1999 Ductile Iron Production - A Comparison of Alternative Treatment Methods
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Ductile Iron Production - A Comparison of
Alternative Treatment Methods
by
Dr. Torbjørn Skaland
Manager of Research
Elkem ASA, Research
P.O. Box 8040, Vaagsbygd
N-4675 Kristiansand
Norway
Tel. + 47 38 01 70 00
Fax. + 47 38 01 74 94Ductile Iron Production - A Comparison of
Alternative Treatment Methods
Dr. T. Skaland
Abstract :
The objective of the present paper is to give an overview of various commercial
ductile iron treatment processes available and in particular, major advantages and
disadvantages of the various methods. The addition of nodularizing agents to molten
iron is probably the most important single step in the production of ductile iron.
Magnesium is the most widely used spheroidizing element, and it is frequently added
with cerium and other rare earth materials. The objective of the founding business is
to produce the quality required at the lowest possible cost. Recovery of the treatment
method and alloy are key factors in this respect.
The list of variables that influence recovery includes: metal temperature, type and
size of nodularizing material, quantity of metal being treated, rate of tapping, and
treatment method. This paper will focus mainly on those treatment methods that have
proven to be the most commercially successful. These are known as the sandwich
process, the tundish cover process, the flow through process, the in-mould process,
and the cored wire process.
An overview of each process is given, and important characteristics, possibilities and
limitations described. Guidelines for how to choose the right treatment method is
given, as well as suggestions for how to improve existing equipment. Finally, effects
of the main element's magnesium, calcium, rare earth’s and silicon, added through
nodularizing agents are described.
1. INTRODUCTION
Magnesium treatment for nodularization of ductile iron can be carried out by several
different treatment processes. Magnesium is the most popular nodularizing agent,
and it is usually added in multi-component alloy form. These components contain
additions to reduce reaction violence, promote graphite spheroidization, neutralize
the effect of impurities on graphite morphology, and control the matrix structure. The
most common alloys for nodularizing iron are ferrosilicon containing 3 to 12 per cent
magnesium. Magnesium and its alloys possess low vapourization temperatures, and
consequently their addition to molten iron must be done with extreme care.
The treatment is a means of modifying the solidification structure so that the graphite
phase precipitates and grows as spherical particles instead of flakes, thus resulting in
a cast iron with significantly improved mechanical properties. The nodularizing action
can be regarded as a simultaneous desulphurization and deoxidation treatment,
where elements having strong affinity to both sulphur and oxygen are added. When
dissolved sulphur and oxygen are removed from the liquid, graphite growth will
proceed as nodules according to its crystallographic features.2. PROCESS HISTORY
During the years of ductile iron history from the early 1950s a range of different
treatment processes has been developed. Some were found acceptable while others
are rejected a long time ago. Some of the early processes involve lance injection and
stirring of the ladle, but most of these are not seen in foundries any more. The most
widespread processes today are the group involving overpouring of an MgFeSi-alloy
in some kind of treatment ladle. Various modified ladles to improve magnesium
recovery has been designed. Some of the best alternatives also involve application of
ladle cover lids. /1/
Another group of ductile iron treatment processes is the continuos methods
represented by the in-mould process and the flow-through process. These processes
differ from others in the sense that iron is treated continuously during overflow of the
alloy. The MgFeSi is placed in a chamber either in a specially designed box (flow-
through) or inside the sand mould as part of the runner system (in-mould).
Finally, the last process to join the family is the cored wire process. This process has
been gaining popularity during the last 10 years, and it represents a kind of process
where the magnesium material (metal or alloy) is packed in a hollow steel tube that is
injected into the liquid iron. The tube dissolves in contact with the hot metal, and the
magnesium reaction goes on for as long as it takes to inject the required length of
wire.
3. MAGNESIUM TREATMENT PROCESSES
This section gives an overview of the most common ductile iron treatment processes
at present. These are:
Overpour or sandwich
Tundish cover
In the mould
Flow through
