Inoculation of Grey and Ductile Iron a Comparison of Nucleation Sites and Some Practical Advises
Transcript of Inoculation of Grey and Ductile Iron a Comparison of Nucleation Sites and Some Practical Advises
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INOCULATION OF GREY AND DUCTILE IRON
A COMPARISON OF NUCLEATION SITES AND SOME PRACTICAL
ADVISES
Svein Oddvar Olsen*, Torbjrn Skaland*, Cathrine Hartung*
Elkem ASA, Foundry Products Division, NORWAY
ABSTRACT
The objective of this paper is to review some important aspects related to cast ironinoculation. Important conditions in the production of cast iron are described and
characteristic microstructures and mechanical properties exemplify the difference
between inoculated and un-inoculated irons.
Principal mechanisms of inoculation and graphite nucleation in grey and ductile
irons are described. The findings are based on advanced electron microscopy
studies of micro-particles as heterogeneous nucleation sites for graphite. Effects of
minor alloying elements such as Ca, Ba, Sr, and Al are explained as well as the
critical role of oxygen and sulphur in the graphite nucleation process. (1)
Finally, the mechanisms of inoculant fading are explained and some practicaladvises for optimized and reproducible inoculation given.
Keywords: Cast iron, inoculation, graphite nucleation, fading
INTRODUCTION
In the production of quality cast irons the inoculation process is of vital
importance. When comparing un-inoculated and inoculated irons, differences in
microstructure are easily revealed, which again will strongly affect the final
mechanical properties of the casting. Through inoculation the graphite nucleation
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and eutectic undercooling of the iron can be controlled and this will be of crucial
assistance in giving the iron its required service properties.
WHAT IS INOCULATION?
Inoculation is a means to control and improve the microstructure and mechanical
properties of cast iron. The inoculation process will provide sufficient nucleation
sites for the dissolved carbon to precipitate as graphite rather than iron carbides
(cementite). The most common inoculant is a ferrosilicon based alloy with small
and defined quantities of either Ca, Ba, Sr, Zr, rare earths, and Al. Examples of
un-inoculated and inoculated irons are shown in Figure 1 and the influence of
inoculation on mechanical properties in Figure 2. Consequently, the effects of
grey and ductile iron inoculation are improved machinability, increased strength
and ductility, reduced hardness and section sensitivity and a more homogeneousmicrostructure. Typically, inoculation also reduces the tendency for solidification
shrinkage formation.
GREY IRON INOCULATION
The grey iron microstructure is normally determined by the base iron
composition, the solidification cooling rate and the inoculation process. Figure 3
shows different grey iron microstructures as a function of solidification
undercooling. Controlled undercooling promote the normally desired type A flakegraphite, characterised by randomly distributed graphite flakes in a fully pearlitic
matrix. The role of inoculation is to provide sufficient nucleation sites for graphite
that is activated at low undercooling, thus promoting the formation of good type A
graphite structures. Hence, inoculation is a means to change the otherwise
undesired graphite forms into a more desired form.
It has been found that balancing manganese and sulphur is important for the
machinability of grey iron. Experiences have also resulted in a recommended ratio
between manganese and sulphur in grey iron. Manganese should be adjusted to
balance the residual sulphur level according to the following relationship:
%Mn = %S x 1.7 + 0.3 [1]
Table 1 shows the influence of Mn:S ratio on eutectic cell count and chill
tendency in un-inoculated condition. This relationship also suggests that MnS
inclusions could act as nucleation sites for graphite flakes. The crystal lattice
match between cubic MnS and hexagonal graphite is actually quite good. It is also
known that if the sulphur content is less than about 0.03%, although balanced
properly by Mn, the number of MnS inclusions will be insufficient to produce
effective nucleation of good type A graphite structures.
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Further, scanning electron microscope (SEM) investigations has shown that in un-
inoculated and inoculated irons the number of MnS inclusions are about the same,
but the distribution tends to be somewhat different. In un-inoculated iron, MnS
inclusions are predominantly found between the primary austenite dendrites while
in inoculated iron these inclusions are found to be more randomly distributedthroughout the iron matrix. This suggests that inoculation is affecting the
formation sequence of MnS particles during cooling and solidification. Figure 4
shows an example of an inclusion that has acted as nuclei for graphite flake. The
figure shows the distribution of relative intensity (X-ray mapping) of the different
constituent elements. From this analysis it can be seen that a Mn(X)S compound
with a core of Al/Ca oxides is present as graphite nucleation site. Further studies
show that Ba and Sr can act the same way as Ca and Al. This means that the
active elements in the inoculant, Ca-Ba-Sr-Al, primarily will form stable oxides
that can act as nuclei for the Mn(X)S phase to precipitate on. The sulphide particle
will again be the preferred nuclei for graphite flakes to grow from upon
solidification. For the foundry it is therefore very important that the Mn:S ratio isadjusted to the right level and that some oxygen is also available for the
inoculating elements to combine with in the production of grey iron. (3, 6, 7, 8)
DUCTILE IRON INOCULATION
Figures 5 shows examples of microstructure in inoculated and un-inoculated
ductile irons. The extensive chill (carbides) in un-inoculated condition will
destroy the mechanical properties of this iron and make it very difficult to
machine such castings. Hence, inoculation is a crucial requirement for mostductile iron processes simply to make machinable castings.
In ductile iron the nodularising treatment will influence inoculation efficiency and
therefore it is important to select the correct treatment process and magnesium
bearing material. Formation of a high number of small micro-inclusions during
magnesium treatment is an advantage, and Figure 6 shows how nodularising
provides the basis for an effective subsequent inoculation. Also, Figure 7 shows
how investigations of micro-inclusions at different magnifications have led to the
discovery of the nucleation site for graphite in ductile iron. During nodularising,
numerous inclusions are formed with a sulphide core and an outer shell containing
complex magnesium silicates. Such micro-inclusions will however not provideeffective nucleation of graphite because the crystal lattice structure of magnesium
silicates does not match well with the lattice structure of graphite. However, after
inoculation with a ferrosilicon alloy containing Ca, Ba or Sr, the surface of the
magnesium silicate micro-particles will be modified and other complex Ca, Sr, or
Ba silicate layers will be produced (see Figure 8). Such silicates have the same
hexagonal crystal lattice structure as graphite, and due to very good lattice mach
will therefore act as effective nucleation sites for graphite nodules to grow from
during solidification. (1)
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FADING OF INOCULATION EFFECT
The gradual loss of inoculation effect during liquid metal holding is well known
to the foundry people, and this fading of inoculation will eventually result in
carbide formation and poor graphite structures if the iron is held for prolongedtimes before pouring. The reason for this fading loss is coarsening and growth of
micro-inclusions, also called the Ostwald Ripening Effect. The driving force for
this coarsening is a reduction in the specific surface area of inclusions, thus
reducing the total energy of the system. The volume fraction of non-metallic
inclusions will however remain unchanged due to the high particle phase stability.
(10) This fading effect is very fast just after inoculation when distances between
micro-particles are short, and is much more severe to the iron quality than fading
losses of residual magnesium. Figures 9 and 10 show this inoculation fading
effect by particle coarsening and a reduction in the number density of potential
nucleation sites during time. The fading rate of inoculation is directly related to
the diffusion rate of reactive elements through the liquid metal.
INOCULATION METHODS
The required addition rate of an inoculant to liquid iron is very much depending
on where and when it is to be introduced. Figure 11 shows an example of
substantial reductions in addition rate when going from an early addition to the
transfer ladle to a late addition to the metal stream. At transfer, the required
inoculant addition rate may be as high as 1 wt%, while the alternative late in-
stream inoculation may require only 0.1 wt% addition still providing sufficient oreven better inoculation effectiveness. This is primarily due to the late addition
giving much less time available for particle coarsening and fading effects. (2, 4, 5)
INOCULATION ELEMENTS
The main finding from studies of micro-inclusions as nucleation sites for graphite
is that the key nucleating elements in the inoculant are Ca, Ba, Sr and Al. The
ferrosilicon alloy itself is only the carrier material of these critical active elements,
but is also needed in order to give these minor elements the right concentrationand solubility for an optimum inoculation performance.
COMPARISON OF ACTIVE MICRO-INCLUSIONS
In grey iron it is found that small oxide particles will acts as the nuclei for
Mn(X)S that again will be the decisive nuclei for graphite flakes to grow from at
small undercoolings. In ductile iron however, a stable sulphide core is found to be
the nuclei for complex silicates that again will be modified by the active elements
in the inoculant before it can act as a potent nuclei for graphite. However, the
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same specialty ferrosilicon inoculant materials are still being used for both grey
and ductile irons and the main reason is that key elements are highly reactive and
can form various types of micro-inclusions, some of them being favourable sites
for graphite to grow from during solidification.
SUMMARY
The principal inoculation mechanisms are quite different in grey and ductile irons.
In grey iron, a stable oxide will be the primary nuclei for manganese sulphide
precipitation that again will nucleate graphite flakes of good type A form. In
ductile iron, a sulphide is the nuclei for complex silicates that again will nucleate a
high number of graphite nodules. The same inoculant materials can however be
used successfully in both type of irons, since the reactive elements such as Ca, Ba,
Sr and Al are all strong oxide, sulphide and silicates formers in both grey orductile irons.
The inoculant fading effect is connected to diffusion rate, growth and coarsening,
and a general reduction in the number density of micro-inclusions as nucleation
sites for graphite.
In order to obtain a sound and reproducible iron production process some critical
inoculation factors will have to be controlled properly. For grey iron one should
pay special attention to the following factors:
1) The Mn:S ratio should be maintained at the same level every time and sulphurshould preferentially be kept at minimum 0.05%.
2) Aluminium is found to be an important part of the nucleus core and should beadjusted and kept at controlled levels every time. Recommended residual Al-
level in grey iron is 0.005% - 0.01% for optimum inoculation effectiveness.
3) There should be a certain oxygen level in the base iron from fresh metalprocessing. The use of some rusty raw materials may assist in providing a
good oxygen potential.
4) Pouring time after inoculation should be minimized in order to keep fadinglosses under control.
5) Use an inoculant with defined chemical composition and sizing.
For ductile iron, the following factors must be controlled:
1) The magnesium treatment process reactivity should be controlled andminimized. A violent treatment process will provide less potential nucleation
sites and more difficult conditions for powerful inoculation effectiveness.
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2) There should be a certain oxygen level in the base iron from fresh metalprocessing. The use of some rusty raw materials may assist in providing a
good oxygen potential.
3) The sulphur content should be kept low and constant. Preferential range forductile iron is 0.005 to 0.015% base iron sulphur content.
4) Pouring time after inoculation should be minimized in order to keep fadinglosses under control.
5) Use an inoculant with defined chemical composition and sizing.
REFERENCES
1) T.SKALAND A model for graphite formation in ductile iron. Ph.D Thesis1992 : 33, The Norwegian Institute of Technology, Norway (1992)
2) R.ELLIOTT Cast Iron Technology, 1988, London, UK, Butterworths
3) I.RIPOSAN, M.CHISAMERA, S.STAN, T.SKALAND, M.ONSOIEN Analysis of possible nucleation sites in Ca/Sr over-inoculated grey irons. AFS
Transactions vol. 109, 2001, pp. 1151-1162
4) S.I.KARSAY Ductile Iron Production, QIT, 1976
5) Elkem Technical Information Sheets No. 1 34
6) I.RIPOSAN, M.CHISAMERA, S.STAN, T.SKALAND Graphite nucleants(micro-inclusions) characterization in Ca/Sr inoculated grey irons. SPCI 7
Science and Processing of Cast Iron International Conference, Barcelona,
Spain, 2002
7) J.K.SOLBERG, M.ONSOIEN Nuclei for heterogeneous formation ofgraphite spheroids in ductile cast iron. Material Science and Technology, vol
17, October 2001, pp. 1238
8) F.NEUMANN Theorien ber das Impfen. Giesserei, No.14, July 1996, pp.
9) ASM Metals Handbook, vol 1, tenth edition, 1990, pp. 6
10) J.D.VERHOEVEN, Fundamentals of Physical Metallurgy, Chapter 8 and 10,John Wiley & Son, Inc, 1975
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UnUn--inoculatedinoculated InoculatedInoculated
Grey Iron
Ductile Iron
Figure 1: Examples of structures in un-inoculated and inoculated irons. (5)
Control ofstructure andproperties by
minimizingundercooling
andproviding
nucleation ofgraphite during
solidification
UnUn-inoculated-inoculated
InoculatedInoculated
Hardness: 700 HB
Elongation: 0 %
Tensile: 200 MPa
Example:
Tensile: 450 MPa
Hardness: 180 HB
Elongation: 10 %
Example:
Figure 2: Effects of inoculation on typical mechanical properties of ductile iron.
(5)
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Figure 3: Graphite structures as a function of eutectic undercooling in grey iron.
Table 1: Experimental results showing effects of Mn and S contents and Mn:S
ratio on eutectic cell count and chill level in grey iron.
% Mn % S Mn:S Cell Count
[mm]
Chill [mm]
0.8 0.012 67 15 13
1.0 0.022 46 15 10
0.8 0.065 12 21 7
0.3 0.20 1.5 69 23
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a) SEM micrograph of (Mn,X)S
compound and graphite flake
b) Distribution of Carbon
c) Distribution of Manganese d) Distribution of Sulphur
e) Distribution of Aluminium f) Distribution of Calcium
g) Chemical composition along a cross line through the (Mn,X)S compound.
Figure 4: X-ray mapping showing composition of micro-inclusion as nuclei for
graphite flake in grey iron. (3, 6)
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Good Inoculation
Improved Recovery
Reduced Mg-Addition
Poor InoculatedPoor Inoculated InoculatedInoculated
P r o p e r ty U n in o c u la te d I n o c u la te d
P r o o f S t r e n g t h R p0 . 2 N o t d e tec ted 2 0 0 - 4 0 0 M P a
T e n s i l e S t r e n g t h R m < 3 0 0 M P a 3 5 0 - 8 0 0 M P a
E lo n g a t io n A 5 N o t d e tec ted 3 - 3 0 %
B rin e ll H ar d n es s H B > 6 0 0 1 4 0 - 3 0 0
N o d u le C o u n t 1 0 m m sec t io n < 5 0 p e r m m 2 > 1 5 0 p e r m m 2
M ic r o s tru c tu r e A S T MClas s i f i ca t ion
C a rb id ic F e r ri t ic an d /o rP ea r l i t i c
Figure 5: Examples of microstructure and mechanical properties in un-inoculated
and inoculated ductile irons. (5)
SlagNuclei
Size D istribution
TreatmentReactivity
5 m
Figure 6: Schematic representation of size distribution of inclusions as micro-
nuclei and slag in treated ductile iron.
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a) 100x (optical)
d) Schematic compositionc) 70,000x (TEM)
b) 1,000x (SEM)
XO SiO2 or
XO Al2O3 2SiO2
Where X = Ca, Sr or Ba
Figure 7: Ductile iron micro-inclusions at different magnifications and the
schematic composition of nucleation sites for graphite. (1)
Inoculation
XO SiO2 or
XO Al2O3 2SiO2
Where X = Ca, Sr or Ba
MgO SiO22MgO 2SiO2
Core: MgSCaS
Shell:
Major constituent phases:
Mg-treatment
Figure 8: Schematic representation of micro-inclusion composition in treated
ductile iron before and after inoculation. (1)
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Time
Nucleation Sites
Fading of Inoculation = Coarsening of InclusionsFading of Inoculation = Coarsening of Inclusions
OstwaldOstwald - Ripening - Effect- Ripening - Effect
Figure 9: Fading of inoculation described as a coarsening phenomenon causing
reduction in the number density of potential nucleation sites.
Figure 10: Calculated reduction in number density of micro-inclusions as a
function of holding time after inoculation. (1)
In-stream
444
333
Transfer Ladle
Position 1 2 3 4
Addition rate [wt%] 0.3 1.0 0.3 0.5 0.05 0.2 0.04 0.2
Sizing [mm] 0.5 15 0.5 10 0.2 1 0.5 5
Examples:Examples:
Pouring Ladle
222
111
Figure 11: Schematic representation of different methods for inoculant addition to
the transfer ladle, pouring ladle or mould.