China Foundry (May 2015)
Iron casting skin management in no-bake mould – Effects of magnesium residual level and mould coating
Abstract
The relative performance of coatings for furan resin sand moulds [P-toluol sulphonic acid (PTSA) as hardener] [FRS-PTSA moulds], was compared by analyzing the surface layer for degenerated graphite in Mg treated iron with 0.020wt.% to 0.054wt.% Mgres. It was found that the iron nodularising potential (Mg, Ce, La content) and whether the mould coatings contained S, or were capable of desulphurizing were important factors. These moulds have S in the PTSA binder, which aggravates graphite degeneration in the surface layer, depending strongly on the Mgres with lower Mgres increasing the layer thickness. The application of a mould coating strongly influenced graphite deterioration in the surface layer of castings. It either promoted graphite degeneration to less compact morphologies when using S-bearing coatings, or conversely, limited the surface layer thickness using desulphurization type coatings. Independently of the S-source at the metal – mould interface, the presence of sulphur had an adverse effect on graphite quality at the surface of Mg-treated irons, but its negative effect could also reach the graphite phase within the casting section. If the coatings employed desulphurization materials, such as MgO, or a mixture (CaO + MgO + Talc) or Mg-bearing FeSi, they protected the graphite shape, improving graphite nodularity, at the metal – mould interface, and so decreased the average layer thickness in FRS-PTSA moulds. FeSiMg was highly efficient in minimizing the casting skin by improving graphite nodularity. It is presumed that the MgO or (MgO + CaO + Talc) based coatings acted to remove any S released by the mould media. The Mg-FeSi coatings also reacted with S from the mould but additionally supplemented the Mg nodularising potential prior to solidification. This dual activity is achievable with coatings containing active magnesium derived from fine Mg-FeSi materials.
