A New Approach to Graphite Nucleation Mechanism in Gray Irons

A complex research program involving the Scanning Electron Microscopy (SEM) has been undertaken in order to achieve a more detailed understanding of graphite nucleation in un-inoculated and High Purity (HP)-FeSi, Ca-FeSi and Sr-FeSi treated irons at different residual aluminum(Al) levels (0.001…0.05% ) in iron melt. (Mn,X)S compounds, usually less than 5.0 µm in size, with an average 0.4-2.0 µm well-defined core (nucleus), were found to be important sites for graphite nucleation in gray irons. Al contributes to the formation of Al2O3-based sites which act as nucleation sites for (Mn,X)S compounds, even at a very low Al level (<0.003%) in iron melt. Increasing the residual Al content in iron melt helps the initiation of graphite nucleation at a lower undercooling degree. A 0.005-0.01% Al content in melt appears to be beneficial, without the detrimental effect on pinhole occurrence in gray irons. Calcium (Ca) is present in most (Mn,X)S compounds and at similar levels in the core and shell (higher level in Ca-FeSi treated irons), while strontium (Sr) was found mainly in the core and only in Sr-FeSi treated irons. A three-stage model for the nucleation of graphite is proposed: 1) small Al2O3-based sites are initially formed in the melt; 2) complex (Mn,X)S compounds nucleate on these microinclusions and 3) graphite nucleates on the sides of the (Mn,X)S particles which have a lower crystallographic misfit with the graphite. In inoculated irons, the (Mn,X)S compound is more complex, at a lower Mn/S ratio and a higher compatibility to graphite nucleation, especially as inoculating elements (Ca, Sr, etc.) contribute.