Foundry basics – A brief introduction to the foundry
The history of the cast iron foundry: a continuous evolution
The history of the foundry, particularly with regard to cast iron melting, has very distant origins. In Greek and Latin mythology, Vulcan and Hephaestus are depicted as gods who are capable of producing and working metals and alloys with fire and heat.
The origin of the foundry, which is part of the history of mankind, can be traced back to 4000 BC. The oldest casting, however, discovered so far is a copper frog, made in Mesopotamia around 3200 BC. Numerous other examples of products made using melting techniques date back to 2800 BC, in Egypt.
Sand casting technology was introduced in 1300 BC, under the Shang dynasty in China. The earliest finds of cast iron artefacts produced in foundries date back to 500 BC, also in China, and the earliest examples of weapons and decorations made by cast iron foundry processes date back to around 200 BC.
In much more recent periods, it is worth noting the development of foundry work associated with the building of cathedrals and churches and the production of bronze bells in the Middle Ages.
It was at this time that the foundry stopped being “simply” an art and became a technology, with boundless potential. In 1313, the first cast iron cannon was produced.
About 150 years later, the Vannoccio Biringuccio from Siena, wrote “De Le Pirotechnia”, a manual on the foundry divided into 10 sections (minerals, assaying, melting, alloying, pouring, etc.). The first American foundry was established in 1645, in Massachusetts.
Another important milestone occurred around 1760, with the use of cast-iron rails to transport material extracted from coal mines in the English county of Shropshire.
Then, in the second half of the 19th century, the foundry, essential for the manufacture of steel and cast iron products and components, made a significant contribution to the transformation processes of cities and the development of industrialisation.
The foundry process
A foundry process (or pouring moulding) roughly speaking, consists of pouring a metal alloy in a liquid state into a mould, the cavity of which reproduces the negative shape of the part to be obtained.
Once the mould has been filled, the alloy starts to cool and solidify, thus assuming the desired shape.
However, the range of foundry processes is incredibly wide, and it is the sheer number that bears witness to their versatility.
Versatility means that designers and end users have a wide range of choice and considerable flexibility in how to meet the requirements imposed on the component to be manufactured.
There are several criteria for classifying moulding by pouring processes; the simplest has two categories:
- processes in which the mould, made of refractory material, is “disposable”, i.e. it is destroyed at the end of each production cycle;
- processes in which the mould, made of steel, is “permanent”, i.e. it can be reused for a very large number of production cycles;
Each process has specific advantages and benefits, especially when used bearing in mind the alloy to be poured and the type of component to be produced.
Generally speaking, certain requirements must be taken into account in order to choose a correct foundry process:
- surface quality of the casting;
- required size accuracy;
- number of castings to produce;
- costs of making the mould or die;
- type of moulding equipment (i.e. required to make moulds or dies).
The duly identified process plays a direct and essential part in determining the microstructure, quality and final performance of the castings produced.
The microstructure of cast iron
Cast iron is a compound material, with an iron matrix (which can have various microstructural features: pearlite, ferrite, ausferrite) and the presence of graphite structures. The combination of the ferrous microstructures and the dimensions and morphology of graphite determines the application performance of components made of cast iron.
Cast iron classification
Different types of cast iron are produced precisely because of the operating conditions and solutions made possible by foundry processes:
- white cast iron: containing eutectic cementite
- cast iron: grey, spheroidal, with compact graphite.
Through heat treatments, further control of the evolution of the microstructures is possible, thus obtaining, for example, malleable cast iron and austempered ductile iron.