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Full Record for Micrograph 63

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Micrograph no
63
Brief description
Fe, C 3.6, Si 2.1 (wt%), hypereutectic grey cast iron
Keywords
alloy Link to MATTER Glossary entry for alloy, carbon, cast iron Link to MATTER Glossary entry for cast iron, graphite Link to MATTER Glossary entry for graphite, hypereutectic Link to MATTER Glossary entry for hypereutectic, iron, metal, silicon Link to MATTER Glossary entry for silicon
Categories
Metal or alloy
System
Fe-C-X   Link to phase diagram
Composition
Fe, C 3.6, Si 2.1 (wt%)
Standard codes
Reaction
Processing
Applications
Sample preparation
Technique
Reflected light microscopy
Length bar
40 μm
Further information
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.
Contributor
Prof T W Clyne
Organisation
Department of Materials Science and Metallurgy, University of Cambridge
Date
25/10/01
Licence for re-use
Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales
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