Die & Mould Making - CNC - Computer Numerical Control

Grey cast-iron
There is a large range of grey cast-irons
with varying tensile strengths. The silicon
content/sectional area combinations form
various structures of which the low-silicon,
fine graphite and pearlite make the
strongest and toughest material. Tensile
strength varies considerably throughout
the range. A coarse graphite structure
means a weaker type. A typical cast-iron,
where metal cutting is involved, often has
a silicon content of around 2%. Common
are the austenitic types.
Nodular cast-iron (SG)
The graphite is contained as round nodules.
Magnesium especially is used to deposit
the gobules and added to become a magnesium-nickel
alloy. Tensile strength, toughness
and ductility are considerably improved.
Ferritic, pearlitic and martensitic
types with various tensile strengths occur.
The SG cast-iron is also a graphite structure
with properties in-between that of grey
and nodular cast-iron. The graphite flakes
are compacted into short ones with round
ends through the addition of titanium and
other treatment.
Malleable cast-iron
When white iron is heat treated in a particular
way, ferritic, pearlitic or martensitic
malleable cast-iron is formed. The heat
treatments may turn the cementite into
spherical carbon particles or remove the
carbides. The cast-iron product is malleable,
ductile and very strong. The silicon
content is low. Three categories occur:
ferritic, pearlitic and martensitic and they
may also be categorized as Blackheart,
Whiteheart and pearlitic.
Alloyed cast-iron
These are cast-irons containing larger
amounts of alloying elements and, generally,
these have similar effects on properties of
cast-iron as they do on steel. Alloying elements
are used to improve properties by
affecting structures. Nickel, chromium,
molybdenum, vanadium and copper are
common ones. The graphite-free white
cast-iron is extremely wear resistant while
the graphite-containing cast-iron is also
known as heat resistant ductile cast-iron.
Corrosion resistance is also improved in
some types. Toughness, hardness and heat
resistance are typically improved.
The main difference in these types is the
form in which carbon, mainly graphite
occurs.
The general relative machinability of the
four main kinds of cast-iron is indicated in
a diagram where (A) is grey cast-iron, (B)
malleable, (C) S.G. iron and (D) chilled,
white cast-iron.
100
90
80
70
60
50
40
30
20
10
0
A B C D
Relative machinability
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