Directional heat transfer was investigated by temperature measurements in the casting and in the mould using thermocouples. Measurements were performed in operating conditions during pouring, solidification, and cooling of the casting. Total measurement time was 35,5 hours. After cutting, specimens were extracted for metallographic and hardness testing. Test results provided confirmation of directional heat transfer (directional cooling) that would ensure acquirement of a desired casting structure and mechanical properties.Usmjereni prijenos topline u odljevku i kalupu praćen je mjerenjem temperature termoparovima tijekom ulijevanja taljevine u kalup, skrućivanja i hlađenja odljevka. Ukupno trajanje mjerenja iznosilo je 35,5 sati nakon čega je odljevak izrezan i uzeti su uzorci za metalografsku analizu i mjerenje tvrdoće. Rezultati ovih ispitivanja potvrdili su usmjereno odvođenje topline (usmjereno hlađenje) što osigurava dobivanje željene mikrostrukture i mehaničkih svojstava odljevka

J. Črnko

L. Lazić

P. Jelić

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P. JELI], L. LAZI], J. ^RNKOTHE EFECT OF COOLING RATEON THE PROPERTIES OF ALLOYED CAST-IRON SIZING ROLLReceived – Prispjelo: 2008-12-24Accepted – Prihva}eno: 2009-03-26Preliminary Note - Prethodno priop}enjeINTRODUCTIONOperational conditions in iron and steel rolling millsin Croatia have imposed the need for use of sizing rollsof own production. The rolls are characterized by aworking surface depth of up to 250 mm. Rolling is doneby means of the cylinder shell and lateral surfaces 1.Because of exposure to compression stress at elevatedtemperatures (820–850 °C) during operation the rollsmust possess a microstructure that will ensure high re-sistance to wear and thermal fatigue as well as goodmachinability of the crude cast roll.High resistance to thermal fatigue requires high ther-mal conductivity which is achieved by means of thepresence of flake graphite in the microstructure. Thegraphite presence is also essential for good machina-bility. On the other hand, high wear resistance calls forthe presence of hard carbides in the microstructure.Bearing all this in mind and taking into account the factthat a roll of specific dimensions and weight is expectedto have a machined working surface hardness of about380 HB, which drops gradually to about 280 HB to-wards the roll interior, it was examined the composi-tions, microstructures, and hardness values of the avail-able rolls of comparable size. Based on the test results aconclusion was reached to use alloyed grey cast iron forroll casting in order to generate an indefinite chill struc-ture where the amount of eutectic carbides would gradu-ally decrease going from the roll working surface to theinterior, and conversely, the proportion of flake graphitewould grow with the distance from the working surface.The hardness values along the roll cross-section wouldchange accordingly 2.The fact that the casting microstructure is directlydependent on the average cooling rate has been welldocumented 3–6. It follows that to achieve an adequatecoooling rate for the roll working surface suitable condi-tions have to be generated. The moulding technologyapplied in experimental casting 2 failed to ensure asufficiently high melt cooling rate in the border zones ofthe casting so that the carbide content of the sizing rollworking surface was too low. On the other hand, due to alower cooling rate of the melt in the central castingzones, the concentration of carbide-forming elementsincreased in the last solidified melt volume. As a result,major excretion of carbon in the form of carbides tookplace in the direction of the thermal centre of the casting.Hardness testing carried out along the sizing rollcross-section proved the predicted solidification orderto be correct. Tests showed that the recorded hardnessvalues of the sizing roll working surface exceeded therequired value (400 HB), and that, instead of becomingMETALURGIJA 49 (2010) 1, 45-48 45Directional heat transfer was investigated by temperature measurements in the casting and in the mould usingthermocouples. Measurements were performed in operating conditions during pouring, solidification, and co-oling of the casting. Total measurement time was 35,5 hours. After cutting, specimens were extracted for me-tallographic and hardness testing. Test results provided confirmation of directional heat transfer (directionalcooling) that would ensure acquirement of a desired casting structure and mechanical properties.Key words: alloyed grey cast iron, sizing roll, indefinite chill, moulding technology, thermocouple temperaturemeasurementUtjecaj brzine hla|enja na svojstva kalibrirnog valjka od legiranog `eljeznog lijeva. Usmjereni prije-nos topline u odljevku i kalupu pra}en je mjerenjem temperature termoparovima tijekom ulijevanja taljevine ukalup, skru}ivanja i hla|enja odljevka. Ukupno trajanje mjerenja iznosilo je 35,5 sati nakon ~ega je odljevak izre-zan i uzeti su uzorci za metalografsku analizu i mjerenje tvrdo}e. Rezultati ovih ispitivanja potvrdili su usmjere-no odvo|enje topline (usmjereno hla|enje) {to osigurava dobivanje `eljene mikrostrukture i mehani~kihsvojstava odljevka.Klju~ne rije~i: legirani sivi lijev, kalibrirni valjak, nedefinirano zahla|eni sloj, tehnologija kalupljenja, mjerenjetemperature termoparomISSN 0543-5846METABK 49(1) 45-48 (2010)UDC – UDK 621.74:669.131.6:621.771.073.8:536.24=111P. Jeli}, L. Lazi}, J. ^rnko, Faculty of Metallurgy, University of Zagreb,Sisak, Croatialower, they grew higher towards the interior. This wastaken to be the consequence of discordance between thecast iron chemical composition and the moulding tech-nology applied.The aim of the investigation was to examine, by ex-perimental casting, an altered composition of alloyedgrey cast iron and a modified moulding technology incomparison with those refered to in 2, and also to iden-tify the changes that had taken place in themicrostructure and in the hardness of the the sizing rollworking surface.CASTING TECHNOLOGY MODIFICATIONModifications of the casting technology related tothose of the mould. The chromite sand core was reduced toonly one half of the sizing roll circumference (Figure 1).The core thickness was also diminished. At the op-posite side of the sizing roll circumference there was anexternal metal core – a chill. A direct contact of the meltwith the chill was supposed to enhance the cooling ef-fect by producing a heat release that would help direc-tional solidification and thus make a better control of themicrostructure possible 7. In addition to mould modi-fication the chemical composition of alloyed grey castiron was also altered. The chemical compositions of themelt, the one used earlier 2 and the altered one, arecomparatively shown in Table 1. The idea was to pro-mote further the formation of carbides.MEASURING EQUIPMENT AND RESULTSFor temperature measurements a NI4350 high-pre-cision temperature and voltage meter of the plug-and-play type was used. It was connected to a computer withthe Windows 2000 operating system and a Virtual Benchsoftware package for data processing, displaying andstoring. Thermocouples were connected to a NI4350Data Logger. Table 2 shows their characteristics and lo-cations in the mould. All thermocouples were in directcontact with the melt (TC3, TC4, TC6, TC7) were in-serted in ceramic tubes for protection against damage andto ensure steady measurement during the entire period ofsolidification and cooling of the casting.Figure 2 shows cumulative results of temperaturemeasurements. The curves in Figure 3 refer specificallyto the thermocouples TC1, TC2, TC9, and TC10. An al-most total overlapping of the cooling curves noticeablefor the thermocouples TC3, TC4, TC6, and TC7 was ex-pected, because they were immersed in the melt. Moreprominent differences can be observed between the46 METALURGIJA 49 (2010) 1, 45-48P. JELI] et al.: THE EFECT OF COOLING RATE ON THE PROPERTIES OF ALLOYED CAST-IRON SIZING ROLLFigure 1. Schematic view of the sizing roll mould andthermocouple positionsTable 1. Chemical composition of the alloyed cast iron melt / wt.%Element C Mn P S Si Cr Cu Ni Mo VPrevious composition 2,19 0,58 0,026 0,05 0,96 0,72 0,35 1,20 0,09 -Modified composition 2,55 0,38 0,23 0,063 1,23 0,44 0,17 1,33 0,17 0,14Table 2. Thermocouples used in measurementsNo.Data LoggerChannelThermocoupletypeLocationANSIdesignationTC1 Ch02 Fe-Constantan Chill JTC2 Ch03 Ni-CrNi Chill KTC3 Ch04 PtRh Chill/Melt RTC4 Ch05 PtRh Feeder/Melt RTC5 Ch06 Ni-CrNi Central core KTC6 Ch07 PtRh Feeder/Melt RTC7 Ch08 PtRhChromite/MeltRTC8 Ch09 Ni-CrNiChromite/ChillKTC9 Ch10 Fe-Constantan Chill JTC10 Ch11 Fe-Constantan Chill Jcooling curves for the thermocouples at positions 2 and9 in Figure 3. A steeper rise of the TC2 curve in the firsthalf hour of measurement as compared to the TC9 curvecan be explained by a more intense heat transfer throughthe grey iron chill. After six hours of cooling the heatflow at that position equals the heat flow through thechromite core and the grey iron chill, and then it beginsto decrease. The TC9 curve, on the other hand, reflects auniform and steady heat flow with a slight decrease withtime.Figure 3 also shows the cooling curves for positions1 and 10, i.e. for the thermocouples TC1 and TC10,which were located on the outer part of the grey ironchill. The curves can be taken to best reflect the differ-ences in conduction heat transfer between the right sideof the mould with the chromite core on it and the oppo-site side without the chromite core.The TC1 curve demonstrates a fast increase in tem-perature over the melt casting/mould filling period fol-lowed by a continuous descent. The TC10 curve, on theother hand, reflects a somewhat slower increase in tem-perature in the mould filling period but a much slowertemperature decrease with time than the TC1. This isdue to the effect produced by the chromite core on con-duction heat transfer.The temperature data collected during the castingand solidification of the melt allow to conclude that thepresence of the chromite core on one half of the greyiron chill ensured control over the intensity of heat con-duction as one of the prerequisites for achieving a de-sired sizing roll microstructure.Hardness measurements were conducted using aWolpert-Werk type DTA 2R hardness tester on the sam-ples cut from the sizing roll at different locations i.e.depths. From the diagram in Figure 4 it is evident thatthere is a non-linear decrease in hardness with the dis-tance from the sizing roll working surface. Higher hard-ness values for the sizing roll surface can be accountedfor by iron carbide formation as a result of the high cool-ing rate.The samples prepared metallographically were ex-amined by means of a Reichert MeF2 optical micro-scope at 200  magnification. Figures 5a, 5b, and 5cshow the microstructure of the nital etched samples atdifferent depths.Careful examination of the figures shows the carbonfractions that precipitated partly as carbides and partlyas lamellar graphite to depend on the cooling rate duringeutectic solidification. The shape and the portion of theprecipitated graphite vary with the distance from the siz-ing roll surface. The working surface of the sizing rollhas a mottled microstructure. The fraction of cementitegradually decreases with the distance from the surface,while the graphite fraction increases accordingly. Thematrix structure is fully pearlitic and its constituents tes-tify to a directional, partly metastable solidification ofalloyed cast iron.From the hardness test results and the data concern-ing the sizing roll microstructure it can be concludedthat a good conformity between the chemical composi-tion and the moulding technology has been achieved.CONCLUSIONSFrom the temperature records it is evident that theuse of the chromite core on one half of the chill ensuredcontrol over the conduction heat flow rate as a prerequi-site for obtaining a right microstructure and proper me-chanical properties of the casting. The grey iron chillproved to be advantageous over a steel one owing tobetter heat conduction, higher resistance to thermalMETALURGIJA 49 (2010) 1, 45-48 47P. JELI] et al.: THE EFECT OF COOLING RATE ON THE PROPERTIES OF ALLOYED CAST-IRON SIZING ROLLFigure 2. Measured temperature curvesFigure 3. Measured temperature curves for thermocou-ples 1, 2, 9, and 10Figure 4. Hardness test resultsshocks, and higher dimensional stability at temperaturechanges.Hardness and metallographic tests eventually con-firmed predictions and showed soundness of the modifi-cations made in the casting technology and in the chemi-cal composition of the melt. A high cooling rate inducedcarbide formation in the surface and near-surface areasof the sizing roll. The graphite content increased and thecarbide content decreased with the distance from thesurface, because of a lower cooling rate, i.e. a slowerheat transfer.The resultant matrix structure was a pearlitic one.The quantity of eutectic carbides gradually decreasedwith the distance from the sizing roll surface to the inte-rior, while the proportion of flake graphite increasedwith the distance from the surface. The volume fractionsof microstructural constituents and their distribution en-sured proper hardness value distribution throughout thesizing roll.REFERENCES1 M. ^au{evi}: Obrada metala valjanjem, V. Masle{a, Saraje-vo, 1983, 330-332.2 F. Unki}, Z. Kiva~, B. Mio~: Production Problems of theIndefinite Chill Iron Rolls, Proceedings 6th Conference onMATRIB, Croatian Society for Materials and Tribology –Zagreb, Vela Luka, Croatia, 2001, 277-282.3 L.E. Menawati, R.W. Heine, C.R. Loper Jr.: AFS Transac-tions, 78 (1970) 363.4 D.M. Stefanescu, R. Ruxanda: ASM Handbook „Metallo-graphy and Microstructures“, Vol. 9, ASM International,Materials Park, Ohio, USA, 2004, 98-105.5 R. Elliot: Cast Iron Technology, Butterworths & Co. Ltd.,London, 1988.6 J.M. Radzikowska: ASM Handbook „Metallography andMicrostructures“, Vol. 9, ASM International, MaterialsPark, Ohio, USA, 2004, 580-586.7 P. Jeli}: Analysis of the Cooling Rate Influence on SizingRoll Properties, Master Thesis, Faculty of Metallurgy, Uni-versity of Zagreb, Sisak, 2005.Acknowledgements – The authors wish to thank Felisd.o.o. for production castings, Sisak, Croatia, for pro-viding help during the research.Note: The language lecturer for Englich language isprof. Neda Bani}48 METALURGIJA 49 (2010) 1, 45-48P. JELI] et al.: THE EFECT OF COOLING RATE ON THE PROPERTIES OF ALLOYED CAST-IRON SIZING ROLLFigure 5. Microstructure of etched sizing rollsamples:a) at the surface,b) at the depth of 200 mm,c) at the depth of 400 mm.

Utjecaj brzine hlađenja na svojstva kalibrirnog valjka od legiranog željeznog lijeva

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