Question

In a slowly cooled hypereutectoid iron-carbon steel,
the pearlite colonies are normally separated from each
other by a more or less continuous boundary layer of
cementite. Explain how this microstructure develops. Use
simple sketches to illustrate your answer

Answer 1

Pearlite is a mixture of two phases, ferrite and cementite. The two phases then are able to establish cooperative growth at the common front with the austenite, with much of the solute diffusion happening parallel to this front within the austenite. The distance between the "layers" of cementite and ferrite is known as the interlamellar spacing. The interlamellar spacing within pearlite can be made fine by growing the pearlite at large thermodynamic driving forces.

• Hyper eutectoid region – 0.8 to 2.1 %C
• Consider an alloy of composition C1 in figure that, upon cooling, moves down the line zz’. At point g only the ? phase will be present and the microstructure having only gamma grains.
• Upon cooling into the ?+ Fe3C phase field – say to point h – the cementite phase will began to form along the initial ? grain boundaries, similar to the ? phase in point i. This cementite is called proeutectoid cementite that which forms before the eutectoid reaction.
• As the temperature is lowered through the eutectoid to point i, all remaining austenite of eutectoid composition is converted into pearlite; thus the resulting microstructure consists of pearlite and proeutectoid cementite as microconstituents.