Allotropic Changes in Iron

Allotropic is characterized by changes in crystal structure of iron at a definite transformation temperature. Iron has two allotropic modification alpha, beta and Delta with BCC structure, and gamma iron with FCC structure.

● For knowing allotropic change, molten iron is to cool slowly in a insulated crucible and the cooling curve is plotted. At temperature above 1539℃ iron is in liquid from and below this iron start freezing.

● After completion of solidification, the temperature of the solid iron drops down with uniform rate until a temperature reach up to 1395℃. Between this two temperature (1539 to 1395℃ ) the solidious iron is known as Delta iron with BCC structure. A phase transformation Where Delta iron change to gamma iron with FCC structure.

● On further cooling, Uniform rate of drop of temperature from 1395 to 910℃ indicates no change in crystal structure. In 910℃ another phase change indicated in which gamma iron transformed to alpha iron with BCC structure (which is magnetic and stable below 910℃). Then beta iron which is a non magnetic version of alpha iron, is identical to alpha iron crystal structure in  between 910 to 768℃.

● The temperature at which paramagnetic beta iron transformed to ferromagnetic alpha iron is termed as Curie temperature. It is reversible on heating up to 1400℃ from room temperature.

● Under normal rate of cooling actual transformation takes place at temperature which are lower than the equilibrium temperature. Similarly transformations occurs at temperatures higher than equilibrium temperature while heating is carried out under normal rate. These are observed during magnetization and demagnetization cycle.

Allotropic changes in iron - phase transformation
Heating and cooling curves for pure iron

Transformation Temperature :

Acm = In hepereutectoid steel, the temperature at which the solution of cementation in austenite is completed during heating.
Ac1 = The temperature at which austenite begins to from during heating.
Ac2 = The temperature at which the alpha iron change to non magnetic beta iron.
Ac3 = The temperature at which transformation of alpha iron to austenite is completed during heating.
Ac4 = The temperature at which austenite transforms to Delta ferrite during heating.
Arcm = In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling.
Ar1 = The temperature at which transformation of austenite to ferrite or to ferrite + cementite is completed during cooling.
Ar2 = The temperature at which non magnetic beta iron changes to Gerri magnetic alpha iron (ferrite) during cooling.
Ar3 = The temperature at which Delta ferrite transforms to austenite during cooling.
Ar4 = The temperature at which Delta ferrite transformed to austenite during cooling.
Ar' = The temperature at which transformation of austenite to pearlite starts during cooling.

Cementite :

The cementite Fe3C contain 6.67% C by weight. It is typically hard and brittle hard and brittle. Interstitial compound of low tensile strength but high compressive strength. It is hardest structure. It is in crystal structure is Orthorhombic.

Austenite :

A solid solution if carbon in a non magnetic from of iron stable at high temperature. It is a constitute of some form of steel. It also know as gamma phase iron, in plain carbon steel austenite exists above the upper critical temperature.

Ferrite :

A ferrite is a pure form of iron with a body centred cubic crystal structure, occurring in low carbon steel. It is this crystalline structure which gives steel and cast iron their magnetic properties and is the classic example of a ferromagnetic material.

Pearlite :

Pearlite is a two phased, lamellar structure composed of alternating layers of ferrite and cementite that occurs in some steels and cast irons.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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