By | February 20, 2016
iron carbon diagram

                Fig 1. Iron Carbon Phase diagram

A Critical Study of Iron Carbon Diagram for Advanced Learners

In the present day, we use various household things like cooking utensils, finishing items and flat plates made up of steel. We like to use these things in our everyday life for their hardness, lightness, longevity, and attractive appearance.  Many of us are already aware that the steel is an alloy of iron and carbon.Iron carbon diagram explain below.

There are two types of iron-carbon diagrams. These are “stable iron-graphite Fe-Gr” and “metastable iron-cementite Fe-Fe3C”. The stable condition is used widely for our domestic, commercial and industrial purposes.

The stable condition generally takes a longer period of time to develop. The metastable diagram gets more attention than stable iron-graphite Fe-Gr. Being very hard in nature, it is called cementite.

In this article, we shall discuss on iron carbon alloy and iron carbon diagram.

Iron carbon diagram includes following phases:

  • austenite
  • ferrite
  • pearlite
  • cementite
  • martensite
  • ledeburite.


  1. A3
  2. Acm
  3. A1
  4. A2


  • Hypoeutectoid steels = carbon range from 0 to .83%
  • Eutectoid steels = carbon range .83%
  • Hypereutectoid steels = carbon range from .83 to 2.06%
  • Cast irons = carbon range from 2.06 to 4.3%.


It is known as a α soild solution.the maximum solubility is .025% C at 723 deg. Celsius.

It dissolves only .008% C at room temperature. it has low strength, good toughness.


It is formed at 723 deg. Celsius on very slow cooling .it is eutectoid mixture containing .80% C.

It has high strength and low toughness.


Solubility is 2.0% max. at 1130 deg celsius. it is not stable at room temperature.

Iron having good strength and toughness.


It is the hardest structure. it is hard and brittle substance. it contains 6.67% C. it has high compressive strength. it is iron carbide has low toughness, low tensile strength.


It is fine mixture of austenite and cementite contains 4.3% C. Temperature range is 1147 deg. Celsius.


It is hard and brittle and formed by rapid cooling. interstitial carbon atoms distort the BCC ferrite to BCT structure. it has high strength and hardness, low toughness.

ic diag

                     Fig 2.High Carbon Steel

On the basis of iron-carbon diagram, we can divide iron-carbon alloys into three distinct categories like cast irons, steels, cast steels.

Cast Irons

Cast iron is a group of iron-carbon alloys. The carbon content may be greater than 2%. The alloy may constituent affect its colour while fractured.  These are again two types.

White cast iron

White cast iron has carbide impurities that allow cracks to pass straight through it

Grey cast iron

Grey cast iron includes graphite flakes that deflect a passing crack. It initiates many new cracks as the material breaks.

Uses of cast iron

Cast iron has various uses of making latches, columns, hardware, hinges, baluster, ordnance, stairs, decorative features, fences, tools, stoves, utensils, pipes, firebacks, and  structural connectors in buildings and monuments


Steel is an alloy of iron and carbon. Besides, there are some other elements used as a mixture. Steel is used in construction, household things and other applications. This is used widely in the construction industry for its high tensile strength, lightweight and low cost. Carbon and other elements are used with iron as a mixture to act as hardening agents. These agents prevent the dislocation or movement that generally occur within the crystal lattices of iron atoms. The carbon in typical steel alloys may give up to 2.1% of its weight.

Uses of Steel

Iron and steel are extensively used in the construction industry. It is required for construction of roads, bridge, railways, other infrastructure, skyscrapers, appliances, and buildings. Most of the modern and tall structures like towers, stadiums and airports are supported by a strong steel skeleton. For reinforcing the concrete structure, steel is used.

Cast Steel

Cast Steel is a type of iron alloy including mechanical properties which are lower than wrought steels. The parentage of carbon use may reach up to 1.7wt. %. Mechanical properties of cast steel are much better than cast iron. However, the casting properties are worse.

From the above discussion, it is very clear that a part of Fe-Fe3C diagram referring to steel.

Uses of cast steel

What is the difference between Cast Iron and Cast Steel?

Cast iron usually indicates ductile iron, gray iron, and malleable iron. It is casting iron with carbon. The content can be upper than 2%.

Cast steel refers to normal carbon steel and alloy steel. The steel casting with carbon content is lower than 2%.

Iron-Carbon Equilibrium or Phase Diagram

The infographic given below indicates the iron-carbon equilibrium diagram that represents the complete range of iron-carbon alloys. This diagram shows the transformations taking place in an alloy of iron-carbon. This transformation is from pure iron to cementite (carbon content 6.67%). The carbon percentage is displayed on the horizontal axis. The temperature has been shown on the vertical axis. The names of the phases have been determined by the lines (boundaries) of these areas on the equilibrium diagram shown below.

Any Iron-carbon alloy in the molten state is called solution of Fe3C in iron. When this solution cools, frozen and solidifies, it becomes an iron-carbon alloy. It mainly depends on the amount of carbon used or present after the alloy gets solidified as a solid solution.







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