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Ductile Iron Data

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  1. [1]
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    Ductile Iron Data

    FORWARD
    Over forty years ago, the birth of a new engineering material, Ductile Iron, was announced at the 1948 American Foundrymen's Society Annual Conference. Looking back on the first four decades of Ductile Iron reveals the classical pattern of the research, development and commercialization of a new material. In the early years INCO, the patent holder, introduced Ductile Iron to designers and engineers by distributing technical literature and conducting seminars. As knowledge of the properties and economies of Ductile Iron spread, its usage increased dramatically throughout the fifties and early sixties. After the termination of INCO's promotion of Ductile Iron in 1966, Ductile Iron market growth continued to outperform other ferrous castings but, as the engineers and designers who benefited from the early promotional efforts of INCO retired and were replaced by a new generation, the knowledge gap about Ductile Iron began to widen.
    During the past decade the development and commercialization of austempered Ductile Iron (ADI) has added a new star to the Ductile Iron family. Combining the strength, ductility, fracture toughness and wear resistance of a steel with the castability and production economies of a conventional Ductile Iron, ADI offers the designer an exceptional opportunity to create superior components at reduced cost. Only one factor has detracted from this story of forty years of Ductile Iron technology - the promotion of this material to designers has been a poor second to its technical development. In fact,, the lack of knowledge and understanding among some potential users about the properties and uses of Ductile Iron is astounding.
    In 1985 QIT-Fer et Titane and Miller & Company, two suppliers to the Ductile Iron foundry industry, recognized that a lack of engineering data was inhibiting the sales of Ductile Iron castings. To remedy this lack of information, QIT and Miller & Company formed the Ductile Iron Group (DIG). For the past five years, the DIMG, which also includes the Ductile Iron Society, have conducted market surveys to identify the informational needs of designers and engineers and have addressed these needs through the publication of technical literature and the presentation of technical lectures and seminars.
    "Ductile Iron Data for Design Engineers" (revised edition), produced by Rio Tinto Iron & Titanium for distribution by the Ductile Iron Marketing Group, will help to overcome the lack of information which has persisted, even after forty years of Ductile Iron production. By informing designers and engineers about the impressive mechanical properties and economic advantages of Ductile Iron and ADI, this book should be of significant benefit to both users and producers of this remarkable material.
    Keith D. Millis (deceased)
    Formerly Executive Director
    Ductile Iron Society & Co founder of Ductile Iron


    World's largest Ductile Iron casting produced to date: crosshead for pipe press.
    Net casting weight 230 tonnes. Length: 14m, height: 2m and width: 2.5m.

    This casting contains 80 tonnes of Sorelmetal in order to obtain the excellent
    properties required in the heaviest sections


  2. [2]
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    The Ductile Iron

    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
    .
    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.


    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."




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  3. [3]
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    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.

    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

    .
    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.





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    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 ******* 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.


    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.

    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    INTRODUCTION
    Figure 2.4 Micrograph of Gray Iron showing crack-like behaviour of graphite flakes.


    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    INTRODUCTION
    Figure 2.5 Micrograph of Ductile Iron showing how graphite spheroids can act as "crack-arresters."


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    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?
    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    INTRODUCTION
    Figure 2.6 Worldwide growth of Ductile Iron production, 1950-2000.


    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.
    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    INTRODUCTION
    Figure 2.7 Comparison of the engineering characteristics of Ductile iron versus competitive ferrous cast materials.




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    The Ductile Iron


    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).
    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    INTRODUCTION
    Figure 2.8 Microstructures and tensile strengths for various types of Ductile Iron.
    MatrixFerritic
    Grade 5
    Ferritic-
    pearlitic
    Grade 3
    Pearlitic
    Grade 1
    Martensitic
    (With
    retained
    austenite)
    Tempered
    Martensitic
    ADI
    Grade 150
    ADI
    Grade 230
    Austenitic60,000 p.s.i.
    (414 mPa)
    80,000 p.s.i.
    (552 mPa)
    100,000 p.s.i.
    (690 mPa)
    N.A.*115,000 p.s.i.
    (793 mPa)
    150,000 p.s.i.
    (1050 mPa)
    230,000 p.s.i.
    (1600 mPa)
    45,000 p.s.i.
    (310 mPa)
    *Approximate ultimate tensile strength 87,000 p.s.i. (600 mPa) Hard, Brittle. (Note that the magnifications are different.)



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    The Ductile Iron


    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.

    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.
    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    INTRODUCTION
    Figure 2.9 Examples of typical Ductile Iron castings used in a modern automobile.

    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    INTRODUCTION
    Figure 2.10 Ductile Iron nuclear waste container weighing 85 tons.

    Courtesy: Siempelkamp, Krefeld, Federal Republic of Germany.

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    Tensile Properties cont

    Introduction
    Ductile Iron is not a single material, but a family of versatile cast irons exhibiting a wide range of properties which are obtained through microstructure control. The most important and distinguishing microstructural feature of all Ductile Irons is the presence of graphite nodules which act as "crack-arresters" and give Ductile Iron ductility and toughness superior to all other cast irons, and equal to many cast and forged steels. As shown in Figure 2.8, Section II, the matrix in which the graphite nodules are dispersed plays a significant role in determining mechanical properties.
    Matrix control, obtained in conventional Ductile Iron either "as-cast" through a combination of composition and process control, or through heat treatment, gives the designer the option of selecting the grade of Ductile Iron which provides the most suitable combination of properties. Figure 3.1 illustrates the wide range of strength, ductility and hardness offered by conventional Ductile Iron. The high ductility ferritic irons shown on the left provide elongation in the range 18-30 per cent, with tensile strengths equivalent to those found in low carbon steel. Pearlitic Ductile Irons, shown on the right side, have tensile strengths exceeding 120 ksi (825 MPa) but reduced ductility. Austempered Ductile Iron (ADI), discussed in Section IV, offers even greater mechanical properties and wear resistance, with ASTM Grades providing tensile strengths exceeding 230 ksi (1600 MPa). Special Alloy Ductile Irons, described in Section V, can be selected to provide creep and oxidation resistance at high temperatures, resistance to thermal cycling, corrosion resistance, special magnetic properties, or low temperature toughness.
    The numerous, successful uses of Ductile Iron in critical components in all sectors of industry highlight its versatility and suggest many additional applications. In order to use Ductile Iron with confidence, the design engineer must have access to engineering data describing the following mechanical properties: elastic behaviour, strength, ductility, hardness, fracture toughness and fatigue properties. Physical properties - thermal expansion, thermal conductivity, heat capacity, density, and magnetic and electrical properties - are also of interest in many applications. This Section describes the mechanical and physical properties of conventional Ductile Irons, relates them to microstructure, and indicates how composition and other production parameters affect properties through their influence on microstructure.
    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    ENGINEERING DATA
    Figure 3.1 General relationships between tensile properties and hardness for Ductile Iron.



    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    INTRODUCTION
    Figure 2.8 Microstructures and tensile strengths for various types of Ductile Iron.
    MatrixFerritic
    Grade 5
    Ferritic-
    pearlitic
    Grade 3
    Pearlitic
    Grade 1
    Martensitic
    (With
    retained
    austenite)
    Tempered
    Martensitic
    ADI
    Grade 150
    ADI
    Grade 230
    Austenitic60,000 p.s.i.
    (414 mPa)
    80,000 p.s.i.
    (552 mPa)
    100,000 p.s.i.
    (690 mPa)
    N.A.*115,000 p.s.i.
    (793 mPa)
    150,000 p.s.i.
    (1050 mPa)
    230,000 p.s.i.
    (1600 mPa)
    45,000 p.s.i.
    (310 mPa)
    *Approximate ultimate tensile strength 87,000 p.s.i. (600 mPa) Hard, Brittle. (Note that the magnifications are different.)

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    Tensile Properties

    Tensile Properties
    The tensile properties of conventional Ductile Iron, especially the yield and tensile strengths and elongation, have traditionally been the most widely quoted and applied determinants of mechanical behaviour. Most of the world-wide specifications for Ductile iron summarized in Section XII describe properties of the different grades of Ductile Iron primarily by their respective yield and tensile strengths and elongation. Hardness values, usually offered as additional information, and impact properties, specified only for certain ferritic grades, compolete most specifications. Although not specified, the modulus of elasticity and proportional
    limit are also vital design criteria. Figure 3.2

    illustrates a generalized engineering stress-strain curve describing the tensile properties of ductile engineering materials.

    Modulus of Elasticity

    Figure 3.2 shows that, at low tensile stresses, there is a linear or proportional relationship between stress and strain. This relationship is known as Hooke's Law and the slope of the straight line is called the Modulus of Elasticity or Young's Modulus. As shown in Figure 3.3, the initial stress-strain behaviour of Ductile Iron lies between those of mild steel and Gray Iron. Annealed or normalized mild steels exhibit elastic behaviour until the yield point, where plastic deformation occurs suddenly and without any initial increase in flow stress. In Gray Iron, the graphite flakes act as stress-raisers, initiating microplastic deformation at flake tips at very low applied stresses. This plastic deformation causes the slope of the stress-strain curve to decrease continually and as a result Gray Iron does not exhibit true elastic behaviour.
    Ductile Iron exhibits a proportional or elastic stress-strain relationship similar to that of steel but which is limited by the gradual onset of plastic deformation. The Modulus of Elasticity for Ductile Iron, measured in tension, varies from 23.5 to 24.5 x 106 psi(162 - 170 GPa). In cantilever, three point beam or torsion testing, values as low as 20.5 x 106 have been reported. The Dynamic Elastic Modulus (DEM), the high frequency limit of the Modulus of Elasticity measured by the resonant frequency test, exhibits a range of 23.5 to 27 x 106 psi (162 - 186 GPa).
    DUCTILE IRON DATA
    FOR DESIGN ENGINEERS
    ENGINEERING DATA
    Figure 3.3 Elastic and yielding behavior for steel, Gray Iron and ferritic and pearlitic Ductile Irons.


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    Common Metallurgical Defects in Ductile Cast Iron


    C.M.Ecob
    Customer Services Manager, Elkem AS, Foundry Products Division


    Abstract
    The objective of this paper is to provide an overview of some of the most common metallurgical defects found in the production of ductile cast iron today. The examples shown have all been determined during the examination of samples in Elkem’s Research facility in Norway. Whilst many foundries recognise the defects, an appreciation of the possible causes, and therefore cures, is not always apparent. The causes and cures for the different problems are examined in the paper. Emphasis is made on shrinkage problems, probably the most common problem seen by Elkem’s team of service engineers around the world.

    Introduction
    Metallurgical defects in ductile iron can be very costly to the foundry, not only because the part has to be remade or rectified, but due to the unfortunate fact that many defects are not revealed until after the expensive machining stage. Care in the selection of raw materials, good process control in the melting stage and proper metal handling procedures will go a long way to the prevention of defects. Further, a routine for logging and recording of defect occurrences will reveal which are the major problem areas, allowing for a systematic elimination of the defects. This paper will examine the most common defects, starting with shrinkage. Deterioration of affordable steel scrap qualities, use of incorrect inoculants and nodularisers plus the pressures to get castings out of the door as fast as possible has led to an increase in the incidences of shrink/porosity related cases seen by Elkem’s team of technical service engineers. Indeed, the ductile iron foundry, which truthfully claims not to have shrinkage concerns is the exception to the rule.

    Other common defects may be divided into two basic categories:
    - Those related to nodule shape and size, such as compacted graphite structures, exploded and chunky graphite,
    graphite floatation, spiky graphite and nodule alignment.
    - Those related to inclusions/abnormalities within the matrix, such as flake graphite surfaces, slag inclusions,
    carbides and gas. These problem areas are described to aid recognition of the defect and causes are discussed
    together with possible cures.

    Shrinkage Control
    There are many causes of shrinkage in ductile iron, experience globally has shown that about 50% of shrinkage defects are related to sand systems, feeding and gating. The other 50% may be attributed to metallurgical factors such as carbon equivalent, temperature, inoculation or high magnesium residuals.



    Figure 1: Typical sub-surface shrinkage defect with dendrite arms partly covered with graphite sticking out.

    When a shrink or porosity is detected in a casting, there are several immediate and simple steps that can be taken to identify the cause of the problem.
    Firstly, the geometry of the casting should be examined to determine whether the location of the defect is close to a sharp radius or a potential hot spot. At the same time, the sand in the region of the shrink should be examined to look for any soft spots. Sand integrity accounts for a high proportion of shrinkage defects and a worn seal on the moulding machine, for example, resulting in a lower sand compaction can often be the cause of an unexplained sudden outbreak of shrinkage.
    The second avenue of investigation should be the gating / runner designs and the feeding of the casting. Whilst many foundries have computer aided design systems, patterns are often altered slightly over the years at shop floor level and can be significantly different from the original design. Also, changes to the feeder specification can lead to different burn characteristics and metal solidification patterns. This can affect the amounts of feed metal available to different parts of the casting.
    Metallurgically, there are many factors that can affect the shrinkage tendency.
    Figure 2: Effect of magnesium ******* on shrinkage
    Magnesium, apart from being one of the most powerful carbide stabilizers, has a marked effect on the shrinkage tendency of ductile irons. Foundries operating at the higher end of the magnesium range, 0.05% or above, will find that the iron is more prone to shrink than foundries operating at lower, but very acceptable, levels, say 0.035-0.04%. Both under-inoculation and over-inoculation can cause shrinkage. In the case of under- inoculation, not enough dissolved carbon is precipitated as graphite. Graphite nodules have a far lower density than the matrix and to precipitate the low density, high volume graphite has an overall expansion effect, which helps to counter the natural tendency of the iron to shrink. With over-inoculation, too many nucleation points are active early in the solidification, resulting in an early expansion and sometimes large mould wall movements. Later in the solidification, when feeders become inactive and contraction takes place, there is no graphite coming out from solution to counteract the contraction and the result is shrinkage between the eutectic cells. In many foundries, the microstructure shows even sized nodules (accounting for the fact that the section cuts through nodules in 2-dimensions). Many foundry men still consider this to be a good structure, even though the iron is prone to shrinkage. Nodularizers and specialist inoculants are available these days, which help to counter shrinkage by giving a skewed nodule distribution.
    Figure 3: The same base iron treated with two different nodularisers resulting in a) Skewed nodule distribution b) Unskewed nodule distribution
    A skewed nodule distribution indicates that some nodules are being created late in the solidification process and the drawing of graphite from solution at this stage is a very effective way to counter shrink. Most inoculants act almost instantaneously and this gives the even nodule size effect. Once the potency of the inoculant has gone, then there is no driver to create nodules late in the solidification and shrinkage can be the result. More recently, nodularisers have been developed by Elkem that have the same effect of producing the skewed and shrink reducing nodule distribution curve.
    A low carbon equivalent, or metal that has been held for some time at temperature, due to a mechanical breakdown, for example, is also prone to shrinkage. In these cases, the inherent nuclei within the melt will be low and some preconditioning may be necessary to achieve a good level of nucleation.

    Compacted Graphite within the structure. There are several causes of this, the most common being that the nodularisation process has partly failed. Incorrect weighing of the nodulariser or the use of the wrong nodulariser are possible reasons for the failure, although a long holding time in the ladle or excessive temperatures can be contributory factors.
    Figure 4: Sample with compacted graphite present in the matrix due to partly failed

    Nodularisation process
    Another cause of CG particles in the matrix is an incorrect sulphur level in the base iron. Many foundries melt both grey and ductile charges and segregation of returns is essential. During the nodularisation process, the first reactions that take place are a desulphurisation and deoxidation, these elements combining preferentially with the magnesium. The base sulphur level must be accounted for in the calculation of MgFeSi charge weight. A note of caution here with regard to the addition of the MgFeSi to the ladle or treatment vessel. To add the MgFeSi early to a hot ladle and then hold the ladle for several minutes until the moulding line calls for metal is bad practise as the alloy will be burning or oxidising in the bottom of the ladle during this time. Higher and more consistent recoveries can easily be achieved by adding the alloy just before tap from the furnace.

    Low Nodule Count
    As the compacted graphite mentioned above may commonly be attributed to the nodulariser, then low nodule counts tend to be a function of the inoculant. Figure 5 shows a low count compared to the foundry’s normal practise. Avoiding long holding times in the furnace and prolonged pouring time post-inoculation will help to achieve consistent nodule counts, as will improving the responsiveness of the iron via preconditioning. The use of a specialist powerful inoculant will give the most consistent results.


    Figure 5: Two casting with the same metal treatment resulting in a) low nodule count due to long pouring time and b) normal nodule count with normal practise.

    Exploded graphite
    Characteristically, exploded graphite looks exactly as the name might suggest that the graphite has been blown apart. Most MgFeSi alloys contain some rare earth metals, cerium, lanthanum, neodymium, praesodimium etc and these are beneficial in that they neutralise the effects of some detrimental tramp elements such as lead, bismuth, antimony, titanium etc..Rare earth elements are also nodularisers and aid the effects of the magnesium. In excess, however, rare earths can cause exploded graphite. This is more especially when high purity charges are used which are low in tramp elements. Exploded graphite is normally found in thicker section castings with slow cooling rates or at very high carbon equivalent levels.


    Figure 6: Sample with exploded graphite present due to excess concentration of rare earth metals.
    Care should be taken when using induction melting as rare earths can be cumulative in the iron. They tend to have very high melting points and do not volatilise, although some will be oxidised and come out in the slag. This is important to note if a low/zero RE containing nodulariser is substitutes to eliminate the problem as it may take time to dilute the residual RE out of the system. Should exploded graphite occur, then examination of the rare earth sources should be made – normally the MgFeSi. Melting a virgin charge with steel scrap, pig iron and no returns will quickly show if the returns and/or the MgFeSi are the problem. Latin America and countries in the Far East tend to use high levels of rare earth in the nodulariser. Reductions in the carbon equivalent may help to reduce exploded graphite.

    Chunky graphite
    The causes of chunky graphite are exactly the same as for exploded graphite with the addition that the defect is also found in thinner casting sections and is not as sensitive to the carbon equivalent as exploded graphite.


    Figure 7: Sample with chunky graphite present due to excess concentration of rare earth metals.


    Graphite floatation
    This is caused when large, low density graphite nodules are formed during the solidification of thick section or otherwise slow cooling castings. The nodules, being of a lower density than the matrix, tend to float towards the surface of the casting and thus can have a negative effect on the mechanical properties (and surface finish) in that region. A reduction in the carbon equivalent will help to control this, as will a reduction in the pouring temperature or increasing the cooling rate of the casting by the use of chills. The inoculation system should also be examined, as it is likely that the large graphite nodules have been formed very early during the solidification process and an inoculant, which will generate more, smaller nodules, could be an advantage.

    Figure 8: Sample with graphite floatation present due to high carbon equivalent.
    Nodule Alignment
    This is caused by large dendrites growing during the solidification with the nodules being precipitated between the dendrite arms. Thus the nodules appear to be aligned. Whilst not normally a serious problem, this can have detrimental effects on such properties as tensile strength or impact resistance. The normal causes are low carbon equivalent where not enough graphite is precipitated during the cooling, under inoculation or too high a pouring temperature.


    Figure 9: Sample with nodule alignment caused by large dendrites growing during the solidification with the nodules being precipitated between the dendrite arms.

    Spiky Graphite
    The occurrence of spiky graphite in ductile iron is rare provided that the nodulariser used contains a small amount of rare earths. Normally, the rare earth metals neutralise such elements as lead, bismuth, titanium and antimony, as discussed in the section on exploded graphite, however the use of a rare earth-free nodulariser where traces of the deleterious elements are present results in spiky graphite. This is most commonly found in converter iron where the separate additions of RE have been left out by human error. The effect of spiky graphite is a dramatic reduction in the mechanical properties of the iron, the spikes provide points of weakness in the structure. The only cure for this type of defect is the addition of rare earths with the nodulariser.




    Figure 10 Sample with spiky graphite present in the matrix due to too elevated level of Pb.
    Flake Graphite on the Casting Surface This is commonly seen in foundries, however many ignore the flake graphite on the surface as it forms part of the machining allowance. The defect is illustrated in Figure 11 and clearly shows the thin layer of flake graphite adjacent to the mould. This is found mainly in greensand systems and is caused by a build up of sulphur in the sand, which reacts with the magnesium in the iron to form magnesium sulphides and effectively de-nodularise the iron. A higher Mg or Re in the nodulariser can overcome this, subject to shrinkage restrictions discussed earlier, but the most common remedy is to use an inoculant containing cerium. This has the effect of re-nodularising the iron locally.

    Figure 11 Sample with flake graphite on the surface of the casting due to high sulphur ******* in he moulding sand.
    Carbides In the production of ductile iron, it must be remembered that magnesium is one of the most powerful carbide promoters. Coupled with this, the violence of the magnesium reaction during the nodularisation process tends to destroy nuclei. For these reasons, inoculation requirements are heavier than for grey irons and under-inoculation or the use of the wrong inoculant are amongst the most common causes of chill or carbides in ductile iron. Poor inoculation is not the only cause of carbides, however, and all the potential reasons need to be explored to determine the reason behind carbide formation.


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