Iron Carbon Diagram

Importance of the iron-carbide system

Of all binary alloy systems, the one that is possibly the most important is that for iron and carbon.

Both steels and cast irons, primary structural materials in every technologically advanced culture, are essentially iron-carbon alloys.

Studying the phase diagram for this system gives us an overview on the development of microstructures.

The relationships between heat treatment, microstructure, and mechanical properties are based on the phase diagram of this system.

The Iron Carbon diagram

The Fe-C phase diagram is a fairly complex one, but we will only consider the part of the diagram up to around 7% Carbon.

Reactions in the iron carbon diagram

1. Eutectic reaction

L 4.3%C ↔ γ 2.05%C + Fe3C 6.7%C , T=1146oC

2. Peritectic reaction

L 0.5%C + δ0.1%C ↔ γ 0.18%C , T=1493oC

3. Eutrctoid reaction

γ 0.8%C ↔ α 0.025%C + Fe3C 6.7%C , T=723oC

4. Polymorphic transformations

α-iron(BCC) ↔ γ-iron(FCC) , T=910oC

γ-iron(FCC) ↔ δ-iron(BCC) , T=1390oC

Phases in Fe–Fe3C Phase Diagram

Ferrite (α)

An interstitial solid solution of carbon dissolved in BCC α iron. Carbon solubility – 0.025 wt.% max. at 723 C, 0.006 wt.% min. at 0 C. The softest structure that appears on the iron – iron carbide diagram. Average properties: 40,000 psi TS, 40 % elong.

Austenite (γ)

An interstitial solid solution of carbon dissolved in FCC γ-iron. Carbon solubility – 2.00 wt.% max. at 1130 C, 0.80 wt.% min. at 723 C. Not stable at room temperature; can be made stable under certain conditions. Average properties: 150,000 psi TS, 10 % elong, high toughness.

Cementite (Fe3C)

An interstitial intermetallic compound of iron carbide with an orthorhombic structure. Its chemical formula is Fe C and contains 6.67 wt % carbon The hardest and brittle structure that appears on the iron – iron carbide diagram. Average properties: 5,000 psi TS, high compressive strength

Pearlite

The eutectoid mixture of fine plate-like lamellar mixture of ferrite and cementite. Formed from austenite that contains 0.80 wt.% carbon during slow cooling at 723 C. Average properties: 120 000 psi TS 20 % elong

Ledeburite

The eutectic mixture of austenite and cementite. Formed from liquid that contains 4.30 wt.% carbon during slow cooling at 1130 C. Not stable below 723 C, where austenite of ladeburite transformed into pearlite.

The structure is then called “transformed ledeburite.”

A few comments on Fe–Fe3C system

C is an interstitial impurity in Fe. It forms a solid solution with α, γ, δ phases of iron Maximum solubility in BCC α-ferrite is limited (max. 0.022 wt% at 727 °C) - BCC has relatively small interstitial positions

Maximum solubility in FCC austenite is 2.14 wt% at 1147 °C - FCC has larger interstitial positions

Mechanical properties: Cementite is very hard and brittle - can strengthen steels. Mechanical properties also depend on the microstructure.

Classification of alloys

There are three types of ferrous alloys:

• Iron: less than 0.006 wt % C in α−ferrite at room T

• Steels: 0.006 - 2.05 wt % C

• Cast iron: 2.05 - 6.7 wt % (usually < 4.5 wt %)

Now let's put the different phases and classifications on the diagram

As mentioned above, alloys can be found in the form of Irons, steels and cast irons. Each has its properties which differ according to different factors that will be discussed later in details.

Cast irons

These are alloys having carbon content above 2.05%, they don't complete their solidification with all carbon soluble in Iron which gives rise to the eutectic reaction.

Cast Irons are considered the cheapest type of commercial alloys, they are easier to melt and cast into large and complex shapes than steels and they have high hardness & great resistance to abrasion wear.

Cast Irons are classified into different types according to:

The carbon content, the form of excess carbon, its shape & distribution. Percentage impurities present in the alloy, specially the silicon content. The heat treatment carried out after melting.

Types of Cast Irons

White Cast Iron (W.C.I.)

Malleable Cast Iron (M.C.I.)

Gray Cast Iron (G.C.I.)

Nodular Cast Iron (N.C.I.)

White Cast Iron

Cementite appears in W.C.I. in large amounts as continuous interdendritic network which makes cast irons very hard. W.C.I. alloys are usually hypo-eutectic in carbon content. They have been given this name because when broken, the fresh fracture is very bright.

W.C.I. alloys are characterized by high hardness, high resistance to abrasion, brittleness and difficulty of machining.

They are used as balls for grinding, screw flights conveying abrasive solids, bearings and abrasive plates.

The following figure shows the microstructure of a W.C.I. alloy

Malleable Cast Iron

Manufactured by the heat treatment of White cast Iron. The undissolved carbon exists as graphite in the form of spheroids.

Cementite undergoes the graphitization reaction producing Iron & Carbon:

Fe3C →3Fe + C

This reaction is enhanced by :

High Carbon Content

High Silicon Content

High temperature

The procedure of heat treatment

1. Heat the W.C.I. very slowly to a temperature between 723OC and 1146OC

2. Leave the alloy at this temp. for (36-48 hrs) to complete the graphitization reaction (soaking period)

3. By changing the cooling rate , Alloys of distinct properties can be obtained.

Types of M.C.I.

1. Ferritic M.C.I.

• Slow cooling rate so that all eutectoid Fe3C decomposes

• Soft , weak and ductile (Just like Ferrite)

2. Bull’s Eye M.C.I.

• Less slow cooling rate so that only part of the eutectoid Fe3C decomposes

• Intermediate between the ferritic and pearlitic types

3. Pearlitic M.C.I.

• A bit rapid cooling rate so that no eutectoid cementite decomposes

• Harder and more brittle than the ferritic type

Gray Cast Iron

Most or all the undissolved carbon exist in the form of thin curved plates or flakes of graphite.

G.C.I. alloys are characterized by:

1. Being weak under tension but Good under compression

2. Cheap and Easy to cast

3. High Damping Capacity

4. Low Notch Sensitivity

Impurities in G.C.I.

1. Sulfur

It makes the alloy very brittle.

That can be cured by adding Manganese which reacts with sulfur forming MnS which has nearly on effect on the alloy

2. Phosphorous:

It forms a ternary eutectic – Steadite- with Austenite and Cementite which makes the alloy very brittle( shown in the following microstrucure)

This effect can be reduced adding Na2CO3 to the alloy

Nodular Cast Iron

These alloys are formed by adding nodulizing agents (Magnesium , Cesium , Sodium) to a gray iron cast melt. Graphite is present as tiny balls.

Advantages of N.C.I.

Improved strength and ductility to all cast irons

Disadvantages of N.C.I.

Formation of Blowholes

High shrinkage on cooling

Nodular Cast Irons may exist in the ferritic or pearlitic forms.

Carbon Steels

In their simplest form, steels are alloys of Iron (Fe) and Carbon (C) having a carbon content between 0.006 & 2.05% carbon by weight.

Steels can be sub-classified into hypo-eutectoid steel, eutectoid steel & hyper-eutectoid steel according to the carbon content.

Eutectoid steel

When alloy of eutectoid composition (0.8 wt % C) is cooled slowly it forms pearlite, a lamellar or layered structure of two phases: α-ferrite and cementite (Fe3C) The layers of alternating phases in pearlite are formed for the same reason as layered structure of eutectic structures: redistribution C atoms between ferrite (0.025 wt%) and cementite (6.7 wt%) by atomic diffusion. Mechanically, pearlite has properties intermediate to soft, ductile ferrite and hard, brittle cementite.

Microstructure of eutectoid steel

Hypoeutectoid steel

They are alloys having compositions to the left of eutectoid point (0.025 - 0.8 wt % C) Hypoeutectoid alloys contain proeutectoid ferrite (formed above the eutectoid temperature) plus the eutectoid perlite that contain eutectoid ferrite and cementite.

Microstructure

Hypereutectoid steel

They are alloys having compositions to the right of eutectoid point (0.8 - 2.05 wt % C) Hypoeutectoid alloys contain proeutectoid cementite (formed above the eutectoid temperature) plus the eutectoid perlite that contain eutectoid ferrite and cementite.

Microstructure

Superheating / Supercooling

Upon crossing a phase boundary on the composition temperature phase

diagram phase transformation towards equilibrium state is induced.

But the transition to the equilibrium structure takes time and

transformation is delayed.

During cooling, transformations occur at temperatures less than predicted

by phase diagram: supercooling.

During heating, transformations occur at temperatures greater than

predicted by phase diagram: superheating.

Degree of supercooling/superheating increases with rate of

cooling/heating.

Metastable states can be formed as a result of fast temperature change.

Microstructure is strongly affected by the rate of cooling.

Below we will consider the effect of time on phase transformations using

iron-carbon alloy as an example by discussing the eutectoid reaction.

The S-shaped curves are shifted to longer times at higher T showing that the

transformation is dominated by nucleation (nucleation rate increases with supercooling)

and not by diffusion (which occurs faster at higher T).

Martensite Transformation

Martensite forms when austenite is rapidly cooled (quenched) to room T.

• It forms nearly instantaneously when the required low temperature is reached. The

austenite-martensite does not involve diffusion → no thermal activation is needed, this

is called an athermal transformation.

• Each atom displaces a small (sub-atomic) distance to transform FCC γ-Fe (austenite) to

martensite which has a Body Centered Tetragonal (BCT) unit cell (like BCC, but one unit

cell axis is longer than the other two).

• Martensite is metastable - can persist indefinitely at room temperature, but will

transform to equilibrium phases on annealing at an elevated temperature.

• Martensite can coexist with other phases and/or microstructures in Fe-C system

• Since martensite is metastable non-equilibrium phase, it does not appear in phase Fe-

C phase diagram

Bainite Transformation

If transformation temperature is low enough (≤540°C) bainite rather than fine pearlite

forms where:

For T ~ 300-540°C, upper bainite consists of needles of ferrite separated by long

cementite particles

For T ~ 200-300°C, lower bainite consists of thin plates of ferrite containing very

fine rods or blades of cementite

In the bainite region, transformation rate is controlled by microstructure

growth (diffusion) rather than nucleation. Since diffusion is slow at low

temperatures, this phase has a very fine (microscopic) microstructure.

Pearlite and bainite transformations are competitive; transformation between

pearlite and bainite not possible without first reheating to form austenite

upper bainite lower bainite

TTT Diagram

T (Time) T(Temperature) T(Transformation) diagram is used to determine when

transformations begin and end for an isothermal (constant temperature) heat treatment

of a previously austenitized alloy. When austenite is cooled slowly to a temperature

below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the

cooling rate increases, the pearlite transformation temperature gets lower. The

microstructure of the material is significantly altered as the cooling rate increases. By

heating and cooling a series of samples, the history of the austenite transformation may

be recorded. TTT diagram indicates when a specific transformation starts and ends and

it also shows what percentage of transformation of austenite at a particular

temperature is achieved.

Figures below show the microstructure of different phases.

Austenite Pearlite

Bainite Martensite

Formation of TTT diagrams

The family of S-shaped curves at different T are used to construct the TTT

diagrams.

The TTT diagrams are for the isothermal (constant T) transformations (material

is cooled quickly to a given temperature before the transformation occurs, and

then keep it at that temperature).

At low temperatures, the transformation occurs sooner (it is controlled by the

rate of nucleation) and grain growth (that is controlled by diffusion) is reduced.

Slow diffusion at low temperatures leads to fine-grained microstructure with

thin-layered structure of pearlite (fine pearlite).

At higher temperatures, high diffusion rates allow for larger grain growth and

formation of thick layered structure of pearlite (coarse pearlite).

At compositions other than eutectoid, a proeutectoid phase (ferrite or

cementite) coexist with pearlite. Additional curves for proeutectoid

transformation must be included on TTT diagrams.

Effect of cooling rate on TTT diagram(Heat Treatment)

Cooling rates in the order of increasing severity are achieved by quenching from

elevated temperatures as follows: furnace cooling, air cooling, oil quenching, liquid salts,

water quenching, and brine. If these cooling curves are superimposed on the TTT

diagram, the end product structure and the time required to complete the

transformation may be found.

If a cooling rate is very high, the cooling curve will remain on the left hand side of the

Transformation Start curve. In this case all Austenite will transform to Martensite. If

there is no interruption in cooling the end product will be martensite

In the following Figure, the cooling rates A and B indicate two rapid cooling processes. In

this case curve A will cause a higher distortion and a higher internal stresses than the

cooling rate B. The end product of both cooling rates will be martensite. Cooling rate B is

also known as the Critical Cooling Rate, which is represented by a cooling curve that is

tangent to the nose of the TTT diagram. Critical Cooling Rate is defined as the lowest

cooling rate which produces 100% Martensite while minimizing the internal stresses and

distortions.

In the following Figure, a rapid quenching process is interrupted (horizontal line

represents the interruption) by immersing the material in a molten salt bath and soaking

at a constant temperature followed by another cooling process that passes through

Bainite region of TTT diagram. The end product is Bainite, which is not as hard as

Martensite. As a result of cooling rate D; more dimensional stability, less distortion and

less internal stresses are created(known as interrupted quenching).

In the following Figure cooling curve C represents a slow cooling process, such as

furnace cooling. An example for this type of cooling is annealing process where all the

Austenite is allowed to transform to Pearlite as a result of slow cooling

Sometimes the cooling curve may pass through the middle of the Austenite-Pearlite

transformation zone. Shown below, cooling curve E indicates a cooling rate which is not

high enough to produce 100% martensite. This can be observed easily by looking at the

TTT diagram. Since the cooling curve E is not tangent to the nose of the transformation

diagram, austenite is transformed to 50% Pearlite (curve E is tangent to 50% curve).

Since curve E leaves the transformation diagram at the Martensite zone, the remaining

50 % of the Austenite will be transformed to Martensite.

Finally the following figure represent the different phases present in the TTT diagram