Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Micrograph Library Full Record for Micrograph 15
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Full Record for Micrograph 15

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Micrograph no
15
Brief description
Fe, C 0.8 (wt%), eutectoid transformation - pearlite (coarse)
Keywords
alloy Link to MATTER Glossary entry for alloy, austenite Link to MATTER Glossary entry for austenite, carbon, cementite Link to MATTER Glossary entry for cementite, eutectoid reaction Link to MATTER Glossary entry for eutectoid reaction, ferrite Link to MATTER Glossary entry for ferrite, iron, lamella Link to MATTER Glossary entry for lamella, metal, pearlite Link to MATTER Glossary entry for pearlite, steel Link to MATTER Glossary entry for steel, supercooling Link to MATTER Glossary entry for supercooling
Categories
Metal or alloy
System
Fe-C-X   Link to phase diagram
Composition
Fe, C 0.8 (wt%)
Standard codes
Reaction
Processing
Slow cooled
Applications
Sample preparation
Technique
Reflected light microscopy
Length bar
40 μm
Further information
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 (g to a + 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.
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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