2013 Volume 53 Issue 10 Pages 1683-1695
The paper reviews original data obtained by the authors, recently disconnected published, concerning the specific solidification pattern of low S (<0.05%) irons, with very low Al (<0.005%), melted and superheated in acid lined, coreless induction furnaces, and how superheating affects the iron quality with effective metallurgical treatments for use in these conditions. Solidification undercooling increased with increasing superheat, associated with significant changes in chemical composition, such as C, Si, Mn, Al and Zr, involved in the nucleation of graphite. The concept in the present paper sustains a three-stage model for nucleating flake graphite [(Mn,X)S type nuclei]. There are three important groups of elements [deoxidizer/Mn, S/inoculating] and three technology stages in electric melt iron [superheat/pre-conditioning base iron/final inoculation]. Different materials were used for pre-treatment of the iron melt, to control oxidization levels and/or to promote active graphite nucleation sites, including carbon materials and metallurgical silicon carbide. Special attention was paid to maintain Al and Zr recoveries in the melting furnace for their effects on the iron structure. A double treatment utilizing strong oxide forming elements, such as Al and Zr for preconditioning, followed by inoculation decreased eutectic undercooling parameters. This treatment improved graphite characteristics and avoided carbides. For foundry application, it is recommended to ensure (Mn,X)S compound formation, compatible for nucleating graphite with less eutectic undercooling. Attention is drawn to ensuring a control factor (%Mn) × (%S) equals 0.03 – 0.06, accompanied by 0.005–0.010% Al and/or Zr content in inoculated grey irons.