The Iron-Iron Carbide Equilibrium Diagram

The Iron-Iron Carbide Equilibrium Diagram

Iron is a substance of allotropy. Alloy components, the most significant of which is carbon, influence the temperature at which the allotropic modifications occur in iron. In this paper is provided information about the part of the iron"carbon alloy scheme that is of concern to technicians.

The Iron-Iron Carbide Diagram

The iron-carbon composite scheme diagram portion between plain iron and an interstitial compound, iron carbide (Fe3C), comprising 6.67 percent by weight is called iron-iron carbide balance diagram. It may be observed that although it is called as a diagram of equilibrium, it is not a real diagram of equilibrium, as equilibrium does not imply a shift of stage with moment. The iron carbide compound actually breaks down into metal and coal (graphite). This decomposition takes a long time to form graphite at room temperature, even at 1300Â ° F. The metastable stage is called the iron carbide. Thus, although the iron-iron carbide diagram technically reflects metastable circumstances, under comparatively gentle heating and cooling circumstances it can be regarded as depicting alterations in equilibrium.

The figure above demonstrates an iron-iron carbide balance diagram marked with Greek letters in particular to depict the strong alternatives. However, giving unique names to most of the constructions appearing on the diagram is prevalent practice. The strong solution from Î3 is called austenite. The diagram displays three horizontal lines indicating isothermal responses.

The extended perspective of the diagram part in the bottom left corner of the diagram is shown in the following picture.

Due to the solid solution of the Î, this part of the diagram is regarded as delta region. A peritectic reaction is the horizontal line at 2720Â ° F. The peritectic response equation can be published as

At point M, the maximum carbon solubility in b.c.c. Î ' Fe is 0.10%. Carbon presence influences the allotropic shift between Î ' and Î3. As the carbon concentration in iron increases, the allotropic change temperature increases at 0.1 percent carbon from 2554 to 2720Â ° F.

On cooling, the NM line represents the start of the change in crystal structure from b.c.c. Î' Fe to f.c.c. Î3 Fe for metals comprising less than 0.1% water. By means of a peritectic response for alloys between 0.10 and 0.18 percent carbon, the MP part of row MPB reflects the start of this crystal structure shift. On drying, the start of the glass shape shift is provided by the row NP for plastics comprising less than 0.18% oxygen. The PB part reflects the start and end of the shift in the crystal structure through the peritectic response.

That is, the allotropic shift starts and finishes at steady temperature for alloys between 0.18 and 0.50 percent carbon. Any metal comprising more than 0.5% oxygen can be seen to trim the diagram to the left of point B and solidify straight to austenite. It will totally bypass the delta solid solution and the allotropic shift.

The following picture indicates the equilibrium diagram of iron-iron carbide marked with the popular names for the constructions.

It can be seen that at 2065Â ° F there is an eutectic response. The eutectic point E is 4.3% biomass and the eutectic wind range CED. Whenever this row is passed by an alloy, there must be an eutectic response. Any liquid present at the time of reaching this line must now solidify into the very good blend of the two stages at either end of the horizontal line, namely austenite and metal carbide (called cement). This eutectic mixture is called ledeburite and can be described as a response

As shown in the figure, each alloy will consist of a mixture of ferrite and cementite below the eutectoid temperature line HJK.

Depending on the carbon content, dividing the iron-iron carbide diagram into two components is prevalent practice. Alloys that contain less than 2% of carbon are known as steels and metals that contain more than 2% of carbon are regarded as cast iron. The variety of metal is further reduced by the quantity of eutectoid carbon (0.8% C). Steels comprising less than 0.8% C are called hypoeutectoid steels, while steels comprising between 0.8% and 2.0% C are called hypereutectoid steels. Eutectic carbon content (4.3 percent C) also subdivides the cast iron variety. Cast iron comprising less than 4.3% C is known as hypoeutectic cast iron and those comprising more than 4.3% C are regarded as hypereutectic cast iron.


Names are given to various micro-constituents for descriptive or commemorative reason. Information about them is given below.

Cementite or metal carbide 

The formation of cementite requires a set quantity of coal and a set quantity of metal. Fe3C is his chemical formula. It includes by weight 6.67 percent of coal. It is a compound of difficult and brittle interstitial poor tensile strength (about 5000 psi) but elevated compressive strength. Its orthorhombic crystal structure. It is the easiest building on the carbide diagram of iron-iron.


The name provided to the strong solution Î3 is Austenite. It is an interstitial strong carbon solution deposited in Î3 metal with a face-centered cubic framework (f.c.c.) of crystal. Maximum solubility at 2065Â ° F (point C) is 2% nitrogen.

Average properties of austenite are as under.
Tensile strength: 150,000 psi
Elongation: 10 % in 2 in. gage length
Hardness: Rockwell C 40
Toughness: High
Austenite at room temperature is usually volatile. Austenite can be obtained at room temperature under certain circumstances (as in stainless steels from austenite). Austenite has no magnetic character.
It is the eutectic mix of austenite and cement. It has a carbon content of 4.3% and is formed at 2065Â ° F (point E). It occurs when the fuel content exceeds 2%, which constitutes the separating line between steel and cast iron on the balance diagram.


The name provided to the α strong solution is ferrite. It is an interstitial solid solution of a small amount of carbon dissolved in α iron with a crystal structure centered on the body (b.c.c.). The maximum solubility at 1333 ° F (point H) is 0.025% oxygen, and at room temperature it dissolves only 0.008% oxygen. It is the weakest iron-iron carbide diagram framework.

Average properties of ferrite are as under.
Tensile Strength: 40,000 psi
Elongation: 40 % in 2 in. gage length
Hardness: Less than Rockwell C 0 or less than Rockwell B 90
Toughness: Low

Post a Comment

* Please Don't Spam Here. All the Comments are Reviewed by Admin.