Volume 04 - Heat Treating Part 6 potx

Fig. 1 Effect of time on case depth at 925 °C (1700 °F) Steel Composition. Any carburizing grade of carbon or alloy steel is suitable for pack carburizing. It is generally agreed that the diffusion rate of carbon in steel is not markedly influenced by the chemical composition of the steel. Chemical composition does have an effect on the activity of carbon and thus can affect the carbon level at saturation for a particular temperature. Depth of Case. Even with good process control, it is difficult to obtain parts with total case-depth variation of less than 0.25 mm (0.010 in.) from maximum to minimum in a given furnace load, assuming a carburizing temperature of 925 °C (1700 °F). Commercial tolerances for case depths obtained in pack carburizing begin at ±0.25 mm (±0.010 in.), and, for deeper case depths, increase to ±0.8 mm (±0.03 in.). Lower carburizing temperatures provide some reduction in case- depth variation because variation in the time required for all parts of the load to reach carburizing temperature becomes a smaller percentage of total furnace time. Because of the inherent variation in case depth and the cost of packing materials, pack carburizing normally is not used on work requiring a case depth of less than 0.8 mm (0.03 in.). Typical pack- carburizing temperatures selected to produce different case depths on a variety of production parts are given in Table 1. Table 1 Typical applications of pack carburizing Dimensions (a) Carburizing OD OA Weight Case depth to 50 HRC Temperature Part mm in. mm in. kg lb Steel mm in. °C °F Mine-loader bevel gear 102 4.0 76 3.0 1.4 3.1 2317 0.6 0.024 925 1700 Flying-shear timing gear 216 8.5 92 3.6 23.6 52.0 2317 0.9 0.036 900 1650 Crane-cable drum 603 23.7 2565 101.0 1792 3950 1020 1.2 0.048 955 1750 High-misalignment coupling gear 305 12.0 152 6.0 38.5 84.9 4617 1.2 0.048 925 1700 Continuous-miner drive pinion 127 5.0 127 5.0 5.4 11.9 2317 1.8 0.072 925 1700 Heavy-duty industrial gear 618 24.3 102 4.0 150 331 1022 1.8 0.072 940 1725 Motor-brake wheel 457 18.0 225 8.9 104 229 1020 3.0 0.120 925 1700 High-performance crane wheel 660 26.0 152 6.0 335 739 1035 3.8 0.150 940 1725 Calender bull gear 2159 85.0 610 24.0 5885 12,975 1025 4.0 0.160 955 1750 Kiln-trunnion roller 762 30.0 406 16.0 1035 2280 1030 4.0 0.160 940 1725 Leveler roll 95 3.7 794 31.3 36.7 80.9 3115 4.0 0.160 925 1700 Blooming-mill screw 381 15.0 3327 131.0 2950 6505 3115 5.0 0.200 925 1700 Heavy-duty rolling-mill gear 914 36.0 4038 159.0 11,800 26,015 2325 5.6 0.220 955 1750 Processor pinch roll 229 9.0 5385 212.0 1700 3750 8620 6.9 0.270 1050 1925 (a) OD, outside diameter; OA, overall (axial) dimension Distortion normally becomes more pronounced as processing temperature is increased. In some instances, carburizing temperature is selected on the basis of the maximum amount of distortion that can be tolerated. In any case, following proper container packing procedures will help minimize distortion. Furnaces for Pack Carburizing The suitability of a furnace for pack carburizing depends on its ability, at reasonable cost, to: provide adequate thermal capacity and temperature uniformity (furnaces must be controllable to within ±5 °C, or ±9 °F, and must be capable of uniform through heating to within ±8 to ±14 °C, or ±14 to ±25 °F); and provide adequate support for containers and workpieces at the required temperatures. Modern heating systems and furnace construction provide ample heating capacity and temperature uniformity over a wide range of temperatures. A variation of ±8 °C (± 14 °F) throughout the entire working section of a large furnace can be easily maintained. Many furnaces incorporate automatic compensation for heat losses at doors or other connection points. Combustion systems that maintain constant pressure or constant flow permit close temperature control on variable loads. Zoning is also a major contributor to control. To maintain good uniformity, it is necessary to load the furnace as uniformly as possible and to allow adequate space between containers 50 to 100 mm (2 to 4 in.) or more to permit circulation of the heating gases. The three types of furnaces most commonly used for pack carburizing are the box, car-bottom, and pit types. Box furnaces are loaded by mechanical devices or by in-plant transportation equipment. Car-bottom furnaces provide for convenient loading of heavy units. A car-bottom furnace with a car at each end allows a second car to be loaded while the furnace is in use, which minimizes the heat loss and downtime between batches. Pit furnaces are general-purpose furnaces that may be used for carburizing and other heat-treating operations that require minimum floor space. Adequate support of containers and workpieces does much to minimize distortion. It also helps maintain the shape and extend the life of carburizing containers. Three or more points of support should be used in car-bottom furnaces. The container should be blocked above the hearth to allow circulation around, and proper shimming of, the container. In box- type furnaces, silicon carbide and certain other hearth materials, provide excellent wear resistance to maintain the shape of the hearth. Their high thermal conductivity helps promote temperature uniformity. Furnaces for pack carburizing have a minimum number of parts that are subject to high wear or that require frequent maintenance. Very few alloy parts inside the furnace are subjected to thermal fatigue, and a minimum of auxiliary equipment is needed. The personnel who operate these furnaces do not need extensive technical training. (For more information on equipment, see the next Section, "Heat-Treating Equipment," in this Volume.) Carburizing Containers Materials. Carburizing containers are made of carbon steel, of aluminum-coated carbon steel, or of iron-nickel- chromium heat-resisting alloys. Although uncoated carbon steel boxes scale severely during carburizing and have short lives, they often are the most economical for processing odd lots and unusual shapes. Aluminum coating can significantly extend the life of a carbon steel container, making this material potentially the lowest in cost per hour per unit weight carburized. In the long run, heat-resisting alloys are the most economical container materials for carburizing large numbers of moderate-size parts. However, because heat-resisting alloys are considerably higher in initial cost than plain or aluminum- coated carbon steel, they must be used continuously if they are to approach the lowest possible prorated cost. Design and Construction. For containers of all three materials, the trend has been toward lighter construction from sheet or plate, rather than the heavier cast construction. These lighter containers require ribbing, corrugating, or other bracing methods to make them rigid enough to withstand long periods at high temperature. Containers often are equipped with braced lifting eyes or hooks, special lid-receiving sections, and test-pin openings. A carburizing container should be no larger than necessary. If possible, it should be narrow in at least one dimension to promote uniform heating of the contents. A properly designed box will provide a cooling rate high enough to minimize formation of a carbide network in the case, but low enough to avoid distortion or excessive hardening. Lid Construction. Lids for carburizing containers vary from simple sheet-metal plates to built-up lids of metal and refractory material. The lid may add rigidity to the container. It must be tight enough to prevent air from entering and burning the compound, yet not so tight as to prevent easy expulsion of excess gas generated within the container. Lids must be capable of venting the container, and the venting means must be able to withstand the intense heat liberated by combustion of flammable gas. Lids that fit too loosely can be partly sealed with clay-base cements. Conditioning. Before new alloy carburizing containers are placed in service, they may be conditioned by "precarburizing" without a work load. This pretreatment eliminates the possibility of the container, rather than the work load, being carburized during the first production carburizing cycle. Packing Intimate contact between compound and workpiece is not necessary; however, when properly packed, the compound will provide good support for the workpiece. The layer of compound surrounding the work must be heavy enough to allow for shrinkage and to maintain a high carbon potential during the entire cycle, but not so heavy as to unduly retard heating of the workpiece to carburizing temperature. If the container can be designed to conform to the shape of the workpiece, the compound will be of both uniform and minimum thickness. Work-load density that is, net weight (piece weight) divided by gross weight (weight of the carburizing container, compound, and workpieces) is an important factor in the efficiency of pack carburizing, because it affects heating and cooling time. The smaller this percentage, the lower the relative efficiency of the process. Table 2 shows work-load densities for three different carburized parts. Table 2 Work-load densities in pack carburizing Dimensions (a) Weight per piece OD OA Net Total (b) Part mm in. mm in. kg lb kg lb Net weight, % of gross weight Roll 75 3 1220 48 37 82 72 159 51 Crane wheel 560 22 125 5 130 287 150 331 87 Gear 660 26 205 8 285 628 440 970 65 (a) OD, outside diameter; OA, overall (axial) dimension. (b) Total weight of work plus packing material plus container, divided by number of pieces in pack Procedure. Packing of the workpieces in a compound is a dusty and disagreeable operation (one of the reasons this process is losing favor in industry). For this reason, grouping of boxes, workpieces, and compound should be carefully planned so as to minimize handling of the compound. If possible, workpieces should come to the packer already stacked and sorted, preferably on open trays or in pans. First, a layer of compound from 13 to 50 mm ( 1 2 to 2 in.) deep is placed in the empty box. The part or parts are then stacked in the container, and, if necessary, metal or ceramic supports or spacers are applied and internal container supports are inserted. Whenever possible, workpieces should be packed with the longest dimension vertical to the base of the container. This is extremely important in processing long parts such as shafts and rolls because it minimizes the tendency of these parts to sag. Suspension of the work within the container or within the furnace is useful in minimizing distortion in relatively thin or delicate parts. For applications where small teeth or small holes are to be uniformly carburized, a 6- or 8-mesh material should be used to ensure good filling. After the compound is sufficiently tamped, a final layer is placed on top of the parts. The thickness of the top layer varies according to the type of work, depth of case, type of container, and shrinkage rate of the compound, but it should be adequate to ensure that the work will be covered after shrinkage and other movements have occurred. A minimum depth of 50 mm (2 in.) is recommended. In the final step, the lid is put in place. Process-Control Specimens. In order to control and evaluate the carburizing process, test pins or shims normally are included in the charge. To provide valid results, section sizes and locations of test specimens must closely approximate those of the workpieces. Placing a test pin close to a workpiece often will produce a thermal history identical to that of the workpiece. For control purposes, many containers are equipped with a test-pin section that can be removed from the load during the carburizing cycle. After the pins have been quenched and fractured, case-depth readings made on them aid in evaluating whether satisfactory carburizing results are being obtained and in determining when the prescribed case depth has been attained. Selective Carburizing Stop-off techniques described in the article on gas carburizing in this Volume apply to selective carburization by pack carburizing. In addition, it may be possible to permit any portion of a part that is not to be carburized to protrude from the carburizing container. Alternatively, an inert or lightly oxidizing material may be packed around those areas of a part that are not to be carburized. Liquid Carburizing and Cyaniding of Steels Revised by Arthur D. Godding, Heatbath Corporation Introduction LIQUID CARBURIZING is a process used for case hardening steel or iron parts. The parts are held at a temperature above Ac 1 in a molten salt that will introduce carbon and nitrogen, or carbon alone, into the metal. Diffusion of the carbon from the surface toward the interior produces a case that can be hardened, usually by fast quenching from the bath. Carbon diffuses from the bath into the metal and produces a case comparable with one resulting from gas carburizing in an atmosphere containing some ammonia. However, because liquid carburizing involves faster heat-up (due to the superior heat-transfer characteristics of salt bath solutions), cycle times for liquid carburizing are shorter than those for gas carburizing. Most liquid carburizing baths contain cyanide, which introduces both carbon and nitrogen into the case. One type of liquid carburizing bath, however, uses a special grade of carbon, rather than cyanide, as the source of carbon. This bath produces a case that contains only carbon as the hardening agent. Liquid carburizing may be distinguished from cyaniding (which is performed in a bath containing a higher percentage of cyanide) by the character and composition of the case produced. Cases produced by liquid carburizing are lower in nitrogen and higher in carbon than cases produced by cyaniding. Cyanide cases are seldom applied to depths greater than 0.25 mm (0.010 in.); liquid carburizing can produce cases as deep as 6.35 mm (0.250 in.). For very thin cases, liquid carburizing in low-temperature baths may be employed in place of cyaniding. Cyanide-Containing Liquid Carburizing Baths Light case and deep case are arbitrary terms that have been associated with liquid carburizing in baths containing cyanide. There is necessarily some overlapping of bath compositions for the two types of case. In general, the two types are distinguished more by operating temperature or by cycle times than by bath composition. Therefore, the terms low temperature and high temperature are preferred. Both low-temperature and high-temperature baths are supplied in different cyanide contents to satisfy individual requirements of carburizing activity (carbon potential) within the limitations of normal dragout and replenishment. In many instances, compatible companion compositions are available for starting the bath or for bath make-up, and for regeneration or maintenance of carburizing potential. Low-temperature cyanide-type baths (light-case baths) are those usually operated in the temperature range from 845 to 900 °C (1550 to 1650 °F), although for certain specific effects this range is sometimes extended to 790 to 925 °C (1450 to 1700 °F). Low-temperature baths are best suited to formation of shallower cases. Low-temperature baths are generally of the accelerated cyanogen type containing various combinations and amounts of the constituents listed in Table 1 and differ from cyaniding baths in that the case produced by a low-temperature bath consists predominantly of carbon. Low-temperature baths are usually operated with a protective carbon cover; however, when the carbon cover on a low-temperature bath is thin, the nitrogen content of the carburized case will be relatively high. Cyaniding baths produce cases that are about 0.13 to 0.25 mm (0.005 to 0.010 in.) deep and that contain appreciable amounts of nitrogen. Table 1 Operating compositions of liquid carburizing baths Composition of bath, % Constituent Light case, low temperature Deep case, high temperature 845-900 °C (1550-1650 °F) 900-955 °C (1650-1750 °F) Sodium cyanide 10-23 6-16 Barium chloride . . . 30-55 (a) Salts of other alkaline earth metals (b) 0-10 0-10 Potassium chloride 0-25 0-20 Sodium chloride 20-40 0-20 Sodium carbonate 30 max 30 max Accelerators other than those involving compounds of alkaline earth metals (c) 0-5 0-2 Sodium cyanate 1.0 max 0.5 max Density of molten salt 1.76 g/cm 3 at 900 °C (0.0636 lb/in. 3 at 1650 °F) 2.00 g/cm 3 at 925 °C (0.0723 lb/in. 3 at 1700 °F) (a) Proprietary barium chloride-free deep-case baths are available. (b) Calcium and strontium chlorides have been employed. Calcium chloride is more effective, but its hygroscopic nature has limited its use. (c) Among these accelerators are manganese dioxide, boron oxide, sodium fluoride, and sodium pyrophosphate. In a low-temperature cyanide-type bath, several reactions occur simultaneously, depending on bath composition, to produce various end products and intermediates. These reaction products include the following: carbon (C), alkali carbonate (Na 2 CO 3 or K 2 CO 3 ), nitrogen (N 2 or 2N), carbon monoxide (CO), carbon dioxide (CO 2 ), cyanamide (Na 2 CN 2 or BaCN 2 ), and cyanate (NaNCO). Two of the major reactions believed to occur in the operating bath are the cyanamide shift and the formation of cyanate: 2NaCN ↔ Na 2 CN 2 + C (Eq 1) and either 2NaCN + O 2 → 2NaNCO (Eq 2) or NaCN + CO 2 ↔ NaNCO + CO (Eq 3) Reactions that influence cyanate content proceed as follows: NaNCO + C → NaCN + CO (Eq 4) and either 4NaNCO + 2O 2 → 2Na 2 CO 3 + 2CO + 4N (Eq 5) or 4NaNCO + 4CO 2 → 2Na 2 CO 3 + 6CO + 4N (Eq 6) Equations 5 and 6 deplete the activity of the bath and lead to an eventual loss of carburizing effectiveness unless suitable replenishment practice is followed. Equations 1 and 3 are at least partly reversible. Reactions that produce either carbon monoxide or carbon are beneficial in producing the desired carburized case, as for example: Fe + 2CO → Fe[C] + CO 2 (Eq 7) and Fe + C → Fe[C] (Eq 8) Low-temperature (light-case) baths are usually operated at higher cyanide contents than high-temperature (deep-case baths). The preferred operating cyanide contents shown in Table 2 provide a case that is essentially eutectoidal (>0.80% C). If a hypoeutectoid (<0.80% C) case is desired, the bath is operated at the lower end of the temperature/cyanide range. Conversely, operation at the higher end of the suggested range favors formation of a hypereutectoid surface carbon content. Table 2 Relation of operating temperature to sodium cyanide content in barium- activated liquid carburizing baths Temperature NaCN, % °C °F min Preferred max (a) 815 1500 14 18 23 845 1550 12 16 20 870 1600 11 14 18 900 1650 10 12 16 925 1700 8 10 14 955 1750 6 8 12 (a) The maximum limits are based on economy. If 30% NaCN is exceeded, there is danger that NaCN will break down, with production of carbon scum and attendant frothing. To correct such a condition, the bath temperature should be lowered and the NaCN content should be adjusted to the preferred value. High-temperature cyanide-type baths (deep-case baths) are usually operated in the temperature range from 900 to 955 °C (1650 to 1750 °F). This range may be extended somewhat, but at lower temperatures the rate of carbon penetration decreases, and at temperatures higher than about 955 °C (1750 °F), deterioration of the bath and equipment is markedly accelerated. However, rapid carbon penetration can be obtained by operating at temperatures between 980 and 1040 °C (1800 and 1900 °F). High-temperature baths are used for producing cases 0.5 to 3.0 mm (0.020 to 0.120 in.) deep. In some instances, even deeper cases are produced (up to about 6.35 mm, or 0.250 in.), but the most important use of these baths is for the rapid development of cases 1 to 2 mm (0.040 to 0.080 in.) deep. These baths consist of cyanide and a major proportion of barium chloride (Table 1), with or without supplemental acceleration from other salts of alkaline earth metals. Although the reactions shown for low-temperature liquid carburizing salts apply in some degree, the principal reaction is the so- called cyanamide shift. This reaction is reversible: Ba(CN) 2 ↔ BaCN 2 + C (Eq 9) In the presence of iron, the reaction is: Ba(CN) 2 + Fe → BaCN 2 + Fe[C] (Eq 10) Cases produced in high-temperature liquid carburizing baths consist essentially of carbon dissolved in iron. However, sufficient nascent nitrogen is available to produce a superficial nitride-containing skin, which aids in resisting wear and which also resists softening during tempering and other heat treatments requiring higher than normal operating temperatures. Combination Treatment. It is not uncommon for the carburizing cycle to be initiated in a high-temperature bath and then for the work load to be transferred to a low-temperature carburizing bath. Not only does this practice provide a maximum rate of carburizing, but quenching the work from the low-temperature bath reduces distortion and minimizes retained austenite. Cyaniding (Liquid Carbonitriding) Cyaniding, or salt-bath carbonitriding, is a heat-treating process that produces a file-hard, wear-resistant surface on ferrous parts. When steel is heated above Ac 1 in a suitable bath containing alkali cyanides and cyanates, the surface of the steel absorbs both carbon and nitrogen from the molten bath. When quenched in mineral oil, paraffin-base oil, water, or brine, the steel develops a hard surface layer, or case, that contains less carbon and more nitrogen than the case developed in activated liquid carburizing baths. Because of greater efficiency and lower cost, sodium cyanide is used instead of the more expensive potassium cyanide. The active hardening agents of cyaniding baths carbon monoxide and nitrogen are not produced directly from sodium cyanide. Molten cyanide decomposes in the presence of air at the surface of the bath to produce sodium cyanate, which in turn decomposes in accordance with the following chemical reactions: 2NaCN + O 2 → 2NaNCO (Eq 11) 4NaNCO → Na 2 CO 3 + 2NaCN + CO + 2N (Eq 12) 2CO → CO 2 + C (Eq 13) NaCN + CO 2 → NaNCO + CO (Eq 14) The rate at which cyanate is formed and decomposes, liberating carbon and nitrogen at the surface of the steel, determines the carbonitriding activity of the bath. At operating temperatures, the higher the concentration of cyanate, the faster the rate of its decomposition. Because the rate of cyanate decomposition also increases with temperature, bath activity is greater at higher operating temperatures. A fresh cyaniding bath must be aged for about 12 h at a temperature above its melting point to provide a sufficient concentration of cyanate for efficient carbonitriding activity. For the aging cycle to be effective, any carbon scum formed on the surface must be removed. To eliminate scum, the cyanide content of the bath must be reduced to the 25 to 30% range by addition of inert salts (sodium chloride and sodium carbonate). At the bath aging temperature usually about 700 °C (1290 °F) the rate of its decomposition is low. Bath Composition. A sodium cyanide mixture such as grade 30 in Table 3, containing 30% NaCN, 40% Na 2 CO 3 , and 30% NaCl, is generally used for cyaniding on a production basis. This mixture is preferable to any of the other compositions given in Table 3. The inert salts sodium chloride and sodium carbonate are added to cyanide to provide fluidity and to control the melting points of all mixtures. The 30% NaCN mixture, as well as the mixtures containing 45, 75, and 97% NaCN, may be added to the operating bath to maintain a desired cyanide concentration for a specific application. Table 3 Compositions and properties of sodium cyanide mixtures Composition, wt% Melting point Specific gravity Mixture grade designation NaCN NaCO 3 NaCl °C °F 25 °C (75 °F) 860 °C (1580 °F) 96-98 (a) 97 2.3 Trace 560 1040 1.50 1.10 75 (b) 75 3.5 21.5 590 1095 1.60 1.25 45 (b) 45.3 37.0 17.7 570 1060 1.80 1.40 30 (b) 30.0 40.0 30.0 625 1155 2.09 1.54 (a) Appearance: white crystalline solid. This grade also contains 0.5% sodium cyanate (NaNCO) and 0.2% sodium hydroxide (NaOH); sodium sulfide (Na 2 S) content, nil. (b) Appearance: white granular mixture The carbon content of the case developed in cyanide baths increases with an increase in the cyanide concentration of the bath, thus providing considerable versatility. A bath operating at 815 to 850 °C (1500 to 1560 °F) and containing 2 to 4% cyanide may be used to restore carbon to decarburized steels with a core carbon content of 0.30 to 0.40% C, while a 30% cyanide bath at the same temperature will yield a 0.13 mm (0.005 in.) case containing 0.65% C at the surface in 45 min. Sodium cyanide concentration also has some effect on case depth, as shown for 1020 steel in Table 4. Table 4 Effect of sodium cyanide concentration on case depth in 1020 steel bars Samples are 25.4 mm diam (1.0 in. diam) bars that were cyanided 30 min at 815 °C (1500 °F). Depth of case NaCN in bath, % mm in. 94.3 0.15 0.0060 76.0 0.18 0.0070 50.8 0.15 0.0060 43.0 0.15 0.0060 30.2 0.15 0.0060 20.8 0.14 0.0055 15.1 0.13 0.0050 10.8 0.10 0.0040 Noncyanide Liquid Carburizing Liquid carburizing can be accomplished in a bath containing a special grade of carbon instead of cyanide as the source of carbon. In this bath, carbon particles are dispersed in the molten salt by mechanical agitation, which is achieved by means of one or more simple propeller stirrers that occupy a small fraction of the total volume of the bath. Agitation is also believed to introduce greater exposure and absorption of oxygen in the air. The chemical reaction involved is not fully understood, but is thought to involve adsorption of carbon monoxide on carbon particles. The carbon monoxide is generated by reaction between the carbon and carbonates, which are major ingredients of the molten salt. The adsorbed carbon monoxide is presumed to react with steel surfaces much as in gas or pack carburizing. Operating temperatures for this type of bath are generally higher than those for cyanide-type baths. A range of about 900 to 955 °C (1650 to 1750 °F) is most commonly employed. Temperatures below about 870 °C (1600 °F) are not recommended and may even lead to decarburization of the steel. The case depths and carbon gradients produced are in the same range as for high-temperature cyanide-type baths (see Fig. 1, 2, 3 for data on carbon and low-alloy steels), but there is no nitrogen in the case. The carbon content is slightly lower than that of standard carburizing baths that contain cyanide. [...]... baths Weight Part Depth of case Temperature mm Steel in °C °F Time, h Quench Subsequent treatment Hardness, HRC kg lb Adapter 0.9 2 CR 1.0 0 .040 940 1720 4 AC (a) 6 2 -6 3 Arbor, tapered 0.5 1.1 1020 1.5 0. 060 940 1720 6. 5 AC (a) 6 2 -6 3 Bushing 0.7 1.5 CR 1.5 0. 060 940 1720 6. 5 AC (a) 6 2 -6 3 Die block 3.5 7.7 1020 1.3 0.050 940 1720 5 AC (a) 6 2 -6 3 1.1 2.5 CR 1.3 0.050 940 1720 5 AC (a) 5 9 -6 1 Disk 1.4 3... (a) 6 2 -6 3 1.1 2.5 CR 1.3 0.050 940 1720 5 AC (a) 5 9 -6 1 Disk 1.4 3 1020 1.3 0.050 940 1720 5 (b) (b) 5 6- 5 7 Flange 0.03 0. 06 1020 0. 4-0 .5 0.0150.020 845 1550 4 Oil (c) 55 min(d) 0.09 0.2 1020 1.5 0. 060 940 1720 6. 5 AC (a) 6 2 -6 3 0.9 2 CR 1.0 0 .040 940 1720 4 AC (a) 6 2 -6 3 Carbon steel Gage knurled rings, Hold-down block ... piece of low-carbon steel or iron-nickel-chromium alloy; a composition of Fe35Ni-15Cr is usually preferred for the latter Less-expensive welded pots may be fabricated from either of these materials In a well-designed furnace, life of round alloy pots will vary with maximum operating temperature as follows: Operating temperature Service life, months °C °F 845 1550 9-1 2 870 160 0 6- 9 900 165 0 3 -6 In one... Thermocouple protection tubes Fixtures Baskets Externally heated Submerged 3 5-1 8(c) Carbon steel(d) 4 46 Carbon steel 4 46 3 5-1 8(c) 3 5-1 8(c) 3 5-1 8(c) 3 5-1 8(c) 3 5-1 8(c) Inconel Inconel Cast Immersed Inconel Wrought(b) Internally heated Inconel Inconel HT (e) HT (e) (e) (e) HT (a) When more than one material is recommended for a specific part, each has proved satisfactory in service Multiple choices... Burrows, Durofer A Low-Toxicity Salt-Bath Carburizing Process, Heat Treat Met., Vol 4, 1987 4 P Astley, Liquid Nitriding: Development and Present Applications, Heat Treatment '73, Book No 163 , The Metals Society, 1975, p 9 3-9 7 5 P Astley, Tufftride A New Development Reduces Treatment Costs and Process Toxicity, Heat Treat Met., Vol 2, 1975, p 5 1-5 4 6 H Kunst and B Beckett, Cyanide-Free Regenerator for... Carburizing, Heat Treatment '84, Book No 312, The Metals Society, 1984, p 16. 1-1 6. 5 7 R Engelmann, Paper presented at the 39th Heat Treatment Colloquium, Wiesbaden, West Germany, 5-7 Oct 1983 8 C Skidmore, Salt Bath Quenching A Review, Heat Treat Met., Vol 13, 19 86, p 3 4-3 8 Carbon Gradients Figure 1 shows carbon gradients produced by liquid carburizing 1020 steel bars at 845, 870, and 955 °C (1550, 160 0, and... "Salt Bath Equipment" in this Volume Externally Heated Furnaces Externally heated furnaces may be fired by gas or oil, or may be heated by means of electrical-resistance elements Gas-fired or oil-fired furnaces similar in design to the one shown in Fig 8(a) are commonly used for liquid carburizing These furnaces are generally lower in initial cost than electrode or resistance-heated furnaces and are simple... and Tempered 861 5H Steel Gear with 60 to 62 HRC Surface Hardness The small gear shown in Fig 13(a) closed in along the bore from a minimum dimension of 17.22 mm (0 .67 80 in.) prior to heat treatment to a minimum of 17.14 mm (0 .67 50 in.) after heat treatment In contrast, only slight contraction of the outer bearing surface occurred The gears, made of 861 5H steel, were carburized at 915 °C ( 167 5 °F) to a... Nitrogen, % Cyanided at 760 °C (1400 °F) 1008 0.038 0.0015 0.152 0.0 06 0 .68 0.51 1010 0.038 0.0015 0.152 0.0 06 0.70 0.50 1022 0.051 0.0020 0.203 0.008 0.72 0.51 Cyanided at 845 °C (1550 °F) 1008 0.0 76 0.0030 0.203 0.008 0.75 0. 26 1010 0.0 76 0.0030 0.203 0.008 0.77 0.28 1022 0.089 0.0035 0.254 0.010 0.79 0.27 (a) Carbon and nitrogen contents were determined from analysis of the outermost 0.0 76 mm (0.003 in.)... 0 .64 mm (0.020 to 0.025 in.), reheated to 840 °C (1540 °F), quenched in oil at 55 °C (130 °F), and then tempered at 190 °C (375 °F) to a surface hardness of 60 to 62 HRC Fig 13 Dimensional data relating selected low-alloy steel production parts before and after liquid carburizing and hardening AC, air cooled; OQ, oil quenched Example 2: Carburized, Quenched, and Stress Relieved 862 0 Steel Gear with 61 . 84 5-9 00 °C (155 0-1 65 0 °F) 90 0-9 55 °C ( 165 0-1 750 °F) Sodium cyanide 1 0-2 3 6- 1 6 Barium chloride . . . 3 0-5 5 (a) Salts of other alkaline earth metals (b) 0-1 0 0-1 0 Potassium chloride 0-2 5. 0.0 36 900 165 0 Crane-cable drum 60 3 23.7 2 565 101.0 1792 3950 1020 1.2 0 .048 955 1750 High-misalignment coupling gear 305 12.0 152 6. 0 38.5 84.9 461 7 1.2 0 .048 925 1700. 1025 4.0 0. 160 955 1750 Kiln-trunnion roller 762 30.0 4 06 16. 0 1035 2280 1030 4.0 0. 160 940 1725 Leveler roll 95 3.7 794 31.3 36. 7 80.9 3115 4.0 0. 160 925 1700 Blooming-mill screw