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Microstructure of As-Cast Ferritic-Pearlitic Nodular Cast Irons Jacques Lacaze, Jon Sertucha, Lena Magnusson Åberg

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Microstructure of As-Cast Ferritic-Pearlitic Nodular Cast Irons Jacques Lacaze, Jon Sertucha, Lena Magnusson Åberg Microstructure of as-cast ferritic-pearlitic nodular cast irons Jacques Lacaze, Jon Sertucha, Lena Magnusson Åberg To cite this version: Jacques Lacaze, Jon Sertucha, Lena Magnusson Åberg. Microstructure of as-cast ferritic-pearlitic nodular cast irons. ISIJ international, Iron & Steel Institute of Japan, 2016, vol. 56 (n° 9), pp. 1606-1615. 10.2355/isijinternational.ISIJINT-2016-108. hal-01565250 HAL Id: hal-01565250 https://hal.archives-ouvertes.fr/hal-01565250 Submitted on 19 Jul 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Open Archive TOULOUSE Archive Ouverte ( OATAO ) OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 18081 To link to this article : DOI: 10.2355/isijinternational.ISIJINT-2016-108 URL : http://dx.doi.org/10.2355/isijinternational.ISIJINT-2016-108 To cite this version : Lacaze, Jacques and Sertucha, Jon and Åberg, Lena Magnusson Microstructure of as-cast ferritic-pearlitic nodular cast irons. (2016) ISIJ International, vol. 56 (n° 9). pp. 1606-1615. ISSN 0915-1559 Any correspondence concerning this service should be sent to the repository administrator: [email protected] Microstructure of As-cast Ferritic-pearlitic Nodular Cast Irons Jacques LACAZE,1)* Jon SERTUCHA2) and Lena MAGNUSSON ÅBERG3) 1) CIRIMAT, Université de Toulouse, 31030, Toulouse, France. 2) Engineering and Foundry Process Department, IK4- Azterlan, 48200, Durango (Bizkaia), Spain. 3) Elkem AS Foundry Products R͗D, P.O. Box 8040 Vaagsbygd, NO-4675, Kristiansand, Norway. A review of past works on the formation of ferrite and pearlite in nodular cast iron is proposed. The effects of cooling rate after solidification and of nodule count on the formation of both constituents are stressed, though much emphasis is put on alloying elements and impurities. KEY WORDS: nodular cast iron; as-cast microstructure; ferrite; pearlite; alloying. One of the most relevant advantages of nodular cast iron 1. Introduction is their extensive application in the as-cast condition. This Nodular cast irons are composite materials made of reduces the manufacturing cost and potential problems graphite spheroids embedded in a Fe-rich matrix. Their related to variability from heat-treatment are avoided. The mechanical properties depend both on nodule count and on ¿UVWVHFWLRQEHORZGHDOVZLWKWKHSULQFLSOHRIWKHHXWHFWRLG the matrix constitution resulting from the transformation of transformation of austenite in the stable and metastable high-temperature austenite formed together with graphite V\VWHPVSXWWLQJHPSKDVLVRQWKHUROHRIDOOR\LQJHOHPHQWV GXULQJWKHVROLGL¿FDWLRQVWHS,QDVFDVWPDWHULDOVWKLVWUDQV- (mostly pearlite promoting elements). The following section IRUPDWLRQFDQOHDGWRIHUULWHRUSHDUOLWHRUWRDPL[RIWKHP is devoted to attempts to predict room temperature matrix and sometimes to martensite in case of highly alloyed irons VWUXFWXUHRQWKHEDVLVRIPHOWFRPSRVLWLRQi.e. considering cast in small sections. Bainite is not observed in as-cast FRPSHWLWLRQRIIHUULWHDQGSHDUOLWH,QWKHFRQFOXVLRQVRPH LURQVLWLVREWDLQHGDIWHUDSSURSULDWHKHDWWUHDWPHQWDQGZLOO SURSRVDOVIRUIXUWKHUVWXGLHVLQWKLV¿HOGZLOOEHGHWDLOHG not be considered further here where focus is put on as-cast irons with ferritic-pearlitic structures. The ferritic-pearlitic 2. Eutectoid Transformation of Austenite matrix consists of halos of ferrite around the nodules and SHDUOLWH DZD\ IURP WKHP ODVW WR IUHH]H DUHDV JLYLQJ WKH $PRQJDIHZRWKHUDXWKRUV-RKQVRQDQG.RYDFV1) have so-called bulls-eye structure illustrated in Fig. 1. GHVFULEHG KRZ WKH PLFURVWUXFWXUH LQ )LJ HYROYHV ZLWK austenite starting to decompose in the stable system giving ferrite halos growing symmetrically around graphite nod- ules. Further growth of ferrite involves transfer of carbon from the remaining austenite to graphite nodules by diffu- sion through the ferrite halo (this will hereafter be called the ferritic reaction). The process is thus slower and slower as WKHWKLFNQHVVRIWKHKDORVLQFUHDVHV+HQFHXSRQFRQWLQX- RXV FRROLQJ RI WKH PDWHULDO WKH WHPSHUDWXUH FDQ EHFRPH low enough for nucleation and growth of pearlite in the metastable system (hereafter called the pearlitic reaction). Pearlite growth is comparatively rapid because it proceeds E\ FRRSHUDWLYH FRXSOHG JURZWK RI FHPHQWLWH DQG IHUULWH certainly much like in steels according to Pan et al.2) and Venugopalan.3))XUWKHU-RKQVRQDQG.RYDFV1) could show that pearlite appears at the ferrite/austenite interface and develops as spherical colonies inside the remaining austen- LWH7KXVIHUULWHFDQVWLOOJURZDIWHUWKHSHDUOLWLFUHDFWLRQKDV Fig. 1. Bull-eyes structure of nodular cast irons showing halos of VWDUWHGEXWWKHJURZWKUDWHRIWKLVODWWHULVVXFKWKDWDXVWHQLWH ferrite (white contrast) around graphite nodules (dark con- decomposition is most generally quickly completed. WUDVW WKH UHPDLQLQJ RI WKH PDWUL[ EHLQJ SHDUOLWH GDUN grey contrast). An important number of foundries include thermal analysis for controlling melt preparation before casting * Corresponding author: E-mail: [email protected] using commercial standard rig and cups. Figure 2 shows DOI: http://dx.doi.org/10.2355/isijinternational.ISIJINT-2016-108 an example of such a record when the output has been Fig. 2. Cooling curve and its derivative obtained by casting a standard thermal analysis cup with a nodular cast iron &6L0Q&XZW DIWHU6HUWXFKDet al. TαH[S7SH[S and Ttrans are respectively the experimen- tal temperatures for the start of the ferritic and pearlitic UHDFWLRQVDQGIRUWKHPD[LPXPLQWKHGHULYDWLYHFXUYH Fig. 3. Schematic of ferrite formation as halos around the graph- ite nodules and corresponding radial carbon distribution. pursued down to low enough temperature to register the ΔwC should be positive for carbon to diffuse to graphite solid-state transformation of the alloy. The derivative of the from the ferrite/austenite interface to the graphite nodule. temperature with respect to time (i.e. the cooling rate) is also SORWWHGZKLFKVKRZVWKDWERWKVROLGL¿FDWLRQDQGVROLGVWDWH transformations appearing as plateaus on the cooling curve give sharp changes in the derivative. Focusing on the part of the derivative record corresponding to the solid-state WUDQVIRUPDWLRQLWLVVHHQWKDWLWFRXOGEHGLYLGHGLQWZREHOO VKDSHVLJQDOVZKLFKFRUUHVSRQGWRWKHIHUULWLFDQGSHDUOLWLF reactions. On such curves it is possible to measure the cool- ing rate of the material before the eutectoid transformation and the temperatures for the start of the ferritic and pearlitic reactions. Integration of the area below the derivative peak FRXOG DOVR OHDG WR D ¿UVW HVWLPDWH RI WKH WUDQVIRUPDWLRQ kinetics. The same characterization could be performed as well on differential thermal analysis (DTA) records where the cooling rate is imposed. 2.1. Formation of Ferrite Figure 3 schematically illustrates ferrite growth described above. One particular feature of the ferritic reaction is the fact that no redistribution of substitutional elements at the Fig. 4. ,VRSOHWK)H±&VHFWLRQRIWKH)H±&±6LV\VWHPDWZW ferrite/austenite interface has been reported for as cast mate- Si in the stable eutectoid range. ΔwC shows the difference ULDOV,QRWKHUZRUGVWKHIHUULWHLQKHULWVWKHDOOR\LQJFRQWHQW between the carbon content at the ferrite/austenite inter- α/ γ D /gra RIWKHSDUHQWDXVWHQLWHWKLVLVWKHVRFDOOHGSDUDIHUULWHPHQ- IDFHwC DQGDWWKHJUDSKLWHIHUULWHLQWHUIDFHwC . WLRQHGE\9HQXJRSDODQ3) see appendix. If one assumes the PDWUL[ LV FKHPLFDOO\ KRPRJHQHRXV DQ LVRSOHWK )H±& VHF- WLRQRIWKHUHOHYDQWV\VWHPe.g. the one drawn at 2.5 wt.% carbon distribution in Fig. 3 demonstrates that this diffusion α/ γ D /gra silicon in Fig. 4 LV WKXV UHOHYDQW IRU GHVFULELQJ DXVWHQLWH could proceed only if ΔwC = wC ±wC LVSRVLWLYHZKHUH α/ γ D /gra graphite equilibrium at temperatures above the three-phase wC and wC are the carbon content in ferrite (α) in equi- GRPDLQ DQG IHUULWHJUDSKLWH HTXLOLEULXP DW WHPSHUDWXUHV librium with austenite (γ) and graphite (gra) respectively. below this domain. It is worth noting the very low level of 7KLV FRQGLWLRQ LV IXO¿OOHG DW WHPSHUDWXUHV EHORZ WKH WKUHH carbon that can dissolve in ferrite as compared to that in SKDVH ¿HOG VHH )LJ PHDQLQJ WKDW WKH FDUERQ FRQWHQW WKH SDUHQW DXVWHQLWH 'XULQJ JURZWK RI IHUULWH FDUERQ FDQ in the growing ferrite and in the receding austenite should be rejected partly to austenite but growth of graphite will be described by the metastable extrapolations (shown with proceed mainly by diffusion of carbon to graphite through dashed lines) of the equilibrium lines (solid lines). Note that the ferrite halo. Following Lacaze et al.± the schematic of DV WKH WHPSHUDWXUH GHFUHDVHV WKH FDUERQ FRQWHQW LQ IHUULWH α/ γ LQHTXLOLEULXPZLWKDXVWHQLWHwC DQGWKDWRIDXVWHQLWHLQ )RU DQDO\VLV RI GDWD REWDLQHG RQ DOOR\V FRROHG DW ¿QLWH γ/ α HTXLOLEULXPZLWKIHUULWHwC LQFUHDVH UDWHIRUZKLFKWKHPRGHOLQ)LJDSSOLHVLWLVFRQYHQLHQW Once the temperature of the alloy has reached the three to plot the undercooling for the start of the ferritic reaction
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