Composition: 3.52C-2.51Si-0.49Mn-0.2P-0.02S-0.06Mg (Carbon equivalent = 4.36).

Processing: Casting.

Etching:  10-30s immersion in 2% nital.

The microstructure corresponds to a ductile iron with a ferritic matrix and some residual pearlite. The relatively high ductility of ductile irons is attributed to the spherical or nodular shape of graphite. This is achieved by inoculation with a small amount of Mg. In the industry is very common to obtain a mixed matrix (ferrite-pearlite). However, it is possible to adjust the proportions of ferrite and pearlite by controlling the composition of the material, the cooling rate during solidification and by applying heat treatments. For instance, annealing and slow cooling results in a ferritic matrix, whereas normalizing (air cooling after austenizing) results in a completely pearlitic matrix (see specimen F9). 

Although not very common in this specimen, in some regions there is the characteristic bullseye structure (graphite spheroids surrounded by ferrite). Another constituent that is visible in this specimen is steadite (white). This is a ternary eutectic consisting of ferrite, cementite and iron phosphide that forms for P contents above 0.15% and is known to reduce the toughness of the material due to its low ductility. 

Fe-C diagram

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