Arc Re-Melting of Ductile Cast Iron after Fesimg Modification

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1 Arc Re-Melting of Ductile Cst Iron fter Fesimg Modifiction Dr. Olexiy A. Tchykovsky, Senior Professor, Deprtment of Foundry of ferrous nd non-ferrous metls, Ntionl Technicl University of Ukrine KPI Kyiv, Ukrine, Peremohy v., 37 Astrct The sphericity of grphite grdully disppers fter the exposure of the liquid cst iron in the ldle during csting. This process ws previously investigted nd the influencing fctors such s mgnesium content, exposure time nd temperture were determined. The complete disppernce of the grphite sphericity ws lso oserved fter ductile cst iron re-melting. The im of this work ws to study the kinetics of grphite shpe trnsformtion in ductile iron treted with FeSiMg-7 lloy during the rc re-melting process. The effect of het tretment exposure on the shpe of grphite inclusions ws determined. Keywords Ductile Iron; Sphericl Grphite, Modifiction, Re-Melting, Grphite Nodulrity Degree, Grphite Shpe, Nodulr Grphite Tretment, Exposure Time, Cooling Speed I INTRODUCTION 1.1 Sphericl grphite in ductile cst iron There re mny hypotheses regrding to the origin of sphericl grphite in cst iron, ut none of them provides complex description of this process. The high tempertures complicte oservtions of numer of phenomen tht could revel the mechnism of sphericl grphite formtion during spheroidizing tretment nd the role of mgnesium nd other spheroidizers in this process. The principl industril process of ductile cst iron production is se on using of spheroidizing elements. A rnge of properties of ductile cst iron is define y grphite sphericity. In turn, the grphite sphericity entirely depends on the residul content of spheroidizer, minly mgnesium. Residul mgnesium content, which gives the sphericl grphite shpe, depends on the cooling rte, modifiction process prmeters nd metl mtrix structure. The mount of spheroidizer depends on mny fctors: the mss of cst iron which is processed, the mold filling time, csting wll thickness, sulfur content, temperture, cooling rte etc. Bsed on prctice nd the results otined, it cn e concluded tht the residul mgnesium content which ensure the correct sphericl shpe of grphite in cstings with wll thickness of mm should not e lower thn 0, ,042 %. Spheroidizing tretment technology of cst iron must gurntee stisfctory ssimiltion of mgnesium in liquid metl. According to Voloschenko et l. the residul mgnesium content of cstings, which crystllize with the formtion of metstle structures should not e lower thn 0,035 %. At lower mgnesium content sphericl grphite hs irregulr or mixed shpe, which is uncceptle [1]. The effect of liquid ductile cst iron exposure on kinetics of grphite sphericity reduction is given in Tle 1.1 [3]. In this study the initil cst iron with stle chemicl composition ws smelted in furnce with sic lining t C nd treted with optiml mounts of modifiers. The modified cst iron ws kept in the furnce t C nd the liquid metl proes for nlysis were tken every 3 min. Bsed on the results given in Tle 1.1 nd Figure 1.1 it cn e concluded tht isotherml exposure of modified cst iron reduces the sphericity of grphite, ut the intensity of this process depends on the modifier composition. Immeditely fter the spheroidizing tretment the sphericity rte ws %, fter 10 nd 20 min % nd %, respectively. TABLE EFFECT OF DURATION OF EXPOSURE ON THE GRAPHITE SHAPE IN CAST IRON AFTER MODIFICATION Modifier Grphite sphericity, % fter exposure, min Composition 0, Mg-Ni Cu 2 Mg-Ni REM 3 Mg-Si-Fe Mg-Si Fe-C 5 Mg-C REM-Si- Fe 6 Mg-C REM-Si- Fe 7 Mg-C- REM-Si- Fe-B The lck of proportionlity etween the exposure durtion of liquid modified cst iron nd the grphite sphericity rte is typicl for ll investigted modifiers. The grphite sphericity reduction is slow for short durtions of exposure nd increses with time (Fig. 1.1)

2 Nodulrity grphite rition, % s in the smples tht were modified mgnesium, nd the smples tht hve een modified cerium (fig.1.3) [3]. Fig 1.2 Grphite inclusions in se lloy specimens Exposure time fter modifiction, min Fig. 1.1 Effect of exposure durtion of liquid modified cst iron on grphite sphericity Thus, in ny cse, the spheroidizing effect disppers nd durtion of exposure in liquid stte is the key prmeter of this process. Therefore, the re-melting of ductile iron cstings leds to complete disppernce of sphericl grphite nd further spheroidizing tretment required. Specil ttention ws dedicted to mechnism nd kinetics of dissolution of sphericl grphite t C. It ws estlished tht the heting over the eutectoid trnsformtion temperture significntly ffects the sphericl grphite dissolution kinetics. Lower overheting tempertures inhiit the cron diffusion from grphite throughout the ustenite to ferrite. Cron diffusion occurs directly in the ferrite grins s well s t the ferrite grin oundries. The continuous trnsfer of cron toms to ustenite cretes the gp t the oundry etween grphite nd the metl mtrix. Grphite dissolution in ferrite phse is uneven process nd occurs with continuous ccelertion. The cron dissolution rte in ustenite increses s the temperture increses. Grphite dissolves quickly t tempertures ove 950 C [2]. Thus, there is lot of studies dedicted to the influence of technologicl prmeters on the shpe grphite in cst iron, however the comprehensive dt regrding to the kinetics of grphite sphericity reduction re very limited. The use of mischmetl (consisting of cerium) gives positive results of grphite spheroidizing. Reserch nd industril tests mischmetl used for processing nickel-cron nd iron-cron lloys hve shown tht sphericl grphite formed t high cooling rtes nd low sulfur content [3]. Reserch on re-melting smples of nickel-cron lloy, pre-treted y spheroidizing elements, showed tht the most effective cerium [3, 4]. The se lloy hd included mostly sphericl grphite shpe (fig1.2) [3]. After re-melting nd cooling of smples t speed of 20 degrees per minute sphericl grphite disppers, Fig 1.3 Grphite inclusions in specimens cooled t speed 20 degrees/min under rgon nd hydrogen. After re-melting nd cooling of smples t speed of 40 degrees per minute sphericl grphite is stored only in the smples tht were modified cerium. In the smples tht hve een modified mgnesium grphite sphericl shpe completely lost (fig.1.4). Fig 1.4 Grphite inclusions in specimens cooled t speed 40 degrees/min under rgon nd hydrogen. So, nodulrizsion grphite effect disppers in the smples tht were modified mgnesium nd re-melting t temperture of 1400 C nd exposure in to liquid 10 minutes. Incresing the cooling rte from 20 to 40 deg/min does not ffect the shpe of grphite inclusions. The im of this study is to fill the existing gp in literture nd to investigte the grphite sphericity reduction during the rc re-melting of ductile cst iron modified with FeSiMg-7. 2 EXPERIMENTAL METHODOLOGY 2.1. Cst iron smelting Bse cst iron ws melted in core-less induction furnce with cidic lining crucile, contining 60 kg. The chrge consists of pig iron nd steel scrp. The chemicl composition of pig iron is given in tle

3 TABLE CHEMICAL COMPOSITION OF PIG IRON Element C Si Mn S P Content, % 4,3 1,65 0,6 <0,03 <0,1 TABLE CHEMICAL COMPOSITION OF STEEL SCRAP Element C Si Mn S P Content, % 0,03 0,3 0,06 0,01 0,1 For the cst iron spheroidizing tretment used FeSiMg-7 lloy. FeSiMg-7 chemicl composition is given in Tle 2.3. Cst iron temperture efore pouring into molds ws mesured tungsten- rhenium thermocouple VR5/20 pired with digitl potentiometer A565. TABLE 2.3 CHEMICAL COMPOSITION OF FESIMG-7 Element Mg Si C REM Al Fe Content, % ,2..1 0,3 1 1,2 rest Smple ws molded in mold-ox with dimeter of 200 mm nd height of 300 mm. Schemtic digrm of the experimentl mold is shown in Fig Thermocouples 3, 4 is set in the holes for temperture recording t two points during the smple heting nd melting of the. Thermocouple 3 ws set t the upper prt of the smple, where the heting nd melting occur, for control the desired temperture. Thermocouple 4 ws instlled t the ottom of the smple, is designed to control temperture of solid iron. Melting ws conducted with cron electrode. The temperture ws kept in two rnges 1350 C 1380 C nd 1050 C 1100 C during specified time. The mount of modifier ws 2% of metl weight. Cst iron temperture efore modifiction ws C. For cst iron tretment nd molds pouring ws used lde with volume of 10 kg. After filling, the molds cool to room temperture Smple preprtion The initil csting with 12 smples (fig. 2.1) ws produced in dry snd mold. Fig 2.3 Schemtic digrm of the experimentl mold: 1 smple; 2 sndcly mixture; 3 thermocouple 1; 4 thermocouple 2; 5 contct; 6 cron electrode Fig. 2.1 Csting with smples The molten nd superheted to the tempertures 1450 С cst iron ws treted with mischmetl FeCeMg-5 in the mount of 3% y weight of metl. The molds were poured with modified cst iron, nd cooled to room temperture In ech ductile iron smple with the sizes of mm three through holes with dimeter 3 mm were mde (Fig. 2.2). Fig 2.2 Reserch smple scheme Two holes required for instlltion of thermocouples, nd the third (lower) to supply contct. After re-melting nd exposure smple cooled in mold to room temperture. The smple ws purified y residues of molding mixture nd polished for metllogrphic reserch. 2.3 Metllogrphic nlysis Determintion of grphite nodulrity degree (GND) conducted y the method, developed IPL of Acdemy of Sciences of Ukrine. To determine the mount of grphite inclusions in cst iron used Hlholev point method. The structure of the metl mtrix, grphite nodulrity rte (GNR) [3], the mount nd verge size of grphite inclusions were defined y mens of light opticl microscopy (LOM). 3 EXPERIMENT 3.1 Nodulr iron smples structure chrcteristics fter FeSiMg-7lloy modifiction. For reserch were produced smples with 70 mm length nd 10 mm width ( squre cross-section). Cst iron ws treted with FeSiMg-7 lloy nd pureed into dry snd mold. After smples seprtion the high-temperture nneling ws crried out t temperture C followed y cooling in the furnce

4 Temperture, о С The results of metllogrphic studies found, tht grphite nodulrity degree of se ductile iron modified with FeSiMg- 7 lloy is %, the verge grphite size is in the rnge of m nd the grphite inclusions quntity is in the rnge pcs/mm 2 (fig. 3.1, ). Grphite inclusion fetures of se ductile iron nd metl mtrix structure shown in the tle 3.1. TABLE 3.1 CHARACTERISTICS OF THE BASE NODULAR IRON STRUCTURE AFTER TREATMENT WITH FESIMG-7 ALLOY AND HIGH-TEMPERATURE ANNEALING. Indictor Vlue Grphite inclusion size, m 4 8 Grphite inclusion quntity, pcs/mm Grphite nodulrity degree, % Ferrite/Perlite quntity, % 35/ Melting zone Solid zone Time, sec 115 Fig 3.2 Grph of ductile iron high temperture heting nd short term exposure mode Fig. 3.1 Structure of se nodulr iron modified with FeSiMg-7 lloy. а not etched; etched The longitudinl smple cross-sections were tken for microstructure nlysis. 3.2 Grphite inclusions chrcteristics fter nodulr iron remelting Re-melting ductile iron modified with FeSiMg-7lloy t high temperture nd short-term exposure conditions. The smple ws re-melted y cron electrode electric rc. Grph of heting, exposure nd cooling mode re presented in fig Smples from nodulr iron heted to temperture C nd exposure during seconds. The totl time during which prt of the smple ws in liquid stte is seconds. After heting, melting, exposure, nd cooling the chrcteristics of grphite inclusions in the smple prt, tht ws re-melted, were investigted. Metllogrphic nlysis conducted y 3 mm in direction from temperture control point to melting th (fig.3.3). In generl, the grphite inclusions chnged shpe from nodulr to fully lmellr (fig. 3.3). In solid prt, close to the limit melting, grphite hs nodulr shpe. Dimensions of inclusions re in the rnge 3 5 m, the mount of grphite inclusions is in rnge from 200 to 250 pcs/mm 2 nd the nodulrity (GND) is %. c d e Fig 3.3 Grphite inclusions on the smple length fter re-melting t the temperture C nd exposure time seconds: а - 0 mm (se ductile iron); - 3 mm; c - 6 mm; d -9 mm; e - 12 mm

5 In the trnsition re (fig. 3.4) oserved prtilly shnky grphite with sizes m. In prt of the smple, which is liquid, sphericl grphite inclusions re sent. In the melting re ws found prtilly turulence grphite with length 3 4 m, corl grphite with length 3 5 m, lmellr grphite with length m. Fetures grphite prticles re shown in tle 3.2. Metl mtrix structure long the length of the smple chnges from ferrite-perlite, in solid prt to cementite nd ledeurite in prt, which ws re-melted. TABLE 3.2 GRAPHITE INCLUSIONS CHARACTERISTICS IN THE RE-MELTED SAMPLE AREA OF DUCTILE IRON MODIFIED WITH FESIMG-7 ALLOY. Distnce, mm Grphite shpe Inclusion dimension, GND, % Averge grphite mount, % m Initil (0) Nodulr corl Lmellr, prtilly turulence 6 Lmellr, prtilly turulence, corl 8 Lmellr, prtilly turulence, corl 1, , Along the entire melting line in solid prt of ductile iron smple locl concentrtions of corl nd roken grphite inclusions with length up to15 m re present. These locl concentrtions were oserved t the distnce m from the melting oundry. At the sme distnce, sphericl grphite inclusions re present (fig.3.5). Some of locl concentrtions roken nd corl grphite inclusions, tht re close to the melting oundry, hving contct with the liquid phse (fig. 3.6). All other locl concentrtions of corl nd roken grphite such contcts hven t. Non-nodulr grphite locl concentrtion effect ner the melting zone showed t fig Sphericl grphite includes in ductile iron tht ws modified with FeCeMg-5 lloy during re-melting egin to lose sphericl grphite shpe in solid ner melting oundry. Fig. 3.4 Melting zone of re-melted ductile iron modified with FeSiMg-7 lloy. а not etched; etched Fig. 3.5 The locl concentrtions of non-nodulr grphite in ductile iron modified with FeSiMg-7 lloy fter re-melting t the temperture C nd exposure during sec. а etched; not etched. Fig. 3.6 Enlrge locl concentrtion, connected with melting re, of nonnodulr grphite in ductile iron modified with FeSiMg-7 lloy fter remelting t the temperture C nd exposure during sec. а etched; not etched. The width of the trnsition zone or melting zone is in the rnge m (fig. 3.7). Fig. 3.7 The trnsition (melting) zone of ductile iron modified with FeSiMg- 7 lloy fter re-melting t the temperture C nd exposure during sec (etched) Re-melting ductile iron modified FeSiMg-7lloy t low temperture nd long term conditions. Smple modified with FeSiMg-7 lloy, ws heted to the temperture C (ner mgnesium oiling point) nd exposure during 7 min 10 second (fig. 3.8). Metllogrphic nlysis showed tht fter nodulr iron heting nd exposure t C, during 7 minutes 10 seconds, in the re of the smple tht hs een re-melted, there were no chnges in the shpe of grphite inclusions (fig. 3.9). After heting, exposure, nd cooling t different distnces from the oundry of the heting occurs only chnging the size nd mount of grphite inclusions

6 Теmperture, о С Heting point Solid re Time, seс Fig. 3.8 Grph of ductile iron low temperture heting nd long term exposure mode Dt on the grphite inclusions chnging re shown in tle 3.3. As prt of the smple, which hd temperture elow C grphite inclusions hve nodulr shpe. The nodulr grphite inclusions dimeter decresed to 4 6 m. Grphite nodulrity remined t the sme level %. Grphite inclusions mount decresed from 250 to 200 pcs/mm 2. In the heted re, from the point of control to the het source there is generl pttern of decline grphite sphericity to %. Grphite inclusions mount decresed to 180 pcs/mm 2. Present some grphite inclusion tht hve prtilly roken shpe nd their size re 1,5...3 m. The mount of their grphite inclusions is pcs/mm 2. (рис.3.16) TABLE 3.3 GRAPHITE INCLUSIONS CHARACTERISTICS IN THE RE-MELTED SAMPLE AREA OF DUCTILE IRON MODIFIED WITH FESIMG-7 ALLOY (EXPOSURE TEMPERATURE C, TIME 7 MINUTES 10 SECONDS). Distnce, mm Dimeter, GND, % Averge grphite mount,% m Initil (0) , , ,5...3, Fig. 3.9 Grphite inclusions in the ductile iron re-melted re (vrious view sight) t the temperture C, t 0 mm, 3 mm, 6 mm nd 9 mm, respectively 4 CONCLUSIONS At the repeted re-melting ductile iron modified with FeSiMg-7 lloy it is enough exposure t sec in liquid stte t temperture C for nodulr grphite inclusions destruction. In the re-melted phse oserved includes of corl, some turulence nd lmellr grphite. The size of these includes increse from 150 to 200 m, in the direction from melting zone to the het source. In the solid phse, directly in contct with the melting oundry, oserved locl concentrtions with roken nd corl grphite inclusions. These locl concentrtions re situted long melting oundry t the distnce from 16 to 600 m. Some of the locl concentrtions hve contct with the liquid (re-melted) phse. But, most of the locl concentrtions seprted from the liquid (re-melted) phse with solid ductile cst iron contining spheroidl grphite. Heted, nodulr cst iron, modified with FeSiMg-7 lloy, up to mgnesium oiling temperture C, nd exposure t 7 min nd 10 sec there re no significnt chnges grphite inclusions. However reveled tht towrd the liquid phse reduced verge grphite inclusions size from 5 to 3 m, verge grphite inclusions mount decreses from 300 to 180 pcs/mm 2 nd nodulrity decreses from 95 to 75 %

7 REFERENCES [1] Voloschenko M.V., Toropov A.I., Influence of residul mgnesium on the grphite shpe. // Foundry. Mnuf S [2] Ykovlev F.I, Aout the mechnism of dissolution of grphite during induction heting of the nodulr iron csts. // Foundry. Mnuf S [3] Yuji Kto, Ying Zou, Hideo Nke. Influence of Melting Conditions on Grphite Morphology in Spheroidl Grphite Cst Iron Using Ni-C Alloys. Key Engineering Mterils Vol. 457 (2011) pp [4] Y. Ttsuzw, S. Jung, H. Nke Cooling curve nd grphite morphology in Ni C lloys Interntionl Journl of Cst Metls Reserch 2008 Vol 21 No 1 4 pp