THE CONTINUING EVOLUTION OF MgFeSi TREATMENTS FOR DUCTILE AND CG IRONS
Cathrine Hartung Elkem AS, Foundry Technology Products R&D, Kristiansand, Norway
Douglas White, Kenneth Copi and Matthew Liptak Elkem Materials Inc, Pittsburgh, PA, USA Robert Logan
Elkem Metal Canada, Hamilton, Ontario, Canada Copyright © 2014 American Foundry Society
A version of this paper was previously published in the 2013 Keith Millis Symposium Proceedings Abstract
Production of ductile iron has been linked to the use of Mg and Mg-alloys since its invention. In the early history of ductile iron production, pure or high Mg con- taining alloys were needed due to the higher S level in the base iron.
With the introduction of basic slag cupolas and low S base iron, the high Mg containing alloys were replaced with ma- terials with lower Mg-content providing a calmer reaction with less slag generation and more predictable results and tighter control of the final Mg level.
MgFeSi was recognized early as an excellent material to introduce Mg into the cast iron melt. MgFeSi offered the possibility to design both chemical composition and sizing to accommodate needs such as reaction control for different treatment processes, trace element control, packing density, and boosting of Mg-level.
Introduction
The race to invent a material with higher strength and ductil- ity1
really
took off in the second half of the 1930s with the presentation of a fully spheroidal graphite structure as cast at the sixth International Casting Congress in Düsseldorf in 1936.
In the race were researchers from Germany, United King- dom and the U.S., and they all made it to the finish line in 1948 with their ways to make what we now know as ductile iron. However, it was the production method of the Americans with Keith Millis et. al3
using Mg to obtain
spheroidal graphite that became the commercially viable production method.
International Journal of Metalcasting/Volume 8, Issue 2, 2014
that combined the castability of grey iron and toughness of steel without the heat treatment of malleable iron2
One of the main goals with MgFeSi treatments is to minimize the overall addition of Mg to tight reproducible low levels to reduce the shrinkage tendency observed with high final Mg levels. This can be achieved through a combination of opti- mization of the MgFeSi composition as well as improvement of the treatment process. A well designed MgFeSi-treatment can reduce fume and flare, slag generation, and the need for subsequent inoculation.
The history and evolution of MgFeSi treatment along with relevant recent case studies will be presented in this paper. Examples of how the MgFeSi material can be optimized will be presented both in terms of theory and case studies. Focus of case studies will be on the recent evolutions focusing on maximizing Mg-yield through delayed and calm reaction.
Keywords: ductile iron, Mg-treatment, MgFeSi-alloys, optimized recovery, improved mechanical properties, reduced scrap
Although research that followed in the years after 1948 has shown that several other elements are capable of making spheroidal graphite, it is still Mg that is the most cost ef- fective and preferred element. What has changed is the Mg- sources and how they are introduced into the iron.
Evolution of Treatment Method
Production of ductile iron requires at least one additional step in the process compared to production of grey iron. This step has different names in different parts of the world, but the pur- pose of the step is to ensure graphite formation and growth in the shape of spheroids or nodules. The step is often referred to as magnesium treatment, spheroidization, or nodularization. Magnesium is added to the melt to tie up S and O allowing the graphite to grow as spheres instead of flakes.
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