The iron-carbon phase diagram is a map that shows which phases form in iron-carbon alloys at different temperatures and carbon contents. It is one of the most important tools in metallurgy because it explains why steels and cast irons can have very different strengths, hardness values, and ductility. Engineers use it to choose compositions and heat treatments for machine parts, tools, gears, rails, and structural components.
The diagram connects microscopic structure to real mechanical behavior.
Carbon changes the crystal structure and phase balance of iron, especially between ferrite, austenite, cementite, and mixtures such as pearlite. The eutectoid point, near 0.76 wt% C and 727 °C, marks the composition and temperature where austenite transforms into pearlite during slow cooling. Alloys below about 2.11 wt% C are classified as steels, while higher carbon iron alloys are generally cast irons.
By reading phase boundaries and applying the lever rule, engineers can estimate phase amounts and predict how processing will affect performance.
Key Facts
- The horizontal axis is carbon content in wt% C, and the vertical axis is temperature in °C.
- Steel is usually defined as an iron-carbon alloy with less than about 2.11 wt% C.
- Cast iron usually contains more than about 2.11 wt% C and often lies near 2.5 to 4.0 wt% C.
- The eutectoid reaction is γ austenite -> α ferrite + Fe3C cementite at about 0.76 wt% C and 727 °C.
- Pearlite is a layered mixture of ferrite and cementite formed by slow cooling through the eutectoid temperature.
- Lever rule for a two-phase region: fraction of phase 1 = opposite tie-line length / total tie-line length.
Vocabulary
- Ferrite
- Ferrite is the body-centered cubic form of iron that dissolves very little carbon and is relatively soft and ductile.
- Austenite
- Austenite is the face-centered cubic form of iron that can dissolve much more carbon than ferrite at high temperature.
- Cementite
- Cementite is iron carbide, Fe3C, a hard and brittle compound that increases hardness and wear resistance.
- Pearlite
- Pearlite is a lamellar microstructure made of alternating ferrite and cementite layers formed from eutectoid austenite.
- Eutectoid point
- The eutectoid point is the composition and temperature where one solid phase transforms into two different solid phases.
Common Mistakes to Avoid
- Confusing phases with microstructures is a mistake because ferrite, austenite, and cementite are phases, while pearlite is a mixture of phases arranged in layers.
- Calling all iron-carbon alloys steel is wrong because steels are usually below about 2.11 wt% C, while higher carbon alloys are generally cast irons.
- Ignoring temperature when identifying phases is wrong because the same carbon content can be ferrite, austenite, pearlite, or mixtures depending on temperature.
- Using the lever rule without drawing the tie line is a mistake because phase fractions come from distances along a horizontal line at a fixed temperature.
Practice Questions
- 1 An iron-carbon alloy contains 0.40 wt% C. Is it hypoeutectoid steel, eutectoid steel, hypereutectoid steel, or cast iron? Explain using the eutectoid composition of 0.76 wt% C and the steel limit of about 2.11 wt% C.
- 2 At 727 °C, eutectoid austenite contains about 0.76 wt% C, ferrite contains about 0.022 wt% C, and cementite contains 6.70 wt% C. Use the lever rule to estimate the mass fraction of cementite in pearlite.
- 3 A gear steel is heated into the austenite region and then cooled slowly through 727 °C. Explain why pearlite can form and how increasing carbon content generally affects hardness and ductility.