Volume 04 - Heat Treating Part 8 pptx

Design and Operating Factors. In the design of fuel-fired furnaces, ample space must be provided for combustion so that the flame will not impinge on the pot. If flame impingement is unavoidable, the pot should be rotated slightly at least once a week. Rotating the pot and/or using a sleeve reduces local deterioration in the region of flame impingement and prolongs the service life of the pot. The combustion-chamber atmosphere also has important effects on pot life. A system with a control range from high-fire to low-fire is preferable to an on-off system because the latter allows air to enter the combustion chamber during the "off" portion of the cycle, thereby increasing the rate of sealing of the outer surfaces of the pot. Electrical-resistance-heated furnaces should be equipped with a second pyrometer controller whose thermocouple is located within the heating chamber. This will prevent overheating of the resistance elements, particularly during meltdown, when the thermocouple that controls the temperature of the main bath is insulated by unmelted salt. Because heating elements and refractories are severely attacked by salt, all salt must be kept out of the combustion chamber. For this purpose, a high-temperature refractory fiber rope may be used to seal joints where the pot flange rests on the retaining ring at the top of the furnace. Externally heated pots should be started on low fire (low heat input) regardless of the method of heating. Once the salt appears to melt around the top, heat can be gradually increased to high fire to complete meltdown. Caution: Excessive heating of the sidewalls or pot bottom during startup may create pressures sufficient to expel salt violently from the pot. For added safety, the pot should be covered during meltdown with either a cover or an unfastened steel plate. The waste heat of flue gases may be fed to an adjacent chamber and used to preheat work. Flue gases should always be visible to the operator. The appearance of bluish-white or white fumes at the vent indicates the presence of salts within the combustion chamber; prompt action is required. Advantages and Disadvantages. Because of the ease with which they can be restarted, externally heated furnaces are well suited to intermittent operations. Another advantage of furnaces of this type is that a single furnace can be used for a variety of applications by simply changing the pot for one containing the proper salt composition. Externally heated furnaces do have several characteristics, however, that limit their usefulness in certain operations. They are less adaptable to close and uniform temperature control because the furnace dissipates heat by convection, creating temperature gradients in the bath. Also, the temperature lag of the thermocouple and the recovery time of the furnace may result in overshooting or undershooting the desired temperature by 15 °C (25 °F). In addition to requiring an exhaust system for generated flue gases, externally heated furnaces may overheat at the bottom and sidewalls in restarting, which creates a pressure buildup in the thermally expanding molten salt and may cause an eruption. Finally, externally heated furnaces are seldom practical for continuous high-volume production because of the limitations of pots with respect to size and maximum operating temperature. High maintenance cost is also a factor. Immersed-Electrode Furnaces Ceramic-lined furnaces with immersed (over-the-side) electrodes (Fig. 2), when compared to externally heated pot furnaces, have greatly extended the useful range and capacity of molten salt equipment. The most important of these technical advances are: • The electrodes can be replaced without bailing out the furnace • Immersed electrodes allow more power capacity to be put into the furnace, thus increasing production • Immersed electrodes permit easy startup when the bath is solid. A simple gas torch is used to melt a liquid path between the two electrodes, thus allowing the electrodes to pass current through the salt to obtain operating temperatures Fig. 2 Internally heated salt bath furnace with immersed electrodes and ceramic tiles Immersed-electrode furnaces, however, are not as energy efficient as submerged electrode furnaces. The area in which the immersed electrodes enter the salt bath allows additional heat loss through increased surface area. As exhibited in Table 1, the surface area of the salt bath (A) in the submerged-electrode furnace is smaller than the surface area plus the immersed electrodes (A + B) in the immersed-electrode furnace. However, a good cast ceramic and fiber-insulated cover placed over the bath and electrodes will reduce surface radiation losses up to 60%. Table 1 Service life of electrodes and refractories Operating temperature Service life, years °C °F Electrodes Refractories Submerged-electrode furnaces Furnace A 535-735 1000-1350 15-25 15-25 735-955 1350-1750 6-12 6-12 955-1175 1750-2150 5-7 5-7 1010-1285 1850-2350 2-4 2-4 Furnace B 535-735 1000-1350 10-20 10-20 735-955 1350-1750 4-8 4-8 955-1175 1750-2150 3-4 3-4 1010-1285 1850-2350 1-3 1-3 Immersed-electrode furnaces Furnace C 535-735 1000-1350 2-4 (a) 4-5 735-955 1350-1750 1-2 (a) 2-3 955-1175 1750-2150 1 2 -1 (a) 1-2 1010-1285 1850-2350 1 4 - 1 2 (a) 1 1 2 Furnace D 535-735 1000-1350 2-4 (a) 4-5 735-955 1350-1750 1-2 (a) 2-3 955-1175 1750-2150 1 2 -1 (a) 1-2 1010-1285 1850-2350 1 4 - 1 2 (a) 1 1 2 Furnace E 535-735 1000-1350 2-4 (a) 4-5 735-955 1350-1750 1-2 (a) 2-3 955-1175 1750-2150 1 2 -1 (a) 1-2 1010-1285 1850-2350 1 4 - 1 2 (a) 1 1 2 Note: Service life estimates are based on the assumption that proper rectification of chloride salts is being done, as well as routine unit maintenance and care. (a) Hot leg only Super-duty fireclay brick lines the immersed-electrode furnace. Approximately 130 mm (5 in.) of castable and insulating brick then surrounds the fireclay brick on five sides. Figure 2 is a schematic drawing of an immersed-electrode furnace with interlocking tiles and removable electrodes. The removable electrodes enter the furnace from the top, and a seal tile is located in the front of the electrodes to protect them from exposure to air at the air-bath interface. This protection helps prolong electrode life. Table 1 compares service life of electrodes and refractories for some basic furnace designs. Over-the-top (or over-the-side) electrodes are usually built with laminated cold legs, and water cooling is always required. A typical life expectancy for electrodes operating in such a furnace at 840 °C (1550 °F) is approximately 6 mo to 2 y for over-the-top electrodes, compared to 4 to 8 y for submerged electrodes. The salt is heated by passing alternating current through it with immersed electrodes. As a result of the resistance built up to passage of current through salt, heat is generated within the salt itself. This heat is quickly dissipated by a downward stirring action created by the electrodes. The electrodes are attached by copper connectors to a transformer that converts the line voltage of the plant to a much lower secondary voltage (approximately 4 to 30 V) across the electrodes. Temperature is measured and automatically controlled by a system containing a thermocouple, pyrometer, relay, and magnetic contactor. The energy required by an immersed-electrode furnace is a function of: • Furnace size necessary to hold the load and electrode well • The energy (Q w ) needed to heat the load to the desired temperature. (The value of Q w is a function of load mass, the specific heat of the load, and bath temperature) • Energy losses and safety factors Once energy requirements are determined, then electrode number, size, and spacing can be determined. Microcomputers are used to calculate the rate of heat generation per unit length of the electrode to ensure that the current is uniform from the top and bottom of the electrodes, taking into account the complexity of the current paths between the electrodes, the electromagnetic forces, and the circulation (influenced by the viscosity of the salt). The electrode spacing is usually selected between 25 and 100 mm (1 and 4 in.); the height of the electrode should be smaller than the depth of the pot, the difference depending on electrode spacing. The electrode width is usually 50 to 75 mm (2 to 3 in.) and rarely exceeds 125 mm (5 in.). Transformer voltages usually range from 4 to 30 V, with the ratio of maximum to minimum voltage of a given transformer approximately 4.5 (Ref 1). Steel-Pot Furnaces. Some metal-treating processes are performed in salt compounds that cannot be contained in a ceramic liner. For these applications, furnace manufacturers make use of a welded steel pot with immersed electrodes. This type of furnace is suitable for special applications such as case hardening in straight cyanide baths, tempering, and marquenching. The steel pot often has a sloped back wall, which produces a bottom heating effect resulting in better circulation and uniform temperature. This is accomplished by sloping the electrodes shown in Fig. 3 and 4. As the current passes through the salt between the electrodes, the salt is heated, decreasing its density and causing it to rise toward the bath surface. Control of the rate of rise of the salt is effectively gained by decreasing the distance from the electrodes to the steel pot. At the lower extremity of the electrode, the current enters the metal pot upon leaving the electrode to follow a shorter path to the other electrode. This arrangement ensures current flow through the salt along the entire electrode length. Due to the close proximity of the lower portion of the electrode to the pot, most of the heating is done in the lower part of the bath. This is the desired method of heating any liquid. Typical standard sizes Working dimensions Temperature range (A) Length (B) Width (C) Depth Heating capacity °C °F mm in. mm in. mm in. Input, kW kg/h lb/h 540-150 1000-300 457 18 457 18 610 24 25 45 100 540-150 1000-300 457 18 686 27 610 24 25 68 150 Fig. 3 Metal pot, immersed-electrode salt bath furnace for ferrous tempering and isothermal annealing Typical standard sizes Working dimensions Temperature range (A) Length (B) Width (C) Depth Heating capacity °C °F mm in. mm in. mm in. Input, kW kg/h lb/h 955-650 1750-1200 305 12 305 12 455 18 25 34 75 955-650 1750-1200 305 12 455 18 610 24 40 68 150 955-650 1750-1200 455 18 610 24 610 24 75 159 350 Fig. 4 Metal pot, immersed-electrode salt bath furnace for liquid carburizing, cyaniding, and carbonate baths The metal pots are made of either plain steel or hot-dipped aluminized steel, depending on the application. Thicknesses range from 12 to 38 mm ( 1 2 to 1 1 2 in.). Reinforcing members for light plate, usually angular in shape, are welded from the top. Where depth of the pot so requires, additional members are used at the midsection. The pot is housed in an insulated 230 mm (9 in.) thick wall furnace either with a brick outside wall contained in a rigid welded steel frame or in a steel-clad frame, depending on personal preference. In either type of construction, the frame is self-supporting on a lattice formed by welding channels or beams to the underside of a steel base plate. The pot is supported on an insulated refractory pedestal. Electrode Arrangement. Immersed electrodes are made of either mild steel or an alloy "hot" leg welded to a mild steel "cold" leg. As previously mentioned, these are shaped to follow approximately the slope of the pot wall. The portion of the electrode that crosses over the top of the salt bath and is connected to the plant power source is referred to as the cold leg. This is welded to the hot leg, the portion of the electrode that is immersed in the bath, with sufficient weld cross section to provide necessary current conductor capacity. The shanks are drilled and tapped at the tinned terminal connection end for water cooling when necessary. If the latter is not required, the electrical connection is water cooled. Suitable clamping devices are used to facilitate electrode replacement. Electrode arrangements can vary as follows: • Single-phase operation with metal or ceramic pots: Several electrode arrangements can be used, depending on the size of the bath. If only two electrodes are required, they are normally positioned on the sloped-wall side and at least 125 mm (5 in.) apart. Three electrodes are usually placed so th at the center electrode, equal in size to two of the other electrodes, is used as a common conductor with equal current paths to each of the outer electrodes. More than three electrodes would be arranged in multiple groups • Three-phase operation with metal pots: Three electrodes are used and spaced in a manner similar to the spacing described above. They are connected to three single-phase transformers that have Y- connected secondaries and delta-connected primaries. The current flows from the electrodes to the metal pot, which is the neutral point. Several variations of the three- phase connections are used, depending on the type of furnace and load requirements All accessories, such as starting units, sludging tools, and secondary connectors, are the same for steel-pot immersed- electrode furnaces as for ceramic furnaces. Advantages and Disadvantages. Immersed-electrode furnaces do not require the use of iron-chromium-nickel alloy pots. These furnaces require minimum floor space and maintenance and can be used for all types of neutral salts. Electrodes made of alloy steel should have an average service life equivalent to that indicated for steel pots in the section "Pot Service Life." Worn electrodes can be replaced while the furnace is in operation. Depending on the positioning of electrodes, control to within ±3 °C (±5 °F) is easily obtained with immersed-electrode furnaces. Heat is generated within the bath, and overshooting is readily avoided. These furnaces lend themselves to mechanization and are suitable for high-volume production in the range of 815 to 1300 °C (1500 to 2370 °F). The depth of salt pots for immersed-electrode furnaces is not restricted for ceramic or ceramic-lined pots. Metal pots may be restricted to depths of about 0.6 m (2 ft). Pots may vary in length and width to suit requirements, and multiple pairs of electrodes can be installed to furnish the necessary heating capacity. The immersed-electrode furnace is not recommended for intermittent operation. Depending on furnace size, reheating the salt charge may require a day or more. Pots are not intended to be interchangeable. Removal of the pot usually involves replacement of the surrounding insulation. Reference cited in this section 1. V. Paschkis and J. Persson, Industrial Electric Furnaces and Appliances, Interscience, 1960 Submerged-Electrode Furnaces Submerged-electrode furnaces (Fig. 5 and furnaces A and B in the figure to Table 1) have the electrodes placed beneath the working depth for bottom heating. Many submerged-electrode furnaces are designed for specific production requirements and are equipped with patented features, which offer certain economical and technical advantages. General characteristics of submerged-electrode furnaces include: • Maximum work space with minimum bath area: The electrodes do not occupy any portion of the bath surface, so that they only come in contact with the salt. Bath size is consequently smaller, and electrode life increases many times over by incorporating unidirectional wear and eliminating excessive deterioration at the air-bath interface • Circulation-convection currents: Bottom heating provides more uniform bath temperatures and bath movement through the use of natural convection currents • Triple-layer ceramic wall construction: The temperature gradients through the wall cause any salt penetrating the wall to solidify before it can penetrate the cast refractory material that forms the center portion of the wall construction. The design requires from 5 to 8% of the initial salt charge to fill the ceramic pot. By comparison, in some designs 140 to 150% of the initial charge is needed to seal the ceramic walls of furnaces built with two layers of ceramic brick, backed up and supported by a steel plate. Salt penetrates the ceramic walls of any furnace and distorts the geometry of the walls. Reducing the amount of salt allowed to penetrate the ceramic walls aids in maintaining dimensions and in promoting a longer furnace life • Electrode placement: Enclosing the electrode in a clear rectangular box, free of any protruding obstructions, eliminates any potential hazards to operating personnel during cleaning. A ny sludge formed in the furnace is removed easily by operating personnel Fig. 5 Internally heated salt bath furnace with sub merged electrodes. This furnace has a modified brick lining for use with carburizing salts. Frame Construction. A typical submerged-electrode furnace is made of brick and ceramic material reassembled, regardless of size, in a rigid, self-supporting welded steel frame (see, for example, Table 1). This frame consists of supporting channels or beams welded to the underside of a heavy steel plate that forms the frame base. To this base are welded lengths of heavy angle iron around the outside and on top of the plate. These pieces are notched to permit welding of the heavy angle-iron posts to the plate and vertical sides of the base-plate angle iron. Lengths of heavy angle iron are welded similarly to the top of the posts. When required, additional vertical reinforcing members are welded between the bottom and top pieces of angle iron, and prestressed horizontal members also are used to ensure that the refractory material cannot move after the furnace has been brought to operating temperature. Brick Construction. Three types of refractory materials are commonly used in submerged-electrode furnaces. A typical design is shown by furnace A in Table 1. Submerged-electrode furnace liners are constructed with 230 mm (9 in.) thick high-temperature burned bricks. Consisting of approximately 42% alumina and 52% silica, the brick material is used in standard brick sizes such as 60 by 115 by 230 mm (2 1 2 by 4 1 2 by 9 in.) and in various brick shapes, such as straights, flat backs, and splits. The bricks are laid with a high-quality air-setting mortar that resists abrasion, erosion, and chemical attack by chloride, fluoride, and nitrate-nitrite salts. The mortar offers sufficient wear and corrosion resistance to be economically used with some salts containing cyanide. For straight cyanide or carbonate salts, a welded steel pot or a furnace with a modified brick lining (Fig. 5) is used. The outer wall of the salt bath furnaces is made of a second-quality firebrick with the same dimensions as brick used for the liner. The important qualities of this brick are the strength of the material and uniformity in size and shape. The inner castable wall is constructed with a maximum of refractory cement and aggregate that is poured between the liner and outer wall to form a 240 mm (9.5 in.) thick monolithic wall structure. This dimension provides a temperature [...]... 1 0-3 1 0-3 1 0.013 1 0-4 1 0-4 0.1 1 0-5 1 0-5 0.01 1.3×1 0-4 1 0-6 1 0-6 1 0-3 1.3×1 0-5 1 0-7 1 0-7 1 0-4 1.3×1 0-6 Hard 1.3×1 0-3 Maximum 103 0.13 Soft 1 1.3 Normal range 1 1 0 -8 1 0 -8 1 0-5 (a) Equal to 133.322 387 415 Pa, it differs from torr by one part in 7 × 106 (b) psia = psig + 14.7 psi Comparison of Vacuum and Atmosphere Furnace Processing In most heat- treating. .. 720 13 28 1420 2 588 Boron 1140 2 084 1239 2262 1355 2471 1 489 2712 Cadmium 180 356 220 4 28 264 507 321 610 765 1409 Calcium 463 86 5 5 28 982 605 1121 700 1292 1 487 2709 Carbon 2290 4150 2471 4 480 2 681 485 8 2926 5299 482 7 87 21 Cerium 1091 1996 1190 2174 1305 2 381 1439 2622 Caesium 74 165 110 230 153 307 207 405 690 1274 Chromium 992 181 8 1090 1994 1205 2201 1342 24 48 2 482 4500 Cobalt 1362 2 484 1494... 1447 2637 1602 2916 2735 4955 Lanthanum 1125 2057 1242 22 68 1 381 25 18 1549 282 0 Lead 5 48 10 18 620 11 48 7 18 1324 82 0 15 08 1744 3171 Lithium 377 711 439 82 2 514 957 607 1125 1372 2502 Magnesium 331 6 28 380 716 443 82 9 515 959 1107 2025 Manganese 791 1456 87 8 1612 980 1796 1020 186 8 2151 3 904 Molybdenum 2095 380 3 2295 4163 2533 4591 3009 54 48 5569 10056 Nickel 1257 2295 1371 2500 1510 2750 1679 3054... 483 3 2920 5 288 Palladium 1271 2320 1405 2561 1566 285 1 1759 31 98 Platinum 1744 3171 1 904 3459 2090 3794 2293 4159 4407 7965 Potassium 123 253 161 322 207 405 265 509 643 1 189 Rhodium 181 5 3299 1971 3 580 2150 3900 2357 4274 Rubidium 88 190 123 253 165 329 217 423 679 1254 Ruthenium 20 58 3736 2230 4046 2431 44 08 2666 483 1 Scandium 1161 2122 1 282 2340 1422 2593 1595 2903 Silicon 1116 2041 ... 1 0-4 mm Hg 0.1 μm 1 0-3 mm Hg 1.0 μm 1 0-2 mm Hg 10 μm 1 0-1 mm Hg 100 μm 760 mm Hg 760,000 μm °C °F °C °F °C °F °C °F °C °F Aluminum 80 8 1 486 88 9 1632 996 182 5 1123 2053 2056 3733 Antimony 525 977 595 1103 677 1251 779 1434 1440 2624 Arsenic 220 4 28 310 590 610 1130 Barium 544 1011 625 1157 716 1321 82 9 1524 1403 2557 Beryllium 1029 188 4 1130 2066 1246 2275 1395 2543 Bismuth 536 997 609 11 28. .. desired composition Heat- up time from ambient to operating temperatures of 81 5 to 87 0 °C (1500 to 1600 °F) typically takes 3 to 4 h Fig 10 Fluidized-bed furnace with external heating by electrical resistance elements External-Combustion-Heated Fluidized Beds A fluidized bed contained in a heat- resisting pot can be heated by external gas firing (Fig 11) In this arrangement, a fuel-air mixture is introduced... contents of the bed achieves uniform heat- treating temperature Newer furnace designs extend fluidized-bed technology into the higher temperature ranges (540 to 1040 °C, or 1000 to 1900 °F) required for most common heat treatments Principles of Fluidized-Bed Heat Treating In fluidization, a bed of dry, finely divided particles, typically aluminum oxide in the heat- treating context, is made to behave... Fig 14 Two-stage, gas-fired, internal-combustion fluidized beds (a) Three-chamber design (b) Back-radiation design in heating mode (c) Back-radiation design in cooling mode Internal-resistance-heated fluidized beds are not accepted by users The elements and work load will make contact if insufficient care is taken Applications of Fluidized-Bed Furnaces The potential applications of fluidized-bed technology... Cobalt 1362 2 484 1494 2721 1650 3000 183 3 3331 Copper 1035 189 5 1141 2 086 1273 2323 1432 2610 2762 5003 Gallium 85 9 15 78 965 1769 1093 1999 12 48 22 78 Germanium 996 182 5 1112 2034 1251 2 284 1420 2590 Gold 1190 2174 1316 2401 1465 2669 1646 2995 2996 5425 Indium 746 1375 84 0 1544 952 1746 1090 1990 Iridium 2154 3909 2340 4244 2556 4633 281 1 5092 Iron 1195 2 183 1310 2390 1447 2637 1602 2916... Ruthenium 20 58 3736 2230 4046 2431 44 08 2666 483 1 Scandium 1161 2122 1 282 2340 1422 2593 1595 2903 Silicon 1116 2041 1223 2233 1343 2449 1 485 2705 2 287 4149 Silver 84 8 15 58 920 1 688 1047 1917 1160 2120 2212 4014 Sodium 195 383 2 38 460 291 556 356 673 89 2 16 38 . 1 0-2 0 1 0-2 0 73 5-9 55 135 0-1 750 4 -8 4 -8 95 5-1 175 175 0-2 150 3-4 3-4 101 0-1 285 185 0-2 350 1-3 1-3 Immersed-electrode furnaces Furnace C 53 5-7 35 100 0-1 350 2-4 (a) 4-5 73 5-9 55. Submerged-electrode furnaces Furnace A 53 5-7 35 100 0-1 350 1 5-2 5 1 5-2 5 73 5-9 55 135 0-1 750 6-1 2 6-1 2 95 5-1 175 175 0-2 150 5-7 5-7 101 0-1 285 185 0-2 350 2-4 2-4 Furnace B 53 5-7 35 100 0-1 350. 135 0-1 750 1-2 (a) 2-3 95 5-1 175 175 0-2 150 1 2 -1 (a) 1-2 101 0-1 285 185 0-2 350 1 4 - 1 2 (a) 1 1 2 Furnace D 53 5-7 35 100 0-1 350 2-4 (a) 4-5 73 5-9 55 135 0-1 750 1-2 (a) 2-3