Fabrication of Engineering Materials: Castings & Hot-working

The designer must keep in mind the available fabrication methods and the effects of each on the properties of the finished member.

CASTINGS are used for members of intricate shape that would be difficult to manufacture by other methods. When cast in the metal molds under pressure, the products are called die castings; they are more accurate in size and shape than are ordinary castings, and in many cases they require no machining. Centrifugal castings are made by rapidly revolving the molds during the introduction and solidification of the molten casting material.

   Although castings are desirable for many parts of intricate shape, they present many problems in design. Shrinkage during the cooling period and nonuniform cooling of irregular thick and thin sections, bring residual stresses which may cause some castings to rupture before they can be put into service or to warp during machining operations. Important castings always should be annealed or normalized to relieve these stresses.
   Large castings expose proportionately less cooling surface, cool more slowly and have a coarser grain structure than do small castings of the same material. This usually results in less strength and ductility in the larger castings, unless they are properly heat-treated.

HOT-WORKING of ductile materials by rolling, forging and similar processes refines the grain and generally improves the properties. Rolling, pressing and  extruding processes works the metal throughout and produce nearly uniform structure in all parts of the material. In forging, the working of material is more or less local and the inner part is not affected unless the forging hammer relatively heavy. After hot-working, the material should be allowed to cool slowly and evenly to avoid residual stresses. Brass, lead and other frequently extruded into intricate shapes that cannot be made by rolling or forging.

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