Iron–Carbon Phase: Phase Components, Invariant Reactions, and TTT Diagram

Fundamentals of the Fe–C system

• Diagram type: The Fe–C diagram is an equilibrium phase diagram (usually shown with metastable cementite, Fe₃C). It maps phases present as functions of temperature and carbon content (0–6.67 wt% C).
• Steel vs. cast iron: Steels: up to ≈2.0 wt% C (practically ≤1.2% C for most steels). Cast irons: ≈2.0–4.3 wt% C (eutectic composition at ≈4.3 wt% C), and beyond for hypereutectic irons.
• Key temperatures: A₁ (~727°C) eutectoid; A₃ (α↔γ boundary for hypoeutectoid steels); A_cm (γ↔Fe₃C boundary for hypereutectoid steels). Separate δ-ferrite field exists at very high temperature near melting.
• Metastable vs. stable: Metastable diagram assumes cementite formation; the stable Fe–C diagram (with graphite) is relevant for graphitizing cast irons under slow cooling or with graphitizing elements (e.g., Si).
• Microstructure control: Equilibrium cooling gives ferrite/pearlite in steels; non-equilibrium quenching plus tempering or isothermal holds enable martensite, bainite, or customized pearlite spacing.

Phase components

The principal phases in the metastable Fe–C system (with Fe₃C) include:

α-Ferrite (α)

• Crystal structure: Body-centered cubic (BCC).
• Carbon solubility: Very low; maximum ≈0.02 wt% C at 727°C; near room temperature, solubility is negligible.
• Properties: Soft and ductile; magnetic below Curie temperature (~770°C for pure Fe).

γ-Austenite (γ)

• Crystal structure: Face-centered cubic (FCC).
• Carbon solubility: Much higher than α; up to ≈2.1 wt% C at 1147°C, decreasing with temperature.
• Properties: Non-magnetic at high temperature; key parent phase for many transformations (pearlite, bainite, martensite).

δ-Ferrite (δ)

• Crystal structure: BCC (high-temperature form of ferrite, stable near the melting point).
• Occurrence: Appears in the high-temperature region near liquidus; relevant mainly during solidification and in very low-carbon steels.

Cementite (Fe₃C)

• Composition: 6.67 wt% C (iron carbide).
• Properties: Hard and brittle intermetallic compound; contributes to strength/wear resistance but decreases toughness.

Pearlite (α + Fe₃C lamellar aggregate)

• Nature: A two-phase, lamellar microconstituent formed by eutectoid decomposition of austenite at ~727°C.
• Spacing: Lamellar interspacing (λ) depends on undercooling and transformation rate; finer pearlite forms at lower transformation temperatures (faster nucleation).

Ledeburite (γ + Fe₃C, or α + Fe₃C after further transformation)

• Nature: Eutectic mixture formed at ~1147°C and ~4.3 wt% C from liquid; on further cooling, γ transforms to pearlite at 727°C.
• Relevance: Central to cast iron solidification microstructures.

Martensite (diffusionless product)

• Nature: Supersaturated, body-centered tetragonal (BCT) phase formed by rapid quenching of austenite below M_s; not an equilibrium phase and not part of the equilibrium Fe–C diagram.
• Properties: Very hard and brittle in the untempered state; tempered to adjust toughness/hardness.

Bainite (upper/lower)

• Nature: Non-lamellar transformation product formed isothermally at intermediate temperatures; appears on TTT diagrams (not on equilibrium diagram).
• Properties: Strength–toughness balance often superior to pearlite at similar strength levels; morphology depends on temperature (upper vs. lower bainite).

Invariant reactions

Three key invariant reactions define the characteristic transformation points in the Fe–C system (metastable with Fe₃C):

Eutectoid reaction (A₁ ≈ 727°C, ~0.76–0.80 wt% C)

• Reaction: γ (austenite) → α (ferrite) + Fe₃C (pearlite)
• Composition: Eutectoid steel ~0.76 wt% C (commonly cited ~0.77 wt% C).
• Outcome: Fully pearlitic microstructure at eutectoid composition. Hypoeutectoid steels (C < eutectoid) yield proeutectoid ferrite + pearlite; hypereutectoid steels (C > eutectoid) yield proeutectoid cementite + pearlite.

Eutectic reaction (≈1147°C, ≈4.3 wt% C)

• Reaction: L (liquid) → γ (austenite) + Fe₃C (ledeburite)
• Relevance: Governs the primary solidification of cast irons near the eutectic composition.
• Subsequent cooling: The γ within ledeburite further transforms to pearlite at 727°C.

Peritectic reaction (≈1493–1495°C, low C ~0.16–0.18 wt%)

• Reaction: L (liquid) + δ (delta ferrite) → γ (austenite)
• Occurrence: During solidification of very low-carbon irons/steels; can complicate casting/solidification paths due to diffusion requirements at the solid–liquid interface.

Transformation paths across compositions

• Hypoeutectoid steel (e.g., 0.3 wt% C): On cooling, γ → proeutectoid α along the γ/α boundary (A₃), then remaining γ → pearlite at A₁.
• Eutectoid steel (~0.76 wt% C): Single reaction at A₁: γ → pearlite.
• Hypereutectoid steel (e.g., 1.0 wt% C): On cooling, γ → proeutectoid Fe₃C along the γ/Fe₃C boundary (A_cm), then remaining γ → pearlite at A₁.

Equilibrium microstructures (pearlite and ledeburite)

Pearlite (from eutectoid transformation)

• Formation: Isothermal or slow continuous cooling near 727°C; diffusion-controlled decomposition of γ to alternating α and Fe₃C lamellae.
• Coarse vs. fine pearlite: Higher transformation temperatures produce coarser lamellae (lower strength, higher ductility); lower temperatures produce finer lamellae (higher strength/hardness).
• Distribution: In hypoeutectoid steels, pearlite colonies nucleate in the austenite regions between proeutectoid ferrite; in hypereutectoid, they form between proeutectoid cementite networks.

Ledeburite (from eutectic reaction)

• Formation: At ~1147°C and ~4.3 wt% C; solidifies as an intimate mixture of γ and Fe₃C. On cooling to 727°C, the γ in ledeburite transforms to pearlite, leading to a structure of pearlite + Fe₃C.
• Cast irons: Overall microstructure may include primary austenite or primary cementite depending on hypoeutectic or hypereutectic compositions relative to 4.3 wt% C.

Lever rule (qualitative note)

• Purpose: Estimates phase fractions in two-phase fields (e.g., α + γ, γ + Fe₃C) at equilibrium.
• Practice: Draw the tie line at the temperature of interest and measure segments to compositions of bounding phases.

TTT diagram (Isothermal transformation of austenite)

TTT diagrams map the start/finish times of products formed when austenite is held isothermally at a given temperature, revealing kinetic pathways for pearlite, bainite, and martensite formation. They are composition-specific (commonly drawn for eutectoid steel) and assume prior full austenitization.

Axes and features

• Temperature (vertical axis): Isothermal hold temperature after austenitizing and rapid quench to the hold temperature.
• Time (horizontal axis, logarithmic): Start and finish curves (C-curves) for diffusional transformations.
• Nose of the C-curve: Temperature range where transformation is fastest (shortest incubation time). Avoided during quenching to form martensite.
• Start/finish lines: Typically two S-shaped curves for pearlite at high temperatures and bainite at lower temperatures.
• M_s and M_f lines: Martensite start and finish temperatures on rapid quenching (diffusionless); martensite fraction grows with further cooling below M_s.

Transformation products vs. temperature

• Above A₁: Austenite is stable; no transformation.
• Just below A₁ (~700–650°C for eutectoid steel): Coarse pearlite forms relatively slowly; larger interlamellar spacing.
• Intermediate (~650–550°C): Fine pearlite; higher strength and hardness.
• Lower (~550–350°C): Upper bainite transitioning to lower bainite with decreasing temperature; acicular, non-lamellar morphologies.
• Below M_s: Martensite forms without diffusion; very hard but brittle unless tempered.

Reading and using a TTT diagram

1. Austenitize: Heat to the appropriate γ region (e.g., ~30–50°C above A₃ for hypoeutectoid steel) long enough for solution and homogenization.
2. Quench to hold temperature: Rapidly cool to the chosen isothermal temperature to avoid premature transformation.
3. Isothermal hold: Follow the time axis to intersect the start curve of a product (pearlite or bainite), then continue to the finish curve for full transformation.
4. Quench to room temperature: After the isothermal step, quench or air cool; any remaining austenite may transform to martensite if cooled below M_s.

Factors shifting the TTT curves

• Carbon content: Higher carbon generally lowers transformation temperatures and increases hardenability (curves shift to longer times).
• Alloying elements: Mn, Cr, Mo, Ni, Si, etc., often shift curves rightward (slower transformations), enabling deeper hardening and bainitic/martensitic routes in thicker sections.
• Austenite grain size: Coarser grains reduce nucleation sites (slower transformation), affecting pearlite/bainite start.

Relationship to CCT diagrams

• TTT: Isothermal; idealized holds at constant temperature.
• CCT (Continuous Cooling Transformation): Realistic continuous cooling; curves differ and usually shift to longer times compared to TTT. Cooling rate determines product mix.

Processing examples using TTT

Isothermal pearlite formation (eutectoid steel)

• Process: Austenitize → quench to 650–700°C → hold until fully transformed → air cool.
• Result: Coarse to medium pearlite; moderate strength, good ductility.

Isothermal fine pearlite (higher strength)

• Process: Austenitize → quench to ~600°C → hold to completion → air cool.
• Result: Fine pearlite; higher strength/hardness than coarse pearlite.

Austempering to bainite

• Process: Austenitize → rapid quench to 350–450°C (salt bath) above M_s → hold until bainite forms → cool to room temperature.
• Result: Bainitic microstructure (upper or lower depending on temperature) with good strength–toughness and lower distortion than quenched–tempered martensite.

Quenching to martensite and tempering

• Process: Austenitize → quench to below M_s (e.g., oil/water/polymer/air depending on steel grade) → temper at 150–650°C.
• Result: Martensite (as-quenched) then tempered martensite, allowing tailored hardness and toughness.

Pearlite–bainite mixtures

• Process: Step-quench to a temperature just above the bainite region, hold briefly for partial pearlite formation, then quench to bainite temperature for remainder.
• Result: Mixed microstructures for specific property balances.

Notes on composition, temperature scales, and nomenclature

• Eutectoid composition: Commonly cited near 0.76–0.80 wt% C; many texts use 0.77 wt% C.
• Key temperatures: Eutectoid ~727°C; eutectic ~1147°C; peritectic ~1493–1495°C (values vary slightly by source).
• A₁, A₃, A_cm: On heating, A_c1, A_c3, A_cm; on cooling, A_r1, A_r3, A_rm. Hysteresis is common due to kinetics.
• Metastable assumption: Cementite is treated as stable; in graphitizing conditions (slow cooling, Si additions) graphite may form instead, shifting reactions in cast irons.

Glossary

• Austenitizing: Heating into the γ region to obtain homogeneous austenite prior to transformation.
• Hypoeutectoid steel: Carbon content below eutectoid; forms proeutectoid ferrite + pearlite on cooling.
• Hypereutectoid steel: Carbon content above eutectoid; forms proeutectoid cementite + pearlite on cooling.
• M_s/M_f: Martensite start/finish temperatures, below which diffusionless transformation proceeds.
• Proeutectoid phase: Phase that forms from austenite before reaching the eutectoid temperature (ferrite for hypo-, cementite for hypereutectoid steels).
• TTT (isothermal) vs. CCT (continuous cooling): Idealized isothermal holds vs. realistic continuous cooling; product maps differ.