DUCTILE IRON DATA FOR DESIGN ENGINEERS

 

 

 

Home Page

SECTION 2.  INTRODUCTION

The Casting Advantage
Design Flexibility
Reduced Costs
Materials Advantages
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
References

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.

Back to Top

Design Flexibility
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 performance.

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.

Back to Top

Reduced Costs
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 operations.

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."

Back to Top

Materials Advantages
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.

Back to Top

Cast 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 consumer items.

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.

Graphite
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
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.

Ferrite
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 toughness.

Pearlite
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 engineering applications.

Martensite
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.

Austenite
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
Bainite is a mixture of ferrite and carbide, which is produced by alloying or heat treatment.

Back to Top

Types 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
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
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.

Malleable Iron
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.

Back to Top

History 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?

Back to Top

The 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 further advantage.

Back to Top

The 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 temperature toughness.

Ferritic Pearlitic Ductile Iron
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 applications.

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 V.

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 IV).

Back to Top

A 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 thickness.

Back to Top

References
S. Jeffreys, "Finite Element Analysis - Doing Away with Prototypes", Industrial Computing, September, 1988, pp 34-36.

"NCMS Study Reveals DI Castings May Mean Cost Savings." Modem Casting, May, 1990, p 12.

Jay Janowak, "The Grid Method of Cast Iron Selection". Casting Design and Application, Winter 1990, pp 55-59.

D. P. Kanicki, "Marketing of Ductile Iron," Modern Casting, April, 1988.

A Design Engineer's Digest of Ductil2 Iron, 5th Edition, 1983, QIT-Fer et Titane Inc., Montreal, Quebec, Canada.

S. I. Karsay, Ductile Iron II, Quebec Iron and Titanium Corporation, 1972.

B. L. Simpson, History of the Metalcasting Industry, American Foundrymen's Society.
Des Plaines, IL, 1969.

H. Bornstein, "Progress in Iron Castings", The Charles Edgar Hoyt Lecture,
Transactions of the American Foundrymen's Society, 1957, vol 65, p 7.

G.J. Marston "Better cast than fabricated", The Foundryman, March 1990, 108-113.

Back to Top