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This is a medium carbon steel, consisting of ferrite and
pearlite (a lamellar mixture of ferrite and cementite). It is
banded (dark bands = pearlite; light bands = ferrite) an undesirable
condition. The photomicrograph was taken at 50X
magnification. The etchant was Nital (1% to 3% nitric acid in
ethanol).
This piece was probably hot rolled, followed by air cooling to room
temperature, which permitted the metastable iron - cementite phase
diagram to be followed.
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As the piece cooled below the all-austenite region within
which the hot rolling had been performed, ferrite was first
precipitated at the austenite grain boundaries, thereby causing carbon
to be rejected into the remaining austenite. Once the piece
cooled below the eutectoid temperature (727C) the 0.8% carbon austenite
transformed discontinuously via coupled growth of ferrite and cementite
in a lamellar (layered) morphology called pearlite because of its
appearance to the unaided eye after etching.
The banding in this piece is the result of
microsegregagtion of manganese during solidification of the original
ingot, which has not been homogenized during the subsequent
thermomechanical processing.
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Manganese lowers the eutectoid temperature and eutectoid
composition in the iron - cementite phase diagram and also lowers the
chemical acyivity of carbon in austenite.
As this piece cooled from the all-austenite region, alpha ferrite was
precipitated - more so, the less the local manganese content.
Therefore, the last areas to transform were richer in both manganese
and carbon, so those regions are now mostly pearlite.
Banding can be eliminated by prolonged heating and/or extensive hot
working to homogenize the metal with respect to the manganese, or it
can be circumvented by short time, high temperature austenitization,
which levels out the local carbon content but not the manganese
variations. Proceed to Specimen 2.
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