Liquid metals freeze in much the same way as water. First, snowflake-like crystals form, and as these multiply and grow a solid emerges. However, this solid can be far from homogeneous. Just as a chef preparing icecream has to beat and stir the partially solidified cream to break up the crystals and release any trapped gas, so many steelmakers have to stir partially solidified ingots to ensure a fine-grained, homogeneous product. The preferred method of stirring is electromagnetic, and has been dubbed the ‘electromagnetic teaspoon’. We shall describe this process shortly.

First, however, it is necessary to say a little about commercial casting processes.

Casting, Stirring and Metallurgy

(Leonardo da Vinci on the extraction and casting of metals)

Man has been casting metals for quite some time. Iron blades, perhaps 5000 years old, have been found in Egyptian pyramids, and by 1000 BC we find Homer mentioning the working and hardening of steel blades. Until relatively recently, all metal was cast by a batch process involving pouring the melt into closed moulds. However, today the bulk of aluminium and steel is cast in a continuous fashion, as indicated in Figure 8.1.