenance and Repair Table 10- 12 Characteristics and Applications of High-Velocity Thermal Sprays—cont’d Description Characteristics Application Triboloy 400 Very high strength & good wear resistance Hardness 800 DPH Operating temperature to 1 ,20 0°F Gas turbine bleed air components. .. Carbide 3,800–4 ,20 0 KHN; Erosion and wear resistance Valves: Chokes; Seal rings; Dies; Blast tubes and joints; Orifice plates; Wear plates TMT -24 13 Aluminide Co or Ni bonded WC; Ni, Co, and Cr-Ni Stainless Corrosion and galling resistance Pump plungers; Piston rings; Shafts; Seal rings; Bearings; Threads Protecting Machinery Parts Against Loss of Surface 581 Table 10 -11 Characteristics and Applications... lubrication and corrosion resistance but does not harden the original surface19 ,20 Concluding Comments on Coatings and Procedures 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 * Impreglon® Process 586 Machinery Component Maintenance and Repair. .. high wear and impact resistance Operating temperature to 1 ,20 0°F Hardness >1 ,20 0 DPH Bond strength >25 ,000 psi Gate valves and mill rolls 80 Cr3C2, 20 Ni-Cr Excellent wear resistance to temperatures approaching 1,600°F (1,400°F continuous) Not recommended in corrosive environments Gas turbine hot section components ESD prepared substrate May be used with all TMT WC systems listed 73 WC, 20 CR, 7 NI... Applications TMT -28 13 Nickel Aluminide Carbon, Low-Alloy, Chrome and Cr-Ni steel Corrosion and galling resistance Piston rings; Bearings; Piping TMT-2l3 Chrome Aluminide Carbon and Low-Alloy Steels Galling resistance Bearings EC -114 Complex Aluminide Nickel and Iron-Base Alloys Friction and oxidation resistance Turbine hot section; Oil tools KS-138 Dispersed Phase Aluminide Nickel Alloys Corrosion and erosion... Steel TMT-5 WC TMT-5 Molybdenum Hardness (Vickers) 600–950 700– 820 700– 820 950–1,100 1,600 2, 000 2, 200 2, 350 2, 900–3,100 diffusion alloyed part will also be eight rms Finer finishes require slight lapping Hardnesses of diffusion alloys are shown in Table 10-10 How brittle is such a hard material? Although the hard diffusion alloys cannot stand extensive elongation, they are sufficiently ductile, for example,... engineering applications These properties include: 576 Machinery Component Maintenance and Repair 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... 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 Machinery Component Maintenance and Repair. .. metal-to-metal wear, galling and corrosion to 1,500°F Hardness 600 DPH Gas turbine components Extrusion dies Piston rings Haynes STELLITE 6 High resistance to particle erosion, abrasive wear, and fretting to 1,500°F Hardness 490 DPH Valve and pump components Exhaust valves and seats, conveyor screws, hot crushing rolls Hastelloy C Excellent corrosion resistance Good metal-to-metal wear and abrasion resistance . electrolytic 4.5 0.165 3, 325 + Cast iron 6.6 0. 12 2,500 Steels 6 .2 6.6 0 .11 2, 700 2, 800 Aluminum 12 13 0. 52 1 ,21 6 Copper 9.1 0. 92 1,981 Protecting Machinery Parts Against Loss of Surface 567 Abrasion. 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, 900–3,100 580 Machinery Component Maintenance and Repair 2. Pump impellers. 6 425 135 Type G N EZ Carbon 0.38–0.43 0 .21 –0 .26 0.30–0.40 0 .20 –0 .27 0.30–0.40 Manganese 0.50–0.70 0.50–0.70 0.40–0.70 0.40–0.70 0.50–1.10 Silicon 0 .20 –0.40 0 .20 –0.40 0 .20 –0.40 0 .20 –0.40 0 .20 –0.40 Chromium