THE QUALITY OF CAST IRON SURFACE CREATED AS THE RESULT OF HOT - DIP ZINC GALVANIZING PRECEDED BY HIGH TEMPERATURE OXIDATION

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1 THE QUALITY OF CAST IRON SURFACE CREATED AS THE RESULT OF HOT - DIP ZINC GALVANIZING PRECEDED BY HIGH TEMPERATURE OXIDATION Driusz JEDRZEJCZYK, Mciej HAJDUGA University of Bielsko-Bil, Bielsko-Bil, Willow 2, Polnd, E-mil ddress: djedrzejczyk@th.ielsko.pl Astrct The pper descries the high temperture oxidtion s the method tht llows voiding some prolems rising during hot-dip zinc glvnizing of cst iron. Authors studied the influence of the high-temperture oxidtion, s the preliminry stge previous to coting with zinc on the chnge of surfce lyer structure s well s susurfce lyer of cst iron with flke, vermiculr nd nodulr grphite. Received effects were compred to these otined during cst iron coting without preliminry therml processing. To oservtion oth opticl nd scnning microscope ws pplied. As the consequence of conducted high-temperture oxidtion in susurfce lyer of cst iron pores hve een creted, tht in result of coting in liquid zinc were filled with new phse nd in this wy the new zone with different properties ws otined. To otin such composite susurfce lyer the preliminry chemicl tretment is necessry. It ws dditionlly stted tht the cst iron lyer enriched in zinc is considerly thicker thn lyers got with ppliction of other methods. Thickness of su-surfce lyer where fter-grphite pores re filled with zinc depends directly on kind of grphite. In cst iron with flke nd vermiculr/compcted grphite depth of penetrtion reches 120 m, wheres in nodulr cst iron it reches only 15 m, lthough sometimes single filled with zinc voids re oserved t 75 m depth. Moreover, roughness of cst iron eing previously oxidized mesured fter zinc coting is sometimes even lower thn roughness of zinc coting cst iron fter turning. Keywords: High temperture oxidtion, Hot dip zinc glvnizing, Cst iron. 1. INTRODUCTION There re severl fctors influencing thickness nd properties of Zn coting creted t the Fe-C lloys surfce. Generlly, nlysis of Fe-Zn equilirium digrms shows tht expected coting growth sequence should e s follows: Γ1, δ, ζ, η [1-3]. Coting thickness cn e controlled y: introducing Al ddition to liquid Zn tht cretes lyer locking coting growth; growth mechnisms cn e chnge y introducing to liquid zinc such ddition, like: Ni, Ti, V, Mn [4]; introducing P, Bi nd Sn cn chnge th viscosity nd surfce tension [5]; greter ddition of P, Bi nd Sn results in Zn seprtion from lloyed coting lyer. Aluminum decreses growth of lloyed lyers in consequence of inter-metllic phse cretion Al 2 Fe 5 (Fig. 1) [6]. Nickel concentrtes in Fe-Zn phses nd cuses higher structure compctness. Also temperture level exerts essentil influence on coting qulity. In temperture rnge up to 480 o C nd higher thn 530 o C Zn coting growth is ccording prolic low. Between 490 nd 520 o C, lloyed lyer crystllizes ccording liner dependence cusing rpid nd uncontrolled coting growth. Decresing temperture to 430 o C elimintes Sndeline effect [8]. Tht is why during hot dip Zn coting of steel elements usully tempertures elow 480 o C re used. From the other side high temperture Zn coting is used during cst iron tretment. The creted t cst iron Zn coting should consist of: Γ1, δ nd η phses. So, it follows from the ove dt nlysis tht scientific reserch concentrtes minly on growth model [9,10] nd proper choice of th chemicl composition, temperture, fluxing nd etching thn suitle mteril surfce preprtion. It regrds especilly cst iron zinc coting tht ws poorly spred until recently ecuse of prolems ssocited with: the lrge content of Si nd P in se mteril, etching nd processing of the csting surfce tht demonstrtes the high content of SiO 2 nd grphite precipittions.

2 Fig. 1 Grphite nd grin oundries influence on zinc coting structure: scheme of Fe-Al lyer rekdown mechnism t steel surfce [5]; grphite precipittion inside Zn coting (own investigtions). Generlly the opinion ws formulted tht coting of cst iron cretes greter prolems thn steel with high E equivlent vlue, nd proper cotings (without defects) cn e creted only t decrurized surfce of mllele cst iron (decruriztion llows voiding the dverse influence of coting pollutions y grphite precipittions (Fig 1)) [6]. Therefore using protective zinc covers t cst-iron cstings ws only occsionlly, minly t the production of elements of industril fittings mde from mllele cst iron, which in the lst yers (from technicl nd economic considertions) is replcing with the ductile cst iron. Becuse it follows from the literture dt tht cretion of Zn lyer t crude nodulr cst iron csting not requires ny specil tretment during zinc th preprtion [6], cotings creted t crude surfce of cstings oth in high nd low temperture processing re continuous nd stedy. In industril condition very difficult is to chnge tretment prmeters (th temperture nd chemicl composition) only for single elements. This is the reson why surfce stte stys significnt prmeter influencing the thickness nd qulity of coted lyer (Fig. 2). Fig. 2. Influence of the th temperture nd surfce qulity on the kinetic of zinc coting growth t the cst iron: 1 - lyer creted t the crude csting surfce; 2 - lyer creted t the grinding surfce; λ C - totl lloyed nd zinc lyer thickness [6]. The ssumed investigtion trgets ws: formtion of composite susurfce lyer, incresing of Zn diffusion depth, formtion of outside Zn lyer without grphite precipittions nd impurities, good qulity of coting expressed y reltively low surfce roughness, high corrosion resistnce of creted lyer.

3 2. METHOD OF INVESTIGATION Experiment ws composed of the following stges: proper smple preprtion (csting, oxidtion, zinc coting), smples surfce nlyse nd corrosion tests. Smple preprtion - investigtions refers to cst iron with flke, vermiculr nd nodulr grphite melted in induction furnce of medium frequency. Liquid metl ws cst to snd mould with dimensions φ30x300mm (cst iron with flke nd vermiculr grphite) nd Y2 ingots (cst iron with nodulr grphite).the chemicl composition of pplied cst iron is presented in Tle 1. Tle 1 Chemicl composition of cst iron pplied in experiment Grphite shpe Chemicl composition, wg. % C Si Mn P S Mg Ce Flke 3,32 1,80 0,55 0,065 0, Nodulr 3,63 2,55 0,10 0,025 0,007 0,045 0 Vemiculr 3,65 2,58 0,08 0,023 0,008 0,025 0,015 From such cst ingots smples φ=11mm nd length l=110mm were turned. Then smples were put to silite furnce PKS 600/25 to oxidize. The experiment ws led in five different tempertures - 850, 900, 950, 1000 nd 1050 o C. Smples hve een tken out from the furnce seprtely fter: 2, 4, 6, 8, 10 nd 12 hours. Before oxidtion nd fter cooling dwn smples were exctly mesured, weighed, nd metllogrphic specimens were prepred to oserve surfce perpendiculr to smple s xis. The scle lyer ws removed in two stge procedure: mechniclly y sndlsting, s well s chemiclly, y dipping in solution of oxlic cid. After scle lyer removl the hot dip zinc coting in industril conditions hs crried out. Process consisted of the following ction: degresing dirt nd oil removl; pickling rust, scle nd cron deposit removl; rinsing hydrochloric cid removl; fluxing incresing of zinc dhesion to lloy; glvnizing in temperture 460 o C, within t=1min in th enriched with Ni, Bi nd Al; cooling decresing of smple temperture. Surfce nlyse - to opticl oservtion microscope "NEOPHOT 2 s well s Axiovert A were pplied, wheres further exmintion ws mde with ppliction of scnning microscope Jeol J7 nd X-ry nlyzer JCXA JEOL. Smple s surfce qulity ws descried dditionlly y roughness mesurements mde fter scle lyer removl - MAHR profile mesurement guge with Perthometer Concept softwre. Corrosion tests - corrosion resistnce will e evluted using smples φ=14,7 mm nd thickness 7mm, y ppliction of potentiostt SI 1286, tht enles registrtion of polriztion curves in three electrodes system. Before mesure strt, smples will e plced in corrosion solution 0,5M N 2 SO 4, in temperture 25 o C, within 24h. Smples will e sujected to polriztion in the sme solution, from potentil 1000mVNEK in node direction, with rte 1mV/s. Results will e compred to these mesured for steel tested in similr conditions. 3. RESULTS OF INVESTIGATIONS Cst iron oxidtion leds to severl effects: scle lyer cretion, decruriztion, fter-grphite voids cretion (with dependence on grphite kind). The scle lyer covering cst iron smples cn e divided on two zones - first situted outside the initil surfce of smple where porosity is directed prllel to the smple rdius s well s internl lyer where the direction of porosity is diverse. It cn e stted tht cst iron oxidtion t high temperture leds to similr effect tht is oserved in cse of pure iron - low cron steel t the surfce the scle lyer forms tht is composed of three lyers: wustite (FeO), mgnetite (Fe 3 O 4 ) nd hemtite (Fe 2 O 3 ), dditionlly ecuse of higher silicon content the internl scle lyer in composed of wustite nd fylite mixture. Moreover in susurfce lyer of cst iron the re free from grphite which thickness chieves even 1,6mm is oserved. So, generlly the oserved structure of cst iron cn e divided on three lyers. The conducted process of high-temperture oxidtion leds to cretion not only the scle

4 lyer nd susurfce porous lyer ut lso cuses the chnge of metl mtrix chrcter from perlitic to ferritic. Both mesured prmeters: the dimeter of metl core s well s the smples mss enlrges more intensely in initil stge of oxidtion. This is proly cused y growth of scle lyer, which influence cn e compred to the cot inhiiting the course of oxidtion process. Additionlly, the silicon content interction cn t e neglected; especilly in cst iron where its vlue is much higher thn in steel. Silicon fvours the decrese the speed of oxidtion, oth during externl, s well s internl corrosion. It follows from otined dependence tht porous lyer thickness cn e controlled y suitle selection of temperture nd the time of oxidtion s well s it depends on grphite shpe existing in cst iron. Porous lyer thickness decreses when grphite shpe is chnging from flke to vermiculr nd nodulr. Moreover, wheres in cst iron with flke nd vermiculr fter-grphite voids re filled with new product, in nodulr cst iron chemicl composition of the filler is similr to metllic mtrix (Fig. 3). Chemicl composition of phse filling the fter grphite (flke nd vermiculr) pores is similr to internl scle lyer covering smple surfce. With reference to ove cretion of composite lyer in nodulr cst iron is prcticlly impossile, wheres in cst iron with flke nd vermiculr grphite necessry is not only removl of outside scle lyer ut lso fter grphite pores filler. The ssumed effect cn e relized only y proper chemicl tretment. When the chemicl processing ws used dipping in solution of oxlic cid, the penetrtion depth considerle increse nd chieves 120µm in cst iron with vermiculr nd flke grphite. c d Fig. 3. Structure nd liner chnges of mesured elements concentrtions fter oxidtion:, cst iron with vermiculr grphite; c, d cst iron with nodulr grphite. Differentition of the oxidtion effect results from diverse grphite structure: flke nd vermiculr grphite precipittions re connected to ech other, wheres nodulr grphite precipittions re seprted. Thickness of zinc lyer tht ws creted t the surfce of cst iron smples is stle nd reches µm. Essentil in this cse is fct tht grphite precipittion frequently penetrte in coted zinc lyer nd cn in this wy reduce it tightness (Fig.1, 4).

5 Fig 4 Microstructure of cst iron with flke grphite fter hot dip zinc glvnizing; zinc coting without oxidtion, zinc coting with oxidtion, 950 o C, 8h, scle removed mechniclly. Fig. 5 The chnge of Zn concentrtion t the cross section of creted Zn lyer; points of nlysis, chnge tendency. Such sitution is not oserved in cse of oxidized cst iron. Cst iron, where the scle lyer ws removed only mechniclly using sndlsting, does not revel deep zinc penetrtion inside the fter-grphite pores it rech out 15 m (Fig. 4). It is evident tht oxidtion process results in higher cst iron surfce roughness. Surfce stte is chnging more rpidly in cst iron with flke grphite. In this cse roughness express y R vlue rech even more thn 9µm, fter 12 hour of oxidtion t 950 o C. When grphite shpe chnge from flke to vermiculr nd nodulr, roughness chnges sty smller. Mesured roughness in vermiculr nd nodulr cst iron fter 12 hour oxidtion t 950 o C rech correspondingly 8,2 nd 6,8µm. Although zinc coting mkes surfce more smooth the finl qulity of otined surfce chnges in dependence on prmeters of high-temperture oxidtion. The gretest surfce smoothness - R <6,3 m ws got for smples oxidized t prmeters not higher thn T 1050 o C nd t 8h. For exmple, roughness mesured t smple with flke grphite, tht ws oxidized efore zinc coting (T=950 o C, t = 8h) gined vlue R = 3,62 m. It mens decresing in comprison to oxidized surfce fter snd lsting out 3,26 µm. Although the mesured vlues R not differs essentilly, it is visile tht roughness otining t the surfce glvnized fter turning (R = 4,06 m) is higher tht this one mesured fter glvnizing tretment comined with oxidtion (compre surfce t Fig 4 nd 4). The higher Fe (lower Zn) content ws mesured in internl coting su-lyer, which decreses with incresing of distnce from cst iron surfce. Such results confirm the growth model presented in literture. It follows from mesured Fe, Zn nd C concentrtion tht ζ nd η phses thickness reches out 60 µm wheres the rest of lyer is occupied y δ phse (Fig 5).

6 Reliztion of ssumed results: 1. Formtion of the composite susurfce lyer, 2. Incresing of Zn diffusion depth, 3. Formtion of outside Zn lyer without grphite precipittions nd impurities, 4. Good qulity of coting expressed y reltively low surfce roughness, 5. Higher corrosion resistnce of creted coting.? 4. CONCLUSIONS Fig. 6. Scheme presenting ssumed trgets reliztion 1. During the experiment, s the result of two stge hot-dip zinc glvnizing of cst iron, the following trgets were relized (Fig. 6): formtion of the composite Zn-metllic mtrix susurfce lyer; incresed Zn diffusion depth; formtion of outside Zn lyer without grphite precipittions nd impurities; good qulity of coting expressed y reltively low surfce roughness. 2. Conducted investigtion llows to suppose tht corrosion resistnce of such creted coting should e etter thn resistnce of the lyer otined without high temperture oxidtion. The formulted ssumption will e verified y corrosion tests in 0,5M N 2 SO 4 nd results will e compred to these mesured for steel tested in comprle conditions 3. Cretion of composite lyer in nodulr cst iron is prcticlly impossile, wheres in cst iron with flke nd vermiculr grphite necessry is not only removl of outside scle lyer ut lso fter grphite pores filler. The ssumed effect cn e relized only y proper chemicl tretment, without necessity of pressure processing. The use of chemicl processing cuses, tht depth of zinc penetrtion gets higher - to 120 m level. 4. Roughness mesured fter zinc coting of cst iron eing previously oxidized is sometimes even lower thn roughness of zinc coting of cst iron fter turning. It follows first from negtive influence of grphite precipittions uncovered during turning tht cn chnge loclly the thickness nd structure of coted lyer nd decrese in this wy its tightness. LITERATURE [1] JORDAN C.E., MARDER A.R. Fe-Zn phse formtion in interstitil free steels hot-dip glvnized 450 o C. Journl of Mteril Science, 32, 1997, [2] MITA K., IKEDA T., MAEDA M. Phse digrm study of Fe-Zn intermetllics. Journl of Phse Equiliri, 23, 2000, [3] SU X., TANG N.Y., TOGURI J.M. Thermodynmic evlution of the Fe-Zn system. Journl of Alloys nd Compounds, 325, 2001, [4] REUMOUNT G., FOCT J., PERROT P. New possiilities for the glvnizing process: the ddition of mngnese nd titnium to the zinc th. From Conference Proceedings. Interglv 2000, Berlin 2000, [5] FRATESI R., RUFFINI N. Use of Zn-Bi-Ni lloy to improve zinc coting ppernce nd decrese zinc consumption in hot dip glvnizing. From Conference Proceedings. Interglv 2000, Berlin 2000, [6] MURDER A.R. The metllurgy of zinc coted steel. Progress in Mterils Science, 45, 2000, [7] KOPYCINSKI D. Monogrph, AGH Krkow, [8] KOPYCINSKI D., GUZIK E., WOLCZYNSKI W, Coting Zn formtion during hot dip glvnizing, Mterils Engineering, 4, 2008, [9] KOPYCINSKI D., GUZIK E., Zinc lyer t the nodulr cst iron surfce. Mterils Engineering, 6, 2008,