Machinery Components Maintenance And Repair Episode 2 Part 10 ppt

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 is difficult to control the deposit profile. Gas tungsten-arc welding is also slow, but provides the most accu- rate deposit profile of any of the fusion processes; it has the major disad- vantage of significant dilution, with a corresponding loss in deposit hardness. Gas metal-arc welding is one of the fastest processes for apply- ing 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: metallizing, plasma spray, and detonation gun. Metallizing 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 is 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. Ceram- ics 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 is designed for surfacing large areas quickly and cheaply. The mecha- nization 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 surfac- ing large areas. However, it has the limitation that it is 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 540 Machinery Component Maintenance and Repair Protecting Machinery Parts Against Loss of Surface 541 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 infor- mation. 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 hard- ness. 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 TiO 2 . Since many abrasive materials are harder than the hardest metals, this requires thinking in terms of absolute hardness. As indicated in Figure 10-3, TiO 2 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 TiO 2 . 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 Figure 10-4. To effectively use hard-surfacing, it is imperative that the engineer become familiar with the characteristics of each class. 542 Machinery Component Maintenance and Repair Figure 10-3. Comparison of commercial hardness tester scales. Figure 10-4. 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 nor- mally 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 Protecting Machinery Parts Against Loss of Surface 543 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 parti- cles 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 deto- nation gun. Copper-Base Alloys Brasses (copper and zinc) or bronzes (copper and aluminium, 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. 544 Machinery Component Maintenance and Repair Ceramics Ceramics can be applied as surfacings by plasma, detonation gun spray- ing or with some types of metallizing 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 char- acteristics 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 sat- isfactorily in the welding tests. The compositions of the hard surfacing alloys tested are shown in Table 10-1. Test Results As shown in Figure 10-5, 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 Figure 10-6. 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 Protecting Machinery Parts Against Loss of Surface 545 546 Machinery Component Maintenance and Repair Table 10-1 Compositions of Some Hard-Surfacing Alloys Nominal Composition* Hardness Alloy** C Si Cr W Co Al Mn B Fe Ni Cu Other Other Rockwell C Fe-1 D 0.18 0.30 2.90 1.02 BAL 35 Fe-2 D 0.68 0.47 6.80 1.48 BAL 20 FeCr-1 D 2.20 0.90 30.00 1.30 BAL 3.8 Mo 1.50 Ti 50 FeCr-2 D 0.44 1.20 29.60 1.70 BAL 4.0 Mo 60 FeCr-3 D 4.00 1.00 23.00 BAL 0.5 Mo 7.50 Cb 60 FeCr-4 D 1.00 3.00 13.00 0.70 3.00 BAL 60 FeCr-5 D 6.00 1.00 13.00 2.70 BAL 5.2 Ti 62 FeMn-1 D 0.44 0.52 14.10 16.60 BAL 20 FeMn-2 D 0.64 0.26 0.47 13.60 BAL 20 Co-1 + 1.20 2.06 30.00 4.50 BAL 2.00 3.00 3.00 1.5 Mo 40 Co-2 + 1.80 29.0 9.00 BAL 50 Co-3 + 2.00 0.85 30.90 13.80 BAL 2.30 55 Co-4 + 2.50 32.50 17.50 BAL 58 Co-1A + 0.95 1.20 27.40 5.00 BAL 1.90 30 Co-1C D 1.10 29.00 4.50 BAL 3.00 40 Co-7 D 36 Cu-1 + 14.00 4.00 BAL 20 NiCr-4 + 0.45 2.25 10.00 2.00 2.50 BAL 35 NiCr-5 + 0.65 3.75 11.50 2.60 4.25 BAL 50 NiCr-6 + 0.75 4.25 13.50 3.00 4.75 BAL 56 NiCr-C + 0.04 0.86 15.50 4.10 0.24 5.70 BAL 16 Mo 0.34V 30 COM-1 + BAL 60 WC 62 COM-2 D BAL 60 WC 62 COM-3 + 0.48 0.80 12.00 1.80 10.00 0.80 1.20 1.20 60 WC 60 * Fe—Iron Base; FeMn—Iron manganese; FeCr—Iron Chromlum; Co—Cobalt Base; NiCr—Nick el Chromlum; Cu—Copper Base; COM— Composite. ** + Rod; D Electrode. Protecting Machinery Parts Against Loss of Surface 547 Figure 10-5. Performance of hard-surfacing materials subjected to low-stress abrasive wear. Numbers indicate formulations shown in Table 10-1. Figure 10-6. 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. 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 sur- facings 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 com- posite. 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 do not mean a loss of bond; and thus, in many surfacing applications, cracking ten- dencies can be neglected. In explanation of the results of the adhesive wear tests, it can be hypoth- esized 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 do not have a large volume frac- tion 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 is 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 predom- inant in the system. Once this has been done, the next step will be to select 548 Machinery Component Maintenance and Repair 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, metallizing, 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 do not 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 abra- sive 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 Protecting Machinery Parts Against Loss of Surface 549 [...]... 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 5 62 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... 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... 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... 558 Machinery Component Maintenance and Repair 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... 6 Maintain lapping pressures of 20 25 psi when possible 7 Maintain low lapping speeds of 100 –300 SFPM 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... 10- 4 Cobalt Alloy Applications in a Petrochemical Refinery2 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... 10Co, 4Cr 73WC, 20 Cr, 7Ni 800r3C2, l6Ni, 4Cr Tensile Bond Strength (psi) (b) >10, 000 >10, 000 >10, 000 >10, 000 Modulus of Rupture (psi) 90,000 40,000 70,000 Modulus of Elasticity (psi) 31 ¥ 166 17 ¥ 106 18 ¥ 106 Metallographic Apparent Porosity (Vol %) £1 £1.5 . 6.00 1.00 13.00 2. 70 BAL 5 .2 Ti 62 FeMn-1 D 0.44 0. 52 14 .10 16.60 BAL 20 FeMn -2 D 0.64 0 .26 0.47 13.60 BAL 20 Co-1 + 1 .20 2. 06 30.00 4.50 BAL 2. 00 3.00 3.00 1.5 Mo 40 Co -2 + 1.80 29 .0 9.00 BAL 50 Co-3. BAL 20 NiCr-4 + 0.45 2. 25 10. 00 2. 00 2. 50 BAL 35 NiCr-5 + 0.65 3.75 11.50 2. 60 4 .25 BAL 50 NiCr-6 + 0.75 4 .25 13.50 3.00 4.75 BAL 56 NiCr-C + 0.04 0.86 15.50 4 .10 0 .24 5.70 BAL 16 Mo 0.34V. BAL 50 Co-3 + 2. 00 0.85 30.90 13.80 BAL 2. 30 55 Co-4 + 2. 50 32. 50 17.50 BAL 58 Co-1A + 0.95 1 .20 27 .40 5.00 BAL 1.90 30 Co-1C D 1 .10 29 .00 4.50 BAL 3.00 40 Co-7 D 36 Cu-1 + 14.00 4.00 BAL 20 NiCr-4