Figure 1

The iron-carbon (or iron-iron carbide) phase diagram Benefits of Heat Treating (Part 2): Phases of Steel; Fig. 1tells us that steel containing approximately 0.76 wt% carbon is at the eutectoid composition. If this steel is heated to 1472°F (800°C), austenite ( g), which is a face-centered-cubic (fcc) structure, exists as a solid solution. If the austenite is cooled to below 1333°F (723°C), however, a phase transformation will take place. Iron carbide (Fe3C), also known as cementite, and ferrite (a), a body-centered-cubic (bcc) solid solution containing less than 0.02 wt% carbon, will form. At 1333°F (723°C) a “three-phase equilibrium” exists as seen on the phase diagram. Austenite is in equilibrium with ferrite and cementite. On cooling:

Austenite (0.76 wt %C)àFerrite (a) + Fe3C (0.02 wt%C)

If this phase transformation takes place as we are slow cooling, a microstructural component called pearlite is formed (Fig. 2). Pearlite consists of plates of Fe3C in a matrix of ferrite (a). Pearlite is formed from austenite containing the eutectoid composition of 0.76 wt%C. This microstructure develops as a result of the decomposition of austenite by nucleating carbide plates first and then the ferrite on the sides of the carbide plates.

Figure 2: Ferrite (white) and pearlite (black) formation in steel

Steels that contain less than 0.76 wt %C are called hypoeutectoid steels. For example, the phase diagram tells us that when a hypoeutectoid steel containing 0.40 wt%C is heated to 1650°F (900°C) austenite forms. When this steel is slowly cooled, the first (or primary) phase formed is ferrite. On reaching 1333°F (723°C) on cooling, a microstructure of primary ferrite and austenite develops. Upon cooling below 1333°F (723°C), the austenite, which now contains 0.76 wt%C, transforms to pearlite. The microstructure of this alloy will consist of approximately 50% primary ferrite and 50% pearlite.

Steels containing more than 0.76 wt%C are called hypereutectoid steels. For example, the phase diagram tells us that when a hypereutectoid steel containing 0.90 wt%C is heated to 1650°F (900°C), an austenite solid solution forms. Upon slow cooling, the first (or primary) phase formed is iron carbide (Fe3C). Upon cooling below 1333°F (723°C), austenite, which now contains 0.76 wt%C, transforms to pearlite. The microstructure of this alloy will consist of approximately 2% primary carbide and 98% pearlite.

Upcoming topics…

Part 4:Martensite formation and tempering
Part 5:Time-temperature-transformation (TTT) Diagrams

Glossary of Terms

Austenite:Austenite is the name given any solid solution in which gamma (γ) iron is the solvent. Austenite is the structure from which all quenching heat treatments must start.

Cementite:The common name for iron carbide, Fe3C, the chemical combination of iron and carbon. The stoichiometric phase.

Eutectoid:A eutectoid system is one in which a single-phase solid transforms directly to a two-phase solid.

Ferrite:Ferrite is the name given any solid solution in which alpha (α) iron is the solvent. Theα-phase has a body centered cubic lattice. If you want to be precise, you call itα- ferrite.

Hypereutectoid steels: Hypereutectoid steels are those whose carbon content greater than 0.76 wt%.

Hypoeutectoid steels: Hypoeutectoid steels are those whose carbon content less than 0.76 wt%.

Pearlite:The two-phase mixture obtained right below the eutectoid point at 0.76 % carbon concentration.