Metallurgy for Dummies

Cast Iron – a Definition

Cast irons typically contain 2-4 wt% of carbon with a high silicon concentrations and a greater concentration of impurities than steels. The carbon equivalent (CE) of a cast iron helps to distinguish the grey irons which cool into a microstructure containing graphite and and the white irons where the carbon is present mainly as cementite. 

Cast Iron Making

Cast iron’s properties are changed by adding various alloying elements, or alloyants. Next to carbon, silicon is the most important alloyant because it forces carbon out of solution. Instead the carbon forms graphite which results in a softer iron, reduces shrinkage, lowers strength, and decreases density. Sulfur, when added, forms iron sulfide, which prevents the formation of graphite and increases hardness.

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 What is Cast Iron ?

Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The colour of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due to its carbide impurities which allow cracks to pass straight through. Grey cast iron is named after its grey fractured surface, which occurs because the graphitic flakes deflect a passing crack and initiate countless new cracks as the material breaks.

Cast Iron Making

Carbon (C) and silicon (Si) are the main alloying elements, with the amount ranging from 2.1 to 4 wt% and 1 to 3 wt%, respectively. Iron alloys with less carbon content are known as steel.

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