Gray iron technology has advanced greatly in the past half cen tury. Much has been learned about the mechanisms of nuclea tion and solidification and about solid state transformations. This information has allowed gray iron castings to remain com petitive for structural components. Low cost and excellent castability make them versatile for a wide variety of industrial components including engine blocks, cylinder heads, hous ings, manifolds, hydraulic valve bodies and many other cast ings of similar complexity. Common to many of these compo nents are the requirements of high strength , soundness, good machinability, dimensional stability and uniform properties. These characteristics are dependent on achieving proper and uniform structures throughout all sections of the casting. In many cases, alloy elements added for strengthening cause increased section size sensitivity. The principal function of alloy additions is to control the transformation of austenite to achieve a fine pearlitic structure. Unfortunately, some ele ments that are very effective in doing this have detrimental ef fects on the solidification process, resulting in chill and car bides. Although carbides are not particularly detrimental to strength, they raise hardness, reduce the tensile strength/hard ness ratio and drastically reduce machinability. For example, chromium is very effective in reducing ferrite and producing pearlitic structures in heavy sections; however, it may increase chill and carbides in thinner sections. Additions of silicon can reduce chill in the thinner sections, but can reduce casting strength and structural uniformity by promoting ferrite in the heavier sections. It is clear, then, that foundry metallurgists are often put into a compromising position; they must avoid carbides and chill in thin sections of a casting and yet provide fully pearlitic struc tures in thicker sections. When one considers the full spectrum of section sizes that a single foundry produces, the problem looks worse. Unfortunately, the cheapest remedy for prevent ing ferrite formation in heavy sections is usually the cause of chill and carbides in th'in sections, and the reverse is true as well. In the discussion that follows, the metallurgy of gray irons is discussed in relation to today's material requirements. These metallurgical concepts are used to show the foundry metallur gist a rational approach to alloying gray iron castings for high strength and minimum section sensitivity, keeping in mind four goals: 1) consistent graphite structure; 2) freedom from chill and carbides; 3) freedom from free ferrite; and, 4) fine pearlite with uniform strength and hardness.
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