Effect of Nano-Sized SiO2 Particles Addition on Microstructure and Mechanical Properties of As-Cast Ductile Iron

In order to improve the mechanical properties of ductile iron, different contents of SiO2 particles are added into ductile iron in the casting process. The microstructure and mechanical properties of ductile iron are studied. It is found that SiO2 particles can promote the heterogeneous nucleation of austenite, which can indirectly promote the nucleation of graphite, ferrite and cementite. SiO2 particles can also inhibit the growth of austenite, which can further refine ferrite and cementite during the phase transformation. Finally, the average grain size of ferrite decreased to 27.95 μm and the interlamellar spacing of pearlite decreased to 211 nm in sample 0.5S. A large number of granular pearlite appeared in sample 0.75S. The above changes of microstructure affect together the properties of ductile iron, so that the strength, plasticity and toughness were enhanced simultaneously. Ductile iron has the best tensile strength (495 MPa), elongation (26.1%) and impact absorbed energy (22.06 J) with a SiO2 content of 0.75 wt%. With the increase of SiO2 content, the cleavage steps on the tensile and impact fracture surfaces decreased gradually and more and more dimples appeared, which proved that cleavage fracture gradually transformed into ductile fracture.