Mg bearing coating of resin sand–PTSA moulds to control graphite degeneration in surface layer of ductile iron castings

The relative performance of coatings for furan resin sand (FRS)–P-toluol sulphonic acid (PTSA) mould (FRS–PTSA), compared with no S, Novolak resin sand coated mould (NRS) was evaluated to determine the occurrence of a degenerated graphite surface layer. The FRS–PTSA moulds evidently incited graphite degeneration in the surface layer, as its thickness increased up to 10 times that with NRS moulds. Applying a mould coating containing S aggravated graphite degeneration. Conversely a desulphurising coating could protect the nodular graphite and limit the surface layer thickness. Independently of the S source at the metal/mould interface, sulphur affected graphite degeneration drastically. The coatings incorporating desulphurisation materials, [MgO; (CaO+MgO+Talc); Mg–FeSi] protected the iron from graphite degeneration, with the average layer thickness decreasing from 3·8 to 4·2: a factor of 7–10 reduction in FRS–PTSA moulds. If the first two coatings acted mainly as desulphurisers, the Mg–FeSi coating had an additional role to desulphurisation and provided Mg for the molten iron, to restore some nodularising potential. It is important to combine desulphurisation with supplementary Mg to remove migrating S and restore lost Mg. This complex behaviour could be realised by the application of coatings with active Mg from fine sized Mg–FeSi.