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(12. DERS)

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ELK VE DKME DEMR

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Karbon Edeeri: CE

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http://www.msm.cam.ac.uk/doitpoms/miclib/micrographs/large/000062.jpg

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The carbon equivalent of this alloy (4.37 wt%) is greater than the eutecticcomposition (4.3 wt% C) so this alloy is known as a hypereutectic greycast iron. Si has a very low solubility in cementite and therefore itspresence favours the formation of graphite in preference to cementite.Therefore, upon cooling from the liquid, the primary phase to solidify isfree graphite. Graphite forms directly from the melt as flakes (black) ratherthan dendrites. These flakes are interconnected and align themselves inthe heat flow direction. Upon further cooling the remaining liquid formsinitially as a eutectic mixture of austenite and cementite, known asledeburite. The austenite within the eutectic further transforms to a mixtureof pearlite and cementite. The graphite is eventually embedded in acomplex mixture known as 'transformed ledeburite'. The graphite flakes

are very soft and have low density so compensate for freezing contractionupon solidification, however they also act effectively as cracks making thecast iron is brittle.

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With a carbon equivalent less than the eutectic composition a hypoeutecticwhite cast iron is formed. Primary austenite dendrites (dark) are first toform from the liquid. At the eutectic the remaining liquid transforms to amixture of austenite and cementite (light) with further transformation of theaustenite to ferrite and pearlite.

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An example of a white cast iron, so named due to its relatively lowcarbon content, which means that the carbon present is in the formof cementite. Upon cooling the melt initially forms austenitedendrites and ledeburite. The dendrites then transform to pearlite,and the ledeburite to ferrite and cementite.

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Cementite (white) and pearlite (dark) in white cast iron; 4% picral etch.

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http://www.doitpoms.ac.uk/miclib/micrographs/large/000070.jpg

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This sample shows the typical structure of silal, ironswith high Si content (5.5-7.9 wt%). It is a grey castiron alloyed with 4-6wt% Si to provide good oxidationresistance. The high Si content forms a dense,

adherent iron silicate surface film, which is resistant tooxygen penetration. The flake graphite iron Silal wasone of the first heat resisting cast irons developed.Spheroidal graphite Si irons have higher strength and

improved ductility. The structure shows coreddendrites of ferrite with interdendritic flake graphite. Afew white areas of Si carbide are also present.

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http://www.doitpoms.ac.uk/miclib/micrographs/large/000379.jpg

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http://www.doitpoms.ac.uk/miclib/micrographs/large/000380.jpghttp://www.doitpoms.ac.uk/miclib/micrographs/large/000380.jpg

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The addition of a substantial amount ofsilicon to a relatively low carbon cast ironserves to induce the cementite to

transform to ferrite and graphite flakes.This results in the properties of theresultant metal being more similar to that

of grey cast iron, i.e. making it easier tomachine and improving wear resistance.

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A cast iron with a relatively low carboncontent. The sample has been normalisedat 900 C for 72 hours in order to refine the

grain size. The microstructure consists ofpearlite with graphite.

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Cast iron is a brittle material and it is advantageous to process it in such a wayto improve its ductility. The brittleness is partially due to the graphite flakes

which act as nucleation sites for cracks. Therefore it is an advantage to havethe graphite present as spheres. This can be achieved by a heat treatmentregime or by the addition of a small amount of Mg which poisons the graphitegrowth directions.

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