Features of Cast Iron Smelting in Induction Crucible Furnaces

Transcription

1 A R H I V E S of F U N D R Y E N G I N E E R I N G Publshed quarterly as the organ of the Foundry ommsson of the Polsh Academy of Scences ISSN ( ) Volume 17 Issue 3/ /3 Features of ast Iron Smeltng n Inducton rucble Furnaces V. Grachev A.N. Frumkn Insttute of Physcal hemstry and Electrochemstry of the Russan Academy of Scences, 9 Bldg 1, Bolshaya rdynka St., Sute # 10, Moscow, Russa orrespondng author. E-mal address: cherntsa@vernadsky.ru Receved ; accepted n revsed form Abstract The artcle descrbes the trend towards ncreased use of nducton crucble furnaces for cast ron smeltng. The use of gas cupola s duplex process nducton crucble furnace has been proved the effectve drecton of scentfc and techncal advance n the foundry ndustry. Gas cupolas and nducton furnaces are used for cast ron smeltng at the Penza ompressor Plant where n the 1960s the author developed and ntroduced gas cupolas for the frst tme n the world. In the artcle, the author represents the fndngs of the nvestgaton on thermodynamcs of crucble reducton of slcon, whch s pvotal when choosng the technologcal mode for cast ron smeltng n nducton furnaces. The author proposes a new reacton crucble dagram wth due account of both partal pressure and actvty of the components nvolved nto the process. For the frst tme ever, the electrochemcal mechansm of a crucble reacton has been studed and the correctness of the proposed dagram has been confrmed. Keywords: Innovatve foundry technologes and materals, Heat treatment, ast ron smeltng, Electrc nducton furnace, Electrochemcal mechansm of crucble reacton 1. Introducton In the foundry, cast ron smeltng s performed by gas cupolas and electrc furnaces. Gas cupolas, natural gas-fred blast furnaces, were frst developed and ntroduced wth the partcpaton of the author n Russa and Italy [1, ]. The author has proved that smeltng cast ron n a gas cupola allowed producng a wde range of hgh-qualty castngs [3]. Gas cupolas are stll n operaton at JS Penzkompressormash. The nducton crucble furnaces have been used for smeltng cast ron snce the second half of 0th century. However, such type of smeltng dd not gan wdespread acceptance. Gas cupola stll remaned the prncpal meltng unt. For cast ron smeltng, the applcaton of normal-frequency nducton furnaces expands progressvely. nce the late 1950s, smeltng n electrc furnaces, partcularly n nducton furnaces, has been replacng cupola smeltng n ndustrally developed countres. From 1963 to 1968, n the USA the number of operatng electrc furnaces reached,000 unts. 70% of those were normal-frequency nducton furnaces. Durng the last 6-7 years, the number of cupolas n the USA reduced almost by half, and the number of electrc furnaces ncreased by.5 tmes. Replacement of cupolas by electrc furnaces also takes place n Western Europe []. In the USSR, the frst shop wth normal-frequency nducton furnaces nstead of cupolas was launched n operaton at Kaunas ast Iron Foundry n 1963, almost alongsde gas cupolas. At JS Penzkompressormash, both gas cupolas and electrc nducton furnaces are used. urrently, ts meltng compartment s equpped wth both gas cupolas and normal-frequency electrc nducton furnaces. Smeltng cast ron n nducton furnaces has a set of features, whch are dscussed n the current paper. The man feature s that A R H I V E S o f F U N D R Y E N G I N E E R I N G V o l u m e 1 7, I s s u e 3 / 0 1 7,

2 melt carbon reduces slcon from the lnng, sgnfcantly nfluencng the process. Fg. 1. Layout of the meltng compartment, JS Penzkompressormash foundry. Methods The layout of the meltng compartment s shown n Fgure 1, mages of gas cupolas and nducton crucble furnaces are represented n Fgure. hgh qualty cast ron. The advantages of such furnaces also nclude the possblty of remeltng the non-brquetted cast ron turnngs to 0% of metal charge, and sheet-ron scraps, whch reduces consumpton of pg ron and scrap. Durng smeltng, the loss of and Mn, and the costs of the charge materal are reduced, whle the total loss does not exceed %. However, the mantenance costs ncrease (Table 1). Fg.. Meltng compartment, JS Penzkompressormash The electrc nducton furnace s a rather complcated batchoperated system (Fgure 3). The key advantage of the nducton cast ron smeltng compared to the cupola smeltng les n the possblty to control the process of cast ron superheatng wthn a wde temperaturetme range, perform a much deeper metallurgcal treatment of the melt, accurately mantan chemcal composton, and produce Fg. 3. Inducton crucble furnace [] 6 A R H I V E S o f F U N D R Y E N G I N E E R I N G V o l u m e 1 7, I s s u e 3 / 0 1 7, 5-50

3 Table 1. ast ron smeltng costs ost tems osts, % oke cupola Gas cupola Inducton furnace harge materals Fuel (process energy) Materals (except for the charge) Electrc furnace operaton Total 100% 100% 100% Nevertheless, the nducton furnaces are progressvely replacng the cupolas (Fgure ) [5, 6, 7]. Fg.. Growth of cast ron producton by nducton furnaces n Russa When smeltng cast ron n nducton furnaces, the content of S n the melt decreases drastcally; ths facltates producton of nodular cast ron. The hygene and santary condtons of work sgnfcantly mprove. Less harmful gases and dust s emtted, especally when compared to coke cupolas. Thus s partcularly mportant for ctes and resdental areas, as the nducton furnaces pollute the ar much less than the cupolas that requre bulky and expensve purfcaton facltes. Specfc consumpton of refractores s also reduced: n case of an nducton furnace, t amounts to 3 kg/t; n case of an acd-lned arc furnace, t s 17.6 kg/t; and n case of a gas cupola, t s 7 kg/t. It s lower n a gas cupola, as t can operate wthout repar for 3 weeks, and amounts to 13.6 kg/t. The atmospherc emsson of dust from the gas cupola s 0 tmes lower than from the coke cupola, the content s 50 tmes lower, and the S content s 11 tmes lower. The use of gas cupolas n combnaton wth nducton furnaces s very promsng. 3. Results The meltng compartment equpped wth gas cupolas requres less captal nvestments durng reconstructon of foundry shops and exstng coke cupolas [5]. In the foundry ndustry, the use of nducton electrc furnaces for smeltng cast ron has been progressvely ncreasng. ne of the key features of smeltng cast ron n nducton furnaces s the occurrence of the crucble reacton: The equlbrum of ths reacton plotted aganst the temperature s characterzed by a dagram of equlbrum values of [] and [] (referred to as elsen Roesch rths dagram, Fgure 5а). In Neumann Dötsch study [8], ths dagram s adjusted by value p. At ths, all authors consdered that a = 1. However, the lnng contans 60 65% of, thus, changes. p < 1 may be mportant for vacuum meltng, whle n real-lfe nducton meltng condtons, the СО pressure reaches.5 atm and more. In case of a crucble reacton, the actual values of a n the course of the crucble operaton decrease from 1 to 0.6. The actual values of р, n case of ndustral furnaces, reach the values of 3 atm. For the ntal perod of formaton, when the radus of ts ncpent bubble s small, a sgnfcant resstance to crucble reacton s possble. When analyzng the Neumann Dötsch and our own data, equlbrum of the crucble reacton wthn the pressure range of р = atm and actvty range of a = should be consdered. The adjustment of elsen Roesch rths dagram carred out by Neumann and Dötsch concerned the actvtes and a wth consderaton of mutual nfluence thereof (Fgure 5b) [8]. The author has studed how the actvtes and equlbrum concentratons of carbon and slcon are nfluenced by other elements usually contaned n cast ron (Мn, S, Р), and prncpal alloyng elements (Сг, N, А1, Сu). The calculaton performed wth consderaton of nfluence coeffcents, Mn, etc., S and j a a made t possble to determne correctons to equlbrum values of С and [3]. The changes n concentraton equvalent to the change n actvty are of such low sgnfcance ( Mn + S + P 0.1%) that t s possble to neglect them too, snce ths nfluence equals to only 5 С change n equlbrum temperature [9]. The equlbrum of the crucble reacton may be sgnfcantly nfluenced by the alloyng elements: large quanttes of Сr nhbt the crucble reacton; the reacton s hastened by N, Сu and partcularly large quanttes of Al [3]. Fgure 6 shows a nomographc chart plotted for real-lfe smeltng condtons and allowng precse determnaton of the smeltng mode. j A R H I V E S o f F U N D R Y E N G I N E E R I N G V o l u m e 1 7, I s s u e 3 / 0 1 7,

4 Fg. 5. rucble reacton equlbrum dagram by: a) elsen Roesch rths; b) Neumann and Dötsch Fg. 6. Author s resultant dagram for the crucble reacton The study of the crucble reacton mechansm has been conducted usng samples of sntered wth borc acd and placed nto the melt of cast ron n the nducton furnace. By passng currents of dfferent magntudes and polartes, t has been found that the crucble reacton may be hastened. The sample represented one of the electrodes. The other electrode was metal. nce the passage of electrc current had a notable nfluence on the crucble reacton, we assume that between the electrodes there was an electrolyte. Then the mechansm of the crucble reacton may be represented as follows: x ne ne Ths mechansm can be accepted only as a frst approxmaton, snce the structure hgh-slca layer s much more complcated than that of the onc melt from. The boundary layer of quartz lnng conssts of polyon complexes of varous complexty, wth nclusons of borc compounds and dfferent knds of mpurtes. The development of capllary and other phenomena results n a decrease n content to 60 70%. Accordng to Esn Masson theory [, 5, 6], the polymerzaton of slcon-oxygen anons occurs: These reactons are characterzed by polymerzaton constant К, and, n general case, the slcon ons have the form of 8 A R H I V E S o f F U N D R Y E N G I N E E R I N G V o l u m e 1 7, I s s u e 3 / 0 1 7, 5-50

5 as: ( 1c ) (3 1 ( 1 (31, and the polymerzaton reacton may be represented 1 ( 1 (31 The calculatons provded by.a. Esn for dfferent values of mole fracton of n slcate ( ) have shown that even at К = 0 there were varous knds of complex anons present n the melt, whch, accordng to Esn Masson theory, s explaned by the dsproportonaton reacton: ( 1 31 c 1 ( 3c N 1 ( 3c Table shows the data on dstrbuton of anons n melted slcates for values of N correspondng to 65.6% of. It s logcal to assume that n the boundary layer of the lnng, softened and glassfed by snterng, the followng process shall take place: x ( 1 x ] e ( 1 31 c e 1 (31 Upon contactng wth the Fe С Мn melt, an nterfacal exchange and creaton of a double electrc layer occurs. The electrochemcal behavor of the crucble reacton makes s possble to determne the value of the protectve potental that can nhbt, for example, the cathodc phase wthout ntensfyng the anodc phase. Assumng that the crucble reacton conssts of electrochemcal actons at anode and cathode, a study of electrochemcal phases of ths reacton has been conducted usng a standard potentodynamc method and a P-588 potentostat, wth the reference electrode placed n the boundary layer of the lnng. The cell layout s presented n Fgure 7. The obtaned potentodynamc curves are shown n Fgure 8. n the potentodynamc curve, n case of Fe 3%С melt, there may be clearly seen two horzontal segments correspondng to the lmt currents of the electrode processes (Fgure 8a): + + e = + (segment I) and + + e = (segment II). It was found that the frst phase may be nhbted by an 800 mv voltage. n the curve correspondng to the hgh-slcon melt (Fgure 8b), there may be observed certan oscllatons (apparently, the formaton of mpurtes) n a regon close to the zero value of the current, whch ndcates that the reacton does not occur. Whence, the author has obtaned prevously unknown data on the melt carbon nteracton wth the slcon oxde of the nducton furnace lnng under dfferent thermodynamc condtons. The elsen Roesch rths dagram was presented n a new form based on the real thermodynamc condtons of cast ron smeltng n nducton furnace. The correctness of the adjusted dagram was confrmed by electrochemcal research. The expermental data allowed calculatng the emprcal values of actvaton energy and obtanng the formula for determnaton of the protectve potental: 100 T P P 3 exp ln 1. 1 T 305 a a The possblty of protectng the lnng from the crucble reacton was tested usng a hgh-frequency nducton furnace, wheren two alundum crucbles have been smultaneously lned, n one of whch a protectve current was appled to the nner lnng, whle n the other one no protectve current was appled. The melt n the crucbles had the same composton. The crucble reacton behavor was judged by a change n the slcon content n the samples taken at regular ntervals. It has been found that n the crucble, n whch the protectve current was not appled, the slcon content ncreased, and the carbon content decreased faster than n the crucble, n whch the protectve current was appled. It can be assumed that t was the electrc current that nfluenced the process. However, ths supposton needs further nvestgaton. Table. Dstrbuton of slcon-oxygen anons at varous values of polymerzaton constant Polymerzaton constant К value Mole fracton of slag content ( ( 1) 31 ( 1) and ons at c A R H I V E S o f F U N D R Y E N G I N E E R I N G V o l u m e 1 7, I s s u e 3 / 0 1 7,

6 . onclusons ast ron smeltng development trend attests to the fact that the use of nducton crucble furnaces s expandng. Smeltng n ths type of furnaces has a set of features, whch are prmarly the reacton behavor for melt carbon nteracton wth slca of the lnng accordng to the so-called crucble reacton. The study of these features led to the followng conclusons: 1. For the real condtons of cast ron smeltng, the actvty decreases from 1 to 0.6, and the partal pressure dffers from 1 and can reach kg/cm. The author proposed a nomogram of crucble reacton equlbrum, whch allows determnng the meltng mode for a varety of condtons.. The electrochemcal mechansm of a crucble reacton has been ascertaned emprcally, as evdenced by the horzontal sectons on potentometrc curves obtaned n specal electrochemcal cells. a References Fg. 7. The cell for studyng the electrochemcal processes n the boundary layer of the lnng: 1 cast ron melt, alundum casng, 3 platnum workng electrode, platnum reference electrode, 5 lnng made of wth borc acd, 6 plug made of Al 3, 7 nsulatng casngs, 8 auxlary platnum electrode, 9 nsulaton of auxlary electrode [1] herny, A.A., Marenbakh, L.M., Kurbatsky, I.L. et al. (1963). USSR certfcate of authorshp N Gas cupola. Moscow: State ommttee for Inventons and Dscoveres. [] Grachev, V.A. (016). Development and applcaton of gas cupolas n foundry producton. ARPN Journal of Engneerng and Appled Scences. 11(3), [3] Grachev, V.A. (016). Physcal and chemcal basc prncples of cast ron smeltng. Moscow: IPhKhERAN. [] Fnk,. (011, August 19). Long lnng lfe and long col lfe go hand-n-hand: For coreless nducton meltng, an nformed and effectve mantenance approach wll save thousands n repar and lnng costs. Retreved November 30, 016, from [5] Boldn, A.N. et al. (010). Engneerng ecology of the foundry producton. Moscow: Mashnostroyenye. [6] Dbrov, I.A. (01). State and prospects of the Russan foundry producton. UNID n Russa. Specal ssue, [7] Dbrov, I.A. (015). Development of the foundry engneerng n Russa. Lteshchk Ross. 8, 8-1. [8] Neumann, F. & Dötsch, E. (1975). Thermodynamcs of Fe- - melts wth partcular emphass on the oxdaton behavor of carbon and slcon. Gessereforschung, 7(1), [9] Grachev, V.A. (016). Thermodynamcs and mechansm of crucble reacton durng cast ron smeltng n nducton furnaces. Lteshchk Ross. 10, Fg. 8. Potentodynamc curves representng content n cast ron (coped from the potentostat dagrams): a. reacton occurs; b. reacton does not occur. Horzontal segments 1 and correspond to the lmt currents of the electrode processes: e = +, + + e =. 50 A R H I V E S o f F U N D R Y E N G I N E E R I N G V o l u m e 1 7, I s s u e 3 / 0 1 7, 5-50