Image icon  Fe, C 3.6, Si 2.1 (wt%), hypereutectic grey cast iron

 
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Description: The carbon equivalent of this alloy (4.37 wt%) is greater than the eutectic composition (4.3 wt% C) so this alloy is known as a hypereutectic grey cast iron. Si has a very low solubility in cementite and therefore its presence favours the formation of graphite in preference to cementite. Therefore, upon cooling from the liquid, the primary phase to solidify is free graphite. Graphite forms directly from the melt as flakes (black) rather than dendrites. These flakes are interconnected and align themselves in the heat flow direction. Upon further cooling the remaining liquid forms initially as a eutectic mixture of austenite and cementite, known as ledeburite. The austenite within the eutectic further transforms to a mixture of pearlite and cementite. The graphite is eventually embedded in a complex mixture known as 'transformed ledeburite'. The graphite flakes are very soft and have low density so compensate for freezing contraction upon solidification, however they also act effectively as cracks making the cast iron is brittle.
Keywords: alloy • carbon • cast iron • graphite • hypereutectic • iron • metal • silicon
Categories: Science approaches > Microstructure
Materials > Metals & alloys > Cast irons
Processes > Shaping > Casting
Testing, analysis & experimentation > Metallography
Scale > Micro
Created by: DoITPoMS, University of Cambridge
Prof T W Clyne, Department of Materials Science and Metallurgy, University of Cambridge
Published by: DoITPoMS, University of Cambridge
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Date created: 25 October 2001
Date added: 21 August 2009
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Resource ID: 1129