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Steel Designers' Manual - 6th Edition (2003)
(-)
a
E
V
1600
1400
1200
1000
800
600
400
L+ ô
liquid
L
0 1 2 3 4 5 6 7
weight % carbon
Heat treatment 227
eutectic
Fig. 6.3 Equilibrium phase diagram for iron – iron carbide system (f.c.c., face-centred cubic;
b.c.c., body-centred cubic)
left-hand end of the diagram, with very low carbon contents, the equilibrium structure
at room temperature is ferrite. At carbon contents between these limits the
equilibrium structure is a mixture of ferrite and cementite in proportion depending
on the carbon level. On cooling from the melting temperature, at low carbon levels
a phase known as delta ferrite is formed first, which then transforms to a different
phase called austenite. At higher carbon levels, the melting temperature drops with
increasing carbon level and the initial transformation may be direct to austenite.
The austenite phase has a face-centred cubic lattice crystal structure, which is maintained
down to the lines AE and BE on Fig. 6.3. As cooling proceeds slowly the
austenite then starts to transform to the mixture of ferrite and cementite which
results at room temperature. However, point E on the diagram represents a eutectoid
at a composition of 0.83% carbon at which ferrite and cementite precipitate
alternately in thin laths to form a structure known as pearlite. At compositions less
than 0.83% carbon, the type of microstructure formed on slow cooling transformation
from austenite is a mixture of ferrite and pearlite. Each type of phase present
at its appropriate temperature has its own grain size, and the ferrite/pearlite grains
tend to precipitate in a network within and based on the previous austenite grain