Introduction of Calcium in MgFeSi
Calcium was later added to MgFeSi to decrease reaction and reduce the boiling and loss of Mg.1
Calcium16 was one of
many elements that research later showed to have the abil- ity to produce spheroidal graphite similar to that of Mg and cerium.17
Calcium has a similar strong affinity to sulfur and oxygen as Mg, but compared to Mg, calcium has a higher vapor pressure at the operating temperatures for production of ductile iron.
of this phase, as seen in Fig.8, provided a calmer reaction and thus a positive effect on the Mg-recovery.
By adding calcium to MgFeSi a second Mg-containing phase was introduced into the structure: CaMgSi2
.18 The presence
Research showed that the calcium level should not greatly ex- ceed 2wt% as this would inhibit the reaction between the Mg- FeSi and iron and potentially give undissolved MgFeSi.19,20
Parallel to the development of MgFeSi master alloys in North America, SiCaMg master alloys were developed and dominated as treatment alloy in use in Germany.20
Introduction of Rare Earths in MgFeSi
During the discovery phase of ductile iron, rare earth ele- ments and especially cerium, were shown to be an alternate route to produce nodular structures.17
Rare earth elements
were later incorporated into MgFeSi. Optimized levels were shown to reduce the chance of edge carbides and increase nodule count.21,22,23
Small rare earth additions to ductile iron such as via MgFeSi or NiMgRE were developed as a means to neutralize the damaging effect to the graphite structure that these elements could cause. The content of the rare earth ele-
The importance of trace elements such as Pb, Sb, Bi, As, P and Ti were seen as crucial to control in order to make good ductile iron.24
Figure 7. Comparison of Mg-recovery for 5% containing versus 10% containing MgFeSi at different treatment temperatures.13
The introduction of Ca and rare earths to MgFeSi produced several improvement effects into one alloy. Both Ca and the rare earth elements are strong deoxidizers and desulfurizers and support Mg in tying up S and O allowing for reduction in the Mg content needed to neutralize S and O. In addition,
ments in MgFeSi increased over the years as the amount of MgFeSi required for treatment declined with time.
Mischmetal and rare earth silicides became the most com- mon types of rare earth sources used. These normally con- tained the elements cerium (Ce), lanthanum (La), neodymi- um (Nd), and praseodymium (Pr). Using the individual rare earth elements as pure metals was initially cost prohibitive. Also at that time several rare earth sources were avail- able with different compositions. Studies made by Lalich21 showed that source and relationship between the various main rare earth elements to be of importance.
Figure 8. Overview of the typical structure and phases present in MgFeSi.20 International Journal of Metalcasting/Volume 8, Issue 2, 2014 11
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