In the previous article, “Type of cast iron”, we have introduced the Fe-C diagram to start talking about the analysis of the alloying elements characterizing the secondary fusion cast iron.
Hereafter we define the main chemical elements that identifies the various types of cast iron. Depending on the percentage of such chemical elements in the cast iron composition, the type of cast iron product will vary and it will be possible to manage or modify some specific properties.
Carbon: first characterizing element. As seen in the previous article “Type of cast iron”, the carbon represents the main influential element of the cast iron structure and its influence is studied thorugh the Fe-C diagram. It must be present much more in form of graphite rather then carbide and the graphite needs to segregate forming lamellars in small dimensions or alternatively forming nodules.
Silicon: main graphitised element. The silicon has a key role as graphitised element as it allows the stabilization of the graphite reducing the shrinkage. Shrinkage reduction is really important if you consider that an high presence of shrinkage increases the macroshrinkage phenomenon. The graphite segregation in lamellar form allows to obtain grey cast iron.
Magnesium: fundamental element in case of ductile iron allowing graphite coalescence. It is necessary to carefully control the operating temperature of the furnace during ductile cast iron production in order to reduce the evanescence of magnesium or an excessive slag formation.
Sulphur: presence of sulphur always causes damage. As it allows the cementite stability, it also diminuishes the fluidity of the cast iron and deteriorates the mechanical characteristics as well as the pearlitic structure. Together with Magnesium it allows to neutralize the possible damaging effects of sylicon. The ductile cast iron formation starts only when the sulphur goes down to 0,015% and when an excess of magnesium is present and compared to the sulphur equals to 0.020%.
Phosphorus: the main damaging element that diminuishes the mechanical properties and increases the fragility of the cast iron. As it shows such bad influence on the final properties of cast iron, it is added to the Silicon for the Carbon Equivalent calculation. In certain quantities though the phosphorous increases the cast iron resistance to wear. It decreases as well the solidification temperature of the cast iron keeping it at a liquid state for a longer time increasing the fluidity and the castability.
Manganese: compared to Silicon, Manganese acts in an opposite way. The higher its presence, the higher as well the tendency of cast iron to solidify in white iron. It shows an high compatibility with the sulphur and the oxigen acting as a deoxidizer as well as a desulphurising agent.
Chrome: it enhances the mechanical properties although it prevents the graphitizing process. Being present in big quantity chrome carbides increase the hardness and the cast iron matrix becomes brittle.
Nichel: it allows the graphitizing process, but it is less effective then silicon. It sharpen the graphite such as the pearlitic structure and the mechanical properties. Big quantities of Nichel lead to cast iron hardness.
The percentage with which such elements are detected is really helpful for the qualitative analysis of cast iron and for controlling during the solidification process (following a specific cast iron composition and related treatments) that the cast iron meets the preconfigured qualitative standards.
The thermal analysis given by TCAST becomes useful to the chemical analysis usually conducted in the laboratory with the spectometer or quantometers and to the analysis of the microstructure conducted with the microscope. It allows to focus on the alloying elements that are mostly influential on the composition and it puts them in correlation with the main solidification temperature of the cast iron.