Cast iron

An iron-carbon alloy that usually contains 2-4% carbon, usually also with small quantities of sulphur, phosphorus, silicon, and manganese. Produced from the ore by smelting in a blast furnace from where it was cast directly into finished products, or as 'pig iron'. Traditionally re-processed into wrought iron, or latterly into steel, by removal of most of the carbon content. Generally divided into three groups:

(1) 'Grey cast iron' in which free carbon occurs as flakes of graphite. It has excellent casting properties and can be machined.

(2) 'White cast iron' in which all of the carbon is taken up as cementite, Fe3C and as pearlite. These irons are usually hard and brittle.

(3) 'Malleable cast irons' are usually obtained by heat treatment of white cast irons by converting the combined carbon into free carbon or temper carbon. In the 'whiteheart process', for example, a certain amount of carbon is removed from the surface by oxidation.