Modifying iron grits (MIG) for magnesium treatment of cast irons

摘要

The possibility to increase the graphite nodulizing efficiency was studied by improved capitalization of the specific iron heredity. In this respect, a special Mg-treatment technique is proposed: the utilization of Modifying Iron Grits (MIG) - i.e. supersaturated iron grits, with high active elements content [up to 1.5 wt% (Mg+Ca+Ce)] and up to 35,000 Graphite Nodules/mm{sup}2 at a reduced size (65-70% less than 1.0μm diameter) and/or a metastable structure with a high decomposition capacity (during the heating process) in order to easily generate compact graphite particles. These iron grits (0.3-10 mm) were obtained by a water quenched route (up to 50,000 °C/s), in the Fe-CE (carrier elements -Si, Ni, Cu, Mn) -AE (active elements -Mg, Ce, Ca) Systems, at 4.0-4.7% carbon equivalent. Several alternative MIG-additions (furnace, ladle or inmould) were used in the experiment and a suspension state of potential nucleation substrates for graphite germination were created (MIG included and/or generated compact graphite particles). Several graphite morphologies (nodular, vermicular and coral) were obtained at lower Mg{sub}(res) (NG - 0.02-0.025% Mg; VG- 0.01-0.018% Mg; CG- 0.005-0.010% Mg) and at a higher Mg-recovery (70-98%). MIG addition and re-addition from time to time in the furnace or in the iron stream may be a solution for heavy treated irons production, without violent reaction and with reduced Mg-loss (K=0.02-0.065 as Mg-loss constant value in the induction furnace). NG or VG iron type heavy castings may also be a MIG application field, because of the possibility to introduce these grits into a mould cavity, especially in the critical areas. Even at reduced MIG-addition, treated cast irons show a higher sensitivity to carbide formation but an adequate inoculation develops an increased graphite nodule count by 1.3-1.8 times, in comparison with conventional Mg-treated irons.