366CHINA FOUNDRYVol.6 No.4Colour Metallography of Cast IronBy Zhou Jiyang, Professor, Dalian University of Technology, ChinaTranslated by Ph.D Liu Jincheng, Fellow of Institute of Cast Metal Engineers, UKNote: This book consists of five sections: Chapter 1 Introduction, Chapter 2 Grey Iron, Chapter 3 Ductile Iron, Chapter 4 Vermicular Cast Iron, and Chapter 5 White Cast Iron. CHINA FOUNDRY publishes this book in several parts serially, starting from the first issue of 2009.2.5 Crystallization of the LTFduring nal stage of eutecticsolidication of grey ironIn the nal stage of eutectic solidication, eutectic cells grow gradually into large sizes; the liquid iron between the cells enters the last stage ofsolidication.Atthistime,theregionoftheremainingliquidiron iscalledlasttofreezevolume,LTFinshort,asshowninFig.2-39. Itisdifculttoquantitativelyandstrictlydenetheboundaryofthe LTF,sincecrystallizationconditions(composition,coolingrateand the melt state etc.) in the LTF is obviously different to other regions, leadingtodifferenceinsolidificationstructurewhichinfluences subsequent solid phase transformation. Because the LTF situates in the boundaries of eutectic cells, the structure, purity, inclusions and density or compactness in the LTF exert important effects on the mechanical properties of cast iron. 2.5.1 Crystallisation characteristics of the LTF

ThevolumefractionoftheLTFisverysmall,andtheLTFare surrounded by solidied metal, as solidication in a hot metal mould. Inaddition,thegraphiteprojectingintotheLTFhasgoodthermal conductivity, can transfer heat out the regions; thereby the liquid iron in the LTF has relatively fast solidication velocity. The element concentrations in the LTF are very different from that of mother iron liquid; the carbide formation elements such as Cr, Mn, V and Mo etc. are enriched; while graphitization elements, Si, Cu, Ni and Al are signicantly depleted.

Thenalliquidtosolidifyoftenhasmorenon-metallicinclusions and low melting point metals, which inuence the solidication of the LTF.WhentheLTFislarge,itispronetoformingmicroshrinkage porosity.All the cast irons have such characteristics. 2.5.2 Crystallization of the LTFAccordingtothechemicalcomposition,nucleationstateandcooling condition(sectionthickness),thesolidificationoftheLTFhas following several situations: >Thegraphiteflakesineutecticcellscontinuetoprojectto andbranchesintheLTF;atthesametime,followinggraphite, austenitecontinuestoepitaxiallygrow,formingzigzagshapeand interpenetrating with neighbour eutectic cells, see Fig. 2-40. Hereafter, Chapter 2Grey Iron ()Fig. 2-39: The LTF during the nal stage of eutectic solidicationFig.2-40: Eutectic cell epitaxial growth in the LTFNovember 2009367Serial Reportthe LTF gradually becomes smaller and smaller, until all the liquid is depleted, no any other structure forms. >If the volume fraction of the LTF is larger and there exist graphite nuclei in the region, new graphite nuclei are possible to form and grow incarbonatomricharea.Thegraphitepresentsindependentlyinthe LTF,seeFig.2-41;whiletheformedausteniteisconnectedwiththe austenite in eutectic cells and difcult to be distinguished.

When the carbon is depleted or reduced to a lower value, due to lack of condition for graphite nucleation and growth, graphite stops growing; but austenite continues to grow and extend to remaining liquid until all theliquidisnished.Atthistime,theeutecticcellboundaryiswider, clear and easy to be distinguished, as illustrated in Fig.2-42.

The positive segregation elements (such as Cr, V and Ti) enriched in LTF reduce the temperature interval between TEG TEM, (see Fig.1-21). Whenthecontentofthoseelementsishigh,itispronetoforming intergranular carbides. At this time, thick section thickness seems not tosignicantlyreducethetendencyofcarbideformationinthe LTF, sincetheslowcoolingofthicksectionwillaggravatesegregation. ThicksectioncastingswithoutCr,V,Tiwillnotoccurintergranular Fig.2-41: Graphite precipitation in the LTFFig.2-42: Austenite extends epitaxially to the LTFcarbides,sinceundertheslowcooling,thepointS,theendof solidication, is above the TGM line. Whenthecoolingrateisincreased,thesolidificationprocessis accelerated, the temperatures of the end of solidication, TES, is quickly decreasedbelowTEM,asillustratedinFig.1-17(d),thuspromoting carbide formation.For iron melt containing low carbon equivalent and high content of positive segregation elements, it is prone to forming eutectic carbides between eutectic cells, as shown in Fig. 2-43. Ingreyironcontaininghighcontentofalloyingelements,the carbidesarealsodistributed,exceptineutecticcellboundaries,in honeycomb structure inside eutectic cells, see Fig. 2-44; this is because thereexistsremainingironliquidinthehoneycombstructure,which solidifiesinlaterstage.Allthisisresultedfromthecharacteristics ofeutecticcellsofgreyiron.Besides,carbidesingreycastironare alsosituatedbetweenaustenitedendrites,seeFig.2-45.Therefore, thecarbidesingreyironaremoredispersedthanthatinspheroidal graphite iron. If carbide formation elements are pushed to centre area of castings, a large amount of ledeburite is formed in the centre of castings (see Fig. 2-46).Atthistime,whitesolidicationisformedinsidethecastings, called inverse chill. Fig.2-43: Carbides in the boundaries of eutectic cellsFig.2-44: Carbides in the honeycomb structure of eutectic cellsNovember 2009369Serial Reportload, the tips will develop into crack initiations that cause the frames to fracture prematurely. The author observed the microstructures of thirty tensile samples and found that there occur zigzag shaped metal frames inmostofthesampleshavinglowstrength(3050MPa).Themetal network frames of eutectic cells in type D graphite cast iron are mainly meniscus frames (see Fig. 2-51); maybe, this is the reason that type D graphite cast iron has higher strength. Precipitatephases,suchasgraphite,carbides,phosphorus eutecticsandinclusionsintheLTFallarebrittlephases;their morphology and amount have direct inuence on the strength and brittleness of the metal frames. The isolated particles (such as TiC, TiN) have less detrimental effect; but continuous network, angular orlathyprecipitates(forexamplephosphoruseutecticsand cementite)willcausesignificantdetrimenttoproperties.Figure 2-52 shows crack propagations along carbides. Thepositivesegregationelements(Mn,Cr.MoandVetc) enriched in the LTF increase austenite transformation undercooling andrenetransformedstructure(seeFig.2-53),therebyincrease propertiesofthematerial.However,iftheseelementsaretoo muchenrichedintheLTF,thusleadingtoformationofcarbides andphosphoruseutectics;atthistime,thedetrimenteffectwill exceed positive effect.

Sincethevolumeoffinalsolidifyingliquidintheintercellular regions can not get compensation, small holes, i.e. shrinkage porosity, are formed in site. For low carbon equivalent hypoeutectic grey irons (such as grey iron 250, 300), this type of defect is prone to forming in hot spot area of castings; after machining, small dispersed black spots, i.e. shrinkage holes, can bee seen on the machined surface.2.5.3 Inuence of solidication structure in the LTF on property of grey ironThe structure solidied in the remaining liquid between eutectic cells has following three inuences: Ifnographite,carbidesorphosphoruseutectics,onlyausteniteis formedintheliquidintheLTF,metalnetworkframesconstructed from metal matrix will envelope eutectic cells. When under load, the metal network frames can bear part of the load, release the un-favourite inuence of graphite on the mechanical property; the cutting effect of graphite is restricted within eutectic cells. It is seen from this that metal frame has signicant effect on the mechanical properties of cast iron. It is considered that the strength characteristics of cast iron are mainly dependentontheprimaryausteniteandboundariesofeutecticcells which are not cut by graphite akes [63].The metal network frames can be divided into two types: meniscus Fig. 2-49: Vertical section of stable and meta-stable Fe-C-P systems (the dash line is with addition of Te) [61] (a) Meniscus shape(b) Zigzag shapeFig. 2-50: The types of metal frames formed in the LTFFig. 2-51: Metal frames in type D graphite cast iron (brown colour)Fig.2-52: Crack propagation along intergranular carbides ineutectic cell boundariesand zigzag shape. Spheroidal eutectic cells form meniscus frames and zigzag eutectic cells form zigzag frames, as illustrated in Fig. 2-50.It is obvious that due to the tip effect of zigzag frames, when under 370CHINA FOUNDRYVol.6 No.4KS=Solute concentration in the centre of dendrite (or eutectic cell)Solute concentration in the boundary of dendrite (or eutectic cell)Denition: KS = lno segregation KS 1inverse segregation. Solute concentration in the boundary of dendrite (or eutectic cell) is lower than in the centre of dendrite (or eutectic cell)Similarly, segregation ratio S.R can also be used to describe the segregation degree of solute elements[64, 65].S.R=Solute concentration in the boundary of dendrite (or eutectic cell)Solute concentration in the centre of dendrite (or eutectic cell)S.R >1 positive segregationS.R 1andbelong toinversesegregationelements;carbideformationelementshave KS0.2%, shrinkageporosityoccursbecauseofsolidificationshrinkage whenphosphorusenrichedliquidcoolstophosphoruseutectic temperature.WhenNi,CrandMoelementscoexistincastiron, thecombinedpositivesegregationsincreasetheamountand tendency of macro shrinkage porosity [71]. Some low melting point elements (Pb, Bi, Sn and Sb etc.) often segregatetoboundariesofdendritesoreutecticcells,causingto formspecialstructuresduringsolidication.Traceleadofw(Pb) >0.005%cancausenetworkgraphitetoformroundaustenite dendrites[72]. 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