During the half century which has elapsed since alloy additions La cast irons were first s(Udi~d, an immense amount of work has been done in man~ countries, and the systematic and controlled use of one or mOTC of certain elements has proved to be a potem method for producing engineering castings of h igh quality and predictable and often specified properties. A remarkable range of irons has been evolved \Vilh .characteristics suited La their intended applications. The number of elements used has been widened and thermal treatment may accompany appropriate additions. Alloy additions are usually intended to ensure the presen ce or persistence of a particular metallurgical structure with which the required properties are associated. The loll' alloy irons are basically pearlitic, the high alloy irons austenitic, acicular, martensitic or cementitic. For well defined applications, irons high in chromium and in silicon are llsed, the former in a graphite-free, chromium-ferrite matrix, the latter graphite-ferrite. Alloy additions arc now employed in the malleable cast irons and in the refined pig irons. Although the nodularizing agents are not here regarded as alloy additions in the ordinary sense, the evolution of nodular or ductile iron has had great inAuence on the general situation. It has emphasized the influence of trace or residual elements present in the raw materials or picked up during processing, and the knowledge gained qas been beneficial in the production of other cast irons. Although its precise mechanism still remains obscure, there is greater understanding of the process of inoculation. Developments in metal melting, thermal treatment and other foundry techniques have had repercussions in respect of alloy additions on charge make-up and composition.
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