Effect of inoculant containing rare earth metals and bismuth on microstructure and mechanical properties of heavy-section near-eutectic ductile iron castings

• Chunky graphite formation is influenced by the inoculant chemical composition used in post-inoculation.
• Inoculant containing bismuth and rare earths prevents the chunky graphite formation.
• Sub-micrometric agglomerates of Bi were observed by means of FEG-ESEM.

The effects of inoculation sequence and inoculant chemical composition on heavy-section castings microstructure, with particular attention to chunky graphite, were investigated. The combinations of inoculant types with inoculation sequences were chosen basing on foundry experience.Thermal and chemical analyses were used to control the experimental foundry processes. Metallurgical analyses were performed by means of optical and field-emission gun scanning electron microscope and important microstructural parameters were measured and correlated with the results coming from tensile tests. In-stream inoculation with inoculant containing Bi and rare earths was found to drastically reduce the formation of chunky graphite. This result was attributed to the major fading resistance of such inoculant compared to the standard ones, confirmed both by nodule fraction measurements and sub-micrometric aggregates of Bi detected at the spheroid centre in the central part of the casting.