Iron-Iron carbide Diagram

The part of iron-carbon alloy system diagram between pure iron and an interstitial compound, iron carbide (Fe₃C), containing 6.67 percent carbon by weight is called iron-iron carbide equilibrium diagram. It may be noted that though it is called as equilibrium diagram, it is not a true equilibrium diagram, since equilibrium implies no change of phase with time. In fact, the compound iron carbide decomposes into iron and carbon (graphite). This decomposition takes a very long time at room temperature, even at 1300°F, it takes several years to form graphite. The iron carbide is called metastable phase. Therefore, iron-iron carbide diagram even though technically represents metastable conditions, can be considered as representing equilibrium changes, under conditions of relatively slow heating and cooling.

Above figure shows iron-iron carbide equilibrium diagram labeled in general terms with Greek letters to represent the solid solutions. However, it is common practice to give special names to most of the structures that appear on the diagram. The γ solid solution is called austenite. The diagram shows three horizontal lines which indicate isothermal reactions.

The enlarged view of the portion of the diagram in the upper left hand corner is shown in the figure given below.

This portion of the diagram is known as delta region because of the δ solid solution. The horizontal line at 2720°F is for a peritectic reaction. The equation for the peritectic reaction may be written as

The maximum solubility of carbon in b.c.c. δ Fe is 0.10 percent at point M. The presence of carbon influences the allotropic change between δ and γ. As the concentration of carbon in iron is increased, the temperature of allotropic change increases from 2554 to 2720°F at 0.1 percent carbon.

On cooling, the line NM represents the beginning of the crystal structure change from b.c.c. δ Fe to f.c.c. γ Fe for alloys containing less than 0.1 percent carbon. The portion MP of line MPB represents beginning of this crystal structure change by means of a peritectic reaction for alloys between 0.10 and 0.18 percent carbon. For alloys containing less than 0.18 percent carbon, on cooling, the end of the crystal structure change is given by the line NP. The portion PB represents the beginning and the end of the crystal structure change by means of the peritectic reaction. That is, for alloys between 0.18 and 0.50 percent carbon, the allotropic change begins and ends at constant temperature. It can be seen that any alloy containing more than 0.5 percent carbon will cut the diagram to the right of point B and will solidify to austenite directly. The delta solid solution and the allotropic change will be completely bypassed.

The figure given below shows the iron-iron carbide equilibrium diagram labeled with the common names for the structures.

 It can be seen that eutectic reaction takes place at 2065°F. The eutectic point E is at 4.3 percent carbon and the line CED is the eutectic temperature line. Whenever an alloy passes this line, eutectic reaction must take place. Any liquid that is present when this line is reached must now solidify into the very fine mixture of the two phases that are at either end of the horizontal line, namely austenite and iron carbide (called cementite). This eutectic mixture is given the name ledeburite, and the reaction may be written as

.Depending on carbon content, it is common practice to divide the iron-iron carbide diagram into two parts. Alloys containing less than 2 percent carbon are known as steels and alloys containing more than 2 percent carbon are known as cast irons. The steel range is further subdivided by the eutectoid carbon content (0.8 percent C). Steels containing less than 0.8 percent C are called hypoeutectoid steels while those containing between 0.8 to 2.0 percent C are called hypereutectoid steels. The cast iron range is also subdivided by eutectic carbon content (4.3 percent C). Cast irons that contain less than 4.3 percent C are known as hypoeutectic cast irons and those containing more than 4.3 percent C are called hypereutectic cast irons.

Micro-constituents/Structures

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

Cementite or iron carbide

A fixed amount of carbon and a fixed amount of iron are needed to form cementite. Its chemical formula is Fe₃C. It contains 6.67 percent carbon by weight. It is a hard and brittle interstitial compound of low tensile strength (approximately 5000 psi) but high compressive strength. Its crystal structure is orthorhombic. It is the hardest structure that appears on the iron-iron carbide diagram.

Austenite

Austenite is the name given to the γ solid solution. It is an interstitial solid solution of carbon dissolved in γ iron having a face centered cubic (f.c.c.) crystal structure. Maximum solubility is 2 percent carbon at 2065°F (point C).

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 is normally unstable at room temperature. Under certain conditions it is possible to obtain austenite at room temperature (as in austenite stainless steels). Austenite is non-magnetic.

Ledeburite

It is the eutectic mixture of austenite and cementite. It contains 4.3 percent carbon and is formed at 2065°F (point E). It exists when the carbon content is greater than 2%, which represents the dividing line on the equilibrium diagram between steel and cast iron.

Ferrite

Ferrite is the name given to the α solid solution. It is an interstitial solid solution of a small amount of carbon dissolved in α iron having a body centered cubic (b.c.c.) crystal structure. The maximum solubility is 0.025 percent carbon at 1333°F (point H), and it dissolves only 0.008 percent carbon at room temperature. It is the softest structure on the iron-iron carbide diagram.

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

Pearlite

It is the eutectoid mixture containing 0.80 % carbon and is formed at 1333°F on very slow cooling. It is a 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 (black).

Average properties of pearlite are as under.

Tensile Strength: 120,000 psi
Elongation: 20 % in 2 in gage length
Hardness: Rockwell C 20 or BHN 250-300

Pearlite needs fixed amounts of cementite and ferrite.

If enough carbon is not there, that is carbon is less than 0.80 %, 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 called hypoeutectoid steel.

If, however, there is an excess of carbon above 0.80% in the austenite, pearlite will form, and the excess carbon above 0.80% will form cementite. The excess cementite deposits in the grain boundaries. This excess cementite is also known as proeutectoid cementite. The steel that contains proeutectoid cementite is called hypereutectoid steel.

Slow Cooling of Steel

The various changes that take place during the very slow cooling from the austenite range for hypoeutectoid steel and hypereutectoid steel are discussed below.

Hypoeutectoid steel

Alloy 1 shown in above figure is a hypoeutectoid steel containing 0.2 percent carbon. In the austenite range it is a uniform interstitial solid solution. Upon slow cooling, nothing happens until the line GJ is crossed at point x₁.This line is known as the upper-critical-temperature line on the hypoeutectoid side and is labeled A₃.

The allotropic change from f.c.c. to b.c.c. iron takes place at 1666°F for pure iron and decreases in temperature with increasing carbon content, as shown by A₃ line. Therefore, at x₁, ferrite must begin to form at the austenite grain boundaries. Since ferrite can dissolve very little carbon, in those areas that are changing to ferrite the carbon must come out of solution before the atoms rearrange themselves to b.c.c. The carbon which comes out of solution is dissolved in the remaining austenite, so that, as cooling progresses and the amount of ferrite increases, the remaining austenite becomes richer in carbon. Its carbon content is gradually moving down and to the right along the A₃ line. Finally, the line HJ is reached at point x₂. This line is called the lower-critical-temperature line on the hypoeutectoid side and is labeled A₁. The A₁ line is the eutectoid-temperature line and is the lowest temperature at which f.c.c. iron can exist under equilibrium conditions. Just above A₁ line, the microstructure consists of approximately 25 percent austenite and 75 percent ferrite. The remaining austenite, about 25 percent of the total material and containing 0.8 percent carbon, now undergoes the eutectoid reaction.It may be noted that it is only austenite which is changing at A₁ line. Therefore, when the reaction is complete the microstructure will show approximately 25 percent Pearlite and 75 percent ferrite.

The changes just described would be the same for any hypoeutectoid steel. The only difference would be in the relative amount of ferrite and pearlite.