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 different 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 difference is also reflected 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.

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 (\(Fe_3C\)) is 6.67 %, the metastable iron-carbon phase diagram ends at this concentration.

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

At a carbon concentration of less than 2.06 %, the steel initially solidified 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 solidification 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 solidifies 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 solidified as a crystal mixture are generally more suitable for casting processes (so-called casting alloys). In comparison, however, the solidified 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 solidified 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 solidification or cooling.

The precipitation of graphite instead of cementite affects the transformation temperatures in the phase diagram. Accordingly, a distinction must be made between the stable and the metastable iron-carbon phase diagram. The diagram below compares the metastable phase diagram (blue) and the stable phase diagram (supplemented in red).

White cast iron

In white cast iron, the cast iron solidifies 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 solidification 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 solidifies as usual at a thermal arrest. Due to the strong supercooling, a fine mixture of austenite and cementite is formed. This eutectic microstructure of finely distributed austenite and cementite is also called ledeburite-I immediately after solidification.

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 solidification, 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 eutectoid composition of 0.8 % carbon. Now the austenite crystals in the eutectic of ledeburite-I begin to change into pearlite at a constant temperature. The former austenite crystals have thus 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 finally risen to 4.3 % at 1147 °C, the residual melt crystallizes at a constant temperature to form the eutectic (ledeburite-I). Immediately after solidification, 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 finally 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 \(\gamma\) solid solutions, which initially grew dendritically, can be seen, which finally turned into pearlite (dark spots). As an example, a dendrite is shown in the figure, 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 \(\gamma\)-\(\alpha\)-transformation and thus finally is present in the microstructure as leedeburite-II (dark speckled areas).

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

Hypereutectic cast iron

In hypereutectic cast iron, only primary cementite with a strip-like structure crystallises out initially during solidification. 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 finally reached in the residual melt, it solidifies to the eutectic ledeburit-I.

Immediately after solidification, the microstructure consists of the primary precipitated strip-cementite, which is embedded in the surrounding ledeburite-I. The austenite contained in the eutectic finally 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 the eutectic ledeburit-I becomes ledeburit-II.

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 (finely patterned) and the primarily precipitated cementite needles, which due to the etching during sample production appear as white elongated stripes.

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, fine stripes).

Cast iron with lamellar graphite has excellent casting properties and therefore offers 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 effect. 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 effect 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. 4 times higher than the tensile strength!

In many applications, however, the casting material has to withstand high tensile loads. Since the graphite lamellae obviously have a disturbing effect, the lamellar graphite precipitation must be specifically prevented during the solidification 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 specifically treated with additives such as aluminium before solidification. 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.

The notch effect 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 significantly 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 offers (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.

The micrograph (unfortunately not yet available!) shows vermicular graphite cast iron. The graphite (black), which is precipitated like a worm, can be seen, some of which is still precipitated 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 flake graphite cast iron (or malleable iron), the carbon is formed into individual graphite flakes. In order to obtain this flaky microstructure, the preliminary stage of malleable iron initially solidifies 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 final flake 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 flakes (black areas) can be seen. Carbon-free areas (ferrite) often form around the flakes, which therefore appear white. There, the carbon from the lattice has accumulated into a flake structure.

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, fittings, etc.

Introduction

In the article Phase transformations in the solidified state the microstructural changes of steels during cooling were explained in great detail. Since these transformations are very complex, a brief overview of the microstructural transformations is given in this summarizing article. More detailed information can be found in the article Phase transformations of steels in solidified state (metastable system).

Solidification

The actual solidification process in steels takes place independently of the carbon content as in a solid solution alloy. This is shown in the phase diagram as a typical lenticular region between the liquidus and the solidus line. The carbon is completely soluble in the face-centered cubic γ-iron lattice structure immediately after solidification. This solid solution of face-centered cubic iron and embedded carbon therein is called austenite.

In the solidified state, the iron-carbon phase diagram shows the typical horizontal “K” of a crystal mixture, in which the respective components are insoluble in one another. Note that carbon in iron is actually almost insoluble at room temperature. The phase transformations that the steel undergoes in the further cooling process can therefore be considered in analogy to a crystal mixture alloy. However, the phase transformations take place in an already solidified state!

Phase transformations in solidified state

Hypereutectoid steels

In hypereutectoid steels with a carbon content of more than 0.8 %, carbon in the form of cementite precipitates at the grain boundaries when the solubility limit is reached (grain boundary cementite). This leads to a depletion of carbon in the remaining austenite. Depletion finally progresses until the retained austenite reaches the eutectoid composition of 0.8 % carbon at 723°C.

Now, at a constant temperature of 723 °C, the face-centered cubic austenite begins to convert completely into the body-centered cubic ferrite structure. However, the carbon can no longer be dissolved in the ferrite lattice. Therefore the carbon precipitates directly out of the ferrite in the form of cementite lamellae. This eutectoid phase mixture of ferrite grains with the cementite lamellae embedded therein is also known as pearlite.

The microstructure of a hypereutectoid steel at room temperature consists of the previously precipitated grain boundary cementite and the pearlite formed.

Hypoeutectoid steels

For hypoeutectoid steels with a carbon content of less than 0.8 %, ferrite is precipitated from the austenite lattice when the temperature falls below the γ-α-transformation line, as the face-centered cubic austenite begins to transform into the body-centered cubic ferrite.

The carbon that can no longer be dissolved in the ferrite lattice formed diffuses into the surrounding austenite lattice, as it can still absorb carbon (under-saturated state). This leads to an accumulation of carbon in the remaining austenite. The enrichment finally progresses until the retained austenite has reached the eutectoid composition of 0.8 % carbon at 723 °C.

Now the residual austenite again transforms into pearlite. Note, that the formation of pearlite is always identical regardless of the carbon content of the steel.

At room temperature, the microstructure of a hypoeutectoid steel thus consists of the previously separated ferrite grains and the pearlite formed.

Eutectoid steels

In a eutectoid steel with exactly 0.8 % carbon, the austenite has the eutectoid composition from the very beginning. Thus, pearlite will form directly from the austenite without precipitation processes.

The microstructure of a eutectoid steel consists only of pearlite grains at room temperature.

Note that the microstructure of the steel is always composed of the two phases ferrite and cementite, regardless of whether it is a hypoeutectoid (hypopearlitic) steel or a hypereutectoid (hyperpearlitic) steel. This is precisely the characteristic of the metastable system.

The determination of the microstructure fractions of pearlite and ferrite is explained in the article Determination of microstructure and phase fractions.

The figure below shows steel part of the iron-carbon phase diagram of the metastable system. A closer look at the transformation lines below the solidus line shows the horizontal “K” typical of alloys, whose components are insoluble in one another.

And indeed, the phase transformations of the solidified steel can be considered in analogy to such an alloy, in which the components involved are insoluble in the solid state. After all, the carbon in the \(\alpha\)-iron lattice is also (almost) insoluble at room temperature and is therefore basically a mixed crystal alloy.

The analogy of both phase diagrams and their differences are explained in more detail below.

An essential difference between the two phase diagrams is that the transformation processes in the iron-carbon phase diagram take place not in the liquid state but in the solid state. For this reason, a distinction is made between the terms eutectic (“originating from the melt”) and eutectoid (“originating from the solid state”).

While the A/B crystal mixture alloy is initially present as a homogeneous liquid solution (\(Li\)), the steel is initially also present in the form of the homogeneous solid solution austenite (\(Au\)). These are single-phase regions, as only the melt or austenite is present.

Finally, in the case of hypoeutectic A/B alloys, primary crystals only consiting of atoms (\(A\)) are precipitated when the temperature falls below the corresponding phase line, while ferrite (\(Fe\)) is precipitated in the case of hypoeutectoid steels. These are two-phase regions. Note that the components that are applied to the far left of the concentration axis, i.e. the pure substance \(A\) or pure iron \(Fe\), are precipitated.

Conversely, primary crystals only consisting of B are precipitated in hypereutectic alloys and cementite (\(Ce\)) in hypereutectic steels. Again, these are the components that are applied to the far right of the concentration axis. Note that the phase diagram of the steel was broken off at 2.06 % carbon. Normally the phase diagram on the right ends with 100 % cementite (more on this in the article here).

For hypoeutectic alloys, the residual melt is enriched with B atoms by the precipitation of the primary crystals A until the eutectic composition is finally reached. In the analogous way, the retained austenite is enriched with carbon atoms up to the eutectoid composition by the precipitation of ferrite.

Conversely, in the case of hypereutectic alloys, the precipitation of primary crystals B in the residual melt leads to a reduction in the B concentration down to the eutectic composition. By analogy, in hypereutectoid steels, cementite precipitates at the grain boundaries until the carbon content in the retained austenite has fallen to the eutectoid composition. Both in the case of the A/B crystal mixture alloy and in the case of steels, these precipitation processes take place within the two-phase region at a thermal arrest.

When the eutectic composition in the residual melt is reached, it is finally transformed at a constant temperature into the eutectic, i.e. into a finely distributed mixture of components A and B which are insoluble in one another. In the same way, the retained austenite in steels transforms into the eutectoid pearlite after reaching the eutectoid composition, i.e. into a finely distributed mixture of the components ferrite and cementite, which are insoluble in one another.

In the solidified state, hypoeutectic alloys finally consist of the precipitated primary crystals A and eutectic formed from the residual melt. In hypereutectic alloys, on the other hand, the microstructure contains the precipitated primary crystals B, between which the eutectic is again located. By analogy, the microstructure of hypoeutectoid steels consists of the precipitated ferrite crystals and the eutectoid pearlite, which has formed from the residual austenite. In hypereutectoid steels, however, the microstructure shows the precipitated cementite at the grain boundaries in addition to the eutectoid.

Introduction

For many applications it is important to know exactly what microstructure or phase fractions a steel is composed of at a certain carbon content. This ultimately necessitates a calculation. In order to carry out this, however, the entire iron-carbon phase diagram must be considered. Therefore, the complete phase diagram of the metastable system is briefly described below, before the determination of the microstructure and phase fractions is finally explained.

Up to now, the iron-carbon phase diagram has only been considered up to a carbon content of 2 % (steel part). At higher carbon concentrations, further phase transformations occur, which lead to a different microstructure. Such ferrous materials are then no longer referred to as steels but as cast iron. In the corresponding article on cast iron, the microstructure formation of such materials is described in more detail.

In principle, however, the iron-carbon phase diagram of the metastable system ends at a carbon content of 6.67 %, since the microstructure consists of 100 % cementite. Chemically, the cementite consists of three iron atoms (each with an atomic mass of 56 u) and one carbon atom (with an atomic mass of 12 u). Thus, the mass-related carbon content in the cementite is 6.67 %:

\begin{align}
&\underline{\text{carbon content}} = \frac{12u}{12u+3 \cdot 56u} \cdot 100 \text{ %} = \underline{6.67 \text{ %}} \\[5px]
\end{align}

Determination of the microstructure fractions and phase fractions

In principle, the microstructure and phase fractions are determined by applying the lever rule. The lever arms must always be pulled to the corresponding microstructural or phase boundaries. In the following, the microstructure and phase fractions at room temperature for an hyper- and hypoeutectoid steel will be determined as an example.

Hypoeutectoid steels

In a hypoeutectoid (hypoperlitic) steel, the microstructure consists of ferrite and pearlite grains at room temperature. In order to determine the respective microstructure fractions, the lever arms are drawn accordingly from the considered state point up to the ferrite phase region (at 0 % carbon) and to the pearlite limit (at 0.8 % carbon).

For a steel with, for example, 0.3 % carbon, this results in a ferrite content of 62.5 % at room temperature and a corresponding pearlite content of 37.5%:

\begin{align}
&\underline{\text{ferrite}} = \frac{0.8-0.3}{0.8} \cdot 100 \text{ %} = \underline{62.5 \text{ %}}   \\[5px]
&\underline{\text{pearlite}} = \frac{0.3}{0.8} \cdot 100 \text{ %} =  \underline{37.5 \text{ %}}   \\[5px]
\end{align}

Due to the lever rule, there is generally a linear relationship between the carbon content and the microstructure fractions. For a hypoeutectoid steel, the fraction of pearlite increases steadily with a higher carbon content until it finally reaches 100 % at 0.8 % carbon. Accordingly, the ferrite content decreases to 0 %. The explicit relationship is shown in a microstructure diagram below the phase diagram.

The term microstructure fraction (“grain fraction”) should not be confused with the term phase fraction at this point! After all, the microstructural component pearlite consists of a phase mixture consisting of ferrite as well as cementite. The steel can thus also be characterized by the phase components ferrite and cementite instead of the microstructural components ferrite and pearlite. The procedure for determining the phase fractions is basically identical, but it must be noted that the lever arms must then be drawn up to the respective phase boundaries ferrite and cementite.

For the hypoeutectoid steel with 0.3 % carbon, the total phase content of ferrite is 95.5%. The remaining 4.5 % is finally accounted for by the cementite phase:

\begin{align}
&\underline{\text{ferrite}} = \frac{6.67-0.3}{6.67} \cdot 100 \text{ %} = \underline{95.5 \text{ %}}   \\[5px]
&\underline{\text{cementite}} = \frac{0.3}{6.67} \cdot 100 \text{ %} =  \underline{4.5 \text{ %}}   \\[5px]
\end{align}

Hypereutectoid steels

The microstructure constituents of hypereutectoid steels can be determined in the same way as for hypoeutectoid steels. The lever arms are pulled to the respective microstructure components of the pearlite (at 0.8 % carbon) and the grain boundary cementite (at 6.67 % carbon).

For a steel with, for example, 1.4 % carbon, this results in a pearlite fraction of around 89.8 % at room temperature and a corresponding grain boundary cementite fraction of 10.2 %:

\begin{align}
&\underline{\text{pearlite}} = \frac{6.67-1.4}{6.67-0.8} \cdot 100 \text{ %} = \underline{89.8 \text{ %}}   \\[5px]
&\underline{\text{grain boundary cementite}} = \frac{1.4-0.8}{6.67-0.8} \cdot 100 \text{ %} =  \underline{10.2 \text{ %}}   \\[5px]
\end{align}

For a hypereutectoid steel, the proportion of pearlite decreases steadily with increasing carbon content to a minimum of 78.5 % (at 2.06 % carbon). Accordingly, the fraction of grain boundary cementite increases to a maximum of 21.5 %. The more detailed relationship is shown in the corresponding microstructure diagram below the iron-carbon phase diagram.

For a hypereutectoid steel, too, the term microstructure fraction must again be distinguished from the term phase fraction. Finally, the phase cementite is not only at the grain boundaries but also in the pearlite microstructure, which also consists of ferrite. The corresponding phase proportions of ferrite and cementite can be determined after pulling the lever arms to the respective phase boundaries.

For the hypereutectoid steel with 1.4 % carbon, the total phase fraction of ferrite is 79.0 %. The remaining 21.0 % are finally accounted for by the cementite phase:

\begin{align}
&\underline{\text{ferrite}} = \frac{6.67-1.4}{6.67} \cdot 100 \text{ %} = \underline{79.0 \text{ %}}   \\[5px]
&\underline{\text{cementite}} = \frac{1.4}{6.67} \cdot 100 \text{ %} =  \underline{21.0 \text{ %}}   \\[5px]
\end{align}

As the carbon content increases, the proportion of cementite in the steel also increases. Since the cementite is relatively hard, the hardness of the steel increases accordingly. This results in an almost linear relationship between the carbon content and the hardness of the (unalloyed) steel.

Furthermore, the fine-lamellar cementite precipitation in the microstructure makes the dislocation movement more difficult, which results in a corresponding increase in strength (see hardening mechanisms). The fine lamellas serve as “barriers” for the migrating dislocations, so to speak. Since more cementite is precipitated with increasing carbon content, the fraction of fine lamellar pearlite structure also increases. As the carbon content increases, so does the strength of the steel.

From a carbon concentration of 0.8 %, however, additional precipitation of cementite takes place at the grain boundaries, which in turn leads to embrittlement. This of course only applies to unalloyed steels, i.e. steels that contain no other alloying elements apart from carbon. However, additional alloying elements such as chromium, nickel, manganese, titanium, etc. can also significantly increase strength beyond a carbon content of 0.8 %.

The strength of the steel is therefore decisively determined by the lamellar cementite structure. The finer the pearlite microstructure, the higher the strength. A very fine lamellar structure can be achieved by increased undercooling. It should be noted, however, that as the cooling rate increases, no thermodynamic equilibrium can ultimately be established between or within the phases. Both the transformation temperatures and the types of microstructures that occur change and the iron-carbon phase diagram loses its validity in its actual form.

Phase diagrams only apply at (infinitely) slow cooling speeds!

This means, for example, that a rapid cooling during the \(\gamma\)-\(\alpha\)-transformation leaves no time for the carbon to diffuse out. The result is a new distorted microstructure called martensite. This is used, among other things, for hardening and tempering steel.

Introduction

As explained in the article Microstructure formation of steels during solidification, carbon affects the temperature of the γ-α-transformation. As the carbon concentration increases, the start of the transformation decreases from 911 °C for pure iron to lower temperatures and finally remains constant at a value of 723 °C from a carbon concentration of 0.8 %.

Accordingly, three different types of steel can be distinguished, each of which undergo typical microstructural changes during cooling:

• eutectoid steels with a carbon content of exactly 0.8%!
• hypereutectoid steels with a carbon content greater than 0.8%!
• hypoeutectoid steels with a carbon content of less than 0.8%!

The different phase transformations during cooling from the austenitic state are described in more detail in the following sections.

Eutectoid phase transformation

The changes in the microstructure during the γ-α-transformation are explained in more detail below using the so-called eutectoid steel C80 with a carbon content of 0.8%.

After the steel has solidified in a temperature range and the solid solution austenite structure has formed, the steel is finally subjected to the γ-α-transformation at a temperature of 723 °C. Now the face-centered cubic lattice structure of the austenite begins to transform into the body-centered cubic lattice structure of ferrite at constant temperature (thermal arrest).

Since the centre of the cube is already occupied by an iron atom in the ferrite grid, the carbon atom can no longer be dissolved in it. During this lattice transformation, the carbon is precipitated in the metastable form of iron carbide Fe₃C (cementite). Due to the relatively low temperature of 723 °C, the precipitated atoms are slow and can therfore not travel long distances. They are therefore precipitated directly from the lattice structure and are deposited next to each other in a lamellar structure.

Once austenite has completely transformed into ferrite, the carbon is (almost) completely separated from the iron lattice structure. The former austenite grains have now become ferrite grains with embedded cementite lamellae. This lamellar two-phase mixture of ferrite and cementite is also known as pearlite due to the pearlescent sheen under the microscope.

Pearlite is the eutectoid phase mixture formed at 723 °C, consisting of ferrite and embedded cementite lamellae!

Note that during the γ-α-transformation the outer shape of the grains changes too! While austenite is more polyhedral and thus has an angular grain structure, the pearlite grains are rather roundish. The polyhedral form of austenite is caused by the increased formation of so-called twin grain boundaries. The change of the grain shape during the γ-α-transformation is used e.g. during so-called normalizing in order to eliminate unevenly large grains in a microstructure and thus achieve a homogeneous grain refinement.

Due to its lamellar structure, the pearlite structure is very similar to the eutectic structure of an alloy with components insoluble in one another. The only difference that an eutectic forms from the liquid state while the pearlite structure forms from the already solidified state. In contrast to this, this phase mixture is therefore not called eutectic but eutectoid.

In the present case, the steel with a carbon content of 0.8 % thus has a purely eutectoid structure. Such a steel is therefore also called eutectoid steel or pearlitic steel.

Eutectoid steels have a purely pearlitic structure with 0.8 % carbon at room temperature (ferrite grains with embedded cementite lamellae)!

The micrograph below shows a pearlitic steel with 0.8 % carbon. The strip-shaped embedded cementite lamellae (dark stripes) in the ferrite grains (light areas in between) can be seen.

However, a purely eutectoid pearlite structure is only present if the steel has a carbon content of exactly 0.8 %. The effect a higher carbon concentration has on the structure of the steel is examined in more detail in the following section.

Hypereutectoid phase transformation

Limited solubility of carbon in austenite

Even in the case of hypereutectoid steels with a carbon content of more than 0.8 %, the microstructure is initially present as a pure solid solution structure immediately after solidification (austenite).

Although carbon is relatively well soluble in this austenite structure, its solubility is not unlimited. The carbon atoms are relatively large compared to the gaps in the center of the face-centered cubic unit cells. If the iron atoms are assumed to be touching spheres, there is a gap in the middle of the cube into which a sphere with a maximum of 0.4 times the diameter of the iron atoms fits. However, the carbon atoms have about 0.6 times the diameter.

This means that the carbon atoms are actually too large to fit easily into the middle of the unit cells. As a result, lattice distortions occur in the vicinity of the embedded carbon atoms. Finally, no further carbon atom can be incorporated within the distorted lattice area because the lattice distortions are too strong. Only at certain intervals can further carbon atoms be stored again. The solubility of the colon atoms in γ-iron is thus limited.

The maximum number of carbon atoms that can be dissolved in the austenite lattice depends to a large extent on the temperature. For example, a lower temperature also means a reduced lattice vibration and thus the space within the unit cells also becomes smaller with decreasing temperature. As a result, fewer carbon atoms can be dissolved in the austenite microstructure. Consequently, the solubility of carbon atoms decreases with decreasing temperature! Conversely, a higher temperature means a higher solubility.

The solubility of carbon in austenite decreases with decreasing temperature!

The maximum possible solubility of carbon in austenite is shown at a temperature of 1147 °C with 2.06 %. Every 2nd to 3rd elementary cell (unit cell) is occupied with a carbon atom. As the temperature decreases, the solubility decreases continuously and immediately bevore disintegration of the austenite into the body-centered cubic lattice structure at 723 °C the solubility is only 0.8 % at most. The carbon atoms only find a place in every 6th to 7th unit cell.

Using the solubility limit drawn in the iron-carbon diagram (called solvus line), the corresponding maximum carbon content to be dissolved can finally be determined for every other temperature in the austenite. At a temperature of 1000 °C, for example, the maximum solubility of carbon is around 1.6 %, while the solubility at 940 °C is only around 1.4 % and has even fallen to around 1.0 % at 800 °C.

Microstructural change of a hypereutectoid steel

Due to the limited solubility, microstructural changes occur during cooling of hypereutectoid steels as soon as the solubility limit is overshot, since the steel then obviously contains more carbon than the lattice structure can actually dissolve. Using the example of a steel with 1.4 % carbon (C140), the microstructural changes that take place are described in more detail below.

At first, a hypereutectoid steel solidifies like any other steel as solid solution within a temperature range. Due to the high temperatures immediately after solidification, all carbon is initially completely soluble in the austenite microstructure.

Finally, the solubility starts to decrease continuously from a temperature of 1147 °C according to the marked solvus line. At 1000 °C the maximum solubility is only about 1.6 %. However, since the considered steel has a lower carbon content of 1.4 %, all the carbon is still soluble in the austenite lattice. Since the steel could theoretically dissolve even more carbon, this state is referred to as a undersaturated state.

Finally, the maximum solubility decreases further with decreasing temperature and has dropped to 1.4 % at 940 °C. The state point is exactly at the solubility limit. At this temperature, all the carbon contained in the steel can be completely dissolved in the austenite lattice. Since the austenite lattice is completely saturated with carbon, this state also referred to as a saturated state.

If cooling continues, the carbon content of the steel is higher than the maximum solubility. This becomes clear, for example, when looking at the temperature of 800 °C. According to the solvus line, only about 1.0 % carbon can be dissolved in the austenite lattice at this temperature; however, the steel has a carbon content of 1.4 %. The microstructure must obviously change in some way when the solubility limit is overshot (phase transformation). Otherwise there would be more carbon in the austenite lattice than could actually be dissolved in it.

If the steel is in the so-called supersaturated state shortly after falling below the solvus line (i.e. more carbon is dissolved in the lattice than it can actually absorb), the “too much” carbon is precipitated from the austenite lattice. In the metastable system, this segregation of carbon takes place in the form of cementite (Fe₃C).

Cementite precipitation takes place preferably at energetically favourable locations such as grain boundaries, which is why the precipitated cementite is also called grain boundary cementite. The term secondary cementite is also frequently used.

Note that the cementite is not precipitated in lamellar form in the middle of the lattice structure as in the case of pearlite formation, since cementite precipitation during pearlite formation is caused by the transformation of the lattice structure. However, if the solubility limit is exceeded, the austenite lattice structure is retained, i.e. no lattice transformation takes place. The mechanisms of cementite precipitation during pearlite formation and when the solubility limit is overshot are therefore fundamentally different!

In hypereutectoid steels, the insolvable part of carbon in the austenite lattice precipitates in the form of cementite at the grain boundaries when cooled (grain boundary cementite)!

Cementite precipitation at the grain boundaries is ultimately accompanied by a change in the energetic state. Heat is released in the grid to counteract external cooling. Therefore, the cooling rate slows down even if the solubility limit is exceeded (flattened cooling curve).

If the solubility limit is exceeded, less and less carbon can be dissolved in the austenite with further cooling. As the cooling continues, more and more cementite precipitates at the grain boundaries. This ensures that the austenite is always saturated with carbon according to its solubility. Therefore, when the solubility limit is exceeded, the carbon concentration in the austenite always corresponds to the maximum possible solubility. This of course requires that the cooling is so slow that the carbon also has time to precipitate. Only in this way a thermodynamic equilibrium state can always be achieved.

If the steel is now further cooled, the maximum solubility and thus the carbon content in the austenite decreases more and more until it finally reaches the eutectoid composition of 0.8 % carbon at 723 °C. In principle, the austenite then behaves like a eutectoid steel, which contains exactly 0.8 % carbon.

At a constant temperature of 723 °C, the austenite begins to decompose into pearlite as the face-centered cubic austenite lattice is transformed into the completely insoluble body-centered cubic structure of ferrite. The carbon is precipitated directly from the lattice structure in the form of cementite lamellae.

With precipitating of cementite, the carbon content in the austenite decreases until the eutectoid composition is reached at 723 °C and then the austenite converts to pearlite.

After this final microstructural transformation, the cooling process is finally completed and the microstructure of the hypereutectoid steel consists of pearlite grains (ferrite grains with embedded cementite lamellae) and the cementite previously precipitated at the grain boundaries.

At room temperature, hypereutectoid steels have a pearlitic basic microstructure (ferrite grains with embedded cementite lamellae) with additionally precipitated cementite at the grain boundaries!

The micrograph below shows a hypereutectoid steel with 1.0 % carbon (C100). The pearlite grains (dark) and the cementite (white) precipitated at the grain boundaries can be seen. The fine cementite lamellae in the pearlite are difficult to dissolve by light microscopy and therefore often appear monochromatic.

In addition to hypereutectoid steels with a carbon concentration of over 0.8 %, there are also steels with carbon contents below 0.8 %. They are then referred to as hypoeutectoid steels. With such steels, other microstructural transformations take place during the cooling process. These are discussed in more detail in the following section.

Hypoeutectoid phase transformation

In the following, the cooling of a hypoeutectoid steel will be considered. A steel is called hypoeutectoid if it has a carbon content of less than 0.8 %. The microstructural transformations of a hypoeutectoid steel with 0.4 % carbon are to be explained in more detail as an example.

At first, the hypoeutectoid steel solidifies again like any other steel as a pure solid solution. Carbon is initially completely soluble in the austenite structure.

Basically, a hypoeutectoid steel has too little carbon to exceed the maximum solubility limit of the carbon in the austenite. At the lowest possible temperature of 723 °C – above which austenite even exists – the (minimum) solubility for carbon in austenite is already 0.8 %. The carbon solubility is therefore always higher than the carbon content of hypoeutectoid steels.

This can be clearly seen from the phase diagram when entering the alloy as the corresponding line. In principle, all hypoeutectoid steels will never reach or even exceed the solubility limit. All carbon remains soluble in the austenite lattice of hypoeutectoid steels at any time (at least as long as austenite is exists). The austenite microstructure is permanently in an undersaturated state, as more carbon could be dissolved in it than is contained in the steel at all!

With hypoeutectoid steels, all carbon remains soluble in the austenite lattice!

The microstructural transformation of a hypoeutectoid steel is therefore not determined by the solubility limit as with hypereutectoid steels but rather by the γ-α-transformation. Although carbon in iron causes a shift in the γ-α-transformation towards lower temperatures compared to pure iron, it will still begin at a certain temperature. The onset of the lattice transformation can be seen by the red transformation line in the iron-carbon phase diagram, which every hypoeutectoid steel ultimately intersects as it cools.

In a hypoeutectoid steel of 0.4 % carbon, the lattice transformation from the face-centered cubic austenite lattice to the body-centered cubic ferrite structure starts at around 800 °C (instead of 911 °C as in the case of pure iron). The face-centered cubic lattice preferably begins to transform into the body-centered cubic microstructure at the energetically favourable grain boundaries. This lattice transformation spreads to the surrounding austenite structure as it cools further.

This transformation no longer takes place at a constant temperature as in the case of pure iron, but in a temperature range. Therefore, the γ-α- transformation again comprises a two phase region in the diagram in which the microstructure consists in parts of the already converted ferrite and the remaining austenite.

In hypoeutectoid steels, parts of the austenite first transform into carbon-insoluble ferrite when cooling!

Since the carbon in the already converted α-iron can no longer be dissolved, it is displaced from the body-centered cubic ferrite. However, the surrounding austenite structure is still able to absorb the segragated carbon due to its undersaturated state.

At the beginning of the lattice transformation, for example, the austenite can absorb up to approx. 1.0 % carbon at 800 °C; however, the considered steel only has a carbon content of 0.4 %. Thus, there is still enough space in the austenite to absorb the displaced carbon atoms. Therefore, the carbon precipitated from the ferrite lattice diffuses into the adjacent retained austenite.

Unlike hypereutectoid steels, carbon is not deposited as cementite at the grain boundaries, but is absorbed by the surrounding austenite during γ-α-transformation!

With further cooling, the ferrite grains grow, so that more and more carbon diffuses into the surrounding austenite grains. This leads to a corresponding enrichment of the carbon content in austenite. The respective concentrations can be determined – as is usual in two-phase regions – after approaching the phase boundary and subsequently drawing a vertical line onto the concentration axis. At 750 °C, for example, the carbon content in austenite has increased to approx. 0.6 %, while the carbon concentration in ferrite is of course 0 % due to neglected solubility.

With further cooling and thus increasing ferrite formation, the carbon in the retained austenite accumulates more and more. At 723 °C, the carbon content finally rose to 0.8 %. The retained austenite has now reached the eutectoid composition and is completely saturated, i.e. it can no longer absorb any more carbon. The retained austenite now behaves like a eutectoid steel and finally begins to convert to the eutectoid pearlite at a constant temperature.

Due to the transformation of ferrite from the austenite, it is enriched with carbon until the eutectoid composition is reached at 723 °C and the retained austenite is converted to pearlite.

Thus, the remaining austenite also undergoes the lattice transformation into the carbon-insoluble ferrite structure. The carbon formerly dissolved in the austenite lattice forms the iron carbide compound cementite, which precipitates in lamellar form from the residual austenite during the lattice transformation.

At room temperature, hypoeutectoid steels have a pearlitic basic structure (ferrite grains with embedded cementite lamellae) with the previously formed ferrite grains!

The micrograph below shows a hypoeutectoid steel with 0.45 % carbon (quenched and tempered steel C45). The ferrite grains (white) and pearlite grains (dark stripes) can be seen. In comparison, a microstructure of a hypoeutectoid steel with a higher carbon content of 0.60 % (quenched and tempered steel C60) is shown. Due to the higher carbon concentration, the C60 also has a significantly higher pearlite content in the microstructure.

Note

Note that the γ-α-conversion is always completed at 723 °C regardless of the actual carbon concentration of the steel, as the retained austenite has always reached the eutectoid composition of 0.8 % carbon at this temperature. This applies not only to hypoeutectoid steels but also to hypereutectoid steels!

While for hypoeutectoid steels the carbon concentration in residual austenite accumulates up to 0.8 % carbon due to ferrite precipitation, for hypereutectoid steels the carbon concentration in residual austenite decreases due to cementite precipitation at the grain boundaries until 0.8 % carbon is also reached there. In both cases, this eutectoid composition of retained austenite is reached at a temperature of 723 °C and the retained austenite always decomposes to pearlite at this constant temperature.

Introduction

In principle, steels are binary systems consisting of the host element iron and the alloying element carbon with a maximum content of 2 % (above 2% carbon, the iron-carbon alloy is called cast iron!). The carbon provides the necessary strength and hardness because iron alone would be too soft as a construction material. In order to be able to produce steels according to these different requirements (high hardness or high strength, or a compromise of both), a deeper understanding of the alloy system iron/carbon is required.

Steel is an alloy of iron and carbon! With a carbon content of more than 2 % one speaks of cast iron!

In contrast to the binary systems previously considered, phase transformation does not only take place during solidification. Iron also shows an allotropy (polymorphism), i.e. depending on temperature iron exists in different lattice structures. In the solid state, these cause further phase transformations. Therefore, the phase diagram of the iron/carbon alloy system is somewhat more complex.

In order to understand the microstructural processes inside a steel, it makes sense to first take a closer look at the microstructure formation of pure iron. For this reason, the cooling curve of iron is discussed in more detail in the following section.

Microstructure formation of soft iron

In the following, the cooling curve of pure iron will be examined in more detail. Since pure iron is relatively soft in the solidified state, it is also called soft iron.

The cooling curve of pure iron (Fe) has a series of thermal arrests at which different processes take place in the microstructure. The first thermal arrest is at the solidification temperature of 1536 °C. At this point the melt crystallizes in a body-centered cubic lattice structure (bcc). In this state the iron is also called \(\delta\)-iron (\(\delta\)-Fe). Note that the entire microstructure of \(\delta\)-iron is already completely solidified. Thus, all further phase transformations finally take place in the already solidified state!

At a temperature of 1392 °C, the body-centered cubic \(\delta\)-iron transforms into the face-centered cubic structure (fcc) at a constant temperature. In this lattice modification the iron is also called \(\gamma\)-iron. Since the atomic structure and thus the binding energies change during a lattice transformation, this is also associated with an energy conversion. Therefore, the lattice structure changes at a constant temperature (thermal arrest)!

A further lattice transformation finally takes place at 911 °C. At this temperature, the face-centered cubic iron transforms back into the body-centered cubic structure. In this form the iron is also called (\beta\)-iron.

A last thermal arrest finally occurs at a temperature of 769 °C. However, this is not due to a lattice transformation! The reason for the thermal arrest is a quantum mechanical effect, which is responsible for the fact that the iron is magnetic below this temperature and not above! This temperature is also called Curie temperature (apart from iron, only the elements cobalt and nickel are ferromagnetic at room temperature). The magnetic state of iron with its body-centered cubic lattice structure is also called (\alpha\)-iron.

The Curie temperature is the temperature at which a ferromagnetic material loses its magnetic properties!

The micrograph below shows soft iron (\(\alpha\)-iron) in an almost carbon-free state. The iron grains (white areas) and silicate inclusions (dark spots) can be seen.

Now that the microstructural transformations of pure iron have been explained, the following article describes the phase transformations in the presence of carbon (steel) in more detail.

Microstructure formation of steel

In the previous section, the phase transformations of pure iron were examined in more detail. In addition to iron, however, steels also consist of carbon. This leads to a shift in the described phase transformations of the iron! How the carbon influences the phase transitions ist best explained by the corresponding phase diagram (state diagram).

The state diagram of the iron-carbon system is also called the iron-carbon phase diagram. Due to its complexity, the creation of the phase diagram on the basis of selected cooling curves will not be discussed. Furthermore, the iron-carbon diagram in the following sections is initially only considered up to a carbon content of around 2%, as only this range is relevant for steels. This area in the iron-carbon diagram is therefore also referred to as the steel part. Higher carbon concentrations are discussed in more detail in separate sections.

The steel part is the section of the iron-carbon phase diagram up to a carbon content of 2% relevant for steels!

Carbon initially influences the solidification of the steel like a solid solution. The steel part of the phase diagram terefore has the typical lenticular two-phase region during solidification. The start of solidification is described by the liquidus line and the end of solidification by the solidus line. The microstructure is formed between these lines with a correspondingly slower cooling rate. The phase diagram shows that the solidification range shifts towards lower temperatures with increasing carbon content.

Carbon shifts the solidification range of the steel towards lower temperatures!

In addition, even small amounts of carbon (> 0.1%) completely suppress the body-centered cubic phase of \(\delta\)-iron. The steel then immediately crystallizes in the face-centered cubic lattice structure of \(\gamma\)-iron. Since the \(\delta\) phase has no technical significance anyway, the phase diagram is very often presented in simplified form without this phase region.

Steels behave during solidification like solid solutions in which the alloying element carbon is completely soluble in the host material iron.

The good solubility of carbon is due to the face-centered cubic lattice structure of \(\gamma\)-iron. The relatively small carbon atoms find their place in the free centers of the unit cells. In this case, it is a interstitial solid solution in which the carbon atom is embedded in the interstitials of the iron lattice. This face-centered cubic lattice structure of iron with carbon atoms embedded in it is also called austenite.

Austenite is the face-centered cubic lattice structure of \(\gamma\)-iron with carbon atoms embedded therein (solid solution)!

Accordingly, the two-phase region between the liquidus line and the solidus lines contains the phases melt (L) and austenite (A). In the two-phase region, the respective carbon concentrations of the two phases can be determined as usual by drawing a perpendicular line onto the concentration axis. The phase fractions are again determined by means of the lever rule.

In general, the same basic mechanisms take place during solidification of steels as for solid solutions. However, this only applies as long as the temperatures are sufficiently high and the iron is thus in the face-centered cubic state. Only then is the carbon completely soluble in the iron lattice and the alloy can be regarded as a solid solution.

The austenite phase only exists at sufficiently high temperatures as long as the iron is present in the face-centered cubic structure!

However, due to its allotropy, when the temperature drops, iron eventually changes its face-centered cubic structure and transforms into the body-centered cubic \(\alpha\)-iron. With decreasing temperature a further phase transformation is connected, which takes place now however in the already solidified microstructure! This conversion will be discussed in more detail in the next section.

Carbon precipitation (\(\gamma\)-\(\alpha\)-transformation)

Pure iron changes its face-centered cubic lattice structure of \(\gamma\)-iron when the temperature falls below 911 °C and changes to the body-centered cubic lattice structure of \(\alpha\)-iron. In principle, this lattice transformation also occurs in the presence of carbon, but at other temperatures!

As the carbon content increases, this so-called \(\gamma\)-\(\alpha\)-transformation is shifted towards lower temperatures. In addition, the carbon causes this lattice transformation to take place in a temperature range rather than in thermal arrest at a constant temperature. Only from a carbon content of 0.8 % does the \(\alpha\)-iron form again at constant temperature so that the polylines of the beginning and end of the \(\gamma\)-\(\alpha\)-conversion coincide in the phase diagram.

The presence of carbon shifts the \(\gamma\)-\(\alpha\)-transformation towards lower temperatures!

In contrast to the solid solution of \(\gamma\)-iron, the unit cell of the body-centered cubic lattice of \(\alpha\)-iron is already occupied by an iron atom in the center of the cube. \(\alpha\)-iron can therefore dissolve almost no carbon. The maximum solubility at 723 °C is only 0.02 % and even drops below 0.001 % at room temperature (the exact solubility limit is shown in the diagram with a green solvus line). To simplify matters, it is therefore assumed in the following that no carbon is soluble in the lattice of \(\alpha\)-iron.

The carbon atom previously embedded in the austenite is therefore “pressed out” of the lattice structure during the \(\gamma\)-\(\alpha\)-transformation. Thus, it is an almost carbon-free \(\alpha\)-iron lattice. In contrast to the carbon-containing face-centered cubic lattice of \(\gamma\)-iron, which was called austenite, the almost carbon-free body-centered cubic lattice of \(\alpha\)-iron is also called ferrite.

Ferrite is the almost carbon-free cubic space-centered lattice structure of \(\alpha\)-iron!

Stable system

During the \(\gamma\)-\(\alpha\)-conversion, the carbon that is no longer soluble in \(\alpha\)-iron can in principle precipitate from the lattice in two ways. With slow cooling and a relatively high carbon content, a sufficient number of carbon atoms can come together to form their own hexagonal lattice structure. In this lattice modification, carbon is also called graphite.

Such graphite precipitation is not only favoured by relatively slow cooling speeds but can also be specifically promoted by adding silicon. The precipitation of carbon in the form of graphite is also referred to as a stable system, since the carbon in this form can no longer decay further and is therefore stable in the thermodynamic sense.

A microstructure solidified according to the stable system basically consists of iron (Fe) and graphite (C). This applies in particular to cast iron!

Cast iron usually has a relatively high carbon content (> 2 %) and is therefore a typical representative of the stable system. However, some types of cast iron also solidify according to the metastable system described below. This applies in particular to steels.

Metastable system

If the solidified microstructure is no longer cooled relatively slowly but faster and only small amounts of carbon are present, the carbon atoms can no longer attach to a common graphite lattice structure. In this case, the precipitating carbon combines with three iron atoms to form the iron carbide compound Fe₃C and forms a rhombohedral lattice structure. This intermediate (intermetallic) iron carbide compound is also called cementite.

Cementite is a relatively hard but brittle intermetallic compound consisting of three iron atoms and one carbon atom (Fe₃C)!

As the name suggests, cementite is very hard and significantly responsible for the increase in hardness of the steel! Precipitation of cementite can not only be achieved through faster cooling but also by specific additives such as manganese. The precipitation of carbon in the form of cementite is also called a metastable system in the thermodynamic sense, since the iron carbide compound would decompose into the thermodynamically stable graphite form by diffusion processes at sufficiently high temperatures and sufficiently long annealing times.

In contrast to cast iron, steels generally have a relatively low carbon content (< 2 %) and are therefore typical representatives of the metastable system.

A microstructure solidified according to the metastable system basically consists of iron (Fe) and cementite (Fe₃C). This applies in particular to steels!

Depending on the precipitation of carbon in the form of graphite or cementite, the polylines in the iron-carbon phase diagram differ slightly from one another (more on this in the article on cast iron). Since the metastable system with its cementite precipitation is particularly important for steels, only this metastable system will be discussed in more detail in the following articles.