Benefits of Residual Aluminum in Ductile Iron

Abstract

Two main chemistry systems of micro-inclusions can be identified in ductile iron: Ca-S-X and Mg-Si-O-X with the majority of the inclusions regardless of treatment type and location being of the second type, silicates. Laboratory investigations have shown that simple silicates were present in the matrix, while more complex silicates were present in conjunction with graphite, probably acting as graphite nucleation sites. In these more complex silicates, elevated levels of Al, Ca, Ce and La were typical. Comparing micro-particles embedded in iron matrix and graphite nodules of iron treated with pure Mg-metal and iron treated with MgFeSi alloy showed a higher amount of complex silicates with elevated Al-levels in the iron treated with MgFeSi. Further laboratory investigation was undertaken to explore which source of Al and which range of residual Al would have a favorable impact on the graphite nucleation in ductile iron. The work showed that a residual aluminum of 0.005 to 0.020 wt.% appears to be beneficial for improving ductile iron solidification characteristics without the incidence of pinholes. Greatest benefits were achieved when introducing the Al into the iron via an inoculant late during processing or via a pre-conditioner to the base iron. Al added via the MgFeSi provided less benefit. Some case studies illustrating the effect of Al in ductile iron are also presented, as Al-containing pre-conditioner or/and Al-bearing, FeSi inoculant application.

Enhanced Quality in Electric Melt Grey Cast Irons

“…22) In foundry practice grey irons with low S levels, usually solidify with high eutectic undercooling, leading to the formation of carbides and/or undercooled graphite morphologies, especially in thin wall castings, commonly used in the automotive industry. 2,3,18,[23][24][25] Figure 6(a) shows important changes to the eutectic undercooling parameters at different sulphur levels, in both base and inoculated irons, and at low aluminium content (0.003 wt.%Al).…”

“…8(a)). 25,29) The condition of the liquid iron is heavily dependent on the quality of the original charge materials but then change after superheating the bath. Liquid iron evolves from a colloidal liquid state after melting (suspension of graphite particles) through quasi-homogenous up to a quasi-ideal solution.…”

“…8(b)). 25,29) Detailed results from experimental melting and superheating up to 1 600°C in an acid lined, medium frequency coreless induction furnace, on changes to the solidification characteristics, chemistry and chill tendency, were presented. 24) Melting led to oxidation of most of the important elements (C, Si, Mn, Zr, Al, Cr); C, Al and Zr content continued to decrease up to 1 600°C, while Si and in some cases also Mn and N showed a slight increase especially at temperatures above 1 500°C (Fig.…”

“…9). 24) Holding the iron melt at high temperature (1 530°C) contributed to important loss of C, Al and Zr.…”

“…10). 24,30) Eutectic undercooling (ΔTm) is defined as the difference between the eutectic temperature in the stable system (Tst) and the lowest eutectic temperature (TEU). The parameter relative clear chill (RCC) is defined by RCC = 100 [Wc/B], where B is the maximum width of the test wedge.…”