Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
phenomenon known as ‘weld decay’. The problem may be overcome by
employing steel of very low (< 0.06%) carbon content, so that chromium
carbide cannot form. An alternative solution is to employ a ‘stabilized’ stainless
steel containing an element with a higher affinity for C than Cr, such as Ti
or Nb. In the heat-affected zone of the weld, preferential precipitation of
NbC or TiC rather than chromium carbide occurs in the grain boundaries,
which are not depleted in Cr and, therefore, remain impervious to attack.
3.2.9 Cast irons
As their name implies, cast irons are shaped by casting into a mould rather
than by forging in the solid state. They are alloys of iron that usually contain
between 2.5 and 4% carbon (and from 1 to 3% silicon, which tends to
promote the appearance of the carbon as graphite, rather than as carbide,
Fe 3 C). From the phase diagram shown in Fig. 3.21, it is clear that a binary
Fe–4% C alloy will be close to the eutectic composition and, thus, have a
low melting temperature. Cast irons are very fluid when molten and have
good casting characteristics, but the castings usually have relatively low
impact resistance and ductility, which may limit their use.
The mechanical properties of cast irons depend strongly on their
microstructure. There are three basic types: white iron, grey iron and ductile
iron.
White cast iron
White cast iron has a low silicon content and may contain carbide-stabilizing
elements (such as Cr). When cooled rapidly from the melt, graphite formation
does not occur and the microstructure may be predicted from Fig. 3.21
to consist of dendrites of austenite in a matrix of eutectic (iron carbide +
austenite). On cooling to room temperature, the austenite decomposes to
eutectoid pearlite.
The absence of graphite in the microstructure results in a completely
white fracture surface – hence the name of the material. The hardness is high
(400–600 diamond pyramid hardness (DPN)) due to the presence of the
carbide and the pearlite, making white cast iron very suitable for abrasionresistant
castings.
Grey cast iron
In grey cast iron, the eutectic that forms consists of flakes of graphite + austenite.
The formation of graphite rather than iron carbide is promoted by the presence
of silicon and by conditions of slow cooling. If the casting consists of varying
sections, then the thin regions will be ‘chilled’ and cool at a greater rate than
the thick regions, so that only the latter will form grey cast iron.