Alteration of the Graphite Morphology in Solid Solution-Strengthened Ductile Iron Due to High Contents of Cerium and Bismuth

Abstract

The impact of combined addition of high levels of bismuth up to 120 wt ppm and cerium up to 2000 wt ppm on the graphite morphology in GJS 450-18 with 3.2 wt% carbon and 3.2 wt% silicon was studied. Experiments were conducted with insulated keel blocks with a solidification time of 40 min. Samples from the thermal centre of the castings were analysed by optical microscopy, and the forms and sizes of graphite particles were characterised. Bismuth addition, even at 25 wt ppm, resulted in an altered graphite form in the last-to-freeze regions resembling intercellular lamellar graphite (ILG). Additions of 45 wt ppm or more Ce to these Bi-containing melts prevented the formation of ILG and produced chunky graphite (CHG) instead. ILG did not appear for ratios Bi/Ce > 1.5, while CHG could not be found for ratios Bi/Ce < 0.7. Only one type of graphite degeneration (either ILG or CHG) was present in each sample, thus rendering their formation mutually exclusive. Larger amounts of Ce between 300 and 500 wt ppm resulted in the formation of predominantly graphite form V, while no degenerations could be observed. The addition of Ce to a Bi-contaminated cast iron melt to avoid ILG formation and achieve a regular graphite structure with mainly form VI graphite is not possible. Instead, form V replaces form VI as the predominantly formed morphology. Ce levels higher than 1000 wt ppm resulted in the formation of large areas of undercooled graphite in the last-to-freeze regions regardless of the Bi content in the cast iron melt.