colour metallography of cast iron239 may 2011 serial report colour metallography of cast iron by...
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May 2011Serial Report
Colour Metallography of Cast IronBy Zhou Jiyang, Professor, Dalian University of Technology, China
Translated by Ph.D Liu Jincheng, Fellow of Institute of Cast Metal Engineers, UK
*Note: This book consists of five sections: Chapter 1 Introduction, Chapter 2 Grey Iron, Chapter 3 Spheroidal Graphite Cast Iron, Chapter 4 Vermicular Cast Iron, and Chapter 5 White Cast Iron. CHINA FOUNDRY publishes this book in several parts serially, starting from the first issue of 2009.
Chapter 4
Vermicular Graphite Cast Iron (Ⅱ)
4.5 Solidification of LTF regions during final stage of solidification of VG iron4.5.1 Characteristics of the liquid in the LTF regions
(1) Wider rangeFigure 4-24 compares the size of the LTF regions of three types
of cast iron under the same cooling condition. Since the tips of
graphite in grey iron are in contact with the liquid, the two phases
in the eutectic cells grow in a fully cooperative manner, resulting
in less remaining liquid between the cells. The number of eutectic
cells in SG iron is much more than in VG iron, thus the area of the
LTF regions is also less. Compared with grey iron and SG iron, VG
iron has the largest percentage of the LTF area in the field of view.
(a) Grey iron
(2) With more segregation elements In VG iron, except for the modification elements Mg, Ce and
Ca, there also exist interference elements Ti and Al, which are
easily enriched in the LTF regions.
4.5.2 Microstructure formed in the LTF regions
When the temperature of the liquid in the LTF regions drops to its
crystallisation temperature, the liquid solidifies; depending on its
composition, contamination by impurities and the cooling rate, the
structures formed in the LTF regions are different.
Fig. 4-24: Comparison of the LTF areas of three types of cast iron (sample section thickness 25 mm)
(1) ‘Basket’ net-work structure of pearlite The remaining liquid in the LTF regions is enriched in Ce, Ti
and Mn, which increase undercooling of the liquid, thus promoting
nucleation of austenite. Therefore, austenite dendrites often form
in the LTF regions (see Fig. 4-25); these dendrites contact and
integrate with the austenite grown from eutectic cells. Because
VG iron contains more positive segregation elements than SG iron
or grey iron, during solid phase transformation, the austenite in
the LTF regions has a stronger tendency to form pearlite than in
(b) VG iron
(c) SG iron
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grey iron or SG iron, and the spacing of pearlite is finer (see Fig.
4-26). When the pearlitic structures around eutectic cells connect
with each other, a metallic net-work skeleton is formed, called a
‘basket net-work structure’ [34, 35]; this is illustrated in Fig. 4-27.
The ‘basket net-work structure’ consisting of pearlite (or ferrite)
has a favourable effect on the mechanical properties of VG iron
and enables VG iron to have an ideal combination of properties.
(2) Small graphite nodulesIn the LTF regions, small graphite nodules are often observed
(see Fig. 4-28). And in slow-cooled, thick section VG iron, small
graphite nodules are even more in evidence. The formation of small
nodules is related to a higher content of spheroidizing elements,
Ce and Mg. From their location and size, it is known that small
nodules are formed in the last stage of solidification; different from
small nodules, the isolated and randomly distributed large nodules
are formed in the early stage of solidification. The small nodules
have a beneficial effect on the properties of VG iron.
Fig. 4-25: Austenite dendrites formed in the LTF regions
Fig. 4-26: Fine lamellar pearlitic structure formed in the LTF regions (SEM observation)
Fig. 4-27: The ‘basket’ net-work structure formed in the LTF regions
Fig. 4-28: Small graphite nodules in the LTF regions (section thickness 100 mm)
(3) Flake graphiteIn the LTF regions, when the spheroidizing elements are
insufficient, anti-spheroidizing elements are excessive, or residual
sulphur is too high, flake graphite is easily formed, as illustrated
in Fig. 4-29. This flake graphite markedly weakens the strength of
the metal matrix and is unfavourable to the mechanical properties
of VG iron.
Fig. 4-29: Flake graphite in the LTF regions
(4) CarbidesSince the completion temperature of eutectic solidification
of VG iron, Tes, is higher than that of SG iron and is above the
metastable eutectic temperature, the carbide forming tendency
in the LTF regions of VG iron is weaker than that of SG iron.
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thermal curve reaches point ‘d’ and the liquid in the LTF regions
solidifies; small nodules, vermicular graphite and austenite form,
and solidification is complete.
a – TEN (eutectic begins); b – TEU (a large quantity of eutectic
begins to form); c – TER (eutectic reaction almost complete);
d – TES (eutectic solidification ends)
Fig. 4-31: Formation process of solidification structures of VG iron (corresponding to the points on the curve shown in Fig. 4-30
Nevertheless, if the liquid in the LTF regions contains a too
high level of positive segregation elements, their corresponding
compounds can also form in the LTF regions; TiC is a commonly
observed carbide in VG iron, as shown in Fig. 4-21c.
4.6 Solidification morphology of VG iron4.6.1 Solidification process of VG ironUsing thermal analysis, liquid quenching and colour metallurgy
to observe and analyse the formation, distribution and relationship
of solidification structures, it is possible to effectively study
the solidification process of VG iron. Figure 4-30 shows the
thermal analysis curve of a VG iron with eutectic composition;
the microstructure corresponding to the points on the curve are
shown in Fig. 4-31. It can be seen from these figures that at the
beginning of eutectic solidification (point ‘a’), the structure,
similar to that of SG iron at this point, consists of small graphite
nodules and divorced austenite dendrites. When reaching point
‘b’, the nodule ‘germs’ degenerate towards vermicular graphite,
the eutectic graphite nodule count decreases gradually, a large
number of eutectic cells begin to form, and recalescence appears
on the cooling curve. On reaching point ‘c’, eutectic reaction nears
the final stage, but eutectic cells continue to grow. Finally, the
Fig. 4-30: Thermal analysis curve of a VG iron with eutectic composition
(a) VG ‘germs’ and austenite dendrites precipitate (b) Eutectic cells form
(c) Eutectic cells grow (d) LTF regions solidify
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4.6.3 Solidification morphology of VG iron
Although the morphology characteristics of VG eutectic cells
are closer to those of grey iron, the solidification does not totally
follow the mode of grey iron: i.e. gradually increasing the
thickness of its solid layer (a successive layer-type solidification
mode). The author considered that in comparison with grey
iron, VG iron has 3-5 times wider LTF regions around the
eutectic cells, a smoother outer contour of the eutectic cells and
slightly more eutectic cells, all of which result in the variety of
4.6.2 Characteristics of solidification cooling curve of VG iron
VG iron is a type of intermediate cast iron between grey iron and
SG iron; the characteristic freezing parameters are also in between
grey iron and SG iron, as shown in Fig. 4-32(a). However, for a
VG iron with hypoeutectic composition, the measured cooling
curve shows other characteristics, as illustrated in Fig. 4-32(b).
The obvious difference is that the chilling tendency of the eutectic
reaction of a hypoeutectic VG iron is higher than that of grey
iron and SG iron as well. The reasons for this difference are: (1)
hypoeutectic composition; (2) the vermicularizing elements Mg
and Ce remarkably increase undercooling; (3) highly efficient
inoculants cannot be used because they will increase nodule count
and reduce the percentage of vermicular graphite.
solidification processes [36].
At the edge of a VG iron casting, because of fast cooling, rapid
solidification does not allow the LTF regions to develop and
solidification occurs in a successive layer-type mode, whilst in
the centre of the casting, solidification occurs mainly in a mushy
mode. The influence of wall section thickness has a similar effect
to the above; experimental measurements have shown that for
castings of less than 10 mm thick, solidification is mainly the
successive layer-type mode; for castings with a section thickness
above 25 mm, the solidification occurs mainly in the mushy mode;
see illustrations in Fig. 4-33. In addition, it was found that for
castings with a section thickness up to 50 mm, the size of eutectic
cells increases with increasing section thickness; for castings
thicker than 50 mm, the size of eutectic cells does not show a
significant increase with increasing section thickness. Figure 4-34
shows the solidification microstructure in sections with thicknesses
of 10, 25 and 50 mm, respectively. The outer contour of eutectic
cells in thin-walled VG iron is not very round, whilst in thicker
sections above 25 mm, the eutectic cells show a clear and round
outer contour.
The degree of vermicularization (percentage of vermicular
graphite) has an important influence on the solidification
morphology; see Fig. 4-35. For under-vermicularized VG iron in
which vermicularization is insufficient, there exists more flake
graphite and insufficient vermicular graphite; the solidification is
similar to grey iron, solidifying in the successive layer-type mode.
Conversely, for over-vermicularized VG iron, the nodularity is
too high and in addition to the VG eutectic cells there exists some
SG eutectic cells; under these conditions, the solidification occurs
as a mixture of successive layer-type mode plus mushy mode, as
shown in Fig. 4-35(c). The dendritic structure, vertical to mould
wall, is shown in Fig. 4-36. The picture shows that at the edge
of the casting the structure is austenite with graphite spheroids
enveloped in it, whilst at the centre of the casting, the structure
consists of VG eutectic cells.
4.7 Segregation in VG ironThe micro-segregation of VG iron is more complicated than in
SG iron or grey iron. This is because VG iron contains more
and complex solute elements than either SG iron or grey iron.
The distribution of elements in various structures follows the
common rules of segregation: negative segregation elements
preferentially concentrate in the early-formed austenite, whilst
positive segregation elements are enriched in the LTF regions;
this causes the formation of pearlites, carbides or small graphite
nodules around eutectic cells. The non-uniform distribution of
various elements and structures in thick-section VG iron is more
evident, as shown in Fig. 4-37. Table 4-3 lists the element content
in corresponding regions; it can be seen from the table that the
non-uniform distribution of positive and negative elements is quite
severe. VG iron normally contains a higher Si content than grey
iron and often reaches the level of as-cast ferritic SG iron. The
segregation of such high Si in the centre of eutectic cells causes
ferrite to form, but around the eutectic cells the structure is pearlite.
(a) Hyper-eutectic curves
(b) Hypo-eutectic curves
Fig. 4-32: Cooling curves of grey, VG and SG irons [1]
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References[1] Stefanescu D M, Martinez F and Chen I G. Solidification Behavior
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Fig. 4-33: Influence of section thickness on the macro-solidification structure of VG iron [36]
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Fig. 4-34: Influence of section thickness on the micro-solidification structure of VG iron (Each group has two pictures of the same field of view, the left one is un-etched)
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(a) Hot alkaline etched (b) Nital etched [the same field of view with (a)]Fig. 4-37: The non-uniform phenomena of composition and microstructure distribution in a VG iron
Table 4-3: Element distribution in the solidification structures of VG iron (mass %)
Region in Fig. 4-37Negative segregation element Positive segregation element
Si Ni Cu Cr Mg
A (in the austenite around primary nodules) 2.75 0.062 0.034 0.030 0.0049
B (in VG eutectic cells) 2.68 0.058 0.030 0.027 0.0037
C (in chunky graphite eutectic cells) 2.56 0.053 0.025 0.029 0.0027
D (in the LTF regions) 1.90 0.027 0.008 0.173 0.0170
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