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PROCESS


A good understanding of the power of design will make a casting stronger and more


tolerant of the imperfections in castings.


failure occurs by uniform deformation during tensile elongation. As long as the pearlite is the continuous microconstituent and ferrite is discontinuous, uniform deformation continues. When ferrite is confi ned to individual graphite nodules, that


is, in the form of bullseye ferrite, voids form at the poles of the nodules. Voids between neighboring nodules increase the local stress in the surrounding pearlite matrix and eventually, the voids link up when cracks propagate through the pearlite. When ferrite is continuous, forming a path linking nod- ule to nodule, local deformation in the ferrite regions occurs. As a result, premature fracture through the ferrite regions causes a somewhat reduced ductility.


Fatigue Principles Many castings are subjected to cyclic loading and conse-


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quently fatigue strength is the most critical design parameter. Most laboratory testing programs measure cyclic proper- ties and the fatigue strength in fully reversed loading. T e testing is performed on machined and, smooth-walled speci- mens, not castings. For many wrought materials the fatigue strength or endurance limit generally equals one- third to one-half of the tensile strength. In castings, due to the pres- ence of discontinuities, the fatigue strength can be lower, approaching a quarter of the tensile strength. Until recently, the fatigue properties of the various cast metals have been unavailable. AFS recognized this and several years ago began to build a database on the graphitic irons. T e database includes the strain-life properties that the Finite Element Analysis (FEA) models require to predict the fatigue strength of a part. T e database is being expanded to include aluminum and steel casting alloys. Designers are using the database to design castings for structural applications.


Design of Castings In his Hoyt Lecture of 1961, J.B. Caine has said ‘‘the advan-


tage of castings is derived from the ability of liquid metal to assume any shape, shapes that cannot be formed effi ciently by any other forming process. T is fl exibility enables the design of shapes as castings that will uniformly distribute the load … no part of the casting is overloaded … and stress concentration is at a minimum.’’ T e means to compete with wrought metals of higher


Brokk Inc. | Monroe WA | 1-360-794-1277 info@brokkinc.com | www.brokk.com


40 | MODERN CASTING November 2016


strength and internal integrity comes from careful analysis of the design of a part. Modifi cations of the design usually can result in the use of a lower strength cast metal and arrive at a cast part with equal or higher strength. Castings can be designed with the most favorable geom-


etries to reduce stress concentration by adding stock at high stress areas, increasing radii, using curved surfaces and add-


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