When talking about different types of cast iron, we usually associate the most common distinction known to the foundry “insiders”, in particular to the foundry men: base cast iron, liquid cast iron, white iron, grey iron, malleable iron, nodular cast iron…
But where does this distinction actually come from? We have talked already a little bit about such distinction (even if in more technical term) in the insight “Different types of cast iron, different types of graphite” evaluating how cast iron can differ starting from the study of its cooling curve and the related derivatives.
We can make though a very first rough distinction between refined iron, usually used for steel production, and secondary fusion of cast iron.
Secondary fusion cast iron as well as steel is the result of a particular chemical composition, Fe-C alloy, in which the carbon plays most of the times a key role determining some first main characteristics of the cast iron.
Really important to note is that carbon is not the only element that influences the cast iron. Other elements, we will talk about later on, regarding the chemical composition, cooling rate and possible thermal treatments.
Two different Fe-C phase diagrams in relation to carbon distribution exist in the metallurgy literature:
- Ferro-cementite also known as metastable
- Ferro-graphite also known as stable
In practice, both diagrams are used to study grey iron. They are as well used in combination as it is possible to observe mixed cast iron structures (graphitic-cementitic). Kindly note that graphite can be identified from the microstructure of the cast iron itself and it can be distinguished in lamellar or vermicular, alternatively spheroidal.
Please note also that also the cooling curve of the cast iron can provide relevant information and clarify whether the solidification of the cast iron takes place following the stable diagram or the metastable diagram.
Graphite can assume different forms and such forms can influence the mechanical properties and the final application of the finished product (casting).
For each type of cast iron it is possible to identify from the Fe-c phase diagram 3 subdivisions by taking into account the carbon percentage in the chemical composition: eutectic cast iron, hypoeutectic cast iron and hypereutectic cast iron.
- Eutectic cast iron with a carbon percentage (%C) equal to 4,3%
- Hypoeutectic cast iron with a carbon percentage (%C) from 2 to 4,3%
- Hypereutectic cast iron with a carbon percentage (%C) above 4,3%
So, to sum up: all cast irons are the result of a particular chemical composition Fe-C. Being Carbon the first main influential element, we can study in what quantity it can actually influence the cast iron by looking at the Fe-C phase diagram and identifying the related percentage.