SECTION 2. INTRODUCTION
Cast Iron: The Natural Composite
Types of Cast Irons
History of Ductile Iron
The Ductile iron Advantage
The Ductile Iron Family
A Matter of Confidence
The Casting Advantage
The casting process has been used for over 5000 years to produce both objects of art and
utilitarian items essential for the varied activities of civilization. Why have castings
played such a significant role in man's diverse activities? For the artist, the casting
process has provided a medium of expression which not only imposed no restrictions on
shape, but also faithfully replicated every detail of his work, no matter how intricate.
Designers use the same freedom of form and replication of detail to meet the basic goal of
industrial design - the matching of form to function to optimize component performance. In
addition to design flexibility, the casting process offers significant advantages in cost
and materials selection and performance.
The design flexibility offered by
the casting process far exceeds that of any other process used for the
production of engineering components. This flexibility enables the
design engineer to match the design of the component to its function.
Metal can be placed where it is required to optimize the load carrying
capacity of the part, and can be removed from unstressed areas to reduce
weight. Changes in cross-section can be streamlined to reduce stress
concentrations. The result? Both initial and life-cycle costs are
reduced through material and energy conservation and increased component
Designer engineers can
now optimize casting shape and performance with increased speed and
confidence. Recent developments in CAD/CAM, solid modelling and finite
element analysis (FEA) techniques permit highly accurate analyses of
stress distributions and component deflections under simulated operating
conditions. In addition to enhancing functional design, the analytical
capabilities of CAD/CAM have enabled foundry engineers to maximum
casting integrity and reduce production costs through the optimization
of solidification behaviour.
Castings offer cost advantages over fabrications and forgings over a
wide range of production rates, component size and design complexity.
The mechanization and automation of casting processes have substantially
reduced the cost of high volume castings, while new and innovative
techniques such as the use of styrofoam patterns and CAD/CAM pattern
production have dramatically reduced both development times and costs
for prototype and short-run castings. As confidence in FEA techniques
increases, the importance of prototypes, often in the form of
fabrications which "compromise" the final design, will
decrease and more and more new components will go directly from the
design stage to the production casting. As shown in Figure
2. 1, as component size and complexity increase, the cost per unit
of weight of fabricated components can rise rapidly, while those of
castings can actually decrease due to the improved castability and
higher yield of larger castings. Near net shape casting processes and
casting surface finishes in the range 50-500 microinches minimize
component production costs by reducing or eliminating machining
Replacement of a
multi-part, welded and/or fastened assembly by a casting offers
significant savings in production costs. Inventory costs are reduced,
close-tolerance machining required to fit parts together is eliminated,
assembly errors cannot occur, and engineering, inspection and
administrative costs related to multi-part assemblies are reduced
significantly. A recent study by the National Center for Manufacturing
Sciences (NCMS) has shown that in certain machine tool applications, the
replacement of fabricated structures by Ductile Iron castings could
result in cost savings of 39-50%. Commenting on the NCMS study, Mr. Gary
Lunger, President of Erie Press Inc., stated:
"We make huge
presses and we have relatively clear specifications for what goes into
each press. We have been able to use Ductile Iron as a substitute
material primarily for cylinders and other parts at a significant cost
saving over cast or fabricated steel."
Castings offer advantages over
forgings in isotropy of properties and over fabrications in both
isotropy and homogeneity. The deformation processes used to produce
forgings and plate for fabrications produce laminations which can result
in a significant reduction in properties in a direction transverse to
the lamination. In fabricated components, design complexity is usually
achieved by the welding of plate or other wrought shapes. This method of
construction can reduce component performance in two ways. First,
material shape limitations often produce sharp corners which increase
stress concentrations, and second, the point of shape change and stress
concentration is often a weld, with related possibilities for material
weakness and stress-raising defects. Figure
2.2 shows the results of stress analysis of an acrylic joint model
in which the stress concentration factor for the weld is substantially
higher than for a casting profiled to minimize stress concentration.
Iron: The Natural Composite
Iron castings, as objects of art,
weapons of war, or in more utilitarian forms, have been produced for more
than 2000 years. As a commercial process, the production of iron castings
probably has no equal for longevity, success or impact on our society. In
a sense, the iron foundry industry produces an invisible yet vital
product, since most iron castings are further processed, assembled, and
then incorporated as components of other machinery, equipment, and
The term "cast
iron" refers not to a single material, but to a family of materials
whose major constituent is iron, with important amounts of carbon and
silicon, as shown in Figure 2.3. Cast
irons are natural composite materials whose properties are determined by
their microstructures - the stable and metastable phases formed during
solidification or subsequent heat treatment. The major microstructural
constituents of cast irons are: the chemical and morphological forms taken
by carbon, and the continuous metal matrix in which the carbon and/or
carbide are dispersed. The following important microstructural components
are found in cast irons.
This is the stable form of pure
carbon in cast iron. Its important physical properties are low density,
low hardness and high thermal conductivity and lubricity. Graphite shape,
which can range from flake to spherical, plays a significant role in
determining the mechanical properties of cast irons. Figures
2.4 and 2.5 show that graphite
flakes act like cracks in the iron matrix, while graphite spheroids act
like "crackarresters", giving the respective irons dramatically
different mechanical properties.
Carbide, or cementite, is an
extremely hard, brittle compound of carbon with either iron or strong
carbide forming elements, such as chromium, vanadium or molybdenum.
Massive carbides increase the wear resistance of cast iron, but make it
brittle and very difficult to machine. Dispersed carbides in either
lamellar or spherical forms play in important role in providing strength
and wear resistance in as-cast pearlitic and heat-treated irons.
This is the purest iron phase in a
cast iron. In conventional Ductile Iron ferrite produces lower strength
and hardness, but high ductility and toughness. In Austempered Ductile
Iron (ADI), extremely fine-grained accicular ferrite provides an
exceptional combination of high strength with good ductility and
Pearlite, produced by a eutectoid reaction, is an intimate mixture of
lamellar cementite in a matrix of ferrite. A common constituent of cast
irons, pearlite provides a combination of higher strength and with a
corresponding reduction in ductility which meets the requirements of many
Martensite is a supersaturated
solid solution of carbon in iron produced by rapid cooling. In the
untempered condition it is very hard and brittle. Martensite is
normally "tempered" - heat treated to reduce its carbon content
by the precipitation of carbides - to provide a controlled combination of
high strength and wear resistance.
Normally a high temperature phase
consisting of carbon dissolved in iron, it can exist at room temperature
in austenitic and austempered cast irons. In austenitic irons, austenite
is stabilized by nickel in the range 18-36%. In austempered irons,
austenite is produced by a combination of rapid cooling which suppresses
the formation of pearlite and the supersaturation of carbon during
austempering, which depresses the start of the austenite-to-martensite
transformation far below room temperature. In austenitic irons, the
austenite matrix provides ductility and toughness at all temperatures,
corrosion resistance and good high temperature proper-ties, especially
under thermal cycling conditions. In austempered Ductile Iron stabilized
austenite, in volume fractions up to 40% in lower strength grades,
improves toughness and ductility and response to surface treatments such
as fillet rolling.
Bainite is a mixture of ferrite and carbide, which is produced by alloying
or heat treatment.
of Cast Irons
The presence of trace elements, the
addition of alloying elements, the modification of solidification
behaviour, and heat treatment after solidification are used to change the
microstructure of cast iron to produce the desired mechanical properties
in the following common types of cast iron.
White Iron is fully carbidic in its
final form. The presence of different carbides, produced by alloying,
makes White Iron extremely hard and abrasion resistant but very brittle.
Gray Iron is by far the oldest and
most common form of cast iron. As a result, it is assumed by many to be
the only form of cast iron and the terms "cast iron" and
"gray iron" are used interchangeably. Gray Iron, named
because its fracture has a gray appearance, consists of carbon in the form
of flake graphite in a matrix consisting of ferrite, pearlite or a mixture
of the two. The fluidity of liquid gray iron, and its expansion during
solidification due to the formation of graphite, have made this metal
ideal for the economical production of shrinkage-free, intricate castings
such as motor blocks.
The flake-like shape of
graphite in Gray Iron, see Figure 2.4,
exerts a dominant influence on its mechanical properties. The graphite
flakes can act as stress raisers which may prematurely cause localized
plastic flow at low stresses, and initiate fracture in the matrix at
higher stresses. As a result, Gray Iron exhibits no elastic behaviour and
fails in tension without significant plastic deformation. The presence of
graphite flakes also gives Gray Iron excellent machinability, damping
characteristics and self-lubricating properties.
Unlike Gray and Ductile Iron,
Malleable Iron is cast as a carbidic or white iron and an annealing or
"malleablizing" heat treatment is required to convert the
carbide into graphite. The microstructure of Malleable Iron consists of
irregularly shaped nodules of graphite called "temper carbon" in
a matrix of ferrite and/or pearlite. The presence of graphite in a more
compact or sphere-like form gives Malleable Iron ductility and strength
almost equal to cast, low-carbon steel. The formation of carbide during
solidification results in the conventional shrinkage behaviour of
Malleable Iron and the need for larger feed metal reservoirs, causing
reduced casting yield and increased production costs.
of Ductile Iron Development
In spite of the progress achieved during the first half of this century in
the development of Gray and Malleable Irons, foundrymen continued to
search for the ideal cast iron - an as-cast "gray iron" with
mechanical properties equal or superior to Malleable Iron. J.W. Bolton,
speaking at the 1943 Convention of the American Foundrymen's Society (AFS),
made the following statements.
"Your indulgence is
requested to permit the posing of one question. Will real control of
graphite shape be realized in gray iron? Visualize a material, possessing
(as-cast) graphite flakes or groupings resembling those of malleable iron
instead of elongated flakes."
A few weeks later, in the
International Nickel Company Research Laboratory, Keith Dwight Millis made
a ladle addition of magnesium (as a copper-magnesium alloy) to cast iron
and justified Bolton's optimism - the solidified castings contained not
flakes, but nearly perfect spheres of graphite. Ductile Iron was born!
Five years later, at the
1948 AFS Convention, Henton Morrogh of the British Cast Iron Research
Association announced the successful production of spherical graphite in
hypereutectic gray iron by the addition of small amounts of cerium.
At the time of Morrogh's
presentation, the International Nickel Company revealed their development,
starting with Millis' discovery in 1943, of magnesium as a graphite
spherodizer. On October 25, 1949, patent 2,486,760 was granted to the
International Nickel Company, assigned to Keith D. Millis, Albert P.
Gegnebin and Norman B. Pilling. This was the official birth of Ductile
Iron, and, as shown in Figure 2.6, the
beginning of 40 years of continual growth worldwide, in spite of
recessions and changes in materials technology and usage. What are the
reasons for this growth rate, which is especially phenomenal, compared to
other ferrous castings?
Ductile Iron Advantage
The advantages of Ductile Iron
which have led to its success are numerous, but they can be summarized
easily - versatility, and higher performance at lower cost. As illustrated
in Figure 2.7, other members of the
ferrous casting family may have individual properties which might make
them the material of choice in some applications, but none have the
versatility of Ductile Iron, which often provides the designer with the
best combination of overall properties. This versatility is especially
evident in the area of mechanical properties where Ductile Iron offers the
designer the option of choosing high ductility, with grades guaranteeing
more than 18% elongation, or high strength, with tensile strengths
exceeding 120 ksi (825 MPa). Austempered Ductile Iron (ADI), offers even
greater mechanical properties and wear resistance, providing tensile
strengths exceeding 230 ksi (1600 MPa).
In addition to the cost
advantages offered by all castings, Ductile Iron, when compared to
steel and Malleable Iron castings, also offers further cost savings. Like
most commercial cast metals, steel and Malleable Iron decrease in volume
during solidification, and as a result, require attached reservoirs
(feeders or risers) of liquid metal to offset the shrinkage and prevent
the formation of internal or external shrinkage defects. The formation of
graphite during solidification causes an internal expansion of Ductile
Iron as it solidifies and as a result, it may be cast free of significant
shrinkage defects either with feeders that are much smaller than those
used for Malleable Iron and steel or, in the case of large castings
produced in rigid molds, without the use of feeders. The reduction or
elimination of feeders can only be obtained in correctly design castings.
This reduced requirement for feed metal increases the productivity of
Ductile Iron and reduces its material and energy requirements, resulting
in substantial cost savings. The use of the most common grades of Ductile
Iron "as-cast" eliminates heat treatment costs, offering a
Ductile Iron Family
Ductile Iron is not a single
material, but a family of materials offering a wide range of properties
obtained through microstructure control. The common feature that all
Ductile Irons share is the roughly spherical shape of the graphite
nodules. As shown in Figure 2.5, these
nodules act as "crack-arresters and make Ductile Iron
"ductile". This feature is essential to the quality and
consistency of Ductile Iron, and is measured and controlled with a high
degree of assurance by competent Ductile Iron foundries. With a high
percentage of graphite nodules present in the structure, mechanical
properties are determined by the Ductile Iron matrix. Figure
2.8 shows the relationship between microstructure and tensile strength
over a wide range of properties. The importance of matrix in controlling
mechanical properties is emphasized by the use of matrix names to
designate the following types of Ductile Iron.
Ferritic Ductile Iron
Graphite spheroids in a matrix of
ferrite provides an iron with good ductility and impact resistance and
with a tensile and yield strength equivalent to a low carbon steel.
Ferritic Ductile Iron can be produced "as-cast" but may be given
an annealing heat treatment to assure maximum ductility and low
Ferritic Pearlitic Ductile
These are the most common
grade of Ductile Iron and are normally produced in the "as cast"
condition. The graphite spheroids are in a matrix containing both
ferrite and pearlite. Properties are intermediate between ferritic
and pearlitic grades, with good machinability and low production costs.
Pearlitic Ductile Iron
Graphite spheroids in a matrix of
pearlite result in an iron with high strength, good wear resistance, and
moderate ductility and impact resistance. Machinability is also superior
to steels of comparable physical properties.
The preceding three types
of Ductile Iron are the most common and are usually used in the as-cast
condition, but Ductile Iron can be also be alloyed and/or heat treated to
provide the following grades for a wide variety of additional
Martensitic Ductile Iron
Using sufficient alloy additions to
prevent pearlite formation, and a quench-and-temper heat treatment
produces this type of Ductile Iron. The resultant tempered
martensite matrix develops very high strength and wear resistance but with
lower levels of ductility and toughness.
Bainitic Ductile Iron
This grade can be obtained through alloying and/or by heat treatment to
produce a hard, wear resistant material.
Austenitic Ductile Iron
Alloyed to produce an austenitic matrix, this Ductile Iron offers good
corrosion and oxidation resistance, good magnetic properties, and good
strength and dimensional stability at elevated temperatures. The unique
properties of Austenitic Ductile Irons are treated in detail in Section
Austempered Ductile Iron (ADI)
ADI, the most recent addition to
the Ductile Iron family, is a sub-group of Ductile Irons produced by
giving conventional Ductile Iron a special austempering heat treatment.
Nearly twice as strong as pearlitic Ductile Iron, ADI still retains high
elongation and toughness. This combination provides a material with
superior wear resistance and fatigue strength. (See Section
Matter of Confidence
The automotive industry has
expressed its confidence in Ductile Iron through the extensive use of this
material in safety related components such as steering knuckles and brake
calipers. These and other automotive applications, many of which are used
"as-cast", are shown in Figure 2.9.
One of the most critical materials applications in the world is in
containers for the storage and transportation of nuclear wastes. The
Ductile Iron nuclear waste container shown in Figure
2.10 is another example of the ability of Ductile Iron to meet
and surpass even the most critical qualification tests for materials
performance. These figures show the wide variety of parts produced
in Ductile Iron. The weight range of possible castings can be
from less than one ounce (28 grams) to more than 200 tons. Section
size can be as small as 2 mm to more than 20 inches (1/2 meter) in
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