A useful first attempt to relate composition and structure was shown in Fig. 3 of the article Cast Irons but it had limited use in the foundry. Figure 1 shows a more useful relationship between CE value, structure, tensile strength in 30 mm dia bars and section size. A cylindrical test bar of given dia cools more rapidly than a flat plate of equivalent thickness, hence the section is expressed as bar diameter or section thickness. Line H is the boundary of unmachinable irons while line P is the boundary between soft and pearlitic irons. Thus an iron of carbon equivalent 4,35 should not be made thicker than 20 mm as a bar or 10 mm as a plate to attain a pearlitic iron. To avoid an unmachinable chilled casting the bar should not be less than 8 mm dia or plate less than 4 mm thick.
A useful first attempt to relate composition and structure was shown in
Fig. 3 of the article Cast Irons but it had limited use in the foundry.
Figure 1 shows a more useful relationship between CE value, structure,
tensile strength in 30 mm dia bars and section size. A cylindrical test
bar of given dia cools more rapidly than a flat plate of equivalent
thickness, hence the section is expressed as bar diameter or section
thickness. Line H is the boundary of unmachinable irons while line P is
the boundary between soft and pearlitic irons. Thus an iron of carbon
equivalent 4,35 should not be made thicker than 20 mm as a bar or 10 mm as
a plate to attain a pearlitic iron. To avoid an unmachinable chilled
casting the bar should not be less than 8 mm dia or plate less than 4 mm
thick.
T S. MPa in 30 mm dia. bar
Figure 1. Diagram relating section size, CE value, tensile strength and
structure (After BCIRA)
A melting furnace usually produces iron of a constant CE value and silicon is the element normally used to control chill. Alloying elements are added to cast iron to confer special properties and also to control the chill.
Formation of
graphite
Flake. Neglecting the effect phosphorus, and the presence of primary
austenite dendrites, the successive stages in the growth from the liquid
of flake graphite is shown in Fig. 2a The eutectic begins to solidify at
nuclei from each of which is formed a roughly spherical lump, referred to
as a eutectic cell. In this cell there has been simultaneous growth of
austenite and graphite, the latter being in continuous contact with the
liquid. The normal appearance of graphite in a micrograph suggests that
the structure is made up of a number of separate flakes, but now it is
considered that within each eutectic cell there is a continuous branched
skeleton of graphite, like a cabbage. The skeleton is branched more
frequently with a rapid radial growth of the cell such as occurs when
increasing the rate of cooling of an iron which produces undercooling, and
therefore finer graphite in the micrograph (Fig. 3).
Figure 2.
Figure 3. Medium size graphite outlining dendrites (x60)
The diameter of a eutectic cell, therefore, has a major effect on
mechanical properties, e.g. the greater the number of cells per unit
volume the higher the tensile strength, but soundness is affected
adversely. Superheating or holding time of the molten iron reduce the
number of nuclei, while inoculants such as ferro-silicon and also sulphur
increase nuclei.
Spheroidal. Fig. 2b show the growth of
spherulitic graphite in a magnesium-treated iron. In this case the
spherulitic graphite is quickly surrounded by a layer of austenite and
growth of the spheroid occurs by diffusion of carbon from the liquid
through the austenite envelope. If diffusion distances become large there
will be a tendency for the remaining liquid to solidify as white iron
eutectic, hence inoculation in this iron is highly desirable in order to
increase the number of graphite centres.
Temper carbon nodules. At the malleabilising
temperature (800-950°C) the solid white iron consists of eutectic matrix
of cementite, austenite and sulphide inclusions. Nucleation of graphite
then occurs at austenite cementite interfaces and at sulphide inclusions.
The cementite gradually dissolves in the austenite and the carbon diffuses
to the graphite nuclei. The MnS tends to form a flake aggregate and the
FeS a spherulitic nodule (Fig. 2c).
Micro-structure of cast iron
In preparing the specimens care is required, otherwise, erroneous
results might arise. The graphite is readily removed during polishing and
in this case the cavities can be either burnished over or enlarged. The
various types of micro-structure can be classified into groups without
considering the presence of phosphorus.
The graphite can vary in size
and form as illustrated in Figs. 3-5. The coarse flaky graphite is found
in common iron, while the fine curly type, frequently outlining the
dendrites, is found in high-class iron, especially when superheated before
casting. Spheroidal graphite is found in magnesium treated irons (Fig. 6).
The nodular form is found in annealed irons in which the cementite has
decomposed at 800-950°C. Thus we have:
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Figure 4. Coarse graphite flakes.
Matrix
unetched (x 60) |
Figure 5. Temper carbon in a malleable iron; ferrite
crystals etched (x 100) |
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Figure 6. Enlarged view of graphite spheroid.
Polarised light (x 600) |
Figure 7. Hypo-eutectic white cast iron, cementite
and pearlite(black)
(x 100) BH =100 |
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Figure 8. Hyper-eutectic white cast iron (x 100).
White primary crystals of cementite in eutectic
(cementite and
pearlite) |
Figure 9. Grey iron. High duty;
pearlite and
graphite (x 200) |
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Pearlite + cementite (i.e. eutectic cementite in hypoeutectic irons (Fig. 7) and primary andeutectic cementite in hyper-eutectic irons (Fig. 8)
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white, hard, unmachinable.
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Cementite + graphite + pearlite
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mottled, difficult to machine.
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Graphite + pearlite (Fig. 9)
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grey, machinable, high strength.
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Graphite + pearlite + ferrite (Fig. 4 from the Cast Iron article)
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grey, soft, weaker,
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Graphite + ferrite
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grey, very soft, easily machined.
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The ferrite is of course much less pure than that in
carbon steels.
Phosphide eutectic
Most cast irons contain phosphorus in amounts varying from 0,03 to 1,5%, consequently another micro-constituent is frequently present in the structure, in addition to those phases mentioned above. It occurs in white irons as a laminated constituent (ternary eutectic), consisting of:
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Iron, 91,19%
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Ferrite (with a little phosphorus).
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Carbon, 1,92%
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Cementite, Fe3C.
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Phosphorus, 6,89 %
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Iron phosphide, Fe3P.
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The melting-point is in the region of 960°C, consequently it is the
last constituent to solidify and forms islands in the interstices of the
dendrites.
Although this constituent is very brittle it does not
unduly weaken the iron when in small amounts (up to 1%)
due to the fact that continuous cells are not formed
round the grains. The structure is illustrated in Fig. 4 from
the Cast Iron article which shows the structure of the phosphide
eutectic, together with graphite, ferrite and pearlite.
Phosphorus will thus form this additional constituent in any
of the "grouped" structures already discussed.