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· If a large casting is formed,
different parts cool at different rates and the overall microstructure tends
to be course-grained.
· Slow
cooling through the austenite
range causes nucleation and growth to occur at
high temperatures. This yields a few fast-growing nuclei that give large grains
of austenite. These transform into a complex pearlite
structure with ferrite on
the grain
boundaries (hypoeutectoid starting composition).
· Re-heating into the temperature range where austenite
is stable, heterogeneously nucleates small austenite crystals. When the
material is taken to room temperature in a controlled way these convert to a
fine grained pearlite (Ferrite
plus Fe3C).
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