Influence of magnesium content and coating type on graphite degeneration in surface layer of iron castings in resin sand - P-toluol sulphonic acid (PTSA) moulds

Abstract

The performance of coatings on furan resin sand moulds [P-Toluol Sulphonic Acid (PTSA) as hardener] [FRS-PTSA], was compared to 'no sulphur' Novolak resin coated sand [NRS] moulds by analysing the graphite characteristics in the surface layer of Mg-treated irons (0.020-0.054%Mg) and over the entire section. The extent of an abnormal surface layer is influenced by the different Mg contents for compacted or nodular graphite irons, the sulphur content of the moulding system and the type of mould coating (containing sulphur or desulphurization materials). With lower Mg content, more graphite degeneration was apparent in the cast surface layer, especially at less than 0.03%Mg res [typical Mg content for vermicular/compacted graphite cast irons] with increasing differences between FRS-PTSA and NRS moulds, and uncoated and coated moulds results. Un-coated FRS-PTSA moulds prompted five times more degenerate graphite in the surface layer than NRS moulds. Sulphur bearing coatings increased the layer up to five times the thickness in NRS moulds and up to 50% in FRS-PTSA moulds, especially for less than 0.030%Mg residual. Desulphurization type coatings based on MgO appear to be efficient for high Mg-content iron (0.05%Mg, ductile iron) but not active enough for less than 0.03%Mg (compacted graphite iron type). The change in graphite characteristics in the centre of samples evolved in a clear relationship with degenerate graphite in the surface layer, for the experimental solidification conditions. © 2012 ISIJ.