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Show FABRICATION AND DESIGN Selection of an alloy for a specific application may involve two considerations: composition and wrought or cast form. Although the latter might be considered a design function, it is Included here because, (1) the basic principles of design discussed under that subject are applicable to structures In general, whether cast, or wrought and (2) the procedure followed in selecting the composition would apply to either cast or wrought form. Figure 5 shows the same design for a cast & wrought tube hanger. The Advantage of Cast Are: (1). Initial cost - Since the casting is essentially a finished product as-cast, Its cost per pound may be less than a fabricated item. (2). St~ength - Similar compositions are Inherently stronger at elevated temperature In cast form than In wrought; this is attributable to the very coarse grained as-cast structure and the fact that most equivalent cast compositions are modified with higher carbon content to improve castability. (3). Shapes - Certain shapes can be produced as a casting are not made as a mill form, or that cannot be fabricated economically from the wrought forms available. (4). Compositions - Some compositions are available in castings that lack sufficient ductility to be worked Into wrought forms. The Advantages of Wrought Are: (1). Section Size - There is practically no limit to the section sizes available In wrought Iron. Thinner sections often permit a weight reduction of 50% or more, making the Initial cost of fabrication no more than, and perhaps less than, a casting; decreased tare weight results in substantial reduction in operating costs, thermal efficiency, in many instances. (2). Thermal Fatigue - Thinner sections that reduce thermal stresses, and the Inherently greater ductility of wrought materials, usually promote better resistance to thermal fatigue, expecially under cyclic conditions. (3). Soundness - Wrought materials are normally free of internal and external defects such as shrinks, porosity, cold shuts, etc. (4). Surface Finish - The smooth surface of wrought materials is often beneficial In avoiding focal pOints of concentrated or accelerated corrosion or carbon attack. 4 |