4: Iron Carbon Phase Diagram

Introduction to the Iron Carbon Phase Diagram

• Focuses on the iron carbon phase diagram, which is crucial for understanding steel properties.

• Steel is primarily an alloy of iron and carbon, which can also include other alloying elements.

• Emphasizes the useful properties derived from a mixture of iron and carbon, particularly in plain carbon steels.

Eutectic Composition

• The iron carbon phase diagram features a eutectic composition at 4.3% carbon.

  • At this point, the substance transitions from a liquid to a two-phase solid region of austenite and cementite.

• Most steels, however, have carbon content below 2%, residing in the high-temperature austenite region (gamma iron phase).

  • Eutectic composition (4.3% carbon) is higher than the typical carbon content in steel.

Eutectoid Transformation

• Identifies a resemblance in the phase diagram where certain points appear similar to eutectic points but are classified differently.

• Introduces the concept of eutectoid transformation:

  • Transition from a single-phase solid (s1) to a two-phase solid region composed of s2 and s3.

• Provides terminology clarification to differentiate between eutectic (4.3% carbon) and eutectoid compositions (approximately 0.77% carbon).

Composition Range for Steels

• Defines carbon content range for steels:

  • Plain Carbon Steels: 0.2% - 2% carbon, encompassing the eutectoid composition.

  • Cast Iron Region: 2% - 4% carbon.

Key Phases in the Iron Carbon Phase Diagram

• Highlights significant phases in the iron carbon phase diagram:

  • Austenite (gamma-phase): face-centered cubic structure.

  • Alpha Iron (Ferrite): body-centered cubic structure, occupying the left-hand boundary of the phase diagram.

  • Cementite (Fe3C): intermetallic compound, typically existing in the lower region of the phase diagram.

Transition from Austenite to Ferrite and Cementite

• Discusses the transformation of steel structure during cooling from high temperature to low temperature:

  • Begins cooling from austenite (FCC structure) at 900 °C with a composition of 0.3% carbon (plain carbon steel).

  • Cooling leads to the growth of ferrite from the austenite phase, nucleating at the austenite grain boundaries.

  • As it cools further to the eutectoid temperature, austenite transforms quickly into a ferrite and cementite mixture when reaching the eutectoid point.

Formation of Pearlite

• Describes the product of eutectoid transformation:

  • Results in pearlite, a lamellar structure of alternating ferrite and cementite plates.

  • This structure is crucial for understanding the mechanical properties of steels.

• Specifies that the equilibrium room temperature structure consists of ferrite grains with a matrix of pearlite.

• Introduces the lever rule to calculate fractions of pearlite within the overall composition.

Microstructural Analysis of Carbon Content in Steels

• Discusses micrograph observations in relation to carbon content:

  • Pure alpha ferrite: characterized by single-phase grains with negligible carbon content.

  • At 0.2% carbon, initiation of eutectoid pearlite structure forms; visible as dark areas in micrographs.

  • 0.6% carbon leads to increased dark pearlite regions, illustrating the effect of carbon content on microstructure.

Conclusion

• Recap of the phases and transformations depicted in the iron carbon phase diagram and their relevance to the properties of steel.