Introduction
In the past 40 years the use of Ductile Iron has grown rapidly, mainly through conversions from Gray and Malleable Iron castings and steelcastings, forgings and fabrications but also through its use in new components designed with Ductile Iron. Ductile Iron has been successful because it has offered the design engineer a combination of versatility and properties not available in any of its rivals. Its castability, machinability, damping properties, and economy of production are almost equal to those for which Gray Iron is famous, but its mechanical properties - strength, wear resistance, fatigue strength, toughness and ductility are competitive with many cast, forged and fabricated steel components. The conversion of Gray Iron castings to higher strength Ductile Iron has given the designer two alternative routes to improved component value: significant weight reduction with improved performance through redesign, or lesser but still substantial improvements in performance while maintaining the significant production and commercial benefits of keeping the existing design. Conversions from steel have offered similar, methods of improving cost effectiveness: new designs to improve performance and manufacturability, or the use of existing designs to provide equivalent performance, improved manufacturability and a 10 per cent reduction in weight. In summary, Ductile Iron has been successful because it has offered the designer superior value - higher quality and performance at lower cost.

The driving force of superior product value is clearly evident in the examples in Table 10.1 of successful designs involving conversions to Ductile Iron castings. These examples, taken from Designs in Ductile Iron and Ductile Iron Castings show that, in addition to improvements in product quality, performance and reliability, the replacement of other materials by Ductile Iron castings is also driven by substantial cost savings gained through lower casting cost and superior manufacturability. The numerous design improvements in this Table, subdivided according to their roles in the product value equation, follow.                                                Back to Top

Many of these advantages have been discussed in the preceding Sections, which have addressed both fitness for purpose and manufacturability issues related to the design of Ductile Iron castings. This Section briefly highlights some of the advantages of designing with castings and points out the additional benefits of making those castings in Ductile Iron. Detailed aspects of casting design, and further information on designing with Ductile Iron can be found in the Section references, which specialize in these subjects.

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Table 10.1  Examples of conversions to Ductile Iron.

Designing with Castings
Designing with castings offers the design engineer numerous methods with which to develop a better product in shorter time at lower cost. In order to take full advantage of these opportunities, the design engineer must follow certain principles of Ductile Iron casting design. The most important of these principles are:

• use the design freedom offered by the casting process to optimize component performance. and
• design for casting soundness and freedom from defects.

Freedom of Design
The freedom of design inherent in the casting process is the ideal complement to the electronic design tools - CADCAM, solid modelling and FEA - which enable "electronic prototyping" to rapidly determine the optimum component shape and convert that shape into patterns for the production of castings. This process not only reduces product development time but also minimizes the need for fabricated prototypes which often "compromise" designs and perpetuate the compromise by becoming the production method.

Figure 10.1 illustrates how freedom of design enables castings to provide superior component performance. In this example, the easily cast box and "U" sections of the lever provide lower outer fibre stresses than the oval and I-beam sections. When produced as castings this lever, and numerous similar components, have more efficient load-bearing capabilities, enabling them to be either up-rated in performance or reduced in weight without increasing tensile or fatigue design stresses.

Casting Soundness
The economical production of castings free from harmful shrinkage is a prequisite of good design. Because most cast metals shrink during solidification, prevention of shrinkage defects involves the use of directional solidification to produce feeding paths from attached feeders (risers) to every part of the solidifying castings and the avoidance of casting geometry which impairs the ability of the mold to extract heat from the soldifying casting. One of the major feeding problems is isolated sections which, due to size and geometry, soldify more slowly and cannot be fed through attached sections. "L", "T" and "X" junctions, with their associated right - and acute - angled surface geometries, are common hot spots in castings which should be avoided or modified to reduce both shrinkage and stress concentrations.

Figure 10.2 illustrates common methods for correcting shrinkage problems in isolated heavy sections. The use of risers and padding increase metal consumption and casting cleaning costs while chills increase molding costs. The ideal solution is to used cored holes to induce cooling, reduce weight and eliminate machining operations. The unique solidification behaviour of Gray and Ductile Irons (see Section II) minimizes shrinkage and feeding problems, offering significant design advantages and cost savings in the production of complex castings.

The inability of mold corners, especially acute angles, to extract heat retards freezing and brings the local thermal centre near the casting surface. These problems, which may occur at any sufficiently sharp change in casting surface direction, and the reduced cooling surfaces of multilegged junctions, require the modification of these junctions to improve casting integrity. Figure 10.3 shows how increasing L-junction radii reduces the stress concentration factor and drives the thermal center of the junction away from the casting surface. Figure 10.4 shows a component was changed from a rectilinear, junction-filled design typical of fabrications, to a more castable and efficient curvelinear design of equivalent or superior strength.

Freedom from Defects
Section XI describes the integration of design and ordering to provide superior value to the end user and profitability to both the manufacturer and foundry. One key aspect of this new concept is simultaneous design, in which component value is optimized through concurrent improvements in performance, quality, supply and manufacturability.  Designing for freedom from defects is a good example of simultaneous design involving the cooperation of the designer and foundry.

Table 10.2  Ductile Iron Cyclic Fatigue Properties. 
(Information furnished courtesy of Meritor Automotive Inc., Troy, Michigan 1997).

Consultation
This Section is only a primer on casting design and the designer is urged to consult the references, or better still, contact a Ductile Iron foundry, the Ductile Iron Group or any of its member companies. Survival and profitability for both the users and suppliers of castings requires not only high quality castings, but increased consultations on all aspects of quality, performance and manufacturability.

References
Designs in Ductile Iron, The Ductile Iron Group, 1467 N. Elston Ave., Suite 200, Chicago IL 60622.

Ductile Iron Castings, Produced by the British Cast Iron Research Association, Alvechurch, Birmingham B48 7QB, England, in cooperation with member companies of BRIDUC. J. B. Caine, Design of Ferrous Castings, American Foundrymen's Society, Des Plaines, IL, 1984.

J. C. Morrison and K.J. Smith, "Cost effective substitution of steel components by SG ductile iron.", The Foundryman, March 1989, pp 121-129.

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

A Design Engineer's Digest of Ductile Iron, 7th Edition, 1990, QIT-Fer et Titane Inc., Montreal, Quebec, Canada.

Casting Design as Influenced by Foundry Practice, Meehanite Worldwide, Chattanooga, TN.