No abstract

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.

A three-stage model for the nucleation of graphite in grey irons has been proposed. Stage 1 involves formation of small oxide based sites (usually <2·0 μm) in the melt. Stage 2 involved precipitation of complex (Mn,X)S compounds (usually <5·0 μm) nucleated by stage 1 microinclusions. Finally, in stage 3, graphite nucleates on the sides of the (Mn,X)S compounds, which have low crystallographic misfit with graphite. Three groups of elements are important to sustain this sequence for effective graphite nucleation. These are strong deoxidising elements, such as Al and Zr, to form a high count of very small stage 1 microinclusions, Mn and S to sustain MnS type sulphide formation, and inoculating elements, which act in the first stage and/or in the second stage of the graphite nucleation sequence. Inoculating elements improve the capability of (Mn,X)S compounds to nucleate graphite. In inoculated irons, the (Mn,X)S compounds are more complex. They have a lower Mn/S ratio and higher capability to nucleate graphite, especially when preconditioning/inoculating elements contribute with a high count of effective stage 1 particles.

No abstract

No abstract