Iron-Iron Carbide Diagram

Figure 1. Iron-ironcarbide diagram

CEMENTITE (Fe3C):

Cementite is also known as iron carbide which has a chemical formula, Fe3C. It contains 6.67 % Carbon by weight. It is a typical hard and brittle interstitial compound of low tensile strength (approximately 5,000 psi) but high compressive strength. Its crystal structure is orthorhombic.

 

AUSTENITE (g iron):

It is also known as (g ) gamma-iron, which is an interstitial solid solution of carbon dissolved in iron with a face centered cubic crystal (F.C.C) structure. Average properties of austenite are:

 

Tensile strength

150,000 psi.

Elongation

10 % in 2 in gage length.

Hardness

Rockwell C 40

Toughness

High

Table 1. Properties of Austenite

Figure 2. Austenite (gamma iron) crystal structure

Austenite is normally unstable at room temperature. Under certain conditions it is possible to obtain austenite at room temperature.

 

FERRITE (a iron):

It is also known as (a ) alpha -iron, which is an interstitial solid solution of a small amount of carbon dissolved in iron with a Body Centered Cubic (B.C.C.) crystal structure. It is the softest structure on the iron-iron carbide diagram. Average properties are:

 

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

Table 2. Properties of Ferrite.

Figure 2. Ferrite (alpha iron) crystal structure

PEARLITE (a + Fe3C)

It is the eutectoid mixture containing 0.83 % Carbon and is formed at 1333oF on very slow cooling. It is very fine platelike or lamellar mixture of ferrite and cementite. The structure of pearlite includes a white matrix (ferritic background) which includes thin plates of cementite. Average properties are:

 

Tensile Strength

120,000 psi

Elongation

20 % in 2 in gage length

Hardness

Rockwell C 20 or BHN 250-300

Table 3. Properties of pearlite.

Figure 3. Pearlite microstructure (Light background is the ferrite matrix, dark lines are the cementite network)

A fixed amount of carbon and a fixed amount of iron are needed to form cementite (Fe3C). Also, pearlite needs fixed amounts of cementite and ferrite.

If there is not enough carbon, that is less than 0.83 %, the carbon and the iron will combine to form Fe3C until all the carbon is consumed. This cementite will combine with the required amount of ferrite to form pearlite. The remaining amount of ferrite will stay in the structure as free ferrite. Free ferrite is also known as proeutectoid ferrite. The steel that contains proeutectoid ferrite is referred to as hypoeutectoid steel.

If, however, there is an excess of carbon above 0.83 % in the austenite, pearlite will form, and the excess carbon above 0.83 % will form cementite. The excess cementite deposits in the grain boundaries. This excess cementite is also known as proeutectoid cementite.

 

LEDEBURITE (a + Fe3C)

It is the eutectic mixture of austenite and cementite. It contains 4.3 % Carbon and represents the eutectic of cast iron. Ledeburite exists when the carbon content is greater than 2 %, which represents the dividing line on the equilibrium diagram between steel and cast iron.

 

(d ) DELTA IRON:

Delta iron exists between 2552 and 2802 oF. It may exist in combination with the melt to about 0.50 % Carbon, in combination with austenite to about 0.18 % Carbon and in a single phase state out to about 0.10 % carbon. Delta iron has the Body Centered Cubic (B.C.C) crystal structure and is magnetic.

 

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Last Update: October 16, 1999

By: Serdar Z. Elgun