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The structural characteristics of inoculated grey irons were studied. As a general rule, the beneficial influence of the three inoculating elements is in the order Ba⩽Ca

Specific investigations concerning La-contributions in graphite nucleation process in Ca,Al,La-FeSi inoculated grey cast irons were performed by using a scanning electron microscopy (SEM, EDS). It was re-confirmed that complex (Mn,X)S compounds act as major nucleation sites for graphite flakes. La was mainly concentrated in the first formed oxide-based micro-inclusions (similarly to Al), but also at an important level in the shell of (Mn,X)S compounds (accompanying Ca). It is assumed that La forms micro-inclusions later than Al, as La-reached phase surrounded Al-reached phase. Complex Al-La small micro-inclusions, as possible better nucleation sites for (Mn,X)S compounds and La-Ca presence in the shell/body of these sulphides, possible better nucleation sites for flake graphite, appear to be the peculiar effects of ferrosilicon based inoculants, which include these active elements, promoting type-A graphite, inclusively in low S-grey iron.

Two main chemistry systems of micro-inclusions can be identified in ductile iron: Ca-S-X and Mg-Si-O-X with the majority of the inclusions regardless of treatment type and location being of the second type, silicates. Laboratory investigations have shown that simple silicates were present in the matrix, while more complex silicates were present in conjunction with graphite, probably acting as graphite nucleation sites. In these more complex silicates, elevated levels of Al, Ca, Ce and La were typical. Comparing micro-particles embedded in iron matrix and graphite nodules of iron treated with pure Mg-metal and iron treated with MgFeSi alloy showed a higher amount of complex silicates with elevated Al-levels in the iron treated with MgFeSi. Further laboratory investigation was undertaken to explore which source of Al and which range of residual Al would have a favorable impact on the graphite nucleation in ductile iron. The work showed that a residual aluminum of 0.005 to 0.020 wt.% appears to be beneficial for improving ductile iron solidification characteristics without the incidence of pinholes. Greatest benefits were achieved when introducing the Al into the iron via an inoculant late during processing or via a pre-conditioner to the base iron. Al added via the MgFeSi provided less benefit. Some case studies illustrating the effect of Al in ductile iron are also presented, as Al-containing pre-conditioner or/and Al-bearing, FeSi inoculant application.