Cast iron, wrought iron and steel are all essentially alloys of iron and carbon.
Although the actual situation is much more complex, cast iron, wrought iron and steel can all be thought of as alloys, principally of iron and carbon. To complicate matters, though, it is worth mentioning that there are no precise definitions of the relative make -up of the three types of steel.
Thomas Rickman combined the structural utility of cast iron columns with delicate ornament in the gallery fronts and ceilings to the nave and aisles of St George's Church, Everton, UK.
Iron is extracted from naturally occurring ores and we can think of these ores as providing the source material, iron oxide (FeO).
In the early days of iron and steel production iron oxide ores were mined but the sources of iron oxide have now been worked out.
When iron oxide is heated at high temperatures it becomes transformed into iron.
When iron oxide is heated at high temperatures of 1600 to 3000°F the oxide is reduced to the metal and the resulting reaction can be expressed as:
Iron Oxide + Carbon heated along with a blast of air yields Iron + Carbon Monoxide (a gas released into the air)
i.e. FeO + C > Fe + CO
In practice, this process does not yield pure iron, but an impure product called pig iron. This pig iron contains impurities such as Iron Carbide (Fe3C) which make the material hard and brittle. It is, however, the raw material from which cast iron, wrought iron and steel can be produced.
Pig iron, which is essentially cast iron, derives its name from the shape of the casting beds which resemble piglets being suckled by the sow and the pigs were the billets as supplied to the foundries.
Cast iron is the material produced by remelting this iron (known as pig iron), possibly along with some scrap iron.
The remelting of pig iron, and scrap iron, whilst blowing air into the molten mass until the Carbon content is between 2.4 and 4.0% produces Contemporary Cast Iron which can exist in two forms: grey (Graphite) cast iron and white (Iron Carbide) cast iron.
In the early days of cast iron production, it was difficult to control the level of carbon and other impurities such as sulphur (which has a particularly detrimental effect on the properties of iron). This means that the strength and properties of the material were very much a hit-and-miss affair. Nor was it possible to be sure that the molten material had been able to flow through all of the mould before setting. Consequently, the early cast iron structural elements were often load tested before being used in a building. And putty was sometimes used to plug holes in large section such as circular columns.
Wrought iron is achieved by simple reprocessing of cast iron.
The strength deficiencies of cast iron were eventually partly addressed by the development of a process termed "puddling". This involved reheating cast iron and manually mixing air in with the molten mass. Because of the nature of the puddling process the volumes that could be produced by this process at any one time were small. This, in turn, limited the size of structural components that could be made of this type of iron.
The material produced this way had reasonably high tensile strengths and was much more ductile than cast iron.
The process of producing wrought iron improves the tensile strength. This made it suitable for beams, and the ductility meant that its behaviour in column elements was more predictable than cast iron. However, its use in columns was rare due to the comparative cost of cast and wrought iron.
The production of true wrought iron in Britain ceased in 1973, so what is marketed today under that name is either old material recycled or a type of mild steel.
The invention of the Bessemer process allowed the oxidisation process after remelting to be carefully controlled and the carbon content could therefore be held at a particular level, providing good tensile strength and ductility.
In what we refer to today as steel the carbon content will typically be below 1%. For most structural steel the actual value will be in the region of 0.2%. It is the addition of elements such as silicon and manganese that allow the carbon levels to be controlled with some accuracy, and the manganese also has the beneficial effect of neutralising the otherwise harmful effects of sulphur.
The resulting material has equally high tensile and compressive strengths along with a high degree of ductility.
The rolling process for shaping steel sections
There is a wide range of steels which can be classified in various ways.
The terminology relating to the classification of different steel types is not precise. Broadly speaking steels are described in the following table.
up to 0.3% Carbon
Medium Carbon Steels
0.3 to 0.6 % carbon
High Carbon Steels
over 0.6% Carbon
To form steel into the kind of sections used in structures ingots are heated and then forged or rolled repeatedly, each repetition getting closer to the desired cross sectional shape.