- lamellar mixture of ferrite and cementite
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.
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.
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.
منبع:
http://www.georgesbasement.com/Microstructures/LowAlloySteels/Lesson-1/Specimen01.htm