iron carbon equilibrium diagram

In this article we will discuss about the Iron Carbon Equilibrium diagram, uses of equilibrium diagram, types of iron carbon equilibrium diagram and various phases and terms associated with the iron carbon phase diagram.

What are Phase Diagrams ?

Phase diagrams are graphical representations of the phases present in an alloy under various temperature, pressure, and chemical composition conditions.

The solidification of metal alloys is clearly understood by means of equilibrium diagrams. These are graphic representations of changes in state due to variations in temperature and concentration. Since this diagram indicates the nature and constitution of alloys, and the amount and composition of phases in a given system, it is also known as constitution diagram or phase diagram.

Equilibrium Diagram Characteristics and Uses

Equilibrium implies that changes occurring in a system as a result of process proceeding in one direction are fully compensated by changes due to the reversal of the process in the system. So it is considered as a dynamic condition of balance between atomic movements where the resultant is zero.

The rates of changes of temperature or of composition have been extremely slow during the experimental work, so that the alloy would “come to rest” before a variable such as temperature, were again changed. The condition, therefore, is one of rest rather than change.

Equilibrium diagram indicate the following:

1. Temperature at which the solid alloy will start melting and finish melting.

2.
Possible phase changes which will occur as the result of altering the composition or temperature.

What Equilibrium Diagram Represent ?

The diagram describes the suitable conditions for two or more phases to exist in equilibrium. For example, the water phase diagram describes a point (triple point) where water can coexist in three different phases at the same time. This happens at just above the freezing temperature (0.01°C) and 0.006 atm.

Uses of Equilibrium Diagram in Metallurgy

  • Development of the new alloys on the basis of application requirements.
  • Production of these alloys.
  • Development and implementation of appropriate heat treatment procedures to improve the chemical, physical, and mechanical properties of these new alloys.
  • Troubleshooting issues that arise during the use of these new alloys, ultimately improving product predictability.

Iron Carbon Equilibrium Diagram

The Iron carbon equilibrium diagram (also called the iron carbon phase diagram) is a graphic representation of the respective microstructure states of the alloy iron – carbon (Fe-C) depending on temperature and carbon content.

The iron carbon phase diagram is commonly used to fully understand the various phases of steel and cast iron. Steel and cast iron are both iron and carbon alloys. In addition, both alloys contain trace elements in small amounts.

The graph is quite complex, but because we are limiting our investigation to Fe3C, we will only be looking at up to 6.67 weight percent carbon.

Types of Iron Carbon Equilibrium Diagram

The binary iron carbon equilibrium diagram is the basis of steel and cast iron. It concerns transformations that occur in alloys having compositions from pure iron to cementite (6.67 per cent carbon). There are two versions of iron carbon equilibrium diagram :

1. Iron-cementite system. 2. Iron-graphite system.

These two systems depend on the rate of cooling. Rapid cooling produces cementite and the system is known as Iron-cementite system. In this system, the structures formed in the solidified phases do not reach sufficiently complete equilibrium. So Iron-cementite system is a metastable one.

While slow cooling produces graphite and the system is known as Iron-graphite system. The structures that are formed in the solidified phase reach sufficiently complete equilibrium. So this is a stable one.

Types of Ferrous Alloy on Iron Carbon Equilibrium Diagram

The weight percentage scale on the X-axis of the iron carbon phase diagram ranges from 0% to 6.67% Carbon. The metal is simply known as iron or pure iron up to a maximum carbon content of 0.008 percent weight of carbon. At room temperature, it exists in the ferrite state.

Steel is an iron carbon alloy with a carbon content ranging from 0.008 to 2.14 percent. Steel grades within this range are known as low carbon steel (or mild steel), medium carbon steel, and high carbon steel.

When the carbon content exceeds 2.14 percent, we reach the cast iron stage. Cast iron is extremely hard, but its brittleness severely limits its applications and forming methods.

IRON-CEMENTITE SYSTEM

Iron carbon equilibrium diagram

If a series of time-temperature heating curves are made for steels of different carbon contents and the corresponding critical points plotted a diagram similar to Fig. 2.14 would be obtained. This diagram, which applies only under slow cooling conditions, is known as a partial Iron carbon phase diagram. By referring to this diagram one may readily observe the proper quenching temperatures for any carbon steel, The critical points in Fig. 2.14 on the line PSK are denoted by A1, those on line GS by A3, and those of line SE by Acm

Iron Carbon Equilibrium Diagram with Explanation

Austenite

Austenite, solid solution of carbon and other constituents in a particular form of iron known as γ (gamma) iron. Let us take the example of a piece of 0.20 per cent carbon steel which has been heated to a temperature around 850°C. Above Ar3, point (GS line) this steel is a solid solution (interstitial type) of carbon in gamma iron and is called austenite. It has a face-centered cubic lattice and is nonmagnetic.

Plain austenite may contain up to about 2 per cent carbon at a temperature of 1130°C. Upon cooling this steel the iron atoms start to form body-centered cubic lattice below the point Ar3 (GS line). This new structure that is being formed is called ferrite or alpha iron and is solid solution of carbon in alpha iron containing up to 0.008 per cent carbon at room temperature.

As the steel is cooled to Ar1, (PSK line), additional ferrite is formed. At the Arı line the austenite that remains is transformed to a new structure called pearlite. The name pearlite is due to its pearly luster. It consists of alternate plates of ferrite and cementite and contains about 87 per cent ferrite. Pearlite may be either fine-to-coarse lamellar or granular structure. This is a strong substance and may be cut reasonably well with cutting tools, i.e., the pearlite constituent in steel is machinable.

Eutectoid Steel

As the carbon content of the steel increases above 0.20 per cent, the temperature at which the ferrite is first rejected from the austenite drops until, at about 0.80 per cent carbon (point S), no free ferrite is rejected from the austenite. This steel is called eutectoid steel and is 100 percent pearlite.

Eutectoid Point

What is Eutectoid Point ?

The eutectoid point in any metal, as said earlier, is the lowest temperature at which changes occur in a solid solution.

If the carbon content of the steel is greater than eutectoid (0.8 per cent carbon), a new line is observed in the iron carbon phase diagram denoted by Acm (S line). The line denotes the temperature at which iron carbide is first rejected from the austenite instead of ferrite.

The iron carbide (Fe3C) is known as cementite. It is extremely hard, brittle and appears as parallel plates (lamellar layers), as rounded particles (spheroids) or as envelopes around the pearlite grains. At point C, the eutectic mixture containing 4.3 per cent carbon is known as ledeburite. This is rarely seen in slowly cooled alloys since it breaks down, due to its unstable nature, to other phases during cooling after solidification.

Hypoeutectoid and Hypereutectoid

Steels containing less than 0.80 per cent carbon are called hypoeutectoid and those which contain more than 0.8 per cent carbon are called hypereutectoid steels. This terminology applies only to plain and low alloys steels. With high alloy steels the eutectoid composition is altered and the structure may not even exist.

It should first be pointed out that the normal equilibrium diagram really represents the metastable equilibrium between iron and iron carbide (cementite). Cementite is metastable, and the true equilibrium should be between iron and graphite.

Although graphite occurs extensively in cast irons (2-4 wt% C), it is usually difficult to obtain this equilibrium phase in steels (0.03-1.5 wt%C). Therefore, the metastable equilibrium between iron and iron carbide should be considered, because it is relevant to the behavior of most steels in practice.

Iron Graphite System

IRON-GRAPHITE SYSTEM

It has already been said that iron carbide or cementite is a metastable, although under normal conditions, it tends to persist indefinitely. When cementite does decompose it does according to the reaction :

Fe3C <——–> 3Fe + C

In the stable phase, free carbon or graphite occurs instead of the phase known as cementite. Upon small degree of supercooling, graphite is formed when cast iron solidifies from the liquid state. Slow cooling promotes graphitisation. Rapid cooling partly or completely suppresses graphitisation and leads to the formation of cementite.

An iron-graphite system (as dotted line) is shown in Fig. 2.14. The case of a carbon alloy containing 3.5 per cent carbon by weight is taken as an illustration

At point 1 the alloy is in the liquid state . At point 2 on the cooling line the reaction that occurs can be expressed as:

Image


> Between points 2 and 3, the excess carbon in the austenite is precipitated out as free graphite and not as cementite. At point 3, the eutectoid reaction occurs. This is expressed as:

Image


The mechanism of eutectoid transformation must transform a single solid phase into two others, both with compositions which differ from the original.

Taking the eutectoid decomposition of iron as an example, austenite containing 0.8% C changes into ferrite (iron containing almost no carbon) and cementite (Fe3C, containing 25 at% carbon). Hence carbon atoms must diffuse together to form Fe3C, leaving ferrite. Nuclei of small plates of ferrite and cementite form at the grain boundaries of the austenite, and carbon diffusion takes place on a very local scale just ahead of the interface (schematic below).

Thus the plates grow, consuming the austenite as they go, to form pearlite. The process of graphitisation is controlled by varying the rate of cooling and by proper alloying of the metallic matrix.

Terms used in Iron Carbon Equilibrium Diagram

Eutectic Point

The eutectoid point in any metal is the lowest temperature at which changes occur in a solid solutions.

Eutectic reactions occur at these points, where a liquid phase freezes into a mixture of two solid phases. This occurs when a liquid alloy of eutectic composition is cooled all the way to its eutectic temperature.

Eutectic alloys are the alloys that form at this point. Alloys on the left and right sides of this point are known as hypoeutectic alloys and hypereutectic alloys (‘hypo’ in Greek means less than, ‘hyper’ means greater than).

Austenite

Austenite, solid solution of carbon and other constituents in a particular form of iron known as γ (gamma) iron.

This phase is a solid solution of carbon in FCC Fe with a maximum solubility of 2.14% C. On further heating, it converts into BCC ferrite at 1395°C. γ-austenite is unstable at temperatures below eutectic temperature (727°C) unless cooled rapidly.

Alpha Iron or Ferrite

Existing at low temperatures and low carbon content, α-ferrite is a solid solution of carbon in BCC Fe. This phase is stable at room temperature. In the graph, it can be seen as a sliver on the left edge with Y-axis on the left side and A2 on the right. This phase is magnetic below 768°C.

It has a maximum carbon content of 0.022 % and it will transform to γ-austenite at 912°C as shown in the graph.

Cementite

Cementite, a metastable phase of this alloy with a fixed composition of Fe3C, is a metastable phase of this alloy. At room temperature, it decomposes extremely slowly into iron and carbon (graphite).

This decomposition time is long, and it will take much longer than the application’s service life at room temperature. Other factors, such as high temperatures and the addition of certain alloying elements, can influence this decomposition by promoting graphite formation.

Cementite is hard and brittle, making it ideal for steel reinforcement. Its mechanical properties are determined by its microstructure, which is determined by how it is mixed with ferrite.

We have tried to cover all the terms related to iron carbon phase diagram including the various phases and terms used in it to understand in a better manner. Hope you liked this article about iron carbon equilibrium diagram. Please give your feedback in the comment below.

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