Full Record for Micrograph 62

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Micrograph no

Brief description

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

Keywords

alloy

, carbon, cast iron

, graphite

, hypereutectic

, iron, metal, silicon

Categories

Metal or alloy

System

Fe-C-X

Composition

Fe, C 3.6, Si 2.1 (wt%)

Standard codes

Reaction

Processing

Applications

Sample preparation

Technique

Reflected light microscopy

Length bar

400 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

Related micrographs

Micrograph 63: Fe, C 3.6, Si 2.1 (wt%), hypereutectic grey cast iron (40 μm)