part beyond utility or by fracture. Depending on the type of service, one failure mechanism will prove to be safer than the other. For example, in a pressure-tight container an unexpected major increase in the internal pressure may burst the casting or it may permanently deform it. A permanent deformation will probably be the safer failure mechanism. This is one example of relatively few applications where ductility is the desired design property. Another example is that of a gear. A sudden unexpected load may break or bend one or more teeth. In this case, breakage will probably be safer since the structure will be able to continue operating, albeit poorly, until replacement is effected.

The indiscriminate use of the high ductility grade (Grade 60-40-18) cannot be upheld even from the point of view of safety. Grade 80-55-06 can withstand 70 to 80% higher loads; Grade 120-90-02 can withstand two-and-a-half times more load, either static or dynamic, than Grade 60-40-18. Beyond these limits, Grades 120-90-02, 100-70-03 and 80-55-06 will fail through fracture while Grade 65-45-12 and especially Grade 60-40-18 will fail through permanent deformation under much lighter loads. If safety so dictates, the low strength, high ductility

grades should be chosen but each application must be evaluated individually.

c. Specifying destructive and non-destructive tests on a given number of castings, and the frequency and thoroughness of testing is directly proportional to the degree of safety expected from the casting. For the so-called "safety critical" parts, 100% non-destructive testing is often required even though such extensive testing is very expensive. Radiographic, eddy current, magnetic and ultrasonic and sonic frequency methods are available for non-destructive testing.

COURTESY SALZGITTER HUTTENWERK, A.G., SALZGITTER, GERMAN

Housing for low pressure section of a 300 MW turbine. Total weight: 48 tons. The success of Ductile Iron for this application is attributed to its good elevated temperature strength, and particularly, its high temperature yield strength. Castings allow the design engineer to generate internal contours, which are nearly impossible to machine.