Microstructures
by
George
Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA,
1966
Copyright©2005 by George Langford
Cast Irons, High Alloy Steels,
and Superalloys - Lesson 1 -
Fifth specimen
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This nodular cast iron has a bullseye microstructure due to
an intermediate cooling rate. The magnification is 500X.
Can you tell whether the white rings are ferrite or cementite ?
Is there a fundamental reason why you wouldn't expect the rings to be
cementite ?
Think about this while looking at the metastable/stable
iron - carbon phase diagrams and then look
at the answers below.
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Explanation: The white
rings have to be ferrite because iron carbide cannot be stable in
direct contact with graphite. Also, the white rings are
continuous with the majority (ferrite) phase in the pearlite. The
cementite lamellae in this pearlite are quite coarsely spaced,
indicating slow cooling during the formation of the pearlite.
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This
microstructure combines wear resistance (due to the relatively high
hardness of the pearlite) with toughness because the ferrite rings
relieve the strain concentrations around the graphite nodules during
deformation.
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