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Graphite Growth Morphologies in Cast Iron
Haji Muhammad Muhmonda and Hasse Fredrikssonb Materials Science
and Engineering department, KTH, Sweden a,b
[email protected] a, [email protected] b
Keywords: Cast iron, graphite morphology, transition, clusters,
micro-segregation, ion etching, and thermodynamics.
Abstract. Graphite growth morphology was studied by using
InLense detector on FEG-SEM after performing ion etching on the
samples. Star like and circumferential growth mechanism of graphite
was observed in the graphite nodules. Pure ternary alloy of hypo
eutectic and hyper eutectic composition was treated with pure Mg,
Ca and Sr, to study the effect of O and S concentration in the
melt, on the transition of graphite morphology from nodular to
vermicular/compacted and flake graphite. The change in the melt
composition between the austenite dendrites due to
micro-segregation of S, O and inoculants and their possible effects
on the transition of graphite morphologies as well as the
nucleation of new oxides/sulfides particles is discussed with the
help of thermodynamics.
Introduction
The transition of graphite morphologies in cast iron from
nodular to compacted/vermicular (CG/VG) and to flake graphite was
not fully understood. Strong de-oxidizers and desulphurizer are
often added to cast iron as inoculants which provide nucleation
sites for the graphite. Certain elements such as Ca, Mg, La and Ce,
are known as nodular graphite (SG) promoter. There have been
reported many types of nuclei, such as Igarashi et. al [1] found
MgO/MgS enveloped by (Mg,Si,Al)N, nucleating a nodule. Also CaS,
MgO and Al2O3 were observed in the core of graphite nodule while
(Al,Mg,Si)N in the shell. FeClx was found together with
(Mg,Al,Si)xOx particle in the core of a graphite nodule in high
purity cast iron [2]. Nakae et. al [3] analysed different types of
compounds as nuclei for nodules such as (Mg,Si, Al)N-oxides,
(La,Ce,Nd)OxSy, FeClx, (Ce,La,Nd)S and SiO2 as core compounds, and
concluded that the formation mechanism of nodular graphite is not
based on the nuclei . In flake graphite, MnS was heterogeneously
nucleated by different types of oxides which became nucleation site
for the flake graphite nucleation [4].
Contrary to the inoculation effect there have been evidences
where nodular graphite was obtained without any inoculation such as
Sadocha et. al [5] found only nodular graphite in a spectroscopic
purity ternary alloy (Fe,C,Si). At slightly lower purity level many
curved and bent graphite crystal appeared. According to Gruzleski
[6] the morphological change is brought about by the ability of
graphite to grow as curved crystals under certain condition while
under other conditions this ability does not exist and only flakes
are formed. According to Double et. al [7] and Johnsson et. al [8]
Mg does not have direct action on graphite spherodization but it
rather acts as a scavenger for those elements which stabilizes the
flake form. According to Velichko et. al [9], vermicular graphite
grows initially as spheroids, but later due to sufficient amount of
O and S in the residual melt [10] develops branches with the
crystal structure similar to flake graphite. Chang et. al [11],
suggested the presence of Fe-C cluster in the melt which creates
different polarity due to difference of the ionization energies in
Fe and C, where C atoms exhibit weak negative polarity, and Fe weak
positive polarity.
Franklin et. al [12] made secondary ion mass spectroscopy (SIMS)
analysis on flake and nodular graphite. Higher oxygen content was
found in the flake graphite but sulfur content was much lower than
oxygen. In the nodular and compacted graphite the oxygen and sulfur
content was lower compared to flake graphite. According to Hofmann
et. al [13] about of the oxygen content was present as SiO2, 8 ppm
as FeO + MnO, the remaining being bound as silicates, mixed oxides
or
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more stable oxides than SiO2 such as Al2O3 or TiO2. Elbel et. al
[14] also recorded the oxygen activity for FG, CG and SG at 1350 C
which are 0.97 ppm, 0.30 ppm and 0.16 ppm respectively.
In this investigation, nodular graphite and flake graphite
growth morphology was examined in Field Emission Gun (FEG) SEM
after performing ion etching on the samples. The effect of O and S
concentration in the melt on the graphite growth morphology was
studied experimentally and theoretically with the help of
thermodynamics.
Experimental procedure
The composition of alloys under investigation is provided in
Table 1. Two alloys A1, A2 were prepared in the laboratory using
pure Fe (99.99), C(99.9999) and Si (99.9), in the ceramic coated
crucibles. The composition of these alloys was measured with Spark
Emission Spectroscopy (SES). T640-6 sample was inoculated with
Vaxoninoculants (Provided by Jaques Lacaze, CIRIMAT, ENSIACET,
Universit de Toulouse, France). High Frequency (HF) induction
furnace was used with a graphite suscepter, placed between the coil
and the crucible, in a protective atmosphere of high purity Argon.
A few de-oxidation experiments were performed by adding pure Mg, Ca
and Sr to the melt and cooling down to room temperature, while
using MgO crucibles. The final composition was not measured after
de-oxidation experiments. Samples were examined along the
perpendicular section in the SEM. Experiments A2-4 and A2-5 were
performed on the sample which was treated with Ca (A2-2), in the
DSC equipment using zirconia crucible, under argon atmosphere. Ion
etching was performed perpendicular to the surface for 5 min (PECS
Gatan: Beam 10 KeV, Etching gun ~380 A, left and right gun 10 and 5
A) on the sample T640-6 and A1-1. The morphology of graphite was
studied with InLense detector attached to FEG-SEM. Cross-Section Ar
ion polishing was used on Mg treated sample, parallel to surface
while removing 75 micron deep surface.
Results
Sample T640-6 was examined with InLense detector in FEG-SEM,
after performing ion etching. A star like growth of the graphite
was observed in the centre of the graphite nodules (Fig.1(a,d)).
The branches were extended outward to some distance from where
onward the circumferential growth mechanism was dominant. Those
nodules with star like growth, emerging from the middle of the
graphite, do not seem to have any inclusion in the middle as the
one shown in the Fig. 1 (except c). The nodules with an inclusion
(nuclei) in the centre had circumferential growth by following the
shape profile of the inclusion. In Fig. 1e, the central part did
not have any inclusion, but the graphite formed in layers as
indicated in the magnified image. There should be no effect of the
argon ions on the surface of graphite due to the usage of low
voltage and for short time.
Table 1. Experiments and experimental conditions.
Fig. 1: T640-6 sample, quenched at eutectic temperature. InLense
detector images after Ar ion etching using PECS.
Table 2. Investigated alloys composition.
Materials Science Forum Vols. 790-791 459
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De-oxidation of the melt using pure Mg. The microstructure
contained combination of nodular, compacted/vermicular and
undercooled fine graphite (Fig. 2a). In Fig. 2c, a bundle of
graphite layer have changed its direction of growth from
circumferencial to tangential direction which at certain distance
again have changed the growth morphlogy towards circumferencial
growth mechanism, resulting in two adjacent nodules. Two step
growth could be observed in many nodules as shown in Fig. 2d, where
the outer shell around the nodule was formed during the
solidification process. The circumferencial profile of the growing
graphite layers exactly followed the shape profile of the
nucleating agent (MgO/S inclusio, Fig. 2d). In Fig. 2(e,f,g,h)
after cirtain growth of the graphite, a transition towards flake
like morphology was occurred.
As shown in Fig. 3, many MgS particles were observed both in the
undercooled graphite region as well as in the nodular graphite
region, with and without any connection to graphite. Most of them
were below 1 m in size. A relatively large round MgO particle was
found in the austenite region, sorrounded by large number of nano
sized MgO particles (Fig. 3c).
De-oxidation of the melt using pure Ca. The microstructure
showed large number of perfectly round nodules of various sizes and
some other forms of compacted graphite which were mostly
precipitated inside the shrinkage pores (Fig. 4d), with a complete
absence of undercooled fine graphite (Fig. 4). The fraction of
graphite in this sample was less due to the formation of CaC2 which
can stabilizes or nucleate cementite. Due to multiple nuclei of
CaO/S, a compacted like graphite was produced as indicated with
arrows in Fig. 4c. C was often found in the middle of the noduels
as shown in Fig. 4(b). Sample A2-2 when re-melted (A2-4), with low
oxygen content resulted in fine undercooled graphite together with
some course graphite flakes (Fig.4e), but with high oxygen content
(A2-5), the microstructure was consist of extremely fine vermicular
type of graphite, with no trace of course flakes (Fig. 4f).
De-oxidation of the melt using pure Sr. The sample treated with
pure Sr contained mostly flake graphite; however a small area
(about 5%) at the top edge of the sample surface, contained large
size irregular graphite particles of various sizes as shown in Fig.
5. After ion ethcing there
Fig. 4: (a,b,c,d) Ca treated sample A2-2, Ar Ion etched, (e)
Re-melted Ca treated samples A2-4, (f) Re-melted Ca treated samples
and 1 wt. % O addition.
Fig. 2: A1-1 sample, FEG-SEM InLense detector images.
Highlighted black
Fig. 3: A1-1, FEG-SEM images of Cross-Section Ion Polished
surface, shows MgS particles.
460 Solidification and Gravity VI
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were some particles which contained amorphous graphite
surrounded by layers of graphite (about 5-10 nm thick) as indicated
in Fig. 5(d,e), also the inner part of Fig. 5(f,g) seems like
amorphous. The growth mechanism in the other large size irregular
graphite was flake like but with many different orientations.
Discussion
By adding different types of elements to liquid melt, one
changes the concentration of dissolved trace elements (O, S, N, P
etc.) in the melt by forming different types of inclusions
depending on the affinity of the element with the trace elements.
Ellingham diagram (updated by the creators) [15] is re-drawn (Fig.
6c) to calculate the energy of formation of some important types of
reactive elements. Those elements which have the highest affinity
towards oxygen such as Y and Ca might be consumed mostly prior to
solidification starts; however if oxygen reduces to the equilibrium
value with those active elements, the remaining elements can form
sulfide particles during solidification. In the melt, the amount of
sulfur is usually higher than oxygen which provides opportunity for
new particles formation. From EDX analysis, only few of the
particles were found MgO, but most of them were MgS. Their smaller
sizes depicts that they have nucleated at the end of
solidification. The steeper is the formation energy line, the
quicker it will reach the limit of homogeneous nucleation, as shown
for MgS and SrS (Fig. 6(a,b)). Detailed formulations and
calculations have been earlier reported by Muhmond et. al [4] for
hypo eutectic iron. By adding pure Mg, it will form MgO due to its
higher affinity towards oxygen, with or without silicates, which
together have served as nuclei for the graphite nodule to grow on
it. Primary graphite nodules formed will float up, causing a change
in the remaining liquid composition towards hypo eutectic. At the
inter-dendritic areas, new MgS particles will form (Fig. 6(a,b)),
due to enrichment of sulfur in the melt, since oxygen was already
reduced to lower level in the melt prior to solidification. SrS
might be formed as well; however in the presence of Ce, Mg and Ca
it is difficult to nucleate SrS due to its lower affinity to Sr. If
the newly sulfide/oxides particles are formed prior to, or at the
beginning of eutectic reaction, they could serve as nuclei for
graphite. The morphology of the graphite growth will depend on the
concentration of dissolved trace elements such as O and S
concentration around the growing graphite. The oxide/sulfide
particles which nucleates close the end of solidification, may or
may not serve as nuclei for graphite. Many MgS particles were found
connected to flake graphite which proves that the dissolved content
of sulfur was above the critical limit for transition of graphite.
The formation of new particles is important because they reduces S
and O concentration, however in the absence of such inclusions, the
concentration of the mentioned dissolved elements would continue to
increase and the graphite morphology will be significantly
modified. From some other results often we found very few MgS
particles in the undercooled graphite region compared to the other
forms of graphite. An example is shown in Fig. 2b marked area A,
larger graphite particles are round but the smaller ones are under
transition towards compacted graphite but in the marked area B ,
the graphite are elongated, shows the difference in composition (S
and O) of the melt locally. In the marked area C, a nodule being
trapped by the growing eutectic graphite is found.
The Ca treated sample was completely free from undercooled fine
graphite or normal flake graphite, due to the effective scavenging
of oxygen and sulfur from the liquid. A pore that might have been
formed during cooling of melt, was later filled by C diffusion
towards it (Fig. 4d).
Fig. 5: A2-3, InLense detector images of the Sr treated sample,
after Ion etching. White marked regions in d, e, f and g
indicates
Materials Science Forum Vols. 790-791 461
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Fig. 6: (a,b) Homogeneous nucleation of new MgS and SrS
particles during solidification, while
assuming hyper eutectic composition, KMg part. = KSr part. =
0.031 and start and end temperature = 1200
and 1120 oC. (c) Ellinghams Updated diagram [15] ,Thermodynamic
energy of formation of different
oxides and sulfides, (except for SrS [16]).
Increasing oxygen content have changed the graphite morphology
drastically from flake graphite towards extremely fine vermicular
type of graphite, in the re-melted Ca treated sample (Fig. 4f). Due
to the presence of about 2 ppm oxygen in the argon gas and melting
in the alumina crucible would have allowed the melt to dissolve
some oxygen in it and Ca might have been faded out, thus causing
the formation of undercooled type of graphite. The opposite
happened in the Sr treated sample where due to higher affinity of
Sr towards O, it reduced oxygen content in the sample while sulfur
remained less affected which played a role in modifying the growth
morphology to normal flake graphite.
If one carefully observe different types of graphite morphology
shown in Fig. 2(a,b) and Fig. 5(a,b), the transition of graphite
morphology from the centre of the austenite dendrite towards the
eutectic changes from nodular to compacted/vermicular and then
undercooled fine graphite, or even flake graphite finally. The
explanation can be given by the O/S content dissolved in the
liquid, which enriches due to micro-segregation from the edges of
the dendrite towards the last solidified region. By getting closer
to the end of solidification, the O/S content increases
exponentially. In the absence of de-oxidizers and de-sulphurizer,
it could easily be above the critical limit and thus causing
transition towards either flake graphite or undercooled graphite
growth morphology, depending on the type of trace element and its
concentration.
Concluding remarks
The growth of graphite in a dendritic or star like manner, at
the centre of nodules, depicts certain relationship of the graphite
growth morphology with the lattice structure of the graphite and/or
the presence of nano sized C-C or C-Si and even C-Fe clusters in
the melt which changes the kinetics of the graphite growth, is
under investigations. MgO and MgS being present in the centre of a
nodule as well as connected to/or close to undercooled/vermicular
graphite gives an indication that the concentration of trace
elements around the growing graphite during cooling and
solidification could affect the graphite growth morphology. The
possibility of trace elements (O, S etc.) being co-valent bonded in
the graphite lattice, stabilizing either flake or
compacted/vermicular growth of graphite, will be carefully analyzed
in future.
462 Solidification and Gravity VI
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Acknowledgment
The authors would like to thank Elkem AS Foundary Technology
Products R&D for providing the financial support.
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Solidification and Gravity VI
10.4028/www.scientific.net/MSF.790-791
Graphite Growth Morphologies in Cast Iron
10.4028/www.scientific.net/MSF.790-791.458