This article provides answers to the following questions, among others:

What is the di erence in microstructure between steels and cast iron?

For which manufacturing processes are steels particularly suitable compared to cast iron?
What is the di erence between the phase diagram of the stable and the metastable system?
What is white and grey cast iron?

1 Introduction
2 Cast iron
2.1 White cast iron
2.1.1 Eutectic cast iron
2.1.2 Hypoeutectic cast iron
2.1.3 Hypereutectic cast iron
2.2 Grey cast iron
2.2.1 Lamellar graphite cast iron (grey cast iron)
2.2.2 Spheroidal graphite cast iron (nodular cast iron)
2.2.3 Vermicular graphite cast iron (compacted graphite iron)
2.2.4 Flake graphite cast iron (malleable iron)

Introduction
Up to now, the iron-carbon phase diagram has only been considered up to a carbon content of
2.06 %. If this carbon content is exceeded, further phase transformations occur. Basically, this is
also connected with a di erent microstructure. Iron materials below 2.06 % carbon consist of a
eutectoid based microstructure (pearlite) and above 2.06 % of a eutectic based microstructure
(ledeburite).

In principle, this also results in other material properties. This di erence is also re ected in the
subdivision into steels and cast iron. For example, ferrous materials with a lower carbon content
than 2.06 % are referred to as steels and ferrous materials over 2.06 % as cast iron.

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 Figure: Classification of steels and cast iron in the iron-carbon phase diagram

STEEL HAS A PEARLITIC (EUTECTOID) BASED MICROSTRUCTURE AND

CAST IRON A LEDEBURITIC (EUTECTIC) BASED MICROSTRUCTURE!

This article is intended to provide more detailed information on this new microstructure of cast
iron.

Cast iron
The phase diagram below shows the complete iron-carbon phase diagram of the metastable
system in which the carbon is present in the microstructure in the form of cementite. The
microstructure in the metastable system can therefore consist of a maximum of 100 % cementite.
Since the carbon content in the cementite (F e 3
C ) is 6.67 %, the metastable iron-carbon phase
diagram ends at this concentration.

Figure: Complete iron-carbon diagram of the metastable system

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 If only the range of the phase diagram above a carbon content of 2.06 % is considered, the
fundamental di erence between steels and cast iron in the solidi cation process becomes
apparent.

At a carbon concentration of less than 2.06 %, the steel initially solidi ed as solid
solution (homogeneous austenite microstructure) within the typical lens-shaped crystallization
range in the state diagram.

In the area of cast iron, however, the phase diagram no longer shows this lenticular solidi cation
area, but shows the typical horizontal “K” of a crystal mixture. The eutectic composition is at 4.3 %
carbon, where the two liquidus lines falling from the left and right meet.

Depending on whether the iron-carbon compound solidi es as a solid solution (carbon content <
2.06 %) or as a crystal mixture (carbon content > 2.06 %), other mechanical properties of the
material also result at room temperature. Alloys solidi ed as a crystal mixture are generally more
suitable for casting processes (so-called casting alloys). In comparison, however, the solidi ed
solid solution can be formed much better and are therefore particularly suitable for various
forming processes such as bending, forging, rolling, deep-drawing, etc. (so-called wrought alloys).

For these reasons of manufacturing processing, iron-carbon compounds with a carbon content
lower or higher than 2.06 % are distinguished. Below 2.06% carbon, the material is called steel.
Above 2.06 % carbon, on the other hand, one speaks of cast iron, as it is particularly suitable for
casting processes. In contrast to this, steels can be formed much better and are therefore
forgeable in contrast to cast iron. Note that the transitions in the mechanical properties at the
2.06 % limit are always smooth!

STEELS INITIALLY CRYSTALLIZE AS SOLID SOLUTIONS, WHILE CAST IRON

SOLIDIFIES AS CRYSTAL MIXTURES.

Compared to steel, cast iron therefore has a eutectic based microstructure! The reason that the
steel does not form an eutectic is ultimately because steels are already solidi ed before the
residual melt could have reached the eutectic composition. Just as steels can be divided into
hypoeutectoid and hypereutectoid steels, cast iron can be divided into hypoeutectic and
hypereutectic cast iron respectively.

While steels generally solidify according to the metastable system due to their relatively low
carbon content, cast iron can crystallize both in the metastable form (white cast iron) and in the
stable form (grey cast iron). The vast majority of cast iron solidify according to the stable system
due to the relatively high carbon content. Instead of the precipitation of cementite, the cast iron is
then subject to graphite precipitation during solidi cation or cooling.

The precipitation of graphite instead of cementite a ects the transformation temperatures in the
phase diagram. Accordingly, a distinction must be made between the stable and the metastable
iron-carbon
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 Figure: Metastable and stable iron-carbon phase diagram in comparison

White cast iron

In white cast iron, the cast iron solidi es in the metastable form and is thus subject to the
formation of cementite. The cementite makes the fracture surface of the cast iron appear shiny
white, to which the term “white” cast iron refers.

Depending on the carbon content, white cast iron can be divided into eutectic cast iron (4.3 % C),
hypoeutectic cast iron (<4.3 % C) and hypereutectic cast iron (>4.3 % C). The microstructure
formation and transformation during solidi cation and cooling of such types of cast iron are
explained in more detail below.

Eutectic cast iron

If the cast iron has the eutectic composition of 4.3 % carbon, the melt solidi es as usual at a
thermal arrest. Due to the strong supercooling, a ne mixture of austenite and cementite is
formed. This eutectic microstructure of nely distributed austenite and cementite is also called
ledeburite-I immediately after solidi cation.

THE EUTECTIC PHASE MIXTURE OF AUSTENITE AND CEMENTITE

IMMEDIATELY AFTER SOLIDIFICATION IS CALLED LEDEBURITE-I!

Note that on the left side of the cast iron phase diagram (at 2.06 %) the austenite phase is applied
and on the right side (at 6.67 %) the cementite phase. These phases austenite and cementite are
thus ultimately the components of an A/B alloy system (A ≙ “austenite”) and (B ≙ “cementite”).

Immediately after solidi cation, the austenite crystals present in the ledeburite are completely
saturated with carbon at 1147 °C, i.e. they show the maximum possible concentration of carbon
that is soluble in the austenite. As the solubility continuously decreases according to the solubility
limit (solvus line) during further cooling, the austenite crystals permanently precipitate cementite.

Finally, at 723 °C so much carbon is precipitated from the austenite that it has reached the
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 transformed into pearlite. This eutectic phase mixture of pearlite and cementite is now called
ledeburite-II due to the changed microstructure.

THE EUTECTIC PHASE MIXTURE OF PEARLITE AND CEMENTITE PRESENT

AT ROOM TEMPERATURE IS CALLED LEDEBURITE-II!

Hypoeutectic cast iron

In the case of hypoeutectic cast iron, only austenite primary crystals are precipitated from the
melt when the liquidus line is reached. This increases the carbon content in the residual melt.
Once the carbon content has nally risen to 4.3 % at 1147 °C, the residual melt crystallizes at a
constant temperature to form the eutectic (ledeburite-I). Immediately after solidi cation, the
microstructure consists of the eutectic and the previously primarily precipitated austenite
crystals.

Both the primary austenite and the austenite crystals contained in ledeburite-I precipitate
cementite as cooling progresses due to the decreasing solubility of the carbon. Consequently, the
microstructure in this state consists of ledeburite-I and the primary austenite embedded therein
as well as the precipitated cementite. At 723 °C the eutectoid composition in the austenite
crystals is nally reached (both in the primary crystals and in the eutectic).

While the ledeburite-I changes to ledeburite-II, the primary austenite grains transform to pearlite
grains. Consequently, the microstructure of hypoeutectic cast iron consists of ledeburite-II with
the pearlite grains embedded therein and the cementite previously precipitated from the
austenite crystals.

The micrograph below shows a sample of hypoeutectic cast iron with 2.7 % carbon. The γ solid
solutions, which initially grew dendritically, can be seen, which nally turned into pearlite (dark
spots). As an example, a dendrite is shown in the gure, which was cut through by the
micrograph in the plane. As usual, this pearlite microstructure consists of ferrite and lamellar
cementite. Between the branches of the pearlitic dendrites is the eutectic, which was also subject
to the γ-α -transformation and thus nally is present in the microstructure as leedeburite-II (dark
speckled areas).

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 Figure: Micrograph of hypoeutectic cast iron with a carbon content of 2.7 %.

Figure: Micrograph of hypoeutectic cast iron with a carbon content of 2.7 % (enlarged image)

In comparison, the following microstructure shows a hypoeutectic cast iron with a higher carbon
content of 3.8 %. The signi cantly larger proportion of eutectic matrix compared to pearlite is
striking. In this case, the very ne, lamellar cementite can no longer be dissolved by light
microscopy in the pearlite – it therefore appears dark as a single surface!

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 Figure: Micrograph of hypoeutectic cast iron with a carbon content of 3.85 %.

Figure: Micrograph of hypoeutectic cast iron with a carbon content of 3.85 % (enlarged image)

Hypereutectic cast iron

In hypereutectic cast iron, only primary cementite with a strip-like structure crystallises out
initially during solidi cation. Due to the associated carbon precipitation from the residual melt,
the carbon content there is reduced. Once the eutectic composition of 4.3 % carbon at 1147 °C is
nally reached in the residual melt, it solidi es to the eutectic ledeburit-I.

Immediately after solidi cation, the microstructure consists of the primary precipitated strip-
cementite, which is embedded in the surrounding ledeburite-I. The austenite contained in the
eutectic nally undergoes cementite precipitation when the temperature is lowered. If the carbon
content in austenite has dropped to 0.8 % at 723 °C, it begins to convert to pearlite. In this way
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 The cooled hypereutectic cast iron microstructure thus consists at room temperature of the
primarily precipitated cementite strips which bed in the eutectic of ledeburit-II.

The micrograph below shows a hypereutectic cast iron with 5.5 % carbon. The eutectic ledeburit-II
( nely patterned) and the primarily precipitated cementite needles, which due to the etching
during sample production appear as white elongated stripes.

Figure: Micrograph of hypereutectic cast iron with a carbon content of 5.5 %

Grey cast iron

Lamellar graphite cast iron (grey cast iron)

Without any major treatment of the melt, the graphite normally crystallises in lamellar form. This
is known as lamellar graphite casting. Since this ist the “normal” type of cast iron, it is simply
referred to as grey cast iron.

The micrograph below shows grey iron with 3.5 % carbon. The lamellar graphite can be seen
(dark, large areas) surrounded by a pearlitic based microstructure (dark, ne stripes).

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 Figure: Microstructure of lamellar graphite cast iron (grey cast iron)

Figure: Microstructure of lamellar graphite cast iron (enlarged image)

Cast iron with lamellar graphite has excellent casting properties and therefore o ers a wide range
of applications. In addition, lamellar graphite casting shows very good machinability, as the
graphite also serves as a solid lubricant. In addition, the graphite lamellae in the cast structure
have a special vibration damping e ect. This is why lamellar graphite casting is used, among
other things, as a material for highly vibration-stressed components such as machine beds or
marine diesel engines.

However, the graphite lamellae have a disadvantageous e ect on the tensile strength, since they
act like notches (“predetermined breaking points”) in the casting structure. Therefore, lamellar
graphite castings should not be subjected to tension but to pressure. The compressive strength is
approx.
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 In many applications, however, the casting material has to withstand high tensile loads. Since the
graphite lamellae obviously have a disturbing e ect, the lamellar graphite precipitation must be
speci cally prevented during the solidi cation or cooling process. An alternative to lamellar
graphite casting is nodular graphite casting as explained below.

Spheroidal graphite cast iron (nodular cast iron)

To ensure that the graphite in grey cast iron does not precipitate in the form of lamellae but
spherically, the melt must be speci cally treated with additives such as aluminium before
solidi cation. A graphite precipitate in spherical form is then called spheroidal graphite cast iron or
nodular cast iron.

The micrograph below shows the microstructure of nodular cast iron with 3.6 % carbon. The
spherically precipitated graphite (dark, roundish areas) can be seen, which has contracted from
the immediately surrounding areas. The surrounding areas are almost carbon-free iron (ferrite),
which therefore appears white.

Figure: Microstructure of nodular graphite cast iron (spheroidal graphite cast iron, ductile iron)

The notch e ect of the globular graphite is greatly reduced by the rounded shape compared to
lamellar graphite. Therefore, the tensile strength of spheroidal graphite cast iron is signi cantly
better. Since nodular cast iron is more ductile than “normal” grey cast iron, this type of cast iron is
also referred to as ductile cast iron.

Vermicular graphite cast iron (compacted graphite iron)

Vermicular graphite cast iron o ers (also referred to as compacted graphite iron) a compromise in
the properties between lamellar and nodular graphite cast iron. The graphite is precipitated like a
worm, whereby spherical graphite may also form in the microstructure to a certain extent.

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The micrograph (unfortunately not yet available!) shows vermicular graphite cast iron. The
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graphite (black), which is precipitated like a worm, can be seen, some of which is still precipitated
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 in a spherical form. The carbon was removed from the surrounding areas, which therefore
appear white (ferrite).

Due to its good thermal shock resistance, vermicular graphite casting is particularly suitable for
engine construction.

Flake graphite cast iron (malleable iron)

In the so-called ake graphite cast iron (or malleable iron), the carbon is formed into individual
graphite akes. In order to obtain this aky microstructure, the preliminary stage of malleable
iron initially solidi es graphite-free. The microstructure of this so-called white cast iron therefore
contains only cementite instead of graphite. Only after subsequent heat treatment, annealing,
does the metastable cementite disintegrate into its nal ake graphite form and then belongs to
group of the grey cast iron.

The micrograph below shows malleable iron with 2.7 % carbon. The graphite precipitated in
akes (black areas) can be seen. Carbon-free areas (ferrite) often form around the akes, which
therefore appear white. There, the carbon from the lattice has accumulated into a ake structure.

Figure: Microstructure of flake graphite cast iron (malleable iron)

The advantage of malleable iron is its good castability with properties similar to steel, such as
good toughness and strength. Malleable cast iron is used for thin-walled components, brake
drums, ttings, etc.

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