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Technical Information 28 Magnesium versus Sulphur in Ductile Iron Magnesium is added to liquid iron through the nodularizing operation mainly to desulphurize and deoxidize the base iron. When the base iron is properly desulphurized, graphite will grow as spheres instead of flakes resulting in good ductile iron. The principal desulphurizing reaction of magnesium in liquid iron is as follows: Mg + S = MgS This directly implies that lower base iron sulphur content will minimize the requirement for magnesium addition to make good ductile iron. The curves below show a schematic relationship between the base iron sulphur content and required residual magnesium to produce ductile iron.
Figure 1. Schematic representation of the relation between base iron sulphur content and required residual magnesium to produce ductile iron.
The curves in Figure 1 can be explained as follows. The lower straight line indicates the minimum theoretical Mg required to give a 1:1 ratio between Mg and S. However, Mg also goes into solution in the iron and evaporates (fades) during time, and the upper three curves shows recommended Mg addition to account for such dissolution and fading effects. When iron of a base sulphur of 0.02%S is faded for about 12-15 minutes, a residual initial Mg of about 0.05% may be required. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway
Web www.foundry.elkem.com Org. no. NO 911 382 008 MVA
Revision No. 2.1 20.03.2004
Technical Information 28
2 Slag Large MgO, MgS and Mg-Si-oxide particles
Mg O
Nucleation Sites Small MgO, MgS and Mg-Si-oxide particles
S
=
Desulphurising
+
Deoxidising
Figure 2. Principal effects of magnesium in tying up sulphur and oxygen to produce slag and nucleation sites.
Figure 2 above shows a schematic representation of the principal reactions between Mg, S and O forming potential slag and nucleation sites for graphite. Figure 3 below shows an example of the effects of two base iron sulphur levels and a low and constant MgFeSi addition. The higher S of 0.018% is excess for the low Mg causing poor nodularity. Base S = 0.018 % 1.0 wt% MgFeSi
Base S = 0.010 % 1.0 wt% MgFeSi
Final Mg = 0.033 % Final S = 0.016 %
Final Mg = 0.030 % Final S = 0.010 %
Nodularity
Nodularity
50 %
90 %
Figure 3. Example of the effects of base iron sulphur at 0.018% and 0.010% respectively, for a constant addition of magnesium at 1 wt% MgFeSi alloy giving a constant residual Mg around 0.03% in the treated iron.
For more information about residual magnesium in ductile iron, please refer to Elkem Technical Information sheet 7.
