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Microstructure of as-cast ferritic-pearlitic nodular cast 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, & 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

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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 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 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 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 in case of highly alloyed irons VWUXFWXUHRQWKHEDVLVRIPHOWFRPSRVLWLRQi.e. considering cast in small sections. 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 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 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 . 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 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 SKDVH¿HOGXSRQFRQWLQXRXVFRROLQJIHUULWHFRXOGSRVVLEO\ (ΔTα) as measured with respect to Tα i.e. ΔTα = Tαí7 nucleate but does not grow because this would involve where T is the temperature at which the ferritic reaction long range redistribution of substitutional elements between is seen to start experimentally. This is illustrated in Fig. 5 ferrite and austenite. Such a redistribution needs diffusion for a series of alloys containing various amounts of copper which is very sluggish for substitutional elements at the studied by Sertucha et al. It is seen that the undercooling UHOHYDQW WHPSHUDWXUHV VHH DSSHQGL[ 7KH IHUULWLF UHDFWLRQ for the start of the ferritic reaction appears to follow a power can thus proceed only when the temperature of the alloy law of the cooling rate that would extrapolate to zero at zero has reached the temperature Tα given by the intersection of FRROLQJUDWHWKXVJLYLQJDQLQGLUHFWVXSSRUWWRWKHPRGHO the extrapolation of the austenite/ferrite boundary with the The undercooling for the start of the reaction shown in ORZHVWOLPLWRIWKHWKUHHSKDVH¿HOG7KHHIIHFWRIDOOR\LQJ Fig. 5 corresponds to the so-called incubation time in the on this temperature has been estimated8) by means of ther- literature on time temperature transformations (TTT) and modynamic calculations using Thermocalc9) and data from continuous cooling transformations (CCT). To get further Uhrenius:10) LQVLJKW LQ WKH YHU\ VWDUW RI WKH IHUULWLF UHDFWLRQ +HOODO et al.13) performed dilatometry experiments where samples T =739 + 18.() 4 ⋅w + 2 ⋅ w2 − 14 ⋅ w ZHUHKHOGLQWKHDXVWHQLWH¿HOGDQGWKHQUDSLGO\FRROHGWR α Si Si Cu ...... (1) and held at a temperature below Tα. During this second −45 ⋅wMn + 2 ⋅w Mo − 24 ⋅ wCr − 27. 5 ⋅ wNNi KROGLQJWKHUHFRUGHGFXUYHVVKRZHG¿UVWDFRQWUDFWLRQDQG where wi is the “i” element content in wt.% and Tα is in then a dilation. The contraction relates to carbon deple- Celsius. This expression has been calculated with silicon tion of austenite because its solubility in austenite strongly FRQWHQW XS WR  ZW PDQJDQHVH FKURPLXP FRSSHU DQG decreases with temperature so that the associated change nickel contents up to 1 wt.% and molybdenum content up in the density of austenite overtakes graphite precipitation. to 0.5 wt.%. The sign of their effect agrees with the role of $IWHUVRPHWLPHIHUULWHJURZWKOHDGVWRWKHREVHUYHGGLOD- VLOLFRQDQGPRO\EGHQXPDVIHUULWHVWDELOL]HUVZKLOHFRSSHU tion. Hazotte et al. observed however that ferrite appears manganese and nickel are austenite stabilizers. Contrary to very early during the second holding and that the apparent ZKDWLVH[SHFWHGFKURPLXPSUHVHQWVDQHJDWLYHHIIHFWRQ7α VWDUWRIWKHIHUULWLFUHDFWLRQi.e.ZKHQGLODWLRQVHWVXSUHODWHV ZKLOHLWLVDIHUULWHVWDELOL]HUDFFRUGLQJWRWKH)H±&USKDVH in fact to a ferrite fraction of more than 10%. diagram. This may be related to the downward shape of the ,Q WKHVH GLODWRPHWU\ H[SHULPHQWV WKH PD[LPXP LQ WKH JDPPDORRSLQWKH)H±&UV\VWHP FRQWUDFWLRQZDVIRXQGWREHVWURQJO\UHODWHGWRQRGXOHFRXQW J Note that the average composition of austenite wi in ele- i.e. the diffusion distance for carbon from austenite to graph- ments other than carbon differs from the nominal composi- LWH7KLVVXJJHVWVWKDWZKHQLQFUHDVLQJWKHFRROLQJUDWHLQD o tion wi of the cast iron because of graphite precipitation FDVWLQJWKHDYHUDJHFDUERQFRQWHQWLQDXVWHQLWHVKLIWVPRUH GXULQJVROLGL¿FDWLRQ(YDOXDWLRQRI7α should thus be done and more to the right of the equilibrium austenite/graphite for corrected compositions using the simple following mass phase boundary (graphite solvus) because there is less and balance: less time for austenite depletion. This is illustrated schemati-

gra gra γ gra γ ρ⋅g + ρ ⋅()1 − g o w = ⋅ wi ...... (2) i ρ γ ⋅()1 − ggra where ρgra and ργ are the densities of graphite and austenite respectively. Putting the appropriate values in Eq. (2) leads J o to wi §Âwi . Note also that using an average composition means not accounting for microsegregation built up during VROLGL¿FDWLRQZKLFKZLOOEHGLVFXVVHGODWHU 6LQFHWKHHDUO\ZRUNE\5HKGHU11) growth of ferrite dur- ing the eutectoid reaction is known to be highly sensitive to cooling rate. Rehder11) showed that at very low cooling UDWHSUHFLSLWDWLRQRIIHUULWHLVQRWRQO\UHODWHGWRQRGXOHVEXW occurs at austenite grain boundaries and proceeds with long- range redistribution of substitutional solutes between ferrite and austenite. The same observation has been reported by Brown and Hawkes12) in case of isothermal holding in the WKUHHSKDVH¿HOG*HUYDODQG/DFD]H8) performed an analysis of data from literature which showed the above model with- RXWORQJUDQJHUHGLVWULEXWLRQ )LJVDQG DSSOLHVZKHQWKH cooling rate is higher than about 0.02°&ÂV í or 1.2°&ÂPLQ í. Fig. 5. Evolution with cooling rate of the start of the ferritic and $W FRROLQJ UDWHV ORZHU WKDQ WKLV FULWLFDO YDOXH IHUULWH ZLOO SHDUOLWLF UHDFWLRQV PHDVXUHG RQ DOOR\V ZLWK ± & form at austenite grain boundaries and its growth will be ±6L±0QDQG±&X LQGLFDWHGLQ controlled by long range diffusion of substitutional solutes. WKHFDSWLRQ LQZWDIWHU6HUWXFKDet al. cally in Fig. 6 where it is suggested that high cooling rates LQDXVWHQLWH$WORZHUWHPSHUDWXUHEXWVWLOODERYHWKHORZHU could lead to conditions where ferrite becomes stable only OLPLWRIWKHWKUHHSKDVH¿HOGGLYHUJHQWRUWKRSHDUOLWHPD\ DWWHPSHUDWXUHVIDUEHORZWKHVWDEOHWKUHHSKDVH¿HOGi.e. at be observed when the carbon activity in austenite is very the intersection of the thick solid line which stands for the high. The two above cases could not apply to cast irons average carbon content in austenite upon cooling with the because of the presence of graphite precipitates that act γ/ α extrapolation of the wC line. as carbon sink and limit any increase of carbon activity The dilatometry experiments mentioned above showed in austenite. For temperature below the lower limit of the also that carbon transfer from austenite to graphite dur- WKUHHSKDVH ¿HOG FRQVWDQW RUWKRSHDUOLWH DQG SDUDSHDUOLWH ing cooling was much slower than calculated on a basis grow at low and high undercooling respectively. In both of RI GLIIXVLRQ FRQWURO ZKHQ WKH RSSRVLWH i.e. dissolution of WKHVH WZR ODWWHU JURZWK SURFHVVHV +LOOHUW20) demonstrated JUDSKLWHLQDXVWHQLWHGXULQJKHDWLQJPDWFKHGYHU\ZHOOGLI- WKDWSHDUOLWHLQKHULWVWKHFRPSRVLWLRQRIWKHSDUHQWDXVWHQLWH fusion control calculations. That carbon transfer to graphite so that its growth kinetics is controlled by carbon diffusion is slower than calculated for diffusion control is in fact in austenite. It is however worth to mention the role of LQ DFFRUGDQFH ZLWK UHVXOWV E\ %LUFKHQDOO DQG 0HDG15) on carbon diffusion in ferrite that Nakajima et al.21) have been graphitization of white cast irons where carbon transfer SRLQWHGRXWE\SKDVH¿HOGPRGHOOLQJRIELQDU\)H±&DOOR\V ZDV IRXQG WR EH ± WLPHV VORZHU WKDQ H[SHFWHG 6XFK 8QIRUWXQDWHO\QRVXFKZRUNLV\HWDYDLODEOHIRUWHUQDU\RU a difference could be due to stress effect as investigated multicomponent alloys. by Hillert16) in the case of graphitization of steels or by an $FFRUGLQJWR+LOOHUW20) constant ortho-pearlite grows with interfacial reaction for carbon transfer from austenite to full partitioning of substitutional solutes between graphite. Simple calculations by Silva et al. showed and ferrite while para-pearlite grows with no such partition- an increased compression towards the graphite/austenite LQJi.e. both cementite and ferrite inherit the austenite com- interface that is compatible with a decreased diffusion rate SRVLWLRQ,QFDVHRIFRQVWDQWRUWKRSHDUOLWHDVLQFDVWLURQV RIFDUERQWKRXJKDPRUHSUHFLVHDSSURDFKZRXOGEHQHFHV- partitioning of substitutional alloying elements is expected sary to conclude quantitatively on the relative role of stress to control lamellar spacing within pearlite. The temperature and interface kinetics. for the transition between constant ortho-pearlite and para- SHDUOLWHLQ)H±&±6LDOOR\VGHFUHDVHVVWURQJO\ZLWKWKHVLOL- 2.2. Formation of Pearlite FRQFRQWHQW,WLVVRORZDW±ZW6LWKDWRQO\FRQVWDQW -RKQVRQDQG.RYDFV1) and later Venugopalan3) considered ortho-pearlite needs being considered in the case of cast pearlite grows in cast irons as it does in steels and this is irons. To check if the assumed similarity between constant DOVRDFFHSWHGKHUH1XPHURXVVWXGLHVRQORZFDUERQ)H±&± RUWKRSHDUOLWHLQVWHHOVDQGLQFDVWLURQVLVYDOLG/DFD]H22) X alloys have been reported describing growth of pearlite analyzed literature data for the start of the pearlitic reaction during isothermal holding after austenitization and quench- during isothermal treatment of various cast irons and com- ing to a temperature where pearlite could nucleate and grow. pared with the results of Al-Salman et al.23)RQD)H±&±6L 2ZLQJ WR VLOLFRQ EHLQJ WKH SUHGRPLQDQW DOOR\LQJ HOHPHQW steel. A strong agreement was found in that results for the LW LV RI ¿UVW LQWHUHVW WR ORRN DW WKH GHWDLOHG GHVFULSWLRQ RI VWHHODQGIRUFDVWLURQVZLWKWRZW6LDOOIDOORQD WKH GLIIHUHQW JURZWK FRQGLWLRQV IRU SHDUOLWH LQ )H±&±6L common pearlite nose with little scattering. alloys described by Hillert.20) $W KLJK WHPSHUDWXUHV i.e. The fact that pearlite forming during continuous cooling DW WHPSHUDWXUHV ORFDWHG ZLWKLQ WKH WKUHH SKDVH ¿HOG RI WKH of cast irons inherits the composition of the parent austenite PHWDVWDEOH V\VWHP PHWDSHDUOLWH FRXOG JURZ WKDW LQYROYHV DOORZV GHVFULELQJ WKH SHDUOLWLF UHDFWLRQ LQ D )H±& LVRSOHWK long range redistribution of substitutional alloying elements section of the metastable system as done before for the fer- ritic reaction in the stable system. Such an isopleth section is illustrated in Fig. 7$VIRUWKHIHUULWLFUHDFWLRQ/DFD]Het al. suggested that the pearlitic reaction takes place only when the temperature of the alloy has reached the lower OLPLWRIWKHWKUHHSKDVH¿HOG7KHUHIHUHQFHWHPSHUDWXUHIRU pearlite formation should thus be given by the intersection of the extrapolated austenite/ferrite boundary with the lower OLPLWRIWKHWKUHHSKDVH¿HOG7KLVWHPSHUDWXUHLVGHQRWHG7p LQ)LJ8VLQJDJDLQWKH7KHUPRFDOFVRIWZDUH9) and data IURP8KUHQLXV10) the following equation was proposed for 8) Tp (°& E\*HUYDODQG/DFD]H

T =727 + 21..(). 6 ⋅w + 0 023 ⋅ w2 − 21 0 ⋅ w p Si Si Cu ... (3) −25. 0 ⋅wMn + 8 . 0 ⋅wMo + 13 .00⋅wCr − 33. 0 ⋅ wNi

)RUDOOHOHPHQWVWKHFRPSRVLWLRQGRPDLQRIXVHRIWKLV equation is the same as indicated for Tα. It is interesting to note that ferrite stabilizers as silicon and molybdenum do Fig. 6. ,VRSOHWK)H±&VHFWLRQRIWKHSKDVHGLDJUDPVKRZLQJWKH possible evolution of the average carbon content in austen- increase TpZKLOHWKHVWURQJQHJDWLYHHIIHFWRIFRSSHUPDQ- ite (very thick lines) during cooling at low and high cool- ganese and nickel is to be related to them being austenite ing rate. VWDELOL]HUV)RUZKDWFRQFHUQVFKURPLXPWKHLQFUHDVHLQ7p rite and cementite even when (low) additions of the above HOHPHQWVKDYHEHHQPDGHWRWKHFDVWLURQ%DVHGRQD¿WRI experimental data by Al Salman et al.23)IRUDVLOLFRQVWHHO the growth rate of pearlite could then be expressed as func- tion of ΔTp as

−5 3 −1 Vp =7 ⋅ 10 ⋅()∆Tp exp[ −20 ×∆ Tp /() T + 273 ] µ m ⋅ s ...  which allows reproducing the pearlite nose. $VVWUHVVHGDERYH(T  ZRXOGQRWZRUNIRUFDVWLURQV containing molybdenum and certainly also chromium which are known to affect the growth rate of pearlite in cast irons DVVHHQIRUH[DPSOHLQWKHH[WHQVLYHFRPSLODWLRQRI777 and CCT curves by Röhrig and Fairhurst.25) The decrease in pearlite growth kinetics observed when these elements are added is in agreement with the fact that they are known to strongly affect carbon diffusion in austenite which has been Fig. 7. ,VRSOHWK)H±&VHFWLRQRIWKH)H±&±6LV\VWHPDWZW stated to control constant ortho-pearlite growth. Si in the metastable eutectoid range. )LQDOO\ DOOR\LQJ ZLWK HOHPHQWV WKDW GR QRW GLVVROYH LQ FHPHQWLWHDQGKDYHOLWWOHVROXELOLW\LQIHUULWHVXFKDVFRSSHU could have been expected to affect pearlite growth rate as should be due to the fact it is a carbide former owing to the they should strongly partition. It does not appear this parti- above remark on the downward gamma loop in this system. WLRQLQJKDVDQ\HIIHFWLQWKHFDVHRIFRSSHUDQGWKLVZRXOG $V GRQH SUHYLRXVO\ IRU WKH VWDUW RI WKH IHUULWLF UHDFWLRQ certainly be worth of further investigation. results for the detected start of the pearlitic reaction may conveniently be represented as undercooling ΔT calculated p 3. Growth Competition of Ferritic and Pearlitic Reac- with respect to Tp expressed as: ΔTp = Tpí7ZKHUH7LVWKH tions observed onset temperature of the transformation. Figure 5 shows there is a minimum undercooling necessary for pearl- )URPDERYHLWZRXOGDSSHDUWKDWWKHUHLVDWHPSHUDWXUH LWHWRQXFOHDWHDW¿QLWHFRROLQJUDWHZKLFKDPRXQWVWRDERXW window in which ferrite could grow without the risk of °C for alloys containing copper and manganese according pearlite appearing for usual cast irons. For a homogeneous  to the results by Sertucha et al. Because results on the fer- PDWUL[WKLVZLQGRZFRXOGEHH[SUHVVHGDVΔT = Tαí7p: ULWLFUHDFWLRQKDYHVKRZQHDV\QXFOHDWLRQRIIHUULWHLWFDQEH claimed that such an undercooling for the pearlitic reaction ∆T =12 − 3..() 2 ⋅w + 1 977 ⋅ w2 + 7 ⋅ w Si Si Cu ...... (5) UHODWHVWRGLI¿FXOWLHVLQQXFOHDWLRQRIFHPHQWLWH −20 ⋅wMn − 6 ⋅ w Mo − 37 ⋅ wCr + 6. 5 ⋅ wNNi 2ZLQJ WR WKH JURZWK PHFKDQLVP RI SHDUOLWH WKH XQGHU- cooling for its formation increases much less when the From Eq. (5) it would be concluded that manganese is a cooling rate increases as compared to the ferritic reac- VWURQJSHDUOLWHSURPRWHUZKLOHFKURPLXPDQGPRO\EGHQXP WLRQ $FFRUGLQJO\ WKH FXUYHV IRU WKH VWDUW RI WKH IHUULWLF are very strong and mid pearlite promoter respectively. and pearlitic reactions intersect for a cooling rate of about Copper and nickel appear as mid ferrite promoters while 180°&ÂPLQ í IRU WKH DOOR\V LOOXVWUDWHG LQ )LJ  DQG QR they are generally reported to be pearlite promoters. It may ferrite would form above this critical cooling rate. As for thus be concluded that considering the temperature differ- WKH IHUULWLF UHDFWLRQ LW LV TXLWH SRVVLEOH WKDW WKH UHSRUWHG ence for the start of ferritic and pearlitic reactions could be undercoolings are overestimated because the transformation YDOLG IRU LVRWKHUPDO WUDQVIRUPDWLRQV EXW LV QRW VXI¿FLHQW ZLOOEHGHWHFWHGE\WKHWKHUPRFRXSOHVRQO\DIWHUDVLJQL¿- for analyzing the effect of these usual alloying elements on cant volume fraction of pearlite has appeared. Comparison as-cast materials which are continuously cooled during the between thermal analysis or DTA experiments and dilatom- eutectoid transformation. HWU\UHVXOWVFRXOGFHUWDLQO\FRQ¿UPWKLVFODLPEXWWKHDXWKRUV are not aware of such a work. 3.1. Effects of Pearlite Promoters %DVHGRQZRUNVRQPXOWLFRPSRQHQWVWHHOV9HQXJRSDODQ3) The way copper and manganese act on the eutectoid described pearlite growth in nodular cast irons with a model WUDQVIRUPDWLRQKDVOHGWRVHYHUDOSRWHQWLDOO\FRQWUDGLFWRU\ ¿WWHGRQ777GLDJUDPV,QWKLVZRUNWKHHTXDWLRQGHVFULE- explanations as reviewed by Pan et al.2) In a later work on ing pearlite growth was the same for all cast irons except the effects of alloying on the eutectoid transformation in those containing molybdenum. This agrees with detailed FDVWLURQV9HQXJRSDODQ3) summarized the role of austenite experimental studies by Lalich and Loper19) and Pan et al.2) SURPRWHUVVXFKDVFRSSHUPDQJDQHVHDQGQLFNHOE\VWDWLQJ which have been analyzed in terms of growth kinetics by WKH\DOOGHFUHDVHWKHWHPSHUDWXUHIRUWKHIHUULWLFUHDFWLRQEXW Lacaze.22) $OOR\V ZLWK XS WR  ZW 0Q  ZW &X have different effect on carbon diffusion: manganese would 0.5 wt.% As and 0.3 wt.% Sn all showed similar pearlite GHFUHDVHLWQLFNHOZRXOGOHDYHLWXQFKDQJHGZKLOHFRSSHU growth rates. Lacaze and Sertucha FRQ¿UPHGWKLVDSSOLHV would form a barrier around the nodules that retards carbon as well for alloys with copper up to 0.9 wt.% and tin up to transfer to graphite as suggested by Lalich and Loper.19) As 0.11 wt.%. It may thus be concluded that pearlite growth in VHHQDERYHWKHVHWKUHHHOHPHQWVGRHIIHFWLYHO\GHSUHVVWKH cast iron is controlled by silicon partitioning between fer- stable eutectoid temperature which then certainly leads to DGHFUHDVHLQWKHGLIIXVLRQFRHI¿FLHQWRIFDUERQLQIHUULWH IRUDFDVWLURQZLWKZW&ZW&UZW +RZHYHUWKLVPD\QRWEHVXI¿FLHQWWRH[SODLQWKHLUSHDUOLWH &XZW0QDQGZW6L promoting effect because it is not expected that additions of From Fig. 8 it would be expected that low addition of VXEVWLWXWLRQDO HOHPHQWV DW WKH OHYHO XVHG IRU FRSSHU PDQ- FRSSHU ZRXOG QRW DIIHFW WKH IHUULWLF UHDFWLRQ DQG /DOLFK ganese and nickel (excluding austenitic cast irons) in cast and Loper19) have effectively reported that small addition LURQVZRXOGGUDPDWLFDOO\DIIHFWFDUERQGLIIXVLRQFRHI¿FLHQW ±ZW RIFRSSHUGRHVQRWDIIHFWLW/DFD]Het al.5) in ferrite. noticed that copper favors slightly the ferritic reaction when Figure 8VKRZVWKHHIIHFWRIZW&XZW0QDQG added at similar low level. The known pearlite promoting 1 wt.% Ni on the ferrite/austenite boundary in an isopleth effect of copper appears at higher content as illustrated in )H±& VHFWLRQ DW  ZW 6L FDOFXODWHG ZLWK 7KHUPRFDOF9) Fig. 9,WLVVHHQWKDWWKHUHH[LVWVDFULWLFDODPRXQWRIFRSSHU DQG WKH 7&)( GDWDEDQN $OO WKH OLQHV LQ WKH VHFWLRQ DUH DERXWZWDWZKLFKWKHIHUULWHIUDFWLRQGURSVVXGGHQO\ VKRZQIRUWKHWHUQDU\)H±&±6LDOOR\ZKLOHRQO\WKHIHUULWH at given cooling conditions. Sertucha et al. then suggested austenite boundary with its extrapolation below the ferrite/ that when the alloy composition is such that the ferritic reac- graphite line (dashed line) is shown for the three Fe-C- WLRQSURFHHGVDWWHPSHUDWXUHEHORZWKH&XULHWHPSHUDWXUHLW 2.5Si-X alloys. Note that the temperature of the intersection is abruptly slowed down because of the drop in the carbon between these two boundaries is the Tα temperature because GLIIXVLRQFRHI¿FLHQWLQIHUULWH WKH ORZHU OLPLW RI WKH VWDEOH WKUHH SKDVH ¿HOG LV QHDUO\ )LJXUHLOOXVWUDWHVDOVRWKHVHQVLWLYLW\RIWKH¿QDOPLFUR- KRUL]RQWDOLQHYHU\FDVH VHH)LJ $GGLWLRQRIZW VWUXFWXUHWRFRROLQJUDWHZKLFKUHVXOWVIURPWKHFRPSHWLWLRQ chromium or molybdenum was found to change very little RIJURZWKLQWKHVWDEOHDQGPHWDVWDEOHV\VWHPV)XUWKHUPRUH 26) the TαWHPSHUDWXUHDQGLWLVZRUWKVWUHVVLQJDOVRWKDWDOORI $VNHODQGDQG*XSWD have shown that the relative amount the above additions did leave the ferrite/graphite boundary of ferrite and pearlite is sensitive to change in nodule counts unchanged. or cooling rate only when the nodule count is low. When ,WLVVHHQLQ)LJWKDWDWJLYHQWHPSHUDWXUHDQ\DGGLWLRQ WKHQRGXOHFRXQWLVKLJKH[WUHPHO\UDSLGFRROLQJUDWHVDUH RIFRSSHUQLFNHODQGPDQJDQHVHZRXOGOHDGWRDGHFUHDVH required to suppress ferrite growth. of the driving force for carbon diffusion from austenite to The analysis of literature data performed by Lacaze et JUDSKLWHH[SUHVVHGKHUHDVΔwCDQGWKXVZRXOGGHFUHDVH the amount of ferrite. At given undercooling ΔTαWKHGULY- Table 1. 'LIIXVLRQFRHI¿FLHQWV P2ÂV í  RI FDUERQ LQ DXVWHQLWH LQJIRUFHLVVHHQWREHOHIWXQFKDQJHGE\DGGLWLRQRIFRSSHU J D DCDQGLQIHUULWHDC. TCurie is the Curie temperature of QLFNHODQGWKHVDPHLVYDOLGIRUFKURPLXPRUPRO\EGHQXP ferrite. 2QWKHFRQWUDU\PDQJDQHVHLVVHHQWRGHFUHDVHWKHFDUERQ  17 767  FRQWHQW LQ IHUULWH DW WKH IHUULWHDXVWHQLWH HTXLOLEULXP WKXV γ −5  DC =2. 343 ⋅ 10 ⋅ exp −  (Liu and Ågren )  T + 273  decreasing ΔwCKHQFHLWVYHU\ZHOONQRZQSHDUOLWHSURPRW- ing effect. α −6  10 115  DC =2 ⋅ 10 ⋅exp −  +RZHYHU DOO WKH DERYH HOHPHQWV EXW PRO\EGHQXP GR  T + 273  decrease TαDQGWKXVOHDGWRDGHFUHDVHRIWKHFDUERQGLI-   2 15 629 15 309  ⋅exp0 . 5898⋅ 1 + ⋅arctan  −  (Ågren50)) IXVLRQFRHI¿FLHQWVGXULQJWKHIHUULWLFUHDFWLRQ Table 1). It   π  TTCurie + 273 T + 273  LVVHHQWKDWLQDGGLWLRQWRWKHXVXDOWHPSHUDWXUHHIIHFWWKH GLIIXVLRQFRHI¿FLHQWLQIHUULWHGURSVVLJQL¿FDQWO\DWWKHIHU- rite Curie temperature. The Curie temperature of pure iron LV°&DQGGHFUHDVHVZLWKDGGLWLRQRI&U&X0Q0R1L and Si. Sertucha et al. IRXQGH[SHULPHQWDOO\LWWREH°C

Fig. 9. Effect of copper addition on the ferrite fraction after cool- ing at different rates after austenitization or casting in a thermal analysis cup (60°C/min). The alloys contained Fig. 8. (IIHFWRIZW&X0QRU1LRQΔwC for an alloy with 2.5 ± ZW & ± ZW 6L ± ZW 0Q wt.% Si. After Sertucha et al. al. showed that the same kind of behavior as seen for of this element to cast irons does shift the ferritic and pearl- copper in Fig. 9 exists for tin with a critical amount at about itic reactions to longer times in TTT and CCT diagrams as 0.05 wt.%. Available results concerning manganese present could be seen in the compendium of Rohrig and Fairhurst.25) a more regular effect though it could not be excluded that a drop exists also for this element. Considering data obtained 3.2. Impurities and Trace Elements Effects by Pan et al.2) RQ WKHUPDO DQDO\VLV FXSV D WLQ HTXLYDOHQW Björkegren has reviewed some of the relations avail- (Sneq FRXOGEHGH¿QHGDV able at the beginning of the 1980’s to relate composition and ferrite or pearlite fraction for given casting conditions. The Sn =0.. 075 ⋅w + 0 125 ⋅ w + w ...... (6) eq Mn Cu Sn still most extensive work available is the one published by For the thermal conditions prevailing in thermal analysis Thielemann38) who proposed the following relation for the FXSV LW ZDV IRXQG WKDW D IXOO\ SHDUOLWLF PDWUL[ FRXOG EH fraction of ferrite in the matrix (%): obtained for Sn equal to or higher than 0.13 wt.%. How- eq f=961. exp( − Px) ......  HYHUDVDOUHDG\VWDWHGE\*XHULQDQG*DJQp28) it could also ferrite be concluded from literature data that addition of copper ZKHUH3[LVDOLQHDUUHODWLRQRIWKHFRQWHQWVLQPDQJDQHVH DQGPDQJDQHVHDORQHi.e.ZLWKRXWWLQZRXOGKDUGO\OHDGWR VLOLFRQ FRSSHU WLQ OHDG ELVPXWK DUVHQLF FKURPLXP DQG a fully pearlitic matrix. DQWLPRQ\(TXDWLRQ  LVPHDQLQJIXOSURYLGHG3[LVKLJKHU -RKQVRQDQG.RYDFV1)DQG.RYDFV29) compared the role of than 2.3 for the fraction of ferrite to be lower than 100%. PDQJDQHVHWLQDQGDQWLPRQ\DVSHDUOLWHSURPRWHUV8VLQJ 7KHH[SRQHQWLDOUHODWLRQLQ(T  DJUHHVZLWKWKHDEUXSW $XJHU DQDO\VLV WKH\ UHSRUWHG WLQ DQG DQWLPRQ\ WR IRUP D GURSRIWKHIHUULWHIUDFWLRQVHHQLQ)LJZKLFKPHDQVLQ thin layer at the surface of graphite that would be a barrier to turn that the work by Thielemann was certainly focused on WKHWUDQVIHURIFDUERQWRQRGXOHV+RZHYHU.RYDFV29) men- pearlitic grades. Though it is worth stressing that a study tions that such a diffusion barrier could not be evidenced at E\0RW]DQG5|UKLJ39) reported quantitative information on WKH PDWUL[ÀDNH LQWHUIDFH LQ JUH\ LURQ ZKLOH WLQ DQG DQWL- WKHHIIHFWRIVXOIXUDQGOHDGWKHZRUNE\7KLHOHPDQQLVWKH PRQ\DUHSHDUOLWHSURPRWHUVIRUWKHVHDOOR\VDVZHOO7KXV only one which considered quantitatively both alloying and DVFRQVLGHUHGE\-RKQVRQDQG.RYDFV1) this layer may have WUDFHHOHPHQWV0DJQXVVRQcEHUJet al. suggested to use formed in the solid state by rejection of tin and antimony manganese and tin equivalents reported in the literature to from graphite. This further suggests that there should be a complement the expression of Px for accounting for other strong interaction between carbon atoms on the basal planes elements and arrived at the following relation: of graphite and tin or antimony that limits carbon transfer from the matrix to graphite and thus leads to their pearlite Px= 3.00 [wMn +⋅ 1.5 ⋅ w Ni + 5.2 ⋅ wTi + 1.2 ⋅ wV + 1.7 ⋅wMo ] promoter effect. −2.65 ⋅ (wSi − 2.0))+ 7.75 ⋅ wCu + 90.0 ⋅ [w Sn − 0.185 ⋅ w Co

$ FRPSDULVRQ RI WKH )H±6L DQG )H±6Q GLDJUDPV ZKLFK +0.138 ⋅ wMg + 0.75 ⋅ w N + 0.067 ⋅ wP ]]+ 357 ⋅ wPb + 333 ⋅ wBi present a similar gamma loop indicates that tin is a ferrite +20.1 ⋅ w + 9.60 ⋅ w + 71.7 ⋅ w VWDELOL]HU$FFRUGLQJO\WLQLVH[SHFWHGWRLQFUHDVHERWKWKH As Cr Sb 2) ...... (8) Tα and Tp WHPSHUDWXUHV D IDFW UHSRUWHG E\ 3DQ et al. on an experimental basis. Looking at the slope of the gamma 0DJQXVVRQcEHUJ et al. applied Eq. (8) to data from  ORRS/DFD]HDQG6HUWXFKD proposed that a term +ÂZSn other authors than Thielemann and found a satisfactory FRXOGEHDGGHGWR(T  $FFRXQWLQJIRUWKLVWHUPLWZDV description of the transition from ferritic-pearlitic to fully found that ΔTp extrapolates to zero at zero cooling rate for SHDUOLWLF PDWUL[ ,Q SDUWLFXODU LW FRXOG EH QRWLFHG WKDW cast irons alloyed with more than 0.05 wt.% Sn. This sug- accounting for minor elements such as magnesium or for gests that Sn acts in strongly decreasing the undercooling for impurities such as phosphorus was important: in any series cementite precipitation and thus for pearlite formation. The RIGDWDIURPDJLYHQVRXUFHDFFRXQWLQJIRUWKHVHHOHPHQWV )H±6ESKDVHGLDJUDPSUHVHQWVDJDPPDORRSYHU\VLPLODU GHFUHDVHG WKH VFDWWHULQJ +RZHYHU WKH VFDWWHULQJ RI WKH WR WKDW RI WKH )H±6Q V\VWHP ZKLFK VXJJHVWV LWV HIIHFW RQ data from various sources remained high which may be due pearlite growth may have the same reason. There is however because cooling rate differed from one study to another. DFOHDUQHHGIRUGHWDLOHGLQIRUPDWLRQRQWKH)H±&±6EDQG Another source of scattering is the nodule count which )H±&±6QV\VWHPVWRYDOLGDWHWKLVFRQFOXVLRQ may greatly vary even when melt preparation and casting Cho et al.30) have reported that small additions (up to 0.3 parameters are expected to be constant. Such a variability ZW RI0RDQG1LSURPRWHIHUULWHZKLOHKLJKHUOHYHOVRI has been observed by Sertucha et al. who reported that the í í WKHVHHOHPHQWVDORQHRULQFRPELQDWLRQSURPRWHSHDUOLWHDV nodule count (NA) varied from 190 mm to 590 mm at observed by Hsu et al.31)IRU&RDQG1L1REXNLet al.32) for given casting and inoculation conditions. The composition 1LDQG0Q/DFD]Het al.33) for Ni and Yu and Loper for ranges of the four series of alloys that were investigated 0R&XDQG1L9HQXJRSDODQDQG$ODJDUVDP\35) related the are given in Table 2 for the main elements and Table 3 pearlite promoter effect of nickel to its effect on the eutec- IRU WKH ORZ OHYHO HOHPHQWV ,QWHUHVWLQJO\ HQRXJK LW FRXOG toid temperature. These authors discussed also the complex be observed a two-fold relation between NA and sulfur as effect of molybdenum in the same line as those followed by seen in Fig. 10. Half of the data showed no sensitivity to the

α Series C Si 0Q P S Ni Cu 0J NA f 3.36 1.51 0.09   0 0.01 0.021 190   ferritic series   0.39  0.02  0.15 0.059 590 100   0.09 0.012 0.003  0.01 0.052    Ni series  1.88 0.13  0.005  0.02 0.058 191   3.02 2.20 0.05 0.016 0.005 0.030 <0.020 0.033 136 89 Si series  3.81 0.11 0.032 0.009   0.052 0.060 323 100 2.32 1.66 0.12 0.012 0.008 0.026    95 0 Pearlitic series   1.06  0.021 0.060 1.18   30

Table 3. Composition ranges in other elements (wt.%) of the four series.

Series B Ti Al Bi N Nb Zr Cr Sn 0R V Sb 0 0 0 0 0.0032 0 0 ferritic series 0.032 0.028   0.005  0.012 0.003 <0.01 0.02 <0.005 <0.01 Ni series 0.05 0.01 0.006 0.029 <0.01 Si series     0.013 0 0 0 0 0 0 0 0 0 0 0 Pearlitic series   0.36 0.02 0.0086  0.005 0.35 0.025 0.98 0.5 0.005

3.3. Microsegregation $VVWUHVVHGIRUH[DPSOHE\'RUD]LO microsegregation LVVXHGIURPWKHVROLGL¿FDWLRQVWHSFDQJUHDWO\DIIHFWVROLG VWDWHWUDQVIRUPDWLRQV+RZHYHU*HUYDODQG/DFD]H used experimental characterization of the microsegregations to calculate the local Tα and Tp temperatures and found these temperatures do not change much for 80% of the matrix. Further these results illustrated clearly that the undercooling of the ferritic reaction increases all along the transforma- WLRQ ZKLOH LW LQFUHDVHV DW WKH EHJLQQLQJ DQG WKHQ UHPDLQV constant for the pearlitic reaction. This is in line with the above description of these reactions. %HFDXVHVROLGL¿FDWLRQLVFRQWUROOHGE\FDUERQGLIIXVLRQ build-up of microsegregation of other solutes depends on it as calculated by Lacaze. +RZHYHUDFDOFXODWLRQEDVHGRQ Scheil’s model for substitutional solutes but assuming rapid solid-state diffusion of carbon may conveniently represent

Fig. 10. Evolution of the nodule count NA with the sulfur content WKH GHYHORSPHQW RI PLFURVHJUHJDWLRQ GXULQJ VROLGL¿FDWLRQ of fully ferritic (large circles) and partly pearlitic (small of nodular cast iron. Such a calculation was performed using FLUFOHV DVFDVWDOOR\VDIWHU6HUWXFKDet al. the Scheil’s module of Thermocalc9)ZLWKWKH7&)HGDWD- base selecting an alloy in the middle range of those studied E\5XQGPDQ QDPHO\ZLWK6L0Q&X 1LDQG0R LQZW 7KHFDUERQFRQWHQWZDVVHW noticed that copper and titanium contents presented also a at 3.6 wt.% to give a eutectic composition and 0.1 wt.% Cr two-fold correlation with the sulfur content. This suggests was also added. In Fig. 11WKHFRPSRVLWLRQRIDXVWHQLWHLV that very complex chemical reactions take place during melt plotted versus solid fraction which is “equivalent” to the preparation that would preclude any possibility of predicting distance from the surface of a nodule (solid fraction equal PDWUL[ PLFURVWUXFWXUH WR D VLJQL¿FDQW GHJUHH RI DFFXUDF\ to zero) to half distance between two neighboring nodules without an appropriate description of these reactions which (solid fraction equal to one). Note the logarithmic scale of is far from being available. the Y axis in Fig. 11. It is seen that the predicted microsegre- the formation of pearlite instead of ferrite. While austenite was considered as chemically homoge- QHRXVLQWKHSUHYLRXVVHFWLRQVWKHJUDSKLQ)LJVKRZV this is not the case and the graph in Fig. 12 demonstrates how this affects the local reference temperatures. Accord- LQJO\LWPD\EHH[SHFWHGWKDWWKHUHIHUHQFHWHPSHUDWXUHIRU SHDUOLWHIRUPDWLRQZLOOGHSHQGRQWKHIDFWWKH¿QDOPDWUL[LV IXOO\SHDUOLWLFRUQRW,QIHUULWRSHDUOLWLFPDWUL[WKHSHDUOLWLF reaction has been observed to start at higher undercooling than in fully pearlitic matrix as reported by Samuel and Viswanathan and Lacaze and Sertucha. This is certainly to be associated with microsegregation.

4. Conclusion Control of the eutectoid transformation of austenite to get Fig. 11. Build-up of microsegregation of some substitutional sol- a fully ferritic or fully pearlitic matrix is achieved by alloy- XWHV LQ WKH DXVWHQLWH PDWUL[ GXULQJ VROLGL¿FDWLRQ RI D ing or by change in the cooling rate (which depends in all HXWHFWLFQRGXODUJUDSKLWHFDVWLURQQRWHWKHORJDULWKPLF scale for plotting austenite composition. The origin of the practicality on the casting section size). The most common ;D[LVFRUUHVSRQGVWRWKHRXWHUVXUIDFHRIQRGXOHVi.e. SHDUOLWHSURPRWHUDOOR\LQJHOHPHQWVDUHPDQJDQHVHFRSSHU the start of the eutectic reaction. DQG WLQ EXW HOHPHQWV SUHVHQW DV LPSXULWLHV RU WUDFHV PD\ JUHDWO\DIIHFWWKH¿QDOPLFURVWUXFWXUH,QFUHDVHRIWKHQRGXOH count may also have a positive effect on ferrite fraction by favoring carbon diffusion from austenite to graphite nodules DQGE\PXOWLSO\LQJWKHSRVVLEOHVLWHVIRUIHUULWHQXFOHDWLRQ but it has been stated this is more effective at low nodule FRXQW 0RUH SUHFLVHO\ WKH PDWUL[ VWUXFWXUH RI ORZ QRGXOH count nodular cast irons will be sensitive to cooling rate while very high cooling rates would be necessary to avoid some ferrite forming at high nodule count. Impurities or trace elements appear to have a marked effect on graphite shape and on room temperature prop- HUWLHV +RZHYHU WKH PDQ\ SRVVLEOH LQWHUDFWLRQV EHWZHHQ WKHP PDNH GLI¿FXOW WR SURSHUO\ GH¿QH UXOHV IRU FRQWURO- ling cast iron melt chemistry. Attempts made in the past such as the extensive work by Thielemann38) should be updated as impurities and their level are not the same in the present days as they were at that time. Supporting such a Fig. 12. Local reference temperatures for the ferritic and pearlitic work with the development of an appropriate databank for UHDFWLRQVFDOFXODWHGDORQJWKHFRPSRVLWLRQSUR¿OHVLQ)LJ thermodynamic calculations of inclusion formation during 11. Both the case of an unalloyed cast iron and of an PHOWSUHSDUDWLRQDQGVROLGL¿FDWLRQLVFHUWDLQO\ZRUWKRIDQ alloyed one are shown. DWWHPSW )LQDOO\ GHGLFDWHG VWXGLHV RI JURZWK NLQHWLFV RI SHDUOLWH ZKHQ HOHPHQWV VXFK DV QLFNHO PRO\EGHQXP DQG chromium are present would be of great interest. gations agree very well with those measured by Rundman and with information from literature he reported. REFERENCES Figure 12 illustrates the local evolutions of Tα and Tp   :&-RKQVRQDQG%9.RYDFVMetall. Trans. A9   for the above alloy and compares them to those for the   (13DQ06/RXDQG&5/RSHUAFS Trans.95   unalloyed cast iron (Fe-3.6 wt.% C-2.5 wt.% Si). Note that 3) D. Venugopalan: Proc. Int. Symp. Fundamentals and Applications of the correction given by Eq. (2) was not applied here. As 7HUQDU\'LIIXVLRQHGE\*53XUG\3HUJDPRQ3UHVV1HZ