Ductile Cast Iron Induction Re-Melting

Transcription

1 Dutile Cst Iron Inution Re-Melting Dr. Olexiy A. Thykovsky, Senior Professor, Deprtment of Founry of ferrous n non-ferrous metls, Ntionl Tehnil University of Ukrine KPI Kyiv, Ukrine, Peremohy v., 37 Oksn V. Klok Mster stuent Deprtment of Founry of ferrous n non-ferrous metls, Ntionl Tehnil University of Ukrine KPI Kyiv, Ukrine, Peremohy v., 37 Astrt High impt n ftigue resistne of utile st iron ompre to nother types of st iron is efine y the spheril shpe of grphite inlusions. The spheriity of grphite grully isppers fter the exposure of the liqui st iron in the lle uring sting. This proess ws previously investigte n the influening ftors suh s mgnesium ontent, exposure time n temperture were etermine. The omplete ispperne of the grphite spheriity ws lso oserve fter utile st iron re-melting. The im of this work ws to stuy the kinetis of grphite shpe trnsformtion from spheril to lmellr in utile st iron uring the re-melting proess. The effet of het tretment exposure on the shpe of grphite inlusions ws etermine. Keywors Dutile st iron; spheril grphite, moifition, re-melting, resiul mgnesium ontent I INTRODUCTION 1.1 Spheril grphite in utile st iron There re mny hypotheses regring to the origin of spheril grphite in st iron, ut none of them provies omplex esription of this proess. The high tempertures omplite oservtions of numer of phenomen tht oul revel the mehnism of spheril grphite formtion uring spheroiizing tretment n the role of mgnesium n other spheroiizers in this proess. Kuznetsov et l. propose the metho, whih llows to otin the spheril shpe of grphite in st iron without ing of spheriity moifiers ontining Mg, Se, Y n REM. Bse on the finl omposition, tretment proess prmeters, the otine utile st iron hs following properties: ϭ в= МPа, δ=3 20%, НВ = Depening on the spheroiizing tretment n het tretment regime, the struture of the metl mtrix n e: ferriti (НВ=60 235), ferriti-perliti (НВ= ), perliti, initi n mrtensiti (НВ= ). The verge grphite size is μm [1]. The prinipl inustril proess of utile st iron proution is se on using of spherioizing elements. A rnge of properties of utile st iron is efine y grphite spheriity. In turn, the grphite spheriity entirely epens on the resiul ontent of spheroiizer, minly mgnesium. Resiul mgnesium ontent, whih gives the spheril grphite shpe, epens on the ooling rte, moifition proess prmeters n metl mtrix struture. The mount of spheroiizer epens on mny ftors: the mss of st iron whih is proesse, the mol filling time, sting wll thikness, sulfur ontent, temperture, ooling rte et. In se of insuffiient quntities of spheroiizer n ifferent ooling rtes mgnesium segregtion ours, using lower mgnesium onentrtion in the prts of the sting, whih rystllize in the lst turn. Mirostruturl n spetrl nlysis estlishe the influene of ifferent quntities of resiul mgnesium on the grphite shpe in stings, oole t the sme rte. Bse on prtie n the results otine, it n e onlue tht the resiul mgnesium ontent whih ensure the orret spheril shpe of grphite in stings with wll thikness of mm shoul not e lower thn 0, ,042 %. Spheroiizing tretment tehnology of st iron must gurntee stisftory ssimiltion of mgnesium in liqui metl. Aoring to Voloshenko et l. the resiul mgnesium ontent of stings, whih rystllize with the formtion of metstle strutures shoul not e lower thn 0,035 %. At lower mgnesium ontent spheril grphite hs irregulr or mixe shpe, whih is uneptle [2]. One of the min hllenges of utile st iron proution is to ensure high grphite spheriity rte in stings otine from moifie st iron. This prolem prtiulrly ours ue to prolonge filling of the mols n t low rystlliztion rtes. The effet of liqui utile st iron exposure on kinetis of grphite spheriity reution is given in Tle 1.1 [3]. In this stuy the initil st iron with stle hemil omposition ws smelte in furne with si lining t C n trete with optiml mounts of given moifiers. The moifie st iron ws kept in the furne t C n the liqui metl proes for nlysis were tken every 3 min. Bse on the results given in Tle 1.1 n Figure 1.1 it n e onlue tht isotherml exposure of moifie st iron reues the spheriity of grphite, ut the intensity of this proess epens on the moifier omposition. Immeitely fter the spheroiizing tretment the spheriity rte ws %, fter 10 n 20 min % n %, respetively. 836

2 Noulrity grphite rition, % TABLE EFFECT OF DURATION OF EXPOSURE ON THE GRAPHITE SHAPE IN CAST IRON AFTER MODIFICATION Moifier Grphite spheriity, % 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 lk of proportionlity etween the exposure urtion of liqui moifie st iron n the grphite spheriity rte is typil for ll investigte moifiers. The grphite spheriity reution is slow for short urtions of exposure n inreses with time (Fig. 1.1). ferrite phse is uneven proess n ours with ontinuous elertion. The ron issolution rte in ustenite inreses s the temperture inreses. Grphite issolves quikly t tempertures ove 950 C [4]. Thus, there re lots of stuies eite to the influene of tehnologil prmeters on the shpe of grphite in st iron, however the omprehensive t regring to the kinetis of grphite spheriity reution re very limite. The im of this stuy is to fill the existing gp in literture n to investigte the grphite spheriity reution uring the re-melting of utile st iron. 2 EXPERIMENTAL METHODOLOGY 2.1. Cst iron smelting Initil st iron ws smelte in oreless inution furne IST-0.06 gre with ruile of 60 kg. Chrge mteril ws pig iron n steel srp, hemil omposition is shown in Tle 2.1 n 2.2. 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 The temperture of st iron in the lle efore sting ws mesure with tungsten-rhenium thermoouple VR5/20. Spheroiizing tretment ws performe in 10 kg lle using omplex moifier FeSiMg-7, the hemil omposition is shown in Tle 2.3. TABLE 2.3 CHEMICAL COMPOSITION OFFESIMG-7 Element Mg C REM Si Al Fe Content, % 6,5 8,5 0,2 1,0 0,3 1, ,2 other The mount of moifier ws 2% of metl weight. Cst iron temperture efore moifition ws C. Exposure time fter moifition, min Fig. 1.1 Effet of exposure urtion of liqui moifie st iron on grphite spheriity Thus, in ny se, the spheroiizing effet isppers n urtion of exposure in liqui stte is the key prmeter of this proess. Therefore, the re-melting of utile iron stings les to omplete ispperne of spheril grphite n further spheroiizing tretment require. Speil ttention ws eite to mehnism n kinetis of issolution of spheril grphite t C. It ws estlishe tht the heting over the eutetoi trnsformtion temperture signifintly ffets the spheril grphite issolution kinetis. Lower overheting tempertures inhiit the ron iffusion from grphite throughout the ustenite to ferrite. Cron iffusion ours iretly in the ferrite grins s well s t the ferrite grin ounries. The ontinuous trnsfer of ron toms to ustenite retes the gp t the ounry etween grphite n the metl mtrix. Grphite issolution in 2.2. Smple preprtion The initil sting with 12 smples (Fig. 2.1) ws proue in ry sn mol. Fig. 2.1 Csting with smples Smples were ut from the sting n the entrl prt of mm ws ut from the eh smple. 2.3 Metllogrphi nlysis The struture of the metl mtrix, grphite noulrity rte (GNR) [3], the mount n verge size of grphite inlusions were efine y mens of light optil mirosopy (LOM). 837

3 3 EXPERIMENT 3.1 Inution heting n melting of st iron with noulr grphite. The experimentl setup is shown on Figure 3.1. Dutile st iron smple ws prtilly ple in inutor n the other prt ws intensively oole with wter. Exposure time, min TABLE RESULTS OF METALLOGRAPHIC ANALYSIS Averge Grphite Averge grphite size, spheriity rte, % grphite μm mount,% initil , , , ,1 Aoring to the results of metllogrphi nlysis it ws foun tht the verge grphite size, spheriity rte n the mount of grphite inlusions in forily oole prt of the smple hs not hnge ompre to the initil utile st iron. However, the initil perlite (45)/ferrite (55) metl mtrix struture hs hnge to perlite (70)/ferrite (30). Fig. 3.1 Sheme of the experimentl setup: 1-smple; 2-steel tue; 3- inutor; 4-sn, 5-vessel fille with wter Three thermoouples were inserte in the holes t 5, 12 n 30 mm from the smple ege. The inutor ws fille with sn. One prt of the smple ws hete to C, the mile prt to C, n the other prt elow euteti tempertures C (Fig. 3.2). Smples were expose for 4, 8, 16 n 20 min n oole own t room tempertures. Fig.3.3 Mirostruture of utile st iron expose for: ) 4 min, ) 8 min, ) 16 min, ) 20 min. Fig. 3.2 Heting n ooling urves for thermoouples: I 30 mm from the smple ege, II 12 mm, III 5 mm The longituinl smple ross-setions were tken for mirostruture nlysis. 3.2 Effet of exposure time on grphite shpe n size It ws oserve tht ll smples mirostrutures n e hrterize y three zones (Fig. 3.3): zone with interenriti ot grphite (prt of the smple overhete ove the melting point); trnsition zone (etween melte n soli phse); spheril grphite zone (forily oole prt). The metllogrphi nlysis results re liste in Tle µm Fig. 3.4 Mirostruture of the initil utile st iron: ) without ething, ) fter ething. Fig. 3.5 Mirostruture of forily oole smple prt: ) without ething, ) fter ething 838

4 The struture of the trnsition zone for nerly ll smples hs shown the exfolition into six speifi su-zones (Fig. 3.3, Fig. 3.6): I the suzone of lrge flke grphite prtilly situte in leeurite; II eplete ron suzone with lmost no grphite; III the suzone with smll flke n vermiulr grphite; IV eplete ron suzone; V the suzone with flke n vermiulr grphite; VI ron eplete suzone orering the zone with spheroil grphite (prt of the smple, whih ws forily oole). Ething of smples hs shown ferrite n perlite mirostruture of the trnsition zone. Ferrite islns roun grphite inlusions inite tht they were forme from the initil spheril grphite. Prtilly white struture with ementite in form of leeurite ws oserve in overhete prts of ll smples. Smple, expose for 4 min hs shown leeurite with perlite n the shunky grphite, surroune y ferrite. All other smples hve ler leeurite with interenriti ot grphite. In orer to etermine the with of the trnsition zone 10 mesurements were me for eh smple. Aoring to the results of mesurements the following ws estlishe (t. 3.2): the trnsition zone of the smple expose for 4 min vries from 502 to 523 μm (verge μm); the trnsition zone of the smple expose for 8 min vries from 223 to 256 μm (verge μm); the trnsition zone of the smple expose for 16 min vries from 534 to 555 μm (verge μm); the trnsition zone of the smple expose for 20 min vries from 615 to 642 μm (verge μm). Fig Struture of the smples etween molten n soli zones fter exposure time: ) 4 min, ) 8 min, ) 16 min, ) 20 min TABLE 3.2 CHARACTERISTICS OF THE TRANSITION ZONE WIDTH The trnsition zone with, μm View sight Expos ure, min without ething fter ething TABLE 3.2A CHARACTERISTICS OF THE TRANSITION ZONE WIDTH Exposure Averge with, µm min 4 513, , , ,8 839

5 Averge grphite size, mirons III µm, IV µm, V µm n VI µm. It ws lso oserve tht the verge grphite length for suzone I vries etween µm, for III µm, V µm. In suzones II, IV n VI grphite is prtilly sent (Tle 3.4, Fig 3.10). Fig. 3.9 Effet of exposure time on the withs of trnsition zone suzones non-ething Fig.3.7 Mirostruture of the melte smple prt expose for: ) 4 min, ) 8 min, ) 16 min, ) 20 min 513,7 µm 241,6 µm ething TABLE 3.4 GRAPHITE LENGTHS IN THE TRANSITION ZONE SUBZONES Exerpt Grphite length mirons time, min I II III IV V VI min 8 min 16 min 20 min Exposure time I III V 542,8 µm 630,8 µm Fig Averge grphite size in the trnsition zone suzones Figures 3.11 n 3.12 show the impt of exposure time on the with of trnsition zone n grphite size, respetively. Fig. 3.8 Trnsition zones of smples expose for: ) 4 min, ) 8 min, ) 16 min, ) 20 min TABLE 3.3 DIMENSIONS OF TRANSITION ZONE SUBZONES Ezposure, Suzone with, µm min I II III IV V VI Bse on the metllogrphi nlysis it ws foun tht suzone I with vries from µm, II µm, Fig Effet of exposure time on the trnsition zone with 840

6 VI ron eplete suzone orering the zone with spheroil grphite (prt of the smple, whih ws forily oole). It ws lso foun tht eresing of the het tretment time from 20 to 8 min ereses the trnsition zone with from 630 μm to 250 μm n grphite size from 98 μm to 50 μm, respetively. Grphite inlusions, surroune y ferrite islns in the trnsition zone inite their initil formtion of spheril grphite. Fig Effet of exposure time on the verge grphite size 4 CONCLUSIONS All utile st iron smples sujete to the het tretment h shown three min zones with respet to the grphite shpe: zone with spheril grphite, trnsition zone n zone with interenriti ot grphite. The trnsition zone struture in lmost ll speimens n e ivie into six suzones: I the suzone of lrge flke grphite prtilly situte in leeurite; II eplete ron suzone with lmost no grphite; III the suzone with smll flke n vermiulr grphite; IV eplete ron suzone; V the suzone with flke n vermiulr grphite; Consiering the proesses, whih our t ifferent urtions of the het tretment s prts of the one proess, the following kinetis of grphite shpe trnsformtion n e suggeste. The ollpse of spheroil grphite n flke grphite formtion etween the hete n oole prts of the speimen ours ue to the evportion of resiul mgnesium t high tempertures. Further inresing of exposure time retes onitions for more omplete iffusion of silion n ron, whih les to exfolition of trnsition zone into the suzones with ifferent ron ontent. The smooth trnsformtion from spheroil to flke grphite ws not oserve t given experimentl onitions. REFERENCES [1] Kuznetsov B.L. Noulr n vermiulr iron proution without spheroiizing moifiers. // Founry. Mnuf S [2] Voloshenko M.V., Toropov A.I., Influene of resiul mgnesium on the grphite shpe. // Founry. Mnuf S [3] Litovk V.I. Improving the qulity of utile iron stings. Kiev, Nukov Dumk, p.. [4] Ykovlev F.I, Aout the mehnism of issolution of grphite uring inution heting of the noulr iron sts. // Founry. Mnuf S