Full Record for Micrograph 18

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Micrograph no
Brief description
Fe, C 0.1 (wt%), hypoeutectoid alloy
allotriomorph Link to MATTER Glossary entry for allotriomorph, alloy Link to MATTER Glossary entry for alloy, austenite Link to MATTER Glossary entry for austenite, carbon, ferrite Link to MATTER Glossary entry for ferrite, hypoeutectoid Link to MATTER Glossary entry for hypoeutectoid, iron, metal, pearlite Link to MATTER Glossary entry for pearlite, proeutectoid steel Link to MATTER Glossary entry for proeutectoid steel, steel Link to MATTER Glossary entry for steel
Metal or alloy
Fe-C-X   Link to phase diagram
Fe, C 0.1 (wt%)
Standard codes
Sample preparation
Etched in 3% nital
Reflected light microscopy
Length bar
40 μm
Further information
This is a hypoeutectoid alloy, which has been air cooled from the austenite phase field at 950 °C. The first solid to form is proeutectoid ferrite, its morphology being determined by the cooling rate. At slow cooling rates (furnace cooling) there is sufficient time for the carbon rejected from the austenite to diffuse and equilibrium solidification occurs. With faster cooling the microstructure also depends on the original austenite grain size.

Fast cooling and large grain size favours ferrite forming as Widmanstätten side plates from the grain boundaries. Small grain sizes imply a high number of nuclei and hence the ferrite grows as grain boundary allotriomorphs. In this case air cooling is sufficiently slow to produce allotriomorphic ferrite. The majority of the austenite has changed to ferrite leaving only a small amount to be transformed to pearlite, therefore the microstructure shows large ferrite grains with small islands of pearlite.
Prof T W Clyne
Department of Materials Science and Metallurgy, University of Cambridge
Licence for re-use
Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales
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