Materials for engineering, 3rd Edition - (Malestrom)

114
Materials for engineering
Tensile strength (N mm –2 in 30 mm diameter bar)
154 185 216 263 309
Bar diameter (mm)
5
10
25
P
Mottled or white
H
(hard and
unmachinable) 5
10
Plate thickness (mm)
Ferrite and
pearlite (soft and
machinable)
Pearlite
(strong and
machinable)
15
38
5.0 4.5 4.0 3.5 3.0
CE value
3.29 Showing the structures of cast irons of various Carbon
Equivalent values and section thicknesses.
rate) and the resultant microstructure. It is seen that white cast iron (hard and
unmachinable) is formed at low CE values and rapid cooling, whereas grey
irons (first pearlitic and then ferritic) are formed as CE values and section
sizes increase.
Ductile iron
Malleable cast iron may be produced by taking a white iron casting and
holding it at 950–1000°C, when breakdown of the iron carbide occurs:
Fe 3 C → 3Fe + C
producing graphite in the form of aggregates known as ‘temper carbon’.
These are not in flake form and are thus much less deleterious to the mechanical
properties; the result is a material which possesses a good measure of strength
combined with ductility. Nodular, or spheroidal graphite (SG) cast iron
contains graphite spheroids in the as-cast state, by the addition of cerium
and/or magnesium to the iron, as illustrated in Fig. 3.30. These SG irons can
be made with a pearlite matrix or ferrite with appropriate heat-treatment, or
with acicular or austenitic matrix when suitably alloyed, and they behave as
more or less normal ductile ferrous materials.
In view of the large number of compositions of steels and cast irons
commercially available and the fact that their properties may be varied over
a wide range by appropriate heat-treatment, it is impossible to present a brief