Refractories For Reheating And Heat Treatment Furnaces
Refractories for reheating andheat treatment furnacesS. S. Ghose, A. K. Bose, S. P. Mohanty, S. N. Mishra and S. DesBelpahar Refractories Ltd.After describing the type of furna-Introductioncess and the service conditions en-The group of furnaces used for reheating and heat-treatment covers acountered in different types of rehea-wide field in the processing of bothting and heat-treatment furnaces theferrous and non-ferrous metals. Aauthors illustrates the different types"re-heating" furnace is utilised to raiseof refractories used in various zones ofthe temperature of the metal to prepare it for hot working (shaping),the furnaces e. g. walls, roofs; hearth,combustion chamber etc. The. im-while the "heat-treatment" furnacesportance of insulation and conserva-are 'used for stress relieving and fortion of energy in various types of fur-changing the physical properties of themetal after the product has attained itsnaces ha.i also been emphasied. Refractories for soaking pits has alsofinal shape.been discussed.The above type furnaces may begrouped as follows :I. Re-heating furnaces.These are divided Into generalclasses a—In re-heating and heat-treatmentfurnaces the essential criteria for refractory performance is uninterruptedservice life and minimum maintenancecost. Unscheduled breakdowns areextremely undesirable since it woulda) Batch typeupset the entire production programmeb) Continuous type, which maybe pusher-type, walking beamof the mill.The selection of the refractories totype, rotary hearth type andachieve a successful performance wouldroller-hearth type.depend upon the areas of application2. Soaking pits.in the furnace and the service condi.tions. One of the major criteria influ-The "heat-treating" furnaces encom-encing the choice would be the over-pass a wide range of furnaces varyingall cost so as to achieve the optimumIn size, design and operating conditions.economic performance.130
In general the refractories, are re-the distinction is to be made for refrac-quired to withstand any of the follow-tories which are used in the immersedThestate I. e. where it is exposed to tem-severity of any of these conditionswould greatly influence the choice ofperature on all sides such as checkers,partition walls etc. On the contraryrefractory.where the refractory Is subjected toing conditions during service.high temperature on one face only,High temperature depending onfurnace operational requirements.2.Localised overheating due toflame misalignment or radiationfrom the flame.3.which is the more usual condition encountered in furnaces, the relativelycooler rigid portion of the brick on thecold face side of the brick work cantake most of the load.Insulation and conservation ofAbrasive action due to natureof the fuel e. g. where solid orenergyConservation of energy is also one ofthe important feature which would In-pulverised fuel is used.fluence the choice of refractories. The4.affect of fuel ash etc.5.Load due to the brick work.6.Structural stresses.7.Stresses due to temperaturefacing lining which has to withstand thegradient.abrasive actions and other mechanicalrefractory quality i. e. the thermal conductivity and the lining thickness woulddetermine the amount of heat flowthrough any structure. Usually theabuses of the furnace, is a dense refrac8.Temperature fluctuations andcyclingtory material backed up by .hot faceHowand cold face insulation bricks.Effect of fumes etc. prevailingever, over insulation is also not bene-In the furnace atmosphere.ficial since it would raise the intereface10.Reactions due to scales, slags etc.I I.Load due to charge in the fur-better grade of refractory to withstandthe relatively more intense tempera-9.temperature, which would require ature conditions.nace.12Mechanical abuses either due toIn recent years, with the develop-movement of the charge or anyment of superior quality hot face refractories increasing use is being made ofsuch refractories for the constructionother factor.The temperature conditions in service Is the major factor to determineof furnaces. 'If other operating parameters are suitable, these refractoriesthe quality of the refractory requiredcan be used as the working face of thefor any particular application.lining which in addition to the lowerHere131
heat losses provide the advantage ofthe lighter construction.The common areas of all furnaces areusually i) Walls ii) Roof Ili)Com-bustion chambers iv) Hearth, which areIn continuousfurnaces, the lossesthrough the walls is Insignificant compared to the total energy consumption'.In case of the batch type furnacesthe conditions are distinctly different.Construction with insulating refractories results in a relatively lowerthermal mass of the furnace brickwork. Such constructions have thedealt separately from the design viewpoint. The designers' concern is notonly to achieve astructurally stableconstruction, but also to ensure thatthe structures would withstand thestress/strain influences during opera.tion.Wallsadvantage of reducing the heat lossesThe quality of the refractory as welldue to heat flow through the refrac-as the wall thickness would depend ontory structures as well as reduction Inthe operating environment of the fur-the residual heat content betweennace, wall height as well as the degreeheating cycles due to the lower ther-of conservation of heat desired.mal capacity. These are ideally suitedIn the continuous reheating furnacesfor the construction of the low Inertiathe walls have to usually accommodaterapid heating furnaces where relativelyquicker cycles become possible to beachieved. The introduction of ceramicfibres which can withstand quite hightemperatures have Immensely increasedthe possibil ity of the extensive use ofsuch materials in furnace constructions.other important element's of the furnacesuch as combusion chamber, the load ofthe roof in some types of constructionsand doors etc.However less severeconditions are In general experiencedin the walls than in the roof and combustion chambers. The introduction ofRefractories for re-heating furnacesIn addition to the suitable refractoryquality required to adequately withstand the furnace operational environ-oil firing and high output rates hasresulted in replacement of the usualmedium and high duty fireclay bricksby higher alumina ( 50-40% A1203 )bricks, specially in the bottom part ofthe furnace where lower shrinkage,ments, the following aspects are equal-higher resistance to creep In compres-ly important to achieve success :sion and greater resistance to thermala)Design of suitable refractoryshocks are necessary. A particularstructuredamage to side walls in bottom firedb)Provision of expansion allowancec)Laying of the refractory brickfurnaces is the effect of flame impingement, which being below chargelevel, is not easily detected beforeserious damage has been effected.work.132
to collapse frequently On the spotThe compressional load at thebottom brick for a 4 meter wail wouldenquiry by the authors and follow upbe about 1,5 Kg/cm2. However, complication arises due to the surface Ir-by intensive laboratory trials and estimates of temperature gradients in theregularities, within the tolerance limitsallowed and present day manufacturingwall revealed that the brick qualityused was not suitable to withstand thepractices, which may result In pointoperating conditions. It was more soloading at the contact of two brick in-since the walls also have to withstandterfacer. This results In developmentthe load of the sprung arch roof. Re-of stresses which far exceeds the com-placement with proper grade highpressional crushing strength of thealumina refractories to meet the opera-bricks. The stresses however can beting conditions was ruled out since themore uniformly redistributed by pro vision of suitable mortar joints. inoperators were equally concerned withcase of bowed bricks, transverse tracksbeing used for heating. The proposedand failure would result due to pointhigh alumina lining would obviouslycontacts where the load would exceedIncrease the heat losses, through thethe strength of the bricks2.conservation of the expensive LPG fuelwalls and also increase the outside walltemperature where buckling of theMouldable refractories are being increasingly used for insitu constructionsteel shell had already occured withthe previous campaigns. Since suitableof furnace walls with suitable anchors.grade hot face insulation bricks wereThese are being extensively used innot available Indigenously, it wasUSA and other countries advantageous-developed in the authors laboratoryly with satisfactory performance.with utmost urgency and translatedIn a 22 meter long continuous stripinto actual plant scale production withannealing furnace operating at 950 Cthe firebricks lining backed up by hottion of the hot face insulation brickface and cold face insulation bricks, usedused in the furnace is given belowthe minimum lead time. The specifica-Maximum service Temperature 1300 C80-85SiO2Refractoriness1500 CBulk Density ( gms/cc )0.9-1.1Apparent Porosity ( Vol. % )55-60C C S (Kg/cm2 )30-40% PLC at 1300 C ( 8 Hrs. ) 0.5Thermal conductivity In400 C-2.0Btu/in/hr/ F at hot face600 C-2.3temperature of800 C-2.8133
such refractories have the advantage ofThe furnace has now been operating;the Inherent lower conductivity overequivalent grade of dense bricks. 3. Somesuccessfully without any interruptionsince over a year. The savings on thethe refractory prices several times over.typical properties of refractory con-,cretes for reheating furnace roof blocks( Hardy and Titterington)4 are givenRoofbelowfuel cost alone has already paid backThe furnace roof is usually made withsprung arch or suspended construction.Recommended service temperature range—0 to 1550 CSuspended roof construction is more inCold crushing strength, Kg/cms2—475practice In recent years. particularly forlarge furnaces.Thermal expansion ( 20-1550 C )Thermal spell resistant and volume— 0.25%stable fireclay shapes have been succes fully used in furnaces with moderateoperating conditions. Semi silica bricks,Permanent linear change, % (Shrinkage)0.47—which posesses relatively better spal-Refractoriness— Orton Cone 32Ing resistance, have also been used.Typical data of fireciay refractorieswhich has proved to be suitable forIn case of the sprung arch constructions conditions of point loading dueroof construction is given in Table I.to surface irregularity may occur.Similarly with bowed bricks transverseHowever, the severe operating con-fracturing may also develop whichditions In large furnaces of high outputpartly explains the slabbing off of themills have exposed the limitations ofhot face of the roof during furnacethese refractories. The introductionheating up.of oil firing has further aggravated thesituation due to faster heating ratesparticularly in the lower temperatureThe creep properties also play asignificantly important role In the per-range and there is a greater risk offormance of roof refractories. On theflame misalignment with risk of local-hot face such expansion is pardally off.ised over heating. The alkalis and vana-set by the creep.dium oxide in the fuel ash may have abehind the hot face stresses develop Instrong corrosive effect on the refractories.But immediatelythe relatively rigid portion of the brick which would crack due to its failureto yield to theThe present trend, particularly inresultant. stresses.These difficulties may be elim:;,ated toUSA and other western countries, is toreplace shaped bricks in the roof bygreat extent by proyisica of suitablefully monolithic roofs installed insitu orexpansion allowance in the construction.use of large precast blocks.Besidesgood thermal shock resistance, use ofIn case of the suspended rocf construction several design with proven134
TABLE-1ll oI \ ractories for use in roof of reheating furnacesThermal shock resistant and volume stable firebrick% A120335-37% 0A2.0Fe2O3RefractorinessSK-32Bulk Density ( gm/cc )2.15App, Porosity ( Vol. % )18C C S ( Kg/cm2 )380R tJ L ( 2Kg/cm2 ) -1-- to C1435% P L Cat 1450 C-1- 0.2Spiting Resistance-I- IS1200 C—waterCyclesperformance over the years are in exis-subsequent collapse can be eliminated totance. The essential features are pro-a great extent by providing a recessvision of suitable anchors and theirfor the toe of the arch over the furnaceprotection against over-heating, properwall. In extreme cases where tem-design of the bricks and thermal spallperature cycling and other mechanicalabuses are characteristic of the parti-resistant characteristics.cular furnace, provision of suitablyIn the construction of the sprungarch furnace roofs the authors haveexperienced that provision of adequatedesigned tongue and groove arrangements In the design of sprung archeshas given improved performances,expansion joints provided with meticulous care, through half the length ofthe arch brick on the hot face side ,Combustion ChambersThecombustion chambers usuallyhave shown promising results. Thecomprise of the burner quarls and theexpansion allowance provided should bejust adequate so as to allow for thenose arch. The burner quarls are sub-arch rise during heating up.jected to the destructive Influence ofIn such acase the stresses are uniformly distri-severe over-heating due to radiationbuted and the arch is less prone toand flame Impingement. During opera-cracking or sagging. It has also beention, carbon build up is frequently en-observed that the problems related tocountered which tend to effect theflame Impingement on the roof due todirection of the flames. High aluminamisalignment or any other reason andrefractories in 60-80 /0 AI,03 range are135rapid heating and cooling as well as
conventionally used. The typical problems encountered are cracking andfor this purpose in the authors laboratory, and is given In Table IIsagging of the burner quarls. In severeoperating conditions where the burnerquarls are to perform at continuoushigh temperature, even 90% aluminarefractories have been used advantageously to avoid saggingand deformations.Recently both precast blocks and InsituThe nose arch is also exposed tomore or less similar operatinng condition viz severe localised over heatingdue to flame Impingement and continuous high temperature by flame radiation and possible misalignment.Incastings are being used for installationcase of self supporting arches, the stress/strain conditions and failure due toof burner quarls. The followingfracturing occur similar to the roofmaterial has given successful result Inseveral applications In the burnerarches. High alumina refractories arerecommended where the nose arch hasquarls, which was specifically developedto support the load of the roof andTA BLE—IITypical data of Ramming mass Burner quarlQualityHigh aluminaMax. Service Temperature1700 CRefractoriness 1780 CA1203 %70-75Fe2 O32.5-3SettingChemicalSintering temperatureCrading mm800 C0–5Linear changeAfter drying at 110 C — 0.2After heating at 500 C — 0.2After heating at 1000 C — 0.2IRat 1450 C — r 0.8at 1550 C — H-0.2C C S Kg/cm 2After drying at 110 C — 200-250Heating at 500 C — 300-300OP81PP at 1000 C --. 350 —400at 1450 C — 400-420at 1550 C -- 350-370136.
wall super structures since higher-hotalso to withstand the thrust of the roofstrength and resistance to compres-arch. Subsequently additional ridersional creeps are highly desirable.arches were provided to relieve theburner arches from the direct thrustIn a walking beam furnace operatingof the roof arch. The performanceat moderate temperature of about1100 C, it was unfortunately exberienc-since then have improved appreciably( Fig. 2 ).ed that the furnace has to be shutdownin a few days after the light up due tocomplete collapse of the combustionchamber arches due to cracking. Afterintensive testing in the authors laboratory It was noticed that the refractoryquality used were suitable to serve thepurpose adequately and the failure wasmainly due to other reasons. Duringthe subsequent repairs, expansionallowances were provided on the hotface and also grannular refractorymaterial were used over the arch tohave the flexibility to marginally accommodate the arch rise (Fig. I). Theproblem have been satisfactorily solvedsince then.Fig. 2 Rider arch to relieve the load onthe burner arch,HearthThe hearth of the reheating furnacesis the total area available to carry the'MORTAR JOINTSstock and is subjected to the maximum,ViANNULARAEFRACTORIFS.mechanical abuse, slag and scale attacks,load at high temperatures etc.In pusher type furnaces the preheating zone hearth is either solid throughout with skid rails to give protectionagainst abrasion or have water cooledskids supported by ordinary fireclaybricks as in the bottom fired pusherFig. 1 Provision of expansion allowanceon the burner archtype furnaces. In the soaking zone theSimilarly in a furnace operating con-.hearth is subjected to abrasion andattack by oxide scale which may resulttinuously over 1500 C, sagging of thein severe fluxing of the refractorycombustion chamber arches was a re-under the prevailing high temperaturecurring problem since these arches havecondition. In this area both basic and137
Over SK-36BRL— CM NO0jcNiosi:cs1O
high alumina ( 80-90% A12 03 ) refractories have been successfully used.Fusion cast bricks give better performance, but the relatively high cost precludes its use extensively. Dense ( 8090% A1203 ) fused grain rebondedbrick developed indigenously has givenhighly promising results. These brickshave been successfully used in the skidsand hearth of a pusher type reheatingfurnace in an integrated steel plant. ASettingHydraulicSintering tern.1200 CGrading(0-5) mmLinearCCSchange Kg/cm2After drying at 110 C250-300 0280-310After heating at 500 C 0200-230After heating at 1000 C 0240-260Aftre heating at 1200 C y 0record Ingot production of over 1.5million tonnes has been achieved without any necessity of repair of the hearthRefractories for soaking pitsInwith the BRL-88 brick ( item I, Tablethe soaking pits, although the111 ). The typical data of the materialsoperating temperatures are relativelymoderate ( 1250 to 1400 C ) the ser-suitable for the reheating furnacehearths are given in Table III.vice conditions to which refractoriesare exposed are frequently severe. Therecent trend in the refractory practicesIn the walking beam furnaces thebeing adopted In the various importantrefractories are required to have highareas of the soaking pitsimpact resistance, resistance to thermalare brieflyreviewed below :—shock and scaling. High alumina bricks(80% A1203 ) and a high strength casta-Hearth—This has to support heavyble refractory specially developed forload of ingots and also to resist thethe purpose have performed well inattack of molten scale.service after careful attention was paidclay bricks were used for lining theto the design of the brick shapes. Thethe hearth, but these were attackedvigorously by iron scales with the for-typical data of 80% A1203 brick is givenin Table IV and that of the castable isgiven belowEarlier, fire-mation of liquid slag. The top refractory layer of the hearth in the presentday soaking pits is usually magnesite,chrome magnesite or high A1203Typical data of tastable used In walk-bricks ( 70-80% A1203 ) with a cover.ing beam furnace :ing of coke breeze. The main require-rp,Max. Service Temperature — 1450 Cments are high impact and load rests-PCE 1600 Ctante, and also resistance to iron oxideA120342-45%Fe2O31-1.5%CaO4.8-5.5%attack at the operating temperature.In the lower walls, the refractoriesare subjected to corrosive action of139
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Refractories for reheating and heat treatment furnaces S. S. Ghose, A. K. Bose, S. P. Mohanty, S. N. Mishra and S. Des Belpahar Refractories Ltd. Introduction The group of furnaces used for re-heating and heat-treatment covers a wide field in the processing of both ferrous and non-fe
Refractories, for reheating furnace present and future status 83 B. V. Appa Rao Bhilai Steel Plant I. Refractories Nay a very vital role In . recovery of waste heat, the refractory needs a change to the present condi-tions. At B
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