Buderus Manual on Trenchless Installation of Ductile Cast ... - Duktus

At x 100 magnification, these graphite lamellae can clearly be seen in metallographic
micro-sections (Fig. 2.2). Under the scanning electron microscope, the space-filling
structure of the graphite lamellae is very clear (Fig. 2.3).
Fig. 2.2 Fig. 2.3
Fig. 2.4
Fig. 2.5
The material
The lamellar graphite, which has no strength of its
own, interrupts the matrix of metal in which it is embedded
and thus causes the relatively low strength
which cast iron has. At the same time, this internal
structure also gives the material poor ductility: its
fracture behaviour is brittle. The modelled representation
shown in Fig. 2. simulates the internal
notch effect which is caused by the concentration
of stress lines at the tips of the lamellae. This is the
reason for the poor ductility of grey cast iron.
Some 0 years or so ago, the form in which the
graphite crystallises was successfully altered by a
metallurgical treatment of the melt with metals with
a high affinity for oxygen (cerium, magnesium). In a
melt of cast iron in which the graphite would originally
have solidified in lamellar form, it solidifies in a
spheroidal form if a small amount (about 0.0 %) of
the above metals is added.
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