Historical development of iron and steel in structures
To appreciate how steel became the dominant structural material that it is today, it is essential to understand how it relates to cast iron and to wrought iron both in its properties and in the way that all three materials evolved.
The properties of the three ferrous metals - cast iron, wrought iron, and steel - are described and the evolution of their production is summarised. The evolution of their structural use is also given and the prospects for further development introduced.
1. PROPERTIES OF THE THREE FERROUS METALS: CAST IRON, WROUGHT IRON, AND STEEL
Cast iron, as the name implies, is 'cast' or shaped by pouring molten metal into a mould and letting it solidify; a wide variety of often very intricate forms is thus possible. It is very strong in compression, but relatively weak in tension, and much stiffer than timber, but brittle.
Wrought iron is strong both in tension and compression and ductile, thus making it a much safer material for beams than cast iron. Its main disadvantage is that, never reaching a fully molten state, it can only be shaped by rolling or forging, limiting its possible structural and decorative forms.
The properties of mild steel are similar to those of wrought iron, but it is generally stronger and can be cast as well as rolled. However, it has a lower resistance to corrosion than wrought iron and is less malleable and thus not so suitable for working into elegant, flowing shapes.
These properties, in terms of strength and carbon content, are shown in Figure 1; the values shown should be considered as indicative rather than absolute limits. They do not include malleable or ductile cast irons which have strengths in tension considerably above those shown.