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AMAZON multi-meters discounts AMAZON oscilloscope discounts Many repairs of worn machinery surfaces can be achieved by hard surfacing. By definition, hard surfacing is the process of applying, by specialized welding techniques, a material with properties superior to the basis metal. Perhaps the chief factor limiting wide acceptance of this process today is the aura of mystery surrounding the properties of the various hard surfacing alloys. There are literally hundreds of hard-surfacing alloys commercially available, each with a strange sounding name and a vendor's claim that it’s the ultimate material for this or that application. Rather than sift through the chaff to determine which should be used on an urgent problem, many machinery maintenance people drop the idea of hard surfacing and rely on more familiar techniques. This section will discuss the various forms of wear, and show how a few hard-surfacing materials can solve most wear problems. The following information is intended to simplify the field of hard surfacing so that maintenance and design engineers can effectively use the process to reduce maintenance and fabrication costs. Basic Wear Mechanisms The first step in solving any wear problem is to determine the mode or modes of wear present. This is of the utmost importance. The same approach must be taken in new designs, except that then the question to be asked is not, "How did this part fail?" but "How might wear occur on this part?" By way of a quick review, there are four basic types of wear: adhesive, abrasive, corrosive, and surface fatigue. Each basic type can also be further categorized, as in FIG. 1. Adhesive Wear The mechanism of adhesive wear is the removal of material from one or both mating surfaces by the action of particles from one of the surfaces bonding to the other. With repeated relative motion between the surfaces, the transferred particles may fracture from the new surface and take on the form of wear debris. Adhesive wear is thus analogous to friction, and is present in all sliding systems. It can never be eliminated-only reduced. FIG. 1. Wear mechanisms. Abrasive Wear Abrasive wear occurs when hard, sharp particles, or hard, rough surfaces, contact soft surfaces and remove material by shearing it from the softer surface. The amount of metal removed is a function of the nature of the abrading substance, and of the loading. For this reason, it’s common to subdivide abrasive wear into high stress, low stress, high-velocity impingement, and filing. Corrosive Wear There is no one mechanism to describe corrosive wear. Fretting, the first form of corrosive wear shown in FIG. 1, occurs in systems that are not supposed to move. One of the most common instances of fretting occurs on shafts, under the press-fitted inner race of a rolling element bearing. Vibration provides a slight relative motion between the shaft and the race. This oscillatory motion causes small fragments of one surface to adhere to the other (adhesive wear), and with repeated vibration or oscillation, the fragments oxidize (corrosive wear) and form abrasive oxides (abrasive wear) which amplify the surface damage. Cavitation occurs in highly agitated liquids where turbulence and the implosion of bubbles cause removal of the protective oxide film on the metal surface, followed by corrosive attack on the base metal. If the implosion of the bubbles is particularly energetic, as is the case in ultrasonic devices, the material removal can be quite rapid. Impingement by high velocity liquids causes removal of protective oxide films. Metal removal then occurs by corrosion of the active metal surface. Erosion involves the same mechanism as impingement. However, the liquid in this instance contains abrasive particles that enhance the removal of surface films. Surface Fatigue This last form of wear to be discussed results from high compressive stress because of point or line-contact loading. These high stresses, with repeated rolling, produce subsurface cracks that eventually propagate and cause particles to be removed from the surface. Once this occurs, the deterioration of the entire rolling surface starts. This is a result of the additional compressive stresses that are generated when the first fragments detached are rerolled into the surface. Surface fatigue of this nature occurs in rolling-element bearings, rails, and other surfaces subjected to point or line-contact loads. Hard-Surfacing Techniques Almost every welding technique can be used to apply a hard-surfacing material. Referring to the definition of hard-surfacing-applying by welding or spraying techniques a material with properties superior to those of the basis metal-it can readily be seen that this can be accomplished in many ways. FIG. 2 illustrates most of the methods used. Each has advantages and disadvantages. Shielded metal-arc welding is the most common and versatile welding technique, but many of the hard-surfacing alloys have not been available in a coated electrode form. FIG. 2. Hard-surfacing techniques. Oxyacetylene welding is the preferred method for applying bare filler metals, in that it minimizes "dilution" or mixing of the filler metal with the basis metal, so that the hardest deposit is achieved with this mode of deposition. Gas welding is slow, and it’s difficult to control the deposit profile. Gas tungsten-arc welding is also slow, but provides the most accurate deposit profile of any of the fusion processes; it has the major disadvantage of significant dilution, with a corresponding loss in deposit hardness. Gas metal-arc welding is one of the fastest processes for applying hard-surfacing; however (once again) not all surfacing alloys are avail able as wire that can be roll-driven through the welding gun. (When used for hard-surfacing, the gas metal-arc process is often used without a shielding gas, and then is referred to as the "open arc" process.) Under the heading of spray surfacing techniques, there are three primary processes: metalizing, plasma spray, and detonation gun. Metalizing is commonly done by spraying a powder at the surface with air pressure. The powder is heated to a highly plastic state in an oxyacetylene flame, coalesces, and mechanically bonds to the substrate. Preparation of the substrate often involves knurling or abrasive blasting. In some systems, a wire, instead of a powder, is fed into the flame, and the molten droplets are sprayed on the surface with a gas assist. In another modification of this process, heating of the wire is accomplished by two carbon electrodes. In the powder system, it’s also possible to spray ceramic materials; as in all spray systems, the deposit does not fuse to the basis metal, and there is a high degree of porosity. Some sprayed alloys may be heated with a larger torch after spraying, and the deposit will thus be fused to the substrate. The plasma system is used only with powders, and its 15,000°F flame provides a denser deposit with improved bonding to the surface. Ceramics are commonly sprayed with plasma. Special techniques used in hard-surfacing include detonation gun, bulk welding, and submerged arc. The detonation gun system is a proprietary process which provides even greater densities and better bonds than can be achieved by plasma spraying. This process is described later. Bulk welding is a process combining tungsten arc and submerged arc. It’s designed for surfacing large areas quickly and cheaply. The mechanization involved in this process makes it more economical than most other fusion processes for hard-surfacing large areas. Submerged arc welding, the last on the list of special techniques, is also used for surfacing large areas. However, it has the limitation that it’s best suited to hard surfacings that are available in a coiled wire form. This rules out use on many of the hard cobalt and nickel-based alloys. One of the principal factors that limit acceptance of hard-surfacing is the confusion surrounding selection of an appropriate alloy for a given service. There are literally hundreds of alloys on the market. The American Welding Society (AWS) has issued a specification on surfacing materials (AWS A5. 13), detailing 21 classes of electrodes and 19 classes of rods. Each class may contain rods from a dozen or so manufacturers, each slightly different. And many proprietary alloys are not included in this specification. Each vendor of surfacing materials has his own selection system; most systems are based on application. If the machinery maintenance man's application is not included in the list, there is no way for him to know which material to choose. One thing that seems to be common to most vendors is a reluctance to supply information on the chemistry of their products. Many claim that this is proprietary and that the user does not need the information. This is like buying a "pig in a poke." No surfacing material should be used unless the plant engineer knows the composition of the alloy, the basic structure of the deposit, and the one and two layer as-deposited hardness. Other considerations of importance are cracking tendencies, bond, all position capability (for electrodes), and slag removal (again, electrodes). Hard-surfacing alloys derive their wear characteristics from hard phases or intermetallic compounds in their structure. For example, due to their microstructure, two hardened steels may each have a Rockwell hardness of RC 60. However, the steel with the higher chromium content will have much greater wear resistance than the plain carbon steel. This is because of the formation of hard intermetallic compounds (chromium carbides) within the RC 60 martensitic matrix. Thus, in selecting tool steels and surfacing alloys, one must consider not only the macroscopic Rockwell type of hardness, but the hardness and volume percentage of the micronconstituents. If the machinery maintenance person wishes to solve an abrasive wear problem involving titanium dioxide, he or she must select a surfacing that is harder than TiO2. Since many abrasive materials are harder than the hardest metals, this requires thinking in terms of absolute hardness. As indicated in FIG. 3, TiO2 has an absolute hardness of approximately 1,100 kg/mm^2. The hardest steel is only 900 on this scale. To solve this wear problem, machinery maintenance personnel must select an alloy that has a significant volume concentration of chromium carbide, vanadium carbide, or some other intermetallic compound which is even harder than the TiO2. Most surfacing alloys have significant amounts of these hard intermetallic compounds in their structure, and this is why they are effective. In an attempt to simplify the subject of surfacing alloys without going into detail on microstructures, nine general classifications based on alloy composition have been established. These are illustrated in FIG. 4. To effectively use hard-surfacing, it’s imperative that the engineer become familiar with the characteristics of each class. FIG. 3. Comparison of commercial hardness tester scales. FIG. 4. Classification of hard-surfacing materials. Tool Steels By definition, hard-surfacing is applying a material with properties superior to the basis metal. In repair welding of tool steels, a rod is normally selected with a composition matching that of the basis metal. When this is done, repair welding of tool steels is not really rod surfacing. However, tool steel rods are available in compositions to match hot-work, air-hardening, oil-hardening, water-hardening, high-speed, and shock resisting steels, and these rods can be applied to basis metals of differing composition. If the expected hardness is achieved, the surface deposit will have the service characteristics of the corresponding tool steel. Tool steel rods are normally only available as bare rod. Iron-Chromium Alloys The iron-chromium alloys are essentially "white irons." For many years, the foundry industry has known that most cast irons will become very hard in the chilled areas if rapidly cooled after casting. If chromium, nickel, or some other alloying element is added to the cast iron, the casting may harden throughout its thickness. These alloy additions also provide increased wear resistance in the form of alloy carbides. Iron-chromium hard-surfacings are based upon the metallurgy of these white irons. Hard nesses of deposit can range from RC 40 to RC 60. Some manufacturers use boron as the hardening agent instead of carbon, but the metallurgy of the deposit is still similar to white iron. Iron-Manganese Alloys These alloys are similar to the "Hadfield Steels." They are steels with manganese contents in the 10 to 16 percent range. The manganese causes the steel to have a tough austenitic structure in the annealed condition. With cold working in service, surface hardnesses as high as RC 55 can be obtained. Cobalt-Base Alloys These materials contain varying amounts of carbon, tungsten, and chromium in addition to cobalt, and provide hardnesses ranging from RC 35 to RC 60. Their wear resistance is derived from complex carbides in a cobalt-chromium matrix. The size, distribution and the types of carbides vary with the alloy content. The matrix can be harder than the austenitic matrix of some of the iron-chromium alloys. Nickel-Chromium-Boron Alloys This family of alloys forms deposits consisting of hard carbides and borides in a nickel eutectic matrix. The macrohardness can be as low as RC 35 and as high as RC 60, but at all hardness levels these alloys will provide good metal-to-metal wear resistance when compared with an alloy steel of the same hardness. These alloys are normally applied by oxy acetylene or gas tungsten-arc welding deposition of bare rod, or by powder spraying. Composites A composite, in hard surfacing, is a metal filler material containing substantial amounts of nonmetals. Typically, these are intermetallic compounds such as tungsten carbide, tantalum carbide, boron carbide, titanium carbide, and others. All of these intermetallics are harder than the hardest metal. Thus, they are extremely effective in solving abrasive wear problems. Composite electrodes usually consist of a steel, or soft alloy, tube filled with particles of the desired compound. During deposition, some of these particles dissolve and harden the matrix, while the undissolved particles are mechanically included in the deposit of welding techniques. Oxyacetylene deposition is the preferred technique for application since fewer particles dissolve. The hard particles are available in various mesh sizes and can be so large that they can readily be seen on the surface. Composites are not recommended for metal-to-metal wear problems since these large, hard particles may enhance this type of wear. However, composites of smaller particle size can be applied by thermal spraying techniques, such as plasma and detonation gun. Copper-Base Alloys Brasses (copper and zinc) or bronzes (copper and aluminum, tin or silicon) can be deposited by most of the fusion welding techniques, or by powder spraying. Oxyacetylene deposition is the most common method. These alloys are primarily used for metal-to-metal wear systems with the copper alloy surfacing being the perishable component. These alloys should run against hardened steel for optimum performance. Ceramics Ceramics can be applied as surfacings by plasma, detonation gun spraying or with some types of metalizing equipment. Coating thicknesses are normally in the range of 0.002 to 0.040 in. Commonly sprayed ceramics include carbides, oxides, nitrides, and silicides. These coatings are only mechanically bonded to the surface, and should not be used where impact is involved. Special Purpose Materials Many times metals are surfaced with austenitic stainless steels or soft nickel-chromium alloys for the sole purpose of corrosion resistance. For some applications, costly metals such as tantalum, silver, or gold are used as surfacings. If a particular application requires a very special material, a surfacing technique probably can be used to put this special metal on only the functional surfaces, with a reduction in cost. In an effort to come up with a viable hard surfacing selection system, a series of wear tests was conducted on fusion surfacing materials from each of the classifications detailed in the preceding pages. Several vendors' products in each classification were tested, and the welding characteristics of each material determined. Ceramics, tool steels, and special purpose materials were not tested. The specific procedure for evaluating the fusion surfacings was to make multilayer test coupons, determine the welding characteristics, and run metal-to-metal and abrasive wear tests on the materials that performed satisfactorily in the welding tests. The compositions of the hard surfacing alloys tested are shown in TBL. 1. TBL. 1 Compositions of Some Hard-Surfacing Alloys FIG. 5. Performance of hard-surfacing materials subjected to low-stress abrasive wear. Numbers indicate formulations shown in TBL. 1. FIG. 6. Adhesive wear graph showing results of running test blocks in contact with 440-C stainless steel shaft of HRC 58. In most applications, neither shaft wear nor block wear is desired; several cobalt alloys gave superior results. Test Results As shown in FIG. 5, the abrasive wear resistances of certain com positions, such as FeCr-5 and Composites 2 and 3 were superior. (The three mentioned are notable for ease of application.) Harder nickel and cobalt-based alloys with macrohardnesses of approximately RC 60 did not perform as well. The manganese steels (FeMn-1 and 2), the low chromium iron alloys (Fe-1 and 2), and the copper-based alloy Cu-1 all had poor abrasive wear resistance. Adhesive wear test results are shown in FIG. 6. Co-2 had the lowest net wear. The composite surfaces, Com-1, 2, and 3 performed very well, but produced more wear on the mating tool steel than did the cobalt-based alloys. This result was also experienced with the nickel-based alloys. The copper-based alloy Cu-1 showed the highest surfacing wear rate, but one of the lowest shaft wear rates. Discussion The results of the abrasive wear tests indicated that the theoretical prediction that the abrasive wear rate is inversely proportional to the hardness of the material subjected to wear held true. However, as was mentioned earlier, this result does not refer to the macrohardness, but to a combination of macrohardness and microconstituent hardness. The surfacings that performed best in the abrasive wear tests-Com-2 and 3, and FeCr-5-all had large volume percentages of intermetallic compounds with hardnesses greater than the abrading substance, which in this case was silicon dioxide. Another significant observation was that the iron chromium alloy FeCr 5 with high carbon (6 percent) and titanium (5.2 percent) concentrations outperformed the arc-welded tungsten carbide deposit Com-1. Thus it was shown that a coated electrode (FeCr-5) could be used to get abrasive wear resistance almost as good as that of gas-deposited tungsten-carbide composite. All of the very hard alloys exhibited cracking after welding, making them unsatisfactory for some applications, such as knife edges. The cobalt-based alloy Co-2 had the best abrasive wear resistance of those alloys that did not crack after welding. Cracking and checking don’t mean a loss of bond; and thus, in many surfacing applications, cracking tendencies can be neglected. In explanation of the results of the adhesive wear tests, it can be hypothesized that the hard microconstituents present in many of the surfacing alloys tested promoted wear of the mating metal surface. The cobalt-based alloys that performed best in this test don’t have a large volume fraction of hard microconstituents. In fact, there are few particles large enough to allow a hardness determination. This may account for the low wear of the cobalt-based alloys on the mating tool steel. In any case, adhesive wear, because it’s a complex interaction between metal surfaces, cannot be predicted by simple property measurements-a wear test is required. Selecting a Surfacing Method The first step is to determine the specific form of wear that is predominant in the system. Once this has been done, the next step will be to select a process for application. The final step will be to select the surfacing material. Here are some guidelines for process selection. • If a large area has to be surfaced, consider the use of open arc, sub merged arc, or bulk welding • If distortion cannot be tolerated in a surfacing operation, consider use of spray surfacing by plasma arc, metalizing, or detonation gun • If optimum wear resistance is required, use oxyacetylene to minimize dilution, or use a spray technique • If accurate deposit profiles are required, use gas tungsten-arc welding • If surfacing must be done out of position, use shielded, metal-arc welding The process of application will limit alloy selection to some extent. For example, if spray surfacing is required because of distortion, many of the iron chromium, iron manganese, or tool steel surfacings cannot be employed because they are not available as powders. Selecting a Surfacing Material Here are some guidelines for choosing the right alloy: • Tool steels should be used for small gas tungsten-arc welding deposits where accurate weld profiles are required • Iron-chromium alloys are well-suited to abrasive wear systems that don’t require finishing after welding • The composite alloys should be used where extreme abrasion is encountered, and when finishing after welding is not necessary • Iron-manganese alloys should be used where impact and surface fatigue are present. Deformation in service must occur to get work hardening. These alloys are not well suited for metal-to-metal wear applications • Cobalt-based alloys are preferred for adhesive wear systems. They have the additional benefit of resistance to many corrosive and abrasive environments • Nickel-chromium-boron alloys are suitable for metal-to-metal and abrasive wear systems, and they are preferred where finishing of a surfacing deposit is necessary • Copper-based surfacing alloys are suitable only to adhesive wear systems. They are resistant to seizure when run against ferrous metal, but may be subject to significant wear • 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. TBL. 2 lists specific alloys likely to give exceptionally good performance, based on the tests summarized in FIG. 5 and 10-6. ========= TBL. 2 Hard-Surfacing Selection Guide (Typical Only) Surfacing Chromium Oxide AISI 431 Stainless Steel NiCr-4 FeCr-1 (Iron- Chromium) -- Form Powder Powder
Powder Electrode -- Deposition Process Plasma Spray Metalize Metalize and Fuse Shielded Metal- Arc Welding --- Characteristics: Excellent resistance L to very low stress E abrasion. Thickness 5-40mils. Can be ground to very good finish. No welding distortion. (>HRC 70) Good adhesive wear F resistance when G lubricated. Poor abrasion resistance. Can be ground to good finish. No welding distortion. (HRC 35) Good adhesive wear M resistance; corrosion W resistant. Coating S thickness to 0.125 in. C with fusion bond. E Distortion may occur I in fusing, but B application is faster than oxyacetylene rod surfacing. Moderate resistance L to low stress abrasion S and adhesive wear. Can a be easily finished by grinding. Low cost. (HRC 50) -- Uses: Low Stress Erosion Fretting, Galling Metal-to-Metal Wear, Galling, Seizure, Cavitation, Erosion, Impingement, Brinelling Low Stress, High Stress, Cylinder and Ball Rolling ========= 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. TBL. 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 accomplished 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 resistance, material compatibility, friction factor, minimum or maximum allowable coating thickness, surface finish attainable, bond strength, coefficient of thermal expansion, coating porosity and the amount of thermal distortion from the repair; all have varying degrees of importance depending 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. === TBL. 3 Physical Properties of Some Detonation Gun Coatings (UCAR D-Gun) === 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 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 economic standpoint. FIG. 7. Detonation gun schematic. 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 FIG. 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 equipment 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 charge of oxygen, acetylene, and coating powder is admitted. The particles 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 don’t need grit blasting before coating. FIG. 8. Detonation gun coated piston rod. 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. FIG. 8 shows the pattern formed by the overlapping circular deposits being built up on the surface of a piston rod. Finished 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 application 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. TBL. 3 lists some of the more popular coatings with their compositions and some key physical 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 conjunction 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 900 kg/mm2 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 1,400HV 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 10,000 psi, 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 25,000 psi. This type of test, however, may change the coating structure. Porosity is less than 2 percent by volume for these coatings. FIG. 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 micro-inches rms when deposited on a smooth base material. Finishing of low tolerance parts, such as bearing journals, is usually accomplished by diamond grinding. Parts that don’t require extremely close dimensional control such as hot gas expander blades can be left as coated or, if a smoother finish is desired, they can be given a nondimensional finishing by means of abrasive belts or wet brushing with an abrasive 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 coatings 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. FIG. 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-0.0002 in. 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. Don’t exceed 0.0005 in. infeed per pass. b. Don’t 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. 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. ======= TBL. 4 Cobalt Alloy Applications in a Petrochemical Refinery ======== 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’s 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 proximity 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. TBL. 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 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 ability to carry high compressive loads make them suitable to correct loose interference fits on impellers and coupling hubs. Addition of chromium to the tungsten carbide and cobalt mixtures adds corrosion resistance and improves wear resistance at high temperature levels. In general, this family of coatings is most frequently used in neutral chemical environments but can be used with many oxidizing acids. Cobalt mixture coatings are usually not used in strongly alkaline environments. Coatings that combine tungsten carbide with chromium and nickel exhibit greater oxidation and alkaline corrosion resistance than the tungsten carbide-cobalt coatings. Their wear resistance capabilities are good up to about 1,200°F, which is about 200°F higher than that of the tungsten carbide-cobalt coatings. This higher temperature capability makes these coatings useful for applications such as coating rotor blades on hot gas expanders used for power recovery from catalytic crackers. Coated blades resist the wear from catalyst fines and have extended life from just a few months, as experienced with uncoated blades, to a life of three to five years. This type of coating is suitable for use in many alkaline environments. Chromium carbide combined with nickel and chromium provides excel lent wear resistance at elevated temperatures and is recommended for temperatures up to about 1,600°F. These coatings don’t have the wear resistance of tungsten carbide coatings at low temperatures, but they do perform well at high temperatures. Such coatings have found numerous applications in hot sections of gas turbines. Cobalt base alloys with excel lent wear resistance to temperatures over 1,800°F are also available. In applications where hydrogen sulfide is present, ferrous base materials should not exceed a hardness of 22 Rockwell "C," per recommendation of the National Association of Corrosion Engineers, in order to avoid sulfide stress cracking. The tungsten carbide-cobalt-chromium coatings and tungsten carbide-chromium-nickel coatings have imparted wear resistance to parts in such service while the base material retains a low hardness to avoid sulfide stress cracking. Application of D-Gun coatings on reciprocating machinery has resulted in extended parts life. Uncoated hardened Monel piston rods in oxygen booster compressors that previously required rod resurfacing in one to two years of service have shown virtually no measurable wear in five to six years of service when coated with tungsten carbide-cobalt coatings. In addition, average life of the gas pressure packings was more than doubled. The high bond strength of the D-Gun coatings has also proven useful on polyethylene hyper-compressors. There have been examples of tungsten carbide-cobalt coated plunger pistons that have operated 16,000 hours at 20,000 psi with a wear of only one mil and without any coating peeling problems. In summary, we find that detonation gun coatings are useful to both designers and machinery maintenance personnel as a means of providing dependable wear and corrosive resistant surfaces on machine components operating under difficult service conditions. Properly selected coatings used within their intended limits are significantly capable of extending wear life of parts. The extended wear life reduces the ratio of parts cost per operating hour, justifying the expenditure for coatings on both new and refurbished equipment. Industrial Plating. Another process that will restore worn or corroded machinery surfaces is industrial plating, usually electroplating. This process is not normally applied on-site but parts in need of restoration have to be shipped to a company specializing in this type of work. Surface preparation for plating is usually achieved by smooth machining or grinding. In some cases, shot or grit blasting may be suitable. A very rough surface before plating is neither necessary nor desirable. Unless a greater thickness of deposit is required for wear, corrosion allowance, or for bearing material compatibility, there is no need to remove more metal than required to clean up the surface. Sharp corners and edges should be given as large a radius or diameter as possible. Areas not requiring resurfacing will be protected by the plating shop. Materials that can be repaired belong to the majority of metals used in normal design practice. It is, however, very important that the plating company be informed of the composition or specification. The properties of steel can be adversely affected by plating unless pre cautions are taken. Such effects become increasingly important with high strength materials, which may become brittle or lose fatigue strength. Heat treatment or shot peening can help to reduce these effects. Plating metals normally used for machinery component salvage are chromium and nickel, either singly or in combination. If needed, other metals may be specified, For example, copper, in cases where heat or electrical conductivity is of importance. In the following we would like to concentrate on chromium as the preferred plating metal for machinery wear parts. Industrial chrome-plating has been applied 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. The process offers two major approaches: Restorative plating, to salvage worn parts, and preventive plating, to condition wear parts for service. The following advantages are usually stressed: • 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’s generally accepted that chromium is four to five times harder than the original cast iron wearing surface • Electroetching can provide porosity in a chrome-plated surface where this is necessary to hold a lubricant These characteristics of chrome-plating are further explained in the next section. Chrome-Plating of Cylinder Liners. In reciprocating engine and compressor cylinders surface finish of the liner is accomplished through smooth turning, grinding, or honing. It’s important that the wearing surfaces get a finish that gives the material a maximum of resisting power against the strains to which it’s subjected, and also offers a low coefficient of friction and the best possible conditions for the retention of the lubricating film. It’s desirable to develop a "glaze" on the wearing surface of the liner, under actual conditions of service. This glaze is produced by subtle structural and chemical changes in the surface of the liner and is not easily achieved; it’s more a product of chance than design, due to the wear promoting factors mentioned earlier. Other surface finishing approaches include chemical and metallic coatings, with substantial reductions in ring wear, but uniform coating of cylinders has not been fully documented as of this writing. Just how good a step toward the solution of cylinder liner wear is chromium? Bonding. First of all, proper chrome-plating is deposited one ion at a time, assuring a molecular bond that approaches the integrity of fusion between the basis metal and the chrome-plated surface. The chromium literally grows roots into the basis metal to make its bond the strongest in the industry. With a bond of that quality, good chromium actually adds to the structural strength of a restandard-sized liner, or even a new one, to make it stronger and tougher than before... and eligible for resalvaging time and time again. The importance of bond in chrome-plating includes its value in strengthening the liner wall, even after successive reborings and replatings. It assures that the chrome layer is "locked" to the basis metal, even under heavy wear conditions that would strip and spall conventional chrome-plating. Low Coefficient of Friction Chromium is known to have the lowest coefficient of friction of any of the commonly used structural metals for engine cylinder liners. The value of the sliding coefficient for chromium on chromium has been given as 0.12, for chromium on steel as 0.16, while steel on steel is 0.206. For rotating shafts, chromium was also found to have the lowest friction of any of the metals tested in a study conducted by Tichvinsky and Fisher. Hardness The hardness of chromium has definite advantages over cast iron for long wear characteristics. The hardness is maintained throughout the thickness of the chrome-plating, while the hardness of metals treated with processes like nitriding and carburizing decreases with depth. The value of hardness in a chrome-plated surface lies in its ability to resist abrasion and scoring. Contaminants in lube oil and fuel and their deposits cannot be eliminated, but their abrasive action on the liner wear surfaces has a negligible effect on chromium due to its extreme hardness. TBL. 5 shows some typical hardness tests, in Brinell notation. Because of the relative immunity of chromium to scuffing, which often occurs in "green" tests, or the initial runs of an engine, ring scuffing and piston seizure are eliminated, and engine production is accelerated. This is an example of the use of preventive plating, through processing of a new liner before it’s installed for the first time. Corrosion Resistance The corrosion resistance of chromium is high, partly due to the dense packing of the chromium molecules during electrodeposition. This resistance makes chromium especially adaptable to cylinder liners. ==== TBL. 5 Hardness Table Brinell hardness (load 3,000kg: 10-mm ball) or equivalent Chromium 800-1,000 Cast-iron cylinder compositions 150-275 Cast-iron cylinder compositions, heat-treated Max. 400 Steel 4140 heat-treated 250-375 Steel, carburized 625 Nitralloy in cylinders after removal of surface stock 650-750 ==== Sulfuric acid, which attacks cast iron vigorously, has little effect on chromium. Hydrogen sulfide is another corrosive agent found in diesel engines running on sour gas. In a controlled test of the effects of hydro gen sulfide on steel and chromium, a steel rod partially chrome-plated was subjected to moist hydrogen sulfide at temperatures ranging from 120° to 200°F for 252 hours. After this exposure, the unplated portion of the rod was blackened and badly corroded, but the chrome-plated portion was not disturbed. Chromium is also unaffected by nitric acid and saturated solutions of ammonia, but it’s susceptible to hydrochloric acid. However, as discussed earlier, modern fresh-water cooling prevents the introduction of HCl producing chlorides into diesel engines now being used. The possibility of the occurrence of sufficient quantities of HCl to attack the chrome plated surface in a given application is negligible, thus even in marine engines. Atmospheric corrosion also has no effect on chromium. Porous chromed cylinders may be stored indefinitely with little or no protection, without detrimental results to the plated area. Lubrication One chrome plating company* has developed a proprietary process for etching channels and pockets in the surface of the chrome layer, to provide tiny reservoirs for lube oil. This porous surface provides high oil retention even under the high temperatures of the combustion area in the 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 won’t 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 forming 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 desirability 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 combustion 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 susceptibility of lube oil to flash point combustion, breakdown into deposits, unnecessarily high lube oil consumption, and the production of air pollutants. Thermal Conductivity Thermal conductivity in chromium is higher than for cast iron and commonly used steels, by approximately 40 percent, as shown in TBL. 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 combustion and exhaust temperatures, helping to reduce incomplete combustion and its products. While the coefficient of expansion, also shown in TBL. 6, of chromium is lower than that of cast iron or steel, there is a decided advantage in the difference. The surface of the cylinder liner has a much higher temperature than the underlying basis metal, because of that sharp temperature gradient through the wall. The effect of this gradient on a homogeneous metal, e.g., distortion, is eliminated with chromium plating, because it’s 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. === TBL. 6 Expansion Coefficients and Thermal Conductivity Metal: Chromium, electrolytic Cast iron Steels Aluminum Copper === TBL. 7 Cylinder Wear === TBL. 8 Piston and Ring Wear In porous-chrome cylinder In cast iron cylinder Aluminum piston loss in weight Top ring lost in weight Second ring loss in weight Oil ring loss in weight === |