550 Machinery Component Maintenance and Repair • Ceramics are the preferred surfacings for packing sleeves, seals, pump impellers, and similar systems involving no shock, but with severe low-stress abrasion These surfacings should not be run against themselves without prior compatibility testing. Table 10-2 lists specific alloys likely to give exceptionally good per- formance, based on the tests summarized in Figures 10-5 and 10-6. Table 10-2 Hard-Surfacing Selection Guide (Typical Only) Deposition Surfacing Form Process Characteristics Uses Chromium Powder Plasma Excellent resistance Low Stress Oxide Spray to very low stress Erosion abrasion. Thickness 5–40 mils. Can be ground to very good finish. No welding distortion. (>HRC 70) AISI 431 Powder Metallize Good adhesive wear Fretting, Stainless resistance when Galling Steel lubricated. Poor abrasion resistance. Can be ground to good finish. No welding distortion. (HRC 35) NiCr-4 Powder Metallize Good adhesive wear Metal-to-Metal and Fuse resistance; corrosion Wear, Galling, resistant. Coating Seizure, thickness to 0.125 in. Cavitation, with fusion bond. Erosion, Distortion may occur Impingement, in fusing, but Brinelling application is faster than oxyacetylene rod surfacing. FeCr-1 Electrode Shielded Moderate resistance Low Stress, High (Iron- Metal- to low stress abrasion Stress, Cylinder Chromium) Arc and adhesive wear. Can and Ball Rolling Welding be easily finished by grinding. Low cost. (HRC 50) Protecting Machinery Parts Against Loss of Surface 551 Table 10-2 Hard-Surfacing Selection Guide (Typical Only)—cont’d Deposition Surfacing Form Process Characteristics Uses FeCr-5 Electrode Shielded Very good resistance Low Stress, (Iron- Metal to low stress abrasive Filing, Chromium Arc wear. Easy to apply Impingement + TiC) Welding (HRC 60) Co-1 Rod Oxyacetylene Very good resistance Metal-to-Metal (Cobalt- to adhesive wear. Wear, Galling, Chromium) Moderate resistance Seizure, Fretting, to low stress abrasion. Cavitation, High Corrosion resistant. Velocity Liquid, The alloy is expensive Erosion, and application is Brinelling slow. (HRC 43) Co-1C Electrode Shielded Good resistance to Metal-to-Metal (Cobalt- Metal- adhesive wear. Easy to Wear, Galling, Chromium) Arc apply, Suitable for all Seizure, Fretting, Welding position welding. The Cavitation alloy is expensive. (HRC 43) NiCr-4 Rod Oxyacetylene Good resistance to Metal-to-Metal metal wear. Machinable. Wear, Galling, Will not rust. Costly Seizure to apply. (HRC35) COM-1 Rod Oxyacetylene Excellent resistance to Low Stress, Filing, (Tungsten low stress abrasion. Use Impingement, Carbide- as deposited. Costly to Erosion Steel apply. (>HRC 60) Matrix) COM-3 Rod Oxyacetylene Excellent resistance to Low Stress, Filing, (Tungsten low stress abrasive Impingement, Carbide- wear. Use as deposited. High Velocity Cobalt- Corrosion resistant. Liquid Chromium Costly to apply. Matrix) (>HRC 60) 552 Machinery Component Maintenance and Repair The Detonation Gun Process* If we look at the repair of rotating machinery shaft bearings, journals, seal surfaces, and other critical areas in the context of hard-surfacing, it becomes apparent that there are numerous methods available. As we saw, one of these methods is by the use of detonation gun coatings. In review, the detonation gun is a device that can deposit a variety of metallic and ceramic coating materials at supersonic velocities onto a workpiece by controlled detonation of oxygen-acetylene gas mixtures. Coatings applied by this method are characterized by high bond strength, low porosity, and high modulus of rupture. Table 10-3 shows some of the physical proper- ties of detonation gun coatings. This section describes the equipment used to apply D-Gun coatings and provides data on coating thicknesses used, surface finish available, and physical properties of some popular D-Gun coatings used in machinery repair. Examples are cited showing increases in operating life that can be achieved on various pieces of equipment by properly selected and applied coatings. Shaft repairs on turbomachinery and other equipment can be accom- plished in many ways. Repair methods include weld deposit, sleeving, electroplated hard chromium, flame spraying, plasma arc spraying, and detonation gun coatings. Each of these methods has its own advantages and disadvantages. Again, factors such as time needed to make the repair, cost, machinability, surface hardness, wear resistance, corrosion resis- tance, material compatibility, friction factor, minimum or maximum allowable coating thickness, surface finish attainable, bond strength, coef- ficient of thermal expansion, coating porosity and the amount of thermal distortion from the repair; all have varying degrees of importance depend- ing on the particular application. In some cases, the repair method to be used is simply based on the availability of a shop in the area that can make the repair within the desired schedule. Sometimes compromise coatings or repair methods are selected. In other cases, a planned, scheduled and engineered solution is used to effect a repair that provides service life that is far superior to the original equipment. A properly chosen method of repair can provide improved durability of the repaired part over that of the original part with properties such as higher hardness, better surface finish, improved wear resistance and improved corrosion resistance. Properly chosen coatings can combine the favorable attributes of several materials, thus lessening the compromises that would have to be made if a single material was used. Equipment users have frequently found that repaired components have withstood service * A proprietary process of Praxair Surface Technologies. Protecting Machinery Parts Against Loss of Surface 553 Table 10-3 Physical Properties of Some Detonation Gun Coatings (UCAR D-Gun) COMMERCIAL DESIGNATION LW-1N30 LW-15 LW-5 LC-1C Nominal Composition 87WC, 13Co 86WC, 10Co, 73WC, 20Cr, 800r 3 C 2 , (Weight %) (a) 4Cr 7Ni l6Ni, 4Cr Tensile Bond >10,000 >10,000 >10,000 >10,000 Strength (psi) (b) Modulus of 90,000 40,000 70,000 Rupture (psi) Modulus of 31 ¥ 16 6 17 ¥ 10 6 18 ¥ 10 6 Elasticity (psi) Metallographic £1 £1.5 <1 £1 Apparent Porosity (Vol. %) Nominal Vickers 1,150 HV 1,100HV 1,100 HV 775HV Hardness (kg/mm 2 , 300 g load) Rockwell “C” 71 70 70 63 Hardness—Approx. Max. Rec. Operating 1,000 1,000 1,400 1,400 Temp. (°F) Avg. Coef. of 4.5 ¥ 10 -6 4.2 ¥ 10 -6 4.6 ¥ 10 -6 6.1 ¥ 10 -6 Thermal Expansion (70 to 1,000°F) (70 to 1,832°F) (70 to 1,400°F) (70 to 1,475°F) (in/in/°F) Characteristics Extreme Wear Good Wear Good Wear Excellent Wear Resistance Resistance to Resistance to Resistance at Approx. Approx. Elevated Temp. 1,000°F. 1,400°F. Greater Greater corrosion oxidation Resistance and corrosion than Resistance WC-Co. than WC-Co. (a) The composition shown represents the total chemical composition, but not the complex microstructural phases present. (b) Measured per ASTM C633-69 modified to use a reduced coating thickness of 10mils. ® UCAR is a registered trademark of the Union Carbide Corporation. 554 Machinery Component Maintenance and Repair better than the original equipment manufacturer’s components. This has led many users to specify specialized coatings on key components of new equipment being purchased. In some cases the use of coatings has led to reduced first cost of components since the special properties of coatings allow the use of lower-cost, less exotic base materials. Comparing repair prices to the purchase price of new parts, assuming that the new parts are available when needed, shows that the price of repaired parts may be only 1 / 5 to 1 / 2 that of new OEM parts. If the repair method eliminates the need for expensive disassembly such as rotor unstacking, the savings become even more dramatic. Coupling these savings with the frequently extended service life of the repaired parts over the original ones, which in turn extends periods between inspections and repairs, the coating repair of parts is extremely attractive from an eco- nomic standpoint. Process Details. In the following we will concentrate on the detonation gun process of coating which is often referred to as the D-Gun process. The system is shown again schematically in Figure 10-7. It consists of a water-cooled gun barrel, approximately three feet long, that is fed with oxygen, acetylene and coating powder. Ignition of the oxygen-acetylene mixture is accomplished by means of a spark plug. The detonation wave in the gun barrel, resulting from the ignition of the gas mixture, travels at ten times the speed of sound through the barrel, and temperatures reach or exceed 6,000°F inside the gun. Noise levels generated by the D-Gun require isolating the process in a noise-attenuating enclosure. The equip- ment operator monitors the coating operation from a control console while observing the operation through a view port. Detonation is cyclic, and sub- sequent to each detonation the barrel is purged with nitrogen before a fresh Figure 10-7. Detonation gun schematic. Protecting Machinery Parts Against Loss of Surface 555 charge of oxygen, acetylene, and coating powder is admitted. The parti- cles of coating powder are heated to plasticity and are ejected at super- sonic speeds averaging approximately 2,500 ft per second. Kinetic energy of the D-Gun particles is approximately ten times the kinetic energy per unit mass of particles in a conventional plasma arc gun and 25 times the energy of particles in an oxyacetylene spray gun. The high temperature, high velocity coating particles attach and conform to the part being coated, giving a very strong coating bond at the interface and low porosity in the coating. This coating does not depend on a severely roughened surface to provide mechanical interlocking to obtain a bond. Surface preparation for hardened steel consists of grinding to the desired undersize plus, in some instances, grit blasting. Titanium parts do not need grit blasting before coating. In spite of the high temperatures generated in the barrel of the D-Gun, the part being coated remains below 300°F, so there is little chance of part warpage and the base material metallurgy is not affected. Coating Details. The D-Gun deposits a very thin coating of material per detonation, so multiple passes are used to build up to the final coating thickness. Figure 10-8 shows the pattern formed by the overlapping cir- cular deposits being built up on the surface of a piston rod. Finished Figure 10-8. Detonation gun coated piston rod. coating thickness may be as low as 1.5 to 2 mils for some high pressure applications such as injection pump plungers or polyethylene compressor piston rods but many typical applications use finished thicknesses of three to five mils. Greater thicknesses may be used for repair jobs. Finished thicknesses greater than is practical for a given cermet or ceramic coating may require prior build-up with metallic coatings such as nickel. A number of ceramic and metallic coatings are available for applica- tion with the D-Gun. These include mixtures or alloys of aluminum oxide, chromium oxide, titanium dioxide, tungsten carbide, chromium carbide, titanium carbide, cobalt, nickel, and chromium. Table 10-3 lists some of the more popular coatings with their compositions and some key physi- cal properties. Tungsten carbide and cobalt alloys are frequently used for coating journal areas and seal areas of shafts. In cases where additional corrosion resistance is required, the tungsten carbide and cobalt alloys have chromium added. Such a powder is often used on the seal areas of rotors. Greater oxidation and corrosion resistance at elevated temperatures is accomplished by using powder with chromium and nickel in conjunc- tion with either tungsten carbide or chromium carbide. Carbide coatings exhibit excellent wear resistance by virtue of their high hardnesses. Chromium carbide coatings have a cross-sectional Vickers hardness number (HV) in the range of 650 to 900kg/mm 2 based on a 300 g-load which is approximately equal to 58 to 67 Rockwell “C.” The tungsten carbide coatings are in the range of 1,000 to 1,400 HV or approximately 69 to 74 Rockwell “C.” Coatings applied by the D-Gun have high bond strengths. Bond strengths, as measured per ASTM C633-69 modified to use a reduced coating thickness of 10 mils, are in excess of 10,000psi, which is the limit of the epoxy used in the test. Special laboratory methods of testing bond strengths of D-Gun coatings by a brazing technique have given values in excess of 25,000psi. This type of test, however, may change the coating structure. Porosity is less than 2 percent by volume for these coatings. Figure 10-9 shows a photomicrograph of a tungsten carbide coating applied to steel. The original photo was taken through a 200 power micro- scope. The markers in the margin denote from top to bottom: the coating surface, tungsten carbide and cobalt coating, bond interface and base metal. The tight bond and low porosity are clearly evident. Low porosity is an important factor in corrosion resistance and it enhances the ability of a coating to take a fine surface finish. The as-deposited surface finishes of carbide coatings are in the range of 120 to 150 microinches rms when deposited on a smooth base mater- ial. Finishing of low tolerance parts, such as bearing journals, is usually accomplished by diamond grinding. Parts that do not require extremely close dimensional control such as hot gas expander blades can be left as 556 Machinery Component Maintenance and Repair Protecting Machinery Parts Against Loss of Surface 557 coated or, if a smoother finish is desired, they can be given a nondimen- sional finishing by means of abrasive belts or wet brushing with an abra- sive slurry. A combination of grinding, honing, and polishing is routinely used to finish tungsten carbide coatings to eight microinches, and finishes as fine as two microinches or better are attainable with these coatings. For many applications however, plasma and D-Gun coatings can be used as coated. In fact, in at least one application, a D-Gun tungsten carbide-cobalt coating is grit blasted to further roughen the surface for better gripping action. Probably in the majority of applications, the coat- ings are finished before being placed in service. Finishing techniques vary from brush finishing to produce a nodular surface, to machining, honing, grinding, and lapping to produce surfaces with surface roughness down to less than microinches rms. Machining can be used on some metallic coatings, but most coatings are ground with silicon carbide or diamond (diamond is usually required for D-Gun coatings). The best surface finish that can be obtained is a function not only of the finishing technique, but also of the coating type and the deposition parameters. Finishing of D- Gun coatings is usually done by the coating vendor, since great care must be exercised to avoid damaging the coatings. Figure 10-9. Photomicrograph of tungsten carbide-cobalt coating. A typical check list for grinding of most hard surface coatings follows: 1. Check diamond wheel specifications. a. Use only 100 concentration. b. Use only resinoid bond. 2. Make sure your equipment is in good mechanical condition. a. Machine spindle must run true. b. Backup plate must be square to the spindle. c. Gibs and ways must be tight and true. 3. Balance and true the diamond wheel on its own mount—0.0002in. maximum runout. 4. Check peripheral wheel speed—5,000 to 6,500 surface feet per minute (SFPM). 5. Use a flood coolant—water plus 1–2 percent water soluble oil of neutral pH. a. Direct coolant toward point of contact of the wheel and the workplace. b. Filter the coolant. 6. Before grinding each part, clean wheel with minimum use of a silicon carbide stick. 7. Maintain proper infeeds and crossfeeds. a. Do not exceed 0.0005in. infeed per pass. b. Do not exceed 0.080 in. crossfeed per pass or revolution. 8. Never spark out—stop grinding after last pass. 9. Maintain a free-cutting wheel by frequent cleaning with a silicon carbide stick. 10. Clean parts after grinding. a. Rinse in clean water—then dry. b. Apply a neutral pH rust inhibitor to prevent atmospheric corrosion. 11. Visually compare the part at 50X with a known quality control sample. Similarly, a typical check list for lapping is: 1. Use a hard lap such as GA Meehanite or equivalent. 2. Use a serrated lap. 3. Use recommended diamond abrasives—Bureau of Standards Nos. 1, 3, 6, and 9. 4. Imbed the diamond firmly into the lap. 5. Use a thin lubricant such as mineral spirits. 6. Maintain lapping pressures of 20–25 psi when possible. 7. Maintain low lapping speeds of 100–300 SFPM. 558 Machinery Component Maintenance and Repair Protecting Machinery Parts Against Loss of Surface 559 8. Recharge the lap only when lapping time increases 50 percent or more. 9. Clean parts after grinding and between changes to different grade diamond laps—use ultrasonic cleaning if possible. 10. Visually compare the part at 50X with a known quality control sample. Limitations. All thermal spray-applied coatings have restrictions in their application since a line of sight is needed between the gun and the surface to be coated. The barrel of a D-Gun is positioned several inches away from the surface to be coated, and the angle of impingement can be varied from about 45° to the optimum of 90°. Coating of outside surfaces generally presents no problem, but small diameter, deep or blind holes may be a problem. It is possible to coat into holes when the length is no more than the diameter. The structure and properties of the coating may vary some- what as a function of the geometry of the part, because of variations in angle of impingement, stand-off, etc. Portions of a part in close proxim- ity to the area being plated may require masking with metal. Applications. Detonation gun coatings have been used in a large number of applications for rotating and reciprocating machinery as well as for special tools, cutters and measuring instruments. References 2 and 3 attest to the success of such coatings. Table 10-4 shows typical applications in a petrochemical plant utilizing tungsten carbide based coatings. The tungsten carbide family of coatings is used principally for its wear resistance. Tungsten carbide is combined with up to 15 percent cobalt by weight. Decreasing the amount of cobalt increases wear resistance, while Table 10-4 Cobalt Alloy Applications in a Petrochemical Refinery 2 [...]...560 Machinery Component Maintenance and Repair adding cobalt increases thermal and mechanical shock resistance Coatings of this type are frequently used to coat bearing journals and seal areas on compressors, steam turbines, and gas turbines These coatings have a high resistance to fretting and they have been used on midspan stiffeners of blades for axial flow compressors Their fretting resistance and. .. successfully whenever metal slides and rubs The excellent wear characteristics of chromium make it well suited for use on liners of power engines, reciprocating compressors and, in some cases, on piston rods 562 Machinery Component Maintenance and Repair The process offers two major approaches: Restorative plating, to salvage worn parts, and preventive plating, to condition wear parts for service The following... electroplating of machinery components is used for corrosion protection, wear resistance, improved solderability or brazing characteristics and the salvaging of worn or mismatched 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 572 4 5 6 7 8 Machinery Component Maintenance and Repair Masking equipment and plating solutions... 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 1,200°F Gas turbine bleed air components... 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. .. 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... 8.6 percent by weight of the lube oil, to accelerate wear 568 Machinery Component Maintenance and Repair Figure 10-10 Cylinder wear on chrome and cast-iron cylinders 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... 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... Chromium is extremely hard and therefore gives longer life to plated parts Chromium withstands acid contamination and corrosive vapors found in engine crankcase oils and fuels Chromium-plated parts possess a very low friction factor coupled with high thermal conductivity while permitting the parts to operate at more efficient temperatures Chrome-plating extends life of engine parts It is generally accepted... 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 . (>HRC 60) 552 Machinery Component Maintenance and Repair The Detonation Gun Process* If we look at the repair of rotating machinery shaft bearings, journals, seal surfaces, and other critical. extremely close dimensional control such as hot gas expander blades can be left as 556 Machinery Component Maintenance and Repair Protecting Machinery Parts Against Loss of Surface 557 coated or, if. SFPM. 558 Machinery Component Maintenance and Repair Protecting Machinery Parts Against Loss of Surface 559 8. Recharge the lap only when lapping time increases 50 percent or more. 9. Clean parts