Solidification of Superfine Graphite Iron Revealed by Interrupted solidification Experiments

The tensile strength of near-eutectic grey iron can be increased from 230–300 to 300–345 MPa, without a significant increase in hardness, through 0.3–0.4%Ti addition to low sulphur (<0.01%S) iron. This is due to the combination of higher primary austenite/eutectic ratio and the precipitation of superfine-interdendritic-graphite (SIG), characterised by a fine (10–20 μm) and highly branched fibrous structure. To reveal the influence of the %Ti on graphite shape evolution during solidification and its relationship to the solid fraction, quenching experiments at successive solidification stages were carried out on hypoeutectic alloys with 0.18% and 0.32%Ti. The graphite shape factors were measured, and their evolution as a function of the titanium content and the solid fraction was analysed. SEM was used to evaluate the change in graphite shape during early solidification, as well as its nucleation and growth. The correlation between the oxygen in the melt and SIG formation was also explored. It was concluded that nucleation of graphite in SIG irons occurs on carbon rich regions at the austenite/liquid interface and, sometimes, on titanium carbides. Solidification velocity-undercooling curves were used to explain the transition from lamellar (type-A), to interdendritic (type-D), to SIG, and then to coral graphite.