Of all methods developed, four have stood the test of time and are seen around the world today: ladle, inmould, cored wire and converter treatment.
Evolution Of MgFeSi Master Alloy
MgFeSi was early recognized as an excellent and cost effec- tive material to introduce Mg to the cast iron melt.3,4,5
Why MgFeSi Master Alloy
As mentioned earlier, adding pure Mg to molten cast iron is not easy, and therefore metals or alloys in which Mg is soluble were developed. Magnesium was found to be soluble in met- als like nickel, copper and silicon. Since the treatment method at that time involved adding the alloy to the top of the melt, it was advantageous to have an alloy with similar or higher den- sity than the molten cast iron to facilitate that the Mg would be submerged in the melt prior to reacting. The development of MgFeSi provided an alloy system with density lower than the NiMg alloys but significantly higher than pure Mg metal. Magnesium is soluble in liquid FeSi and forms stable Mg-rich phases with silicon upon solidification, which are trapped be- tween the normal phases in FeSi. When added to the molten iron, this phase releases more slowly in small doses into the
iron, greatly reducing the violence of the reaction.
To be suitable as a treatment alloy for ductile iron, MgFeSi master alloy should ideally have a high density as well as a low melting point to ensure a reaction with high recovery. Looking at the phase diagram for FeSi in Fig.6 there are two FeSi compositions which could be suitable for production of MgFeSi; one at around 20wt% silicon and the other at around 50wt% silicon. The composition with around 50wt% silicon is the preferred and more common. At this silicon level, Mg can be added without risking an explosion and violent reaction during production of MgFeSi and later use of the MgFeSi.13
The early MgFeSi master alloys contained around 10wt% Mg, but it was soon discovered that the recovery could be improved when the Mg-content was decreased to around 5wt%. At a treatment temperature of 1450°C, the vapor pressure of a 5wt% Mg-containing MgFeSi would be less than 1 atmosphere while for a 10wt% Mg containing Mg- FeSi would be around 1.5 atmosphere .15
For a sandwich treatment process, this change from 10wt% Mg to 5wt% Mg in the MgFeSi could provide a 50% im- provement in Mg-recovery as indicated by Fig.7.
Figure 6. Phase diagram of FeSi indicating common composition for production of MgFeSi in green and inoculants in blue.14
10 International Journal of Metalcasting/Volume 8, Issue 2, 2014
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