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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
DoITPoMS, University of Cambridge
Prof T W Clyne, Department of Materials Science and Metallurgy, University of Cambridge
|Published by:||DoITPoMS, University of Cambridge||License:||This resource is released under the Creative Commons Attribution Non-Commercial Share Alike license (2.0 UK: England & Wales). View the full legal code here.|
|Date created:||25 October 2001|
|Date added:||21 August 2009|