Gray iron or grey iron was the original "cast iron". It is relatively easy and inexpensive to make. Compared to the more modern engineered irons, gray iron has a lower tensile strength and lower ductility. In other words, it will fail more easily, and its mode of failure will be sudden fracture (it will not bend). It is used for housings where tensile strength is non-critical, such as engine blocks, pump housings, valve bodies, electrical boxes and decorative castings.
In grey cast iron, a large part or all of the carbon is in the form of flakes or nodules of graphites. Graphitic cast iron has a dark gray or almost black fracture. Upon small degree of super cooling, graphite is formed when the cast iron solidifies from its liquid state. Slow cooling promotes graphitisation. Rapid cooling partly or completely suppresses graphitisation and leads to formation of cementite. Rapidly cooled gray iron with less then 3% carbon is known as white iron.
Ductile iron, also called ductile cast iron or nodular cast iron, is a type of cast iron invented . While most varieties of cast iron are brittle, ductile iron is much more ductile, as the name implies.
Cast iron contains 2% – 4.0% carbon , 1% – 6% silicon , and small amounts of manganese. Contaminants present in pig iron that negatively affect material properties, such as sulfur and phosphorus, have been reduced to an acceptable level. It has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. Its mechanical properties vary greatly, dependent upon the form carbon takes in the alloy. 'White' cast irons contain their carbon in the form of cementite, or iron carbide. This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken carbide, a very pale, silvery, shiny material, hence the appellation. In grey iron the carbon exists free as fine flakes of graphite, and also renders the material brittle due to the stress-raising nature of the sharp edged flakes of graphite. A newer variant of grey iron, referred to as ductile iron is specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, vastly increasing the toughness and strength of the material.
本文转自：China Industry News
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