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Iron (Fe), derives it name from the Latin word ferrum. In its pure form iron is lustrous, silvery, soft, and ductile. However, pure iron is a poor engineering material, generally not as strong as most plastics. Cast irons are based on the Fe-C system, and the solid-state transformations on which cast iron heat treatments are based are similar to those applied to steels.
Iron is the fourth most abundant element on Earth and is one of the most widely disbursed elements in the Earth's crust. In nature it is found in various compounds with oxygen, sulfur, or more complicated ores such as carbonates and silicates (Table 1). Because iron is so abundant, combines readily with other elements (such as manganese to form steel) and requires relatively little energy to extract it from ore, it is one of the most attractive elements to use for the products we require in everyday life (Fig. 1).
Cast Irons
Cast iron is a generic term used to designate a family of metals with a wide variety of properties. All cast irons contain more than 2% carbon and an appreciable amount of silicon (usually 1-3%). The high carbon and silicon content means that they are easily melted, have good fluidity in the liquid state and have excellent pouring properties. The basic types of cast iron are best differentiated by their microstructure as opposed to their chemical analysis because the various types overlap (Table 2).
The metallurgy of cast iron is more complex than its economics and, in fact, is one of the more complex metallurgical systems [Fig 2]. Iron-carbon alloys with less than 2% carbon are metastable; the true stable system being iron-graphite (Fe-C). The general term cast iron includes pig iron, gray iron, malleable iron, chilled iron, white iron, and nodular or ductile iron.
If an iron alloy exceeds about 2% carbon, the carbon does not have to nucleate from decomposition of austenite, but instead, it can form directly from the melt by a eutectic reaction. Note that cementitc (Fe^sub 3^C) can still nucleate at the eutectic more readily than graphite, but on sufficiently slow cooling, graphite itself is able to form and grow.
Consider the solidification of a 3% carbon cast iron (Fig. 3). At a rapid...