Designing with ductile iron - By Dr. Richard Warda, QIT and CANMET
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 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.
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 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."
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.
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.