The foundry and cast iron, history of a continuous evolution
The history of cast iron and foundry
In history, cast iron and the foundry are inextricably linked. Cast iron components are the direct result of foundry technology, which, when properly managed, allow the microstructure and properties of cast iron to be controlled.
The history of cast iron: an easily melted and poured material
The Italian word “ghisa” is clearly linked to the German term “gusseisen”, which means “cast iron” in English.
In other words, from a semantic point of view, cast iron was already intrinsically and inextricably linked to the foundry process. There are two simple metallurgical reasons for this:
- In the Iron-Carbon system, the melting temperature for steel is around 1500 °C (with relatively little variation, depending on the specific composition); for cast iron, on the other hand, usually has a fairly eutectic composition, with a melting temperature of around 1150 °C. It is, therefore, much simpler (and much more energy efficient) to melt cast iron as opposed to steel;
- Given its intrinsic microstructural features, cast iron does not offer sufficient levels of deformability to allow processes such as rolling or drawing. The only possibility of producing articles in cast iron is, in fact, associated with the foundry processes.
For the temperatures involved, the melting of cast iron requires the use of disposable moulds, usually made of silica sand, the consistency of which is ensured by the use of organic or inorganic binders.
The production of cast iron which gradually improved in quality from the second half of the 1800s, was achieved thanks to an increasing level of microstructural control, such as:
- optimisation of the composition
- inoculation treatments
- spheroidization treatments
- carrying out heat treatments such as malleabilisation and austempering
Historical examples of cast iron parts
The initial attempts at producing articles in cast iron date back to around 4000 BC, probably in Anatolia (Armenia), in Mesopotamia (Iran) e in China. Production techniques were progressively refined, and examples of weapons made of cast iron can be dated to around 3000 BC. The possibility of improving the mechanical performance of these artefacts can instead be traced to around 1000 BC, in Greece.
The “ease” of pouring cast iron made it possible to produce artistic artefacts, even in very large sizes. One such example is the Iron Lion of Cangzhou, almost six metres high and weighing an estimated 40 tonnes, produced in 953 AD (Figure 1).
Equally interesting is the cast iron pipe system (measuring a total of 35 km in length) produced in the second half of the 17th century to supply the fountains of the Palace of Versailles. Also worth mentioning is the so-called “fer de Berlin”, with which Prussia replaced gold in jewellery (in order to finance the war against Napoleon) in the early 1800s (Figure 2).
Fig. 1 – Iron Lion of Cangzhou
Fig. 2 – Bracelet made of “Fer de Berlin”
The Lion of Cangzhou
The Iron Lion of Cangzhou is the symbol of the Chinese city of Cangzhou, located in Hebei Province, about 180 km from Beijing.
It is the largest and oldest iron-cast work of art in China and for the city of Cangzhou, it is a veritable icon of history and culture.
Originally, it was probably displayed inside a Buddhist temple and is thought to have carried a bronze statue of the bodhisattva Manjusri on its basin-shaped throne, which was removed when the value of bronze began to rise.
The Iron Lion of Cangzhou is 5.78 m high, 6.5 m long, 3.17 m wide and weighs about 40 tonnes. In honour of the cast iron statue, Cangzhou is also known as the “city of the lion” and a locally brewed beer, the “Cangzhou Lion Beer”, is named after this work of art.
Cast iron: a material in continual evolution
Cast iron is an iron-carbon alloy with a relatively high carbon content (theoretically between 2.06% and 6.67%.).
The allotropic behaviour of iron (variation, with temperature, of the crystal structure), the different solubility of carbon in the various crystal structures of iron, the possibility that carbon reacts with iron forming a carbide (Fe3C), known as cementite, or that it endures as graphite, determine the presence, in various types of cast iron, of different combinations of microstructures.
In this sense, cast iron can certainly be said to be perhaps the first compound material produced on an industrial scale. Graphite, cementite, austenite, ferrite, and perlite, are the structural constituents which, when appropriately balanced, allow good and sometimes excellent mechanical properties to be obtained.
Cast iron, however, is also a “traditional” material that has evolved like few others, thanks to the continuous improvement of its microstructural properties: just think of the transition from grey cast iron, to malleable cast iron, spheroidal graphite iron and austempered ductile iron.
Tradition and innovation are, therefore, a fundamental combination for the application of cast iron.