Image icon  Fe, C 0.8 (wt%), eutectoid transformation - pearlite (coarse)

 
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Description: This steel is of the eutectoid composition. Once the temperature is lowered below the eutectoid temperature the steel becomes simultaneously supersaturated with both ferrite and cementite. A eutectoid transformation results (γ to α + Fe3C). The resultant microstructure, known as pearlite, comprises lamellae of cementite (dark) embedded in ferrite (white). The platelets are parallel to each other and do not follow a specific crystallographic direction.

Each pearlite colony is made up of a number of subgrains. Thus each pearlite colony consists of two interpenetrating single crystals having an orientation relationship with respect to each other and with respect to the austenite grain they grow from, but not with respect to the austenite grain they have grown into. Changes in the apparent interlamellar spacing from colony to colony in the photograph are due to differences in the lamellae spacing with respect to the polished surface.

The coarseness of the pearlite is determined by the interlamellar spacing. This spacing is inversely proportional to the undercooling. This is primarily because of the increased rate of carbide nucleation with increased undercooling.

The pearlite in this sample is coarse due to it being slowly cooled. The undercooling is low so the lamellae spacing is relatively large resulting in a coarse microstructure.
Keywords: alloy • austenite • carbon • cementite • eutectoid reaction • ferrite • iron • lamella • metal • pearlite • steel • supercooling
Categories: Science approaches > Microstructure
Materials > Metals & alloys > Carbon steels
Testing, analysis & experimentation > Metallography
Scale > Micro
Created by: Prof T W Clyne, Department of Materials Science and Metallurgy, University of Cambridge
DoITPoMS, 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: 1079