Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 113
The iron–graphite phase diagram is similar in form to Fig. 3.21, with C
(graphite) replacing Fe 3 C and it may still be used to account for the
microstructures that develop. Hypoeutectic irons solidify to dendrites of
austenite in an austenite/graphite eutectic, and hypereutectic irons form primary
graphite flakes in the same austenite/graphite eutectic. On further cooling,
the austenite decomposes at the eutectoid temperature, at high cooling rates
to pearlite and at slow cooling rates to a ferrite–graphite eutectoid.
The microstructure of a grey cast iron is illustrated in Fig. 3.28. These irons
are so-called because the presence of graphite in the microstructure leads to
a grey-coloured fracture surface.
Grey cast iron is an attractive engineering material because of its cheapness
and ease of machining. Graphite is devoid of strength, but its formation
tends to compensate the tendency for castings to shrink on solidification. Its
flake-like form effectively means that the microstructure is full of cracks,
with the result that the material exhibits little ductility, although it is strong
in compression. Graphitic irons exhibit a high damping capacity, since
vibrational energy is dissipated at these internal interfaces.
The carbon equivalent value (CE) is an index that combines the effect of
Si and P upon the eutectic of iron and carbon and it is defined as:
CE = Total %C + (%Si + %P)/3
The CE shows how close a given composition of iron is to the theoretical
iron–carbon eutectic (CE = 4.3%) and, therefore, how much free graphite is
likely to be present for a given cooling rate. The mechanical properties of a
cast iron are largely controlled by the graphite content and Fig. 3.29 indicates
the relationship between CE value, UTS and section thickness (i.e. cooling
3.28 Optical micrograph of grey cast iron (unetched) × 500.