Microstructure Changes During Solidification of Cast Irons: Effect of Chemical Composition and Inoculation on Competitive Spheroidal and Compacted Graphite Growth

Amongst the most important graphite shapes, nodules and compacted particles are of particular interest as they can coexist in castings with relevant changes in properties. In this context, it has been often reported that the compacted shape is an intermediate form between nodules and graphite lamellas that may be seen as degeneracy from the nodular one. The present work shows the microstructure evolution of an initially ductile iron alloy with a silicon content of about 2.4 wt% when reducing progressively the magnesium content by holding a large melt batch in a nitrogen-pressurized pouring unit for 8 h. Thermal cups with and without inoculant were cast at a regular time interval together with a sample for chemical analysis.

Interestingly, the thermal records of the inoculated samples show no significant changes with time while the structure evolved from fully spheroidal to half-spheroidal half-compacted graphite. Conversely, the thermal curves of the non-inoculated samples showed two arrests, one at nearly the same temperature as for inoculated alloys and a second one at a temperature decreasing with holding time until being below the metastable eutectic temperature. Microstructure observations showed the presence of a limited number of compacted cells which decreases as well with holding time. These observations suggest that these cells start developing during the temperature interval between the first and second arrests, leading to a bulk eutectic transformation either above or below the metastable eutectic temperature. These results support the view that a fully compacted structure can be obtained only with a controlled inoculation which should not be too high to avoid too high nodularity.