Inoculation of Grey and Ductile Iron a Comparison of Nucleation Sites and Some Practical Advises

8/3/2019 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.

Documents

Inoculation of Grey and Ductile Iron a Comparison of Nucleation Sites and Some Practical Advises