DUCTILE
IRON ALLOY CASTINGS
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A
Guide DUCTILE IRON ALLOY CASTINGS
Ductile iron
is characterized by having all of its graphite occur in
microscopic spheroids. Although this graphite constitutes
about 10% by volume of ductile iron, its compact spherical
shape minimizes the effect on mechanical properties. The
graphite in commercially produced ductile iron is not always
in perfect spheres. It can occur in a somewhat irregular
form, but if it is still chunky as Type II in ASTM Standard
A247, the properties of the iron will be similar to cast
iron with spheroidal graphite. Of course, further degradation
can influence mechanical properties. The shape of the graphite
is established when the metal solidifies, and it cannot
be changed in any way except by remelting the metal.
The difference
between the various grades of ductile iron is in the microstructure
of the metal around the graphite, which is called the matrix.
This microstructure varies with composition and the cooling
rate of the casting. It can be slowly cooled in the sand
mold for a minimum hardness as-cast or, if the casting has
sufficiently uniform sections, it can be freed of molding
sand while still at a temperature above the critical and
normalized.
The
matrix structure and hardness also can be changed by heat
treatment. The high ductility grades are usually annealed
so that the matrix structure is entirely carbon-free ferrite.
The intermediate grades are often used in the as-cast condition
without heat treatment and have a matrix structure of ferrite
and pearlite. The ferrite occurs as rings around the graphite
spheroids. Because of this, it is called bulls-eye ferrite.
The high strength grades are usually given a normalizing
heat treatment to make the matrix all pearlite, or they
are quenched and tempered to form a matrix of tempered martensite.
However, ductile iron can be moderately alloyed to have
an entirely pearlitic matrix as-cast.
Learn
a whole lot more
by reading our complete Ductile Iron
Casting Guide. It's in a PDF, for easy downloading
& printing.
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Iron Alloy Casting Guide |
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