the higher vapor pressure of both Ca and rare earth elements make it possible to work with a higher amount of iron in the treatment ladle, as the reaction is calmer. A third com- mon effect of Ca and rare earths is the improved inoculation effect and the positive influence seen in nodule count and reduced carbide formation. A fourth effect of the rare earth elements is the ability to tie up subversive trace elements.
Thin casting of the MgFeSi alloy was introduced to opti- mize yield on sizing and also to maximize the number and minimize the size of the Mg rich phases. Both effects further reduced the Mg reactivity.
Introduction of Pure La in MgFeSi
Over subsequent years, new applications for rare earth ele- ments were developed in other industries, often requiring a single rare earth rather than a mixture. Demands for sepa- ration of Pr and Nd in particular as pure metals resulted in improved availability of Ce and La as a mixture or pure metals. The first use of pure La in MgFeSi dates back to the early inmould process days, where reduced shrinkage tendency was demonstrated by the introduction of La con- taining MgFeSi.25
The use of pure La in MgFeSi as a ladle alloy was introduced around the turn of the century and grew to a large proportion of the market based upon reduced shrinkage tendency and improved machined surface finish.26
The reduced amount of
La metal required to replace other rare earth types can also provide a cost advantage.
Flexible MgFeSi Design for Different Foundry Circumstances
MgFeSi chemistry and size can be altered to suit foundry requirements. Foundries producing very thin castings that must be treated very hot and are prone to carbides will nor- mally consider a lower Mg content design, perhaps with ele- vated Ca and rare earth. Smaller ladles will normally benefit from smaller MgFeSi alloy sizing.
Foundries producing very thick castings where treatments are long at low treatment temperature may consider higher Mg content alloys. These may be rare earth free to avoid chunky graphite, or contain rare earth if the foundry also in- tends to balance the rare earth content with a known amount of tramp element such as Sb to optimize nodule count and nodule quality. Larger alloy sizes are frequently requested for very large ladles, especially if they do not benefit from having a pocket in the bottom.
Ductile Iron Today and Tomorrow
Ductile iron casting production has grown from 100,000 tons of finished goods in 1953 to above 1,000,000 tons in 1964 and close to 25,000,000 tons in 2012.1, 25
12
Growth of ductile iron has been related to the strength to cost ratio replacing steel castings and weldments as well as the strength to weight ratio to provide lighter components to the transportation industries. Initially this was achieved with various grades controlled by adjusting the matrix from alloy- ing additions such as Cu. Heat treatment to produce austem- pered ductile iron has provided the opportunity to provide new higher levels of properties and maximize those effects.
Unique grades of ductile iron have been developed for spe- cial applications such as SiMo and austenitic grades for high temperature cycling applications.
What are the next improvements to meet the goal of all busi- nesses: “To improve quality and lower costs”?
Continuing Moving Towards 100% Mg-Recovery
The treatment process will be thermally efficient to allow treating at the lowest possible temperature. In addition to en- hanced Mg recovery, energy usage is minimized, furnace lin- ing costs are minimized, and productivity gains are possible.
Process and alloy improvements that move toward 100% Mg recovery will provide more environmentally friendly production. This means reduced dust collection costs and a more worker friendly environment.
At 100% Mg recovery, final Mg targets will be met with less variation, allowing lower Mg targets that minimize shrink- age, the risk for carbides, and improve production.
At 100% Mg recovery slag generation is minimized. This can have the advantage of reduced maintenance and refrac- tory costs for treatment ladles and pouring ladles or auto- pours. Slag defects may be reduced.
MgFeSi master alloy in combination with ladle treatment has come a long way in providing the solutions for the future.
The following examples from foundries in the U.S. and Can- ada demonstrate that it is possible to make large gains mov- ing toward the 100% Mg recovery goal which will provide many opportunities for substantial cost reductions.
These concepts are not new. How foundries are currently pushing them further to new levels is summarized in the fol- lowing foundry examples.
Switching to a MgFeSi Grade with Lower Mg-Content
Since the conversion from 10wt% MgFeSi alloys to 5wt% Mg content, there has been a general trend for foundries to request small increases in wt% Mg in an attempt to use less alloy. More recently we have observed that this simply creates more fume, flare, and slag. Repeatedly tests have been run reducing
International Journal of Metalcasting/Volume 8, Issue 2, 2014
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