Heat treatment for steel

Normalizing- of Steel Revised by Thomas Ruglic, Hinderliter Heat Treating, Ine. NORMALIZING OF STEEL is a heattreating process that is often considered from both thermal and microstructural standpoints. In the thermal sense, normalizing is an austenitizing heating cycle followed by cooling in stilI or slightly agitated air. Typicaiiy, the work is heated to a temperature about 55 oC (100 OF)above the upper critica! line of the iron-iron carbide phase diagram, as shown in Fig i; that is, above Ac) for hypoeutectoid steels and above Acm for hypereutectoid steels. To be properly classed as a normalizing treatment, the heating portion of the process must produce a homogeneous austenitic phase (face-centered cubic, or fcc, crystal structure) prior to cooling. Figure 2 compares the time-temperature cycle of normalizing to that of full annealing. Typical normalizing temperatures for many standard steels are given in Table i. Normalizing is also frequently thought of in terms of microstructure. The areas of the microstructure that contain about 0.8% C are pearlitic (lamellae of ferrite and iron carbide). The areas that are low in carbon are ferritic (body-centered cubic, or bcc, crystal structure). In hypereutectoid steels, proeutectoid iron carbide tirst form s along austenite grain boundaries. This transformation continues until the carbon level in the austenite reaches approximately 0.8%, at which time a eutectoid reaction begins as indieated by the formation of pearlite. Airhardening steels are excluded from the class of normalized steels because they do not exhibit the normal pearlitic microstrueture that characterizes normalized steels. Applications of Normalizing Based on Steel Classification A broad range of ferrous products can be normalized. All of the standard low-earbon, medium-carbon, and high-carbon wrought steeJs can be normalized, as' well as many castings. Many steel weldments are normalized to retine the structure within the weldaffeeted area. Austenitic steels, stainless steels, and maraging steels either cannot be normalized or are not usually normalized. Tool steels are generaiiy annealed by the steel supplier. The purpose of normaliziQg varies co nsiderably. Normalization may increase or decrease the strength and hardness of a given steel in a given product form, depending on the thermal and mechanical history of the product. Actually, the funetions of normalizing may overlap with or be confused with those of annealing, hardening, and stress relieving. Improved maehinabilHeating cycle Cooling cycle 1000 1800 900 ~ 1600 800 ai ol i Q; :J :; LO 1400 1§ a. E 1200 ~ ~ 700 f- ol 600 1000 500 O 0.3 0.6 0.9 "/o 1.2 1.5 Time Carbon. Fig 2 Comparison normalizing of time-temperature and full annealing. cycles for The slower ity, grain-structure retinement, homogenization, and moditication of residual stresses are among the reasons normalizing is done. Homogenization of castings by nonnalizing may be done in order to break up or retine the dendritic structure and facilitate a more even response to subsequent hardening. Similarly, for wrought products, normalization can help reduce banded grain structure due to hot rolling, as weii as large grain size or mixed large and smail grain size due to forging practice. The details of normalizing treatments applied to three typical production parts are given in Table 2, which also lists the reasons for normalizing and gives some of the mechanical properties obtained in the normalized and tempered condition. Comparisons of typical hot-rolled or annealed mechanical properties versus typical normalized properties are presented in Table 3. Figure 3 shows that high-carbon steels with large amounts of pearlite have high transition temperatures and therefore will fail in a brittle manner even well above room temperature. On the other hand, lowcarbon steels have subzero transition temperatures and are quite tough at room temperature (Ref I). Depending on the mechanical properties required, normalizing may be substituted for eonventional hardening when the size 'or shape of the part is such that liquid quenching might result in cracking, distortion, or excessive dimensional changes. Thus, parts. that are of complex shape or that incorporate sharp changes in section may be normalized and tempered, provided that the properties obtained are acceptable. The rate of heating generaiiy is not critical for normalizing; on an atomic scale, it is immateriill. In parts having great variations in section size, however, thermal stress can cause distortion. Time at temperature is criticalonly in that it must be sufficient to cause homogenization. Sufficient time mu st be allowed for solution of thermodynamically stable carbides, or for diffusion of constituen! atoms. Fig 1 steels Parlial iron-iron carbide phase diagram showing typical normalizing range for plain carbon transformalion ihari does and inpearlite and coarser microstructure ~ cooling of an{aling ferrite normalizing. Source: Ref 1 results higher iemperature ) tenitization is allafter therequired. [eeovers, at temperature, that is furnace 2ne hour Generally, time sufficient for CO~61ete aus) 81845 4820 5130 94840 OF 94830 94815 86845 . 94817 Grade 9310 9262 9260 9255 8742 8645 8650 5145 8627 5120 5046 6118 5140 5135 5132 60860 50850 9840 8822 8655 6120 oC oC 1525 1700 870 925 1600 900 870 1600 1600 925 4817 Temperalure(a) 870 8625 1575 925 1550 860 1650 1700 870 1650 925 900 1675 1625 900 830 885 845 1600 915 87(l 50846 50844 8740 8640 8637 8642 5160 5150 8622 5155 50840 8720 8620 8617 above.Typical tempcrature. The sieel should be coolcdfor stili aif from indicaled temperature. indicatcd normalizing temperatures in standard carbon and al/oy malizing temperature may vary from as much as 27 oC (50 OF) below. lo as much as 55 oC ((fOO / / OF) steels Temperalure(a) Table 1 / Heat Treating of Steel 36 Table 2 Typical applications of normalizing and tempering of steel components Part Casl 50 mm (2 in.) valve body. 19 lo in.) 25 mm W. lO in section thickness SI•• i lIeat trea(menl Propcrties .her treatment Tensile strength. 620 MPa (90 ksi); 0.2% yield slrength, 415 MPa (60 ksi); elongation in 50 mm, or 2 in., 20%; reduction in area, 40% Hardness, 200 lO 225 H8 Reason for normalizing Ni-Cr-Mo i Full annealed (1750 OF). normalized al 955 oC at 870 oC To meet mechanicalproperty "requirements (1600 OF). tcmpered at 665 oC (1225 OF) 4 37 Forged flange i Valve-bonnet forging 4140 Normalized at 870 oC (1600 OF). tempered at 570 oC (1060 OF) Normalized al 870 oc (1600 OF) and iempered Hardness, 220 to 240 HB To reline grain size and obtain required hardness To obtain uniform siructure. improved machinability, and required hardness per inch of part thickness, is considered to be standard. Parts often can be austenitized adequateIy in mu ch less time (with a saving of energy). In cases where normalizIng is done to homogenIze segregated structures, longer times may be required. The rate of cooling significantly influences both the amount of pearlite and the sIze and spacing of the pearlite lameIlae. At hIgher coolIng rates, more pearlite forms, and the lamellae are finer and more C10sely spaced. Both the increased amount of peari: Ite and the greater fineness of the pearlite result in higher strength and higher hardness. Conversely, lower cooling rates result in softer parts. The effect of mass on hardness (via its effect on cooling rate) Is illustrated by the data in Table 4. In any part havIng both thick and thin sections, the potential exists for variations in cooling rate, and thus for variatIons in strength and hardness as well. This can also increase the probability of distortion or even cracking. CoolIng rate sometImes Is enhanced with fans to increase strength and hardness of parts or to decrease the time required, following the furnace operatIon, for sufficient caoling of part s to permit convenient handling. After parts have cooled uniformly through their cross section to black heat below Ari (the part s are no longer red, as when they were removed from the fumace), they may be water or oII quenched to decrease the total cooling time. In heavy sectIons, coolIng of the center material to black heat may require considerable time. Thermal shock, residual thermally induced stress, and resultant dIstortions are factors to be considered. The mIcrostructure remains essentIally unaffected by the increased coolIng rate, provided that the entire mass is below the Iower critical temperature, Ari, although changes involving precIpitates may occur. Carbon Steels. Table lists typical normalizIng temperatures for some standard grades of carbon steel. These temperatures can be interpolated to obtain values for carbon contents not listed. Steels containing 0.20% C or less usually receIve no treatment subsequent to normalizing. However, medium-carbon or highcarbon steels are often tempered after normalIzing to obtain specific properties such as a lower hardness for straightenIng, cold working, or machining. Whether tempering is desirable depends on specific property requIrements and not on carbon content and section size requirements. Table 3 presents typIcal mechanical properties of selected carbon and al/oy steels in the hot-rolled, normalized, and annealed conditions. Because of pearlite lamellae and spacIng, a low-carbon or medIum-carbon steel of thin i h.'an a hIgh-carbon steel of large ,ubjected to the same treatment. section size ~ction may be harder after normalIzIng i . e3 ditions i !~~~4320 3812.2137 870 615 (1500 (1575 Mr.75131 4; 58 415 52 51 430 50126 61 88 420 60 139 31 50 20M II 59 62 61 440 262 22 30 17.3 M 460123 42 420 62197" 456 12 38 J" 25 69 95 147 87 9 8 87 450 725 620 26 55 30.2 28.0 71 18.8 85 19 20.7 405 38 60 34 46 59 295194187 Annealed at 815 1160 36.5 79 63 7 39 20.0 77 39.0 28 34.0 '-.22.51614 Normalized 830 119 241 385 235 104 229 5168 55 22.2 101 345095121 Normalized 860 4152 26'197 330 103 48 355121 36 105 87174 505 As-rolled at 740 54 70 269 107 36 22.3 23.0 26.: 35.8 25.0 36.0 90 78 13.0 48 91 52 73 -----l1-LO~v'l.Elongo-_____at860(1600385560 .. -lion(b),207 46Normalizedat 815 '" ___(1575 305445179 Annealed at Mr. A40 (1580 285665518 Annealed at 815 1015 (1600 350524143 325785183 370 135 45 115ll93 41 140 41163 118 34167 109 54170 585 92 63 485 71 46 135 27 17 570 73 66 132 217 112 363 609.5 217 690 293 70 269 38 255 12 302 13 54 460 33 615 100 125 21.8 21.7 33.0 37.0 28.6 75 80 73 83 62 17.0 16.0 7 43 70 83 89 67 490 965 425 815 750 520 595 550 505 360117 'i"\; 110 41 67 ll511.7 57 126 30 M 23 321 530 22.0 69 100 53 40 129 As-roiied 4014.0 45 600 18 61 22.7 24.5 28.2 81 70 380Normalized f\·lbr 420118 58 525 35 62 51 22.5 25.7 35.0 21.0 32.8 31.2 57 65 92 34.5 19.7 98 67 11.0 80 5 53 41 61 .. ------17.-7·--·-··.·. Annealed870 at .•860 ·1020(1600 365100 43 435III 21 430110 34 64 248 6g 380 18 29.0 2~.0 302~ 275 40 312.0 24.0 400 115 31 435 293 52 48 560 24.8 33.5 4 63 81 17.5 18.0 385 i43III 31.3 56 370109 37 136 79 50' 490 201 605 23.5 82 94 149 71 32.0 20.8 86 55 360846149 315383156 ___··47_· 106 M4 44 113 69192 655 18 49 39 7 56 580 20.2 26.8 23.7 91 99 76 74 84 85 89 54 84 95 6 148 655 675 525 430 635 460 705 600 585 625 775 395 17.2 315107 57 102 27 54 103 81 45 121 49 48 32 68 25.5 97 68 46 186 475 470 560 835 670 485 690 890 710 (1525 (1500 lci8 ii 68 795 510 745 580 730 685 570 475 14 58 1 860 212 Normalized at 23 1280(1600 725 760 540 635 665 76 940 960 650 565 820 695 715 (1500 910 775 930 24.7 9.0 870 870 (1520 Annealed at 82587n 1025 at 850 (I ~n ksi k.oçi LO strength oC Condition strength oC oC HB Reduction LoHardness. %Yicld or treatmentoC OF) in'foat'Cs, oC oC oC oC oC oC oC OF) OF) OF) OF) OF) OF) OF) OF)°F) __ OF) -- - - -----_._-- 7J lzod impact Normalizing of Steel / 37 38/ Heat Treating of Steel -238 250 -148 -58 32 Test temperature, oF 122 212 302 392 482 572 662 175 200 0.11%C 150 .., >. 150 f? .o 125 • .i:: ••... c Q) Q) 0.20% C 100 0.31% C i 0.41% C 0.49% C 0.60% C 0.69% C 0.80% C -100 -50 O >. f? u ~ 100 E c Q) Q) t> 75 50 ~ E 50 25 O O -150 50 100 150 oC 200 250 300 350 Test temperature, Fig 3 Change in impact transition curves with increas.ing pearlite content in normalized carbon sleels. Source: Ref 1 Alloy Steels. For alloy steel forgings, roIled products, and castings, normalizing is commonly used as a conditioning treatment before final heat treatment. Normalizing also refines the structures offorgings, rolled products, and castings that have cooled nonuniformly from high temperatures. Table lists typical normalizing temperatures for some standard alloy steels. Alloy carburizing steels such as 33 Lo and 4320 usually are normalized at temperatures higher than the carburizing temperature to minimize distortion in carburizing and to improve machining characteristics. Carburizing steels of the 3300 series sometimes are double normalized with the expectation of minimizing distortion; these steels are tempered at about 650 oC (1200 OF)for intervals of up to 15 h to reduce hardness to below 223 HB for machinability. Carburizing steels of the 4300 and 4600 series usually can be normalized to a hardncss not excecding 207 HB and therefore need not be tcmpered for machinability. Hypereutectoid alloy steels such as 52100 are normalized for partial or complete elimination of carbide networks, thus producing a structure that is more susceptible to 100% spheroidization in the subsequent spheroidize annealing treatment. The spheroidized structure provides improved machinability and a more uniform response to hardening. Some alloy grades require more can; in heating to prevent cracking from thermal shock. They also require longer soaking temperature in order to develop specific mechanical properties. This is a normalizing treatment only in the microstructural sense discussed in the introduction to this artiele. Forgings When forgings are normalized before carburizing or before hardening and tempering, the upper range of normalizing temperatures is used. However, when normalizing is the final heat treatment, use is made of the lower range of temperatures. Furnaces. Any appropriately sized fumace may be utilized for normalizing. Fumace type and size will depend upon the specific need. In a continuous fumace, forgings to be normalized are usually placed in shallow pans, and a pusher mechanism at the Ioading end of the fumace transports the pans through the fumace. Fumace burners located on both sides of the fumace fire below the hearth, and combustion products iise along the walls of the work-zone muffle and exhaust into the roof of the fumace. No atmosphere control is used. Combustion products enter the work zone through ports lining both sides of the entire hearth. A typical fumace is 9 m (30 ft) long and has 18gas bumers (or 9 oil bumers) on each side. For purposes of temperature control, such a furnace is divided into three 3 m (10 ft) zones, each having a vertical thermocouple extending into it through the roof of the fumace. Processing. Smail forgings are usually normalized as received from the forge shop. A typical fumace has five pans in each of the three fumace zones. Heating is adjusted so that the work reaches normalizing te mperaturc in the last zone. After passing through the last zone, the pans are dis- i charged onto a cooling conveyor. The work, while still in the pans, is cooled in stili air to below 480 oC (900 OF); it is then discharged into tote boxes, where it cools to room temperature. Total furnace time is approximately 3Yi h, but during this period the work is hel d at the normalizing temperature for only h. Normalizing of large open-die forgings usually is performed in batch-type fumaces pyrometiically controlled to narrow temperaturc ranges. Forgings are held at the normalizing temperaturc Jong enough to allow complete austenitizing and carbide solution to occur (usually one hour per inch of section thickness), and the n are cooled in still air. Axle-Shaft Forging. In forging an axle shaft made of fine-grained 1049 steeI, only one end of the forging bar was heated to upset the wheel-flange section. When, the part was examined in cross section from the flanged end to the cold end, the metallurgical conditions discussed below were revealed. The hot-worked flanged area of the axle exhibited a fine-grained structure as a result of the hot working at the forging temperature (approximately 1095 oC, or 2000 OF). However, a section adjacent to the flange, which also had been heated to the forging temperature but which had not been hot worked, exhibited a coarse-grained structure. Nearer the coo! end of the shaft, a zone that reached a temperature of about 705 oC (1300 OF) exhibited a spheroidized structure. The cold end of the shaft retained its initial fine grain size throughout the forging operation. ' In subsequent operations, this shaft was to be mechanically straightened, machined, and induction hardened. Because of the mixed grain structure, these operations posed several problems. The coarse-grained area adjacent to the flange was extremely weak in the transverse direction, and there was a possibility that fracture wouJd occur if this section wc re subjected to a severe straightening operation. The spheroidized area would not respond adequateJy to induction hardening because the solution rate of this type of carbide formation was too sluggish for the reIatively rapid rate of induction heating. Furthermore, the mixed metallurgical structure woiild present dimculties in machining. Consequently, normalizing was required in order to produce a uniformly fine-grained structure throughout the axle shaft prior to straightening, machining, and induction hardening. low-Carbon Steel Forgings. In contrast to the medium-carbon axle shaft discussed in the preceding paragraphs, forgings made of i of cooling in air to many a)io,y steeIs, rates solution rates. For room cmperature must be carefully controlled. 'rtain alloy and times because of iower~ustenitizing steels are forced-air cooled froin the normalizing i t1dOmnormalized. Only severe quenching ili have any significant effect on their tructure or hardness. ~arb,on,steeis containing 0.25% C or less ~re , om above the austenitizing temperatu~e /' ";-·1; Normalizing of Steel / 39 Table' 4) Effect of mass on hardness of normalized carbon and alloy steels) oC oF156 925 900 890 895 900 910 955 13(\,:) 890 50(2) 915 1700 1650 860 100(4) 116 126 1630 1670 192 207 229 269 143 192 i21 8620 121 137 248 321 235 311 341 217 375 388 262 137 183 131 126 i37 163 179 143 197 285 248 269 262 156 149 170 201 229 167 201 223 183 1640 235 293 255 302 1750 149 174 1580 212 1660 321 285 179 363 277 179 25(1) 1675 1600302 255 241 143 363 170 197 293 217 1600 187 1575 1650 870 : AU dala are based187 single heals. Souree: Rer 2. 3 on ni ""ith diameter, mm (in.). of Hardness, HB. for bar 3310 1030 1015 atures have been used (Ref 4). Locomotiveaxle forgings made of carbon steel to Association of American Railroads (AAR) Specification M-126, Class F (ASTM A 236, Class F), containing 0.45 to 0.59% C and 0.60 lo 0.90% Mn, are double normalized to obtain a uniformly fine grain structure along with other exacting mechanical-property requirements. Forgings made of a low-carbon st eel (0.18% C) with % Mn intended for low-temperature service are doubIe normalized to me et subzero impact requirements. i Bar and Tubular Products Frequently, the finIshing stages of hotmill operations employed in making stcel bar and tube produee properties that closely approximate those obtained by normalizing. When this occurs, normalizing is unnecessary and may even be inadvisable. Nevertheless, the reasons for normalizing bar and tube products are generally the same as those applicable to other forms of steeI. The machinabi1ity of steel bars and tubular products depends on a combination of hardness properties and microstrueture. For a low-carbon alloy steel, a eoarse pearlitic structure obtained by normalizing or annealing maximizes machinability. In the case of medium-carbon alloy steel, a lameIlar pearlitic structure obtained by annealing is desirable in order to optimize machinability. For a high-carbon alloy steel, a spheroidized structure lowers the hardness and increases the machinability of the alloy. Piior processing, part configuration, and processing foiiowing machining should be taken into consideration when determining the need for annealing or normalization. In general, annealing improves maehinability more than normalization does. Normalizing is used to correct the effects of spheroidization, but the steel bar or tu be stili needs to be annealed. Multiple anneals and tempeiing are normally used on only small-diameter part s sueh as wire gage products. Type 4340 is one of the few steels that is typically delivered to the customer with a normalized heat treatment due to machining specifications standard in the aircraft industry. Tubes are easier to normalize than bars of equivalent diameter because the lighter section thickness of tubes permits more rapid heating and cooling. These advantages help minimize decarburization and promote more nearly uniform mierostructures in tube products. Furnaces Requirements. Continuous furnaces of the roiier-hearth type are widely used for normalizing tube and bar products, especiaiiy in long lengths. Batch-type furnaces or other types of continuous furnaces are satisfactory if they provide some means for rapid diseharge and separation of the load to permit free circulation of air around eaeh tube as it cools. Continuous furnaces .S!ructural Stability. Normalizing and te mpenng is also a preferred treatment for r· ';" romoting the structural stability of low~tcalloy heat-resistant alloys, such as AMS ~t~6304(0.45% C, 1% Cr, 0.5% Mo, and 0.3% ~V),at temperatures up to 540 oC (1000 OF). ~.Wheels and spacer rings used in the cold ~V~ends of airc raft gas-turbine engine compres~;~~sors are typical of part s subjected to such t%~treatm~nt to promote structural stability. ~~( Multiple normalizing treatments are em~ployed to obtain complete solution of all lower-temperature constituents in austenite by the use of high initia! normalizing temperatures (for example, 925 oC, or 1700OF), and to refine final pearlite grain size by the use of a second normalIzIng treatment at a temperature closer to the Ac) temperature (for example, 815 oC, or 1500 OF) without destroyIng the beneficial effects of the initial normalIzing treatment. Double normalizing is usualIy applIed to carbon and 10w-alIoy steels of large dimension where extremely high forging temper- Typical rnechanical properties ofnormalized alloy steel sheet ksi In. MPa 48 HRC Yield strength(a) Thickn •••121 835 % Tensile strength lion(b),1240 7 585 Hardness, 250 180 8514 MPa 1725 0.180 825 0.193 50 Elonga195 40 / Heat Treating of Steel should have at least two zones, one for heating and one for soaking. Cooling facilities should be ample so that uniform cooling can proceed until complete transformation - has occurred. If tubes are packed or bundled during cooling from a high temperature, the purpose of normalizing is defeated, and a semiannealed or a tempered product results. Generally, protective atmospheres are not used in roller-hearth continuous fumaces for normalizing bar or tube products. The scale that forms during normalizing is removed by acid pickling or abrasive blast deaning. which is higher in the preheating and soaking zones than in the cooling zone, is usually built in sections. In most furnaces, both the preheating zone and the cooling zone are heated by the hot gases from the heating Sheet and Strip zone. However, both ofthese zones may be Hot-rolled steel sheet and strip (about equipped with burners for more accurate 0.10% c) are normalized primarily to retine temperature control. Air is excluded by grain size, to minImize directional properties, regulating the draft to maintaIn a slight and to develop desirable mechanIcal proper- pressure within all zones. ties. UnIformly fine equiaxed feriite grains Conveyor-lype Furnaces. In modem furare normally obtained in hot-rolled sheet and naces of the conveyor type (the only type strip by finishing the final hot-rolling opera- suitable for treating short lengths), sheets are tion above the upper transformatIon temper- cariied through each of the three zones on ature. However, if part of the hot-rolling rotating disks made of heat-resistant alloys. operation is performed on steel that has trans- These disks have polished surfaces, which formed partially to feriite, the deformed fer- prevent them from scratching the sheets, and ritegrains usually will recrystallize and form are staggered to ensure uniform heating. Thc abnormally coarse-grained patches during the dIsks are mounted on water-cooled shafts, self-anneal induced by coiling or piling at which are driven by variable-speed motors temperatures of 650 to 730 oC (1200 to 1350 through chains and sprockets or shafts and OF).AIso, relatively thin hot-rolled material, if gears. These fumaces may be up to 2.5 m (100 it is inadvertently finished well below the in.) wide and from 27 to 61 m (90 to 200 ft) upper transformation temperature and coiled long. Fuel consumption is 2.3 to 5.2 x 106 or pIled while it is too cold to self-anneal, may kJ/tonne (2.0 to 4.5 x 106 Btu/ton) of steel possess directional properties. These condi- treated, and production rates vary from 2.7 to tions are unsuitable for some types of severe 10.9 tonnes (3 to 12 tons) per hour. press-drawing applications and may be corNormalizing in a three-zone conveyorrected by normalizing. type furnace equipped with pyrometric conNormalizing also may be used to develop trols is a relatively simple operation. If high strength in alloy steel sheet and strip if scratching of sheets is to be avoided, the the products are sufficiently high in carbon sheets are brought to the charging table and and alloy contents to enable them to transhan d laid, one or more at a time, on arider form to fine pearlite or martensite when or conveyor sheet. Heavy sheets are norcooled in air from the nQrmalizing tempermalized singIy, but lighter sheets may be ature. In general, the hardened material is stacked two in a pile. To control heating and tempered to attain an optimum combination retard scaling, single sheets may be laid on arider sheet and covered with a cover of strength and ductility. Typical mechanical properties of normalized 4130, modified sheet. Sheets are carried by disk-rollers into 4335, and modified 4340 steel sheet are the preheating zone, where they absorb given in Table 5. heat rapidly because of the large temperature differential between the sheets and the Processing. The normalizing operation consists of passing the sheet or strip through interior of the furnace and because of the an open, continuous fumace where the mate- large surface-to-volume ratIo. As the sheets rial is heated to a temperaturc approximately become heated and the temperature differ55 to 85 oC (100 to 150 OF)above its uppcr entIal is reduced, the rate ofheat absorptIon transformatIon temperature, 845 to 900 oC slackens. After traveIing 4Yi to 6 m (15 to 20 (1550 to 1650 OF), thiis obtaIning complete ft), the sheets enter the soaking zone at a solution of the original structure wIth the temperaturc several degrees below the norformatIon of austenite and the n air cooling the malizIng temperaturc. HeatIng is completed material to room temperature. in the soaking zone, which is maintained at Furnace Equipment. Normalizing furnaca constant temperaturc, and sheets are held es are designcd to heat and cool sheets at the required temperaturc for a time sufsingIy or two in a pile. Theyare built in the ficient to convert the microstructure to austenite before theyare passed into the coolform of long, low chambers and usually comprise three sectIons: a preheatIng zone Ing zone. The sheets emerge from the (12 to 20% of the total length); a heating, or cooling zone at a temperature that can be varied between 150 and 540 oC (300 and soaking, zone (about 40% of the total 1000 OF), and are conveyed for a short length); and a cooling zone, which occupies distance on the runout table, where, after the remaining 40 to 50% of the length. Heating Arrangements. Normalizing fur- being cooled rapidly in air, they are caref~inaces usually are heated with gas or oII and ly removed from the rider sheet. The tnp through such a furnace is carried out at a Therefore, sheets are scaled during heat uniform speed of 0.03 to O. Lo mis (5 to 20 ft/min) and requires 5 to 20 min to complete. treatment.employ Burners are arrang ect,jpns may become distorted unless bnft:i~l'!ind support are provided. Accordingly, smail and large castings may be arranged so that they support each other. Loading lemperature. When castings are charged, the temperature of the furnace should be such that the thermal shock will not cause metal failure. For the higher-alloy grade s of steel castings, such as C5, C12, and WC9, a safe furnace temperature for charging is 315to 425 oC (600 to 800 OF).For lower-alloy grades, furnace temperatures may be as high as 650 oC (1200 OF).For east carbon steels and low-all oy steels with low carbon contents (low hardenabiIIty), castings may be charged into a furnace operating at the normalizing temperature. Heating. After the furnace has been charged, the temperature is increased at a rate of approximately 225 °C/h (400°F/h) until the normalizing temperature is reached. Depending on steel composition and eastIng contiguration, a reduction in the rate ofheating to approximately 28 to 55 °C/h (50 to 100 °F/h) may be necessary to avoid cracking. Extremely large castings should be heated more slowly to prevent development of extreme temperaturc gradients. Soaking. After the normalizing temperature has been reached, castings are soaked at this temperature for a period that' will ensure complete austenitization and carbide may be p e etermined by microscopic examination of, specimens held for various times at t~henormalizing temperaturc. period solution. e .. uration of the soaking d Cooling, After the soaking period, the castings are unloaded and allowed to cool in Normalizing of Steel / 41 wound from coils; it does not have rolls or catenary furnaces may incorporate pickling or other descaling equipment for removing any other type of conveyor for supporting surface oxides formed on the stee1 during the mateiial passing through the heating zone. The heating zones of catenary furnac- _ normalizing. es range in length from 6 to 15 m (20 to 50 ft). The preheating and cooling zones usuREFERENCES ally are shorter than those in conveyor-type 1. G. Krauss, Steels: Heat Treatment and furnaces, and for some kinds of work may Processing Principfes, ASM Internationbe omitted entirely. At their exit ends, al, 1990 2. Modern Steefs and Their Properties, 6th ed., Bethlehem Steel Corporation, 1966 3. Modern Steefs and Their Properties, Handbook 3310, Bethlehem Steel Corporation, Sept 1978 4. A.K. Sinha, Ferrous Physicaf Metallurgy, Butterworths, 1989