Tutorial: How to Read and Apply TTT and CCT Diagrams in Steel Heat Treatment

Time-Temperature-Transformation (TTT) and Continuous Cooling Transformation (CCT) diagrams are the working tools of the heat treatment metallurgist. They encode, in a single graphical form, the complete transformation kinetics of austenite for a given steel composition: which microstructural phases form, at what temperatures and times they begin and end forming, and how cooling rate determines the final hardness, strength, and toughness of the treated component. This tutorial walks through how these diagrams are constructed, how to read them step by step, and how to apply them to practical heat treatment design, hardenability assessment, and weld procedure qualification.

What Is a TTT Diagram and How Is It Constructed?

A TTT diagram maps the transformation behaviour of austenite when it is instantaneously quenched from the austenitising temperature to a series of fixed sub-critical temperatures and held isothermally. At each hold temperature, the start and finish of each transformation product (ferrite, pearlite, bainite, martensite) are detected by dilatometry (volume change on transformation), metallography, hardness measurement, or high-resolution in-situ X-ray diffraction. The times at which 1% and 99% transformation occurs define the C-curve start and finish at each temperature.

Construction by Dilatometry

The most common experimental technique for TTT diagram construction is high-speed dilatometry. A cylindrical specimen (typically 3–5 mm diameter, 10–15 mm long) is austenitised in a programmed furnace with inert gas atmosphere, quenched by gas jet to the target isothermal temperature in <1 second, and held while the specimen length is continuously measured by a linear variable differential transducer (LVDT). Austenite-to-product transformations involve volume changes (pearlite and ferrite expand, martensite expands more strongly) that appear as slope changes in the dilatation vs time curve.

The Physical Basis of the C-Curve Shape

The characteristic C-shape of the transformation start curve reflects the competing kinetics of two opposing temperature dependencies:

• Thermodynamic driving force (ΔG): The chemical free energy difference between austenite and the product phase increases with decreasing temperature below the Ae₁ equilibrium temperature. Just below Ae₁, ΔG is small and transformation is sluggish even though diffusion is fast.
• Atomic diffusivity (D): Diffusion of carbon and substitutional alloying atoms follows an Arrhenius relationship and decreases exponentially with temperature. Near M_s, diffusion is so slow that transformation is again sluggish even though the driving force is large.

The nose of the C-curve occurs at the intermediate temperature where the product D × ΔG is maximised — approximately 550–600°C for eutectoid steel and lower in temperature for alloy steels. The transformation rate at the nose defines the critical cooling rate: to avoid any diffusional transformation, the steel must be quenched faster than the rate that passes through the nose.

Nucleation and growth rate (simplified Avrami framework):
  Overall transformation rate ∝ N × G
  where:
    N = nucleation rate (nuclei per unit volume per second)
    G = growth rate (m/s)

  Temperature dependence:
    G ∝ D(T) × ΔG(T)    where D(T) = D₀ exp(−Q/RT) [Arrhenius]
    N ∝ exp(−ΔG*/kT)     where ΔG* = critical nucleation barrier

  At high T (near Ae₁):  ΔG(T) small → slow transformation
  At low T (near Ms):     D(T) small  → slow transformation
  At intermediate T:      product N×G maximised → nose of C-curve

Avrami equation for isothermal transformation fraction X:
  X = 1 − exp(−b × t^n)
  where b and n are temperature-dependent constants
  (n ≈ 3–4 for grain boundary nucleation and growth in 3D)

Reading a TTT Diagram: Step-by-Step Guide

The CCT Diagram: Continuous Cooling Transformation

The CCT diagram is constructed by measuring transformation start and finish temperatures as a function of continuous cooling rate, not isothermal hold. It is the more practically applicable diagram because all real industrial heat treatment operations — quenching, controlled air cooling, furnace cooling — and all welding thermal cycles involve continuous cooling rather than isothermal holds.

How the CCT Diagram Differs from the TTT Diagram

The key difference is that during continuous cooling, the steel spends progressively less time at each temperature as it cools. Compared with isothermal holding at the same temperature, the steel has had less opportunity to transform by the time it reaches that temperature. The result is that CCT curves are displaced to longer times and lower temperatures compared with the corresponding TTT curves. A rough empirical rule is that the CCT start curve falls approximately 10–50°C below the corresponding TTT start curve at the same time, but this shift is composition- and temperature-dependent.

Relationship between TTT and CCT (Scheil additivity rule):
  For a continuous cooling path T(t), the fraction transformed is estimated
  by the additive rule (Scheil, 1935):

  ∫₀^t dt' / τ(T(t')) = 1    [transformation start condition]

  where τ(T) is the incubation time at temperature T from the TTT diagram.
  This integral is evaluated numerically by dividing the cooling curve
  into small time steps and summing fractional incubation periods.

  Practical implication:
  — Fast cooling segments spend little time at each T → small contribution
  — Slow cooling through the nose contributes most to the integral
  — If ∫ reaches 1.0, transformation starts regardless of cooling path

CCT construction method:
  1. Measure start/finish temperatures by dilatometry at multiple cooling rates
     (typically 0.05, 0.1, 0.5, 1, 5, 10, 50, 100, 500°C/s)
  2. Plot temperature vs log(time) for each cooling rate
  3. Mark start and finish points from dilatation deviations
  4. Connect start points: CCT start curve
  5. Add cooling rate lines and phase fraction annotations

How to Overlay a Cooling Curve on a CCT Diagram

The most important practical application of a CCT diagram is predicting the microstructure of a steel component after a specific heat treatment quench. The procedure is to draw the actual cooling curve — temperature versus time — on the same axes as the CCT diagram and read the phase regions it passes through.

Calculating a Continuous Cooling Curve

The cooling rate of a steel component depends on its geometry (thickness, diameter), the thermal properties of the steel, and the heat transfer coefficient of the quench medium. Three simplified models are widely used:

1. Newtonian cooling (thin sections, Bi < 0.1):
   T(t) = T_ambient + (T_initial − T_ambient) × exp(−h×A/(ρ×V×c_p) × t)
   Where: h = heat transfer coefficient (W/m²·K)
          A = surface area (m²), V = volume (m³)
          ρ = density (kg/m³), c_p = specific heat (J/kg·K)
   Valid for: thin sheet, wire, small particles where thermal gradient ≈ 0

2. Approximate cooling rate at surface (thick section):
   T_surface(t) ≈ T_quench + (T_austenitise − T_quench) × erf(x / (2√(α×t)))
   Where: α = thermal diffusivity = λ/(ρ×c_p) ≈ 5×10⁻⁶ m²/s for steel
          x = depth from surface
   Valid for: very thick section, short times

3. Rykalin t8/5 for weld HAZ (most used in practice):
   For 3D cooling (thick plate):
   t8/5 ≈ (6700 − 5×T₀) × Q × [1/(500−T₀)² − 1/(800−T₀)²] / (2π×λ)
   Where: Q = heat input (kJ/mm), T₀ = preheat (°C), λ ≈ 0.04 kJ/mm·s·K

   Typical t8/5 values (Q=1.5 kJ/mm, T₀=25°C):
   ~ 7 seconds → maps to CCR of (800−500)/7 ≈ 43°C/s
   On CCT diagram: draw a cooling line from Ae₃ reaching 500°C at t=7s
   → microstructure read from phase regions traversed

Step-by-Step: Reading the Resulting Microstructure

Ms Temperature Calculation and Retained Austenite

The martensite start temperature is one of the most critical parameters read from a TTT or CCT diagram. It determines how much of the austenite transforms to martensite during quenching to room temperature, and whether retained austenite is a concern. Several empirical equations have been published; the most widely used are:

Andrews (1965) — most widely used for low-to-medium alloy steels:
  Ms (°C) = 539 − 423×%C − 30.4×%Mn − 17.7×%Ni − 12.1×%Cr − 7.5×%Mo

Barbier (1998) — better for high-alloy steels:
  Ms (°C) = 565 − 600×(1−exp(−0.96×%C)) − 31×%Mn − 13×%Si
             − 10×%Cr − 18×%Ni − 12×%Mo − 8×%W + 30×%Al

Koistinen–Marburger martensite fraction at temperature T:
  f_M(T) = 1 − exp(−0.011 × (Ms − T))     [T < Ms]
  f_M(20°C) = 1 − exp(−0.011 × (Ms − 20))  [room temperature quench]

Retained austenite fraction after quench to room temperature:
  γ_R = 1 − f_M(20°C) = exp(−0.011 × (Ms − 20))

Examples:
  1040 steel (Ms ≈ 350°C): f_M(20°C) = 1−exp(−0.011×330) = 1−0.027 = 97.3%
    → Retained austenite ≈ 2.7% (negligible)

  4340 steel (Ms ≈ 300°C): f_M(20°C) = 1−exp(−0.011×280) = 1−0.046 = 95.4%
    → Retained austenite ≈ 4.6% (small but may affect dimensional stability)

  M2 high-speed steel (Ms ≈ −30°C): f_M(20°C) = 1−exp(−0.011×(−50)) = 1−1.73 → 0%
    → Room temperature quench does NOT transform to martensite
    → Sub-zero treatment (−78°C dry ice, or −196°C LN₂) required

Effect of Alloying Elements on TTT/CCT Position

CCT Diagrams for Weld HAZ: Why They Differ from Base Metal

The heat-affected zone (HAZ) of a weld undergoes a rapid, non-equilibrium thermal cycle quite unlike a conventional furnace heat treatment. The peak temperature varies with distance from the fusion boundary: from near the melting point at the fusion line to barely above Ac₁ at the outer edge of the HAZ. The CCT diagram used to predict HAZ microstructure must therefore reflect the specific austenitising conditions of the HAZ sub-zone being analysed.

CGHAZ vs FGHAZ: Different CCT Diagrams

In the coarse-grained HAZ (CGHAZ), peak temperatures of 1200–1400°C dissolve all carbides and nitrides, eliminate grain boundary pinning particles, and allow rapid austenite grain growth to 100–300 μm. This has two effects on the CCT diagram:

1. More carbon and alloy in solution: dissolved carbides increase the effective carbon equivalent, shifting C-curves further right and increasing hardenability.
2. Coarser grain size: fewer grain boundary nucleation sites per unit volume shift C-curves to longer times, further increasing hardenability.

The result is that the CGHAZ is significantly more hardenable than the base metal, meaning a lower cooling rate (longer t_8/5) is required before the ferrite/pearlite transformation dominates. At normal heat input levels (0.5–2.0 kJ/mm), the CGHAZ often transforms partially or fully to martensite or lower bainite, explaining why the CGHAZ is typically the hardest and most crack-susceptible region in a weld. The HAZ microstructure article provides detailed treatment of the sub-zone structures, while the hydrogen-induced cracking guide addresses the consequences of hard CGHAZ microstructures in the presence of diffusible hydrogen.

Weld HAZ CCT application — worked example:
  Steel: API 5L X65 pipeline, CE(IIW) = 0.43
  Process: SMAW (η = 0.8), preheat T₀ = 50°C
  Heat input Q = 1.4 kJ/mm

  t8/5 (Rykalin 3D, thick plate):
  t8/5 = (6700 − 5×50) × 1.4 × [1/(500−50)² − 1/(800−50)²] / (2π×0.04)
       = (6700−250) × 1.4 × [1/202500 − 1/562500] / 0.2513
       = 6450 × 1.4 × [4.938×10⁻⁶ − 1.778×10⁻⁶] / 0.2513
       = 9030 × 3.16×10⁻⁶ / 0.2513
       ≈ 9030 × 1.258×10⁻⁵
       ≈ 11.4 seconds

  Average cooling rate from 800→500°C: 300°C / 11.4s = 26°C/s

  On the X65 CGHAZ CCT diagram, CR = 26°C/s:
  → Intersects bainite start at ~500°C
  → Produces predominantly lower bainite + some martensite
  → Expected hardness: ~290–320 HV10 (below 248 HV NACE limit: PASS ✓)

  If preheat reduced to 0°C (T₀ = 0°C), same Q = 1.4 kJ/mm:
  t8/5 recalculates to ~8.5s → CR = 35°C/s
  → More martensite in CGHAZ, hardness ~340–360 HV (EXCEEDS NACE limit ✗)
  → Minimum preheat is justified and required

Practical Applications of TTT and CCT Diagrams

Heat Treatment Process Design

When designing a quench-and-temper heat treatment for a steel component, the CCT diagram tells you the minimum quench severity (cooling rate at the centre of the thickest section) required to achieve the target microstructure. For a target of >90% martensite in the centre of a 50 mm diameter bar, you read from the CCT diagram the minimum cooling rate for martensite formation, calculate the Jominy position that corresponds to that cooling rate, and verify from the Jominy curve that the bar centre reaches adequate hardness. For the Jominy calculation from composition, use the Jominy hardenability calculator.

Austempering and Martempering

TTT diagrams are directly applicable to austempering (isothermal transformation in the bainite region to produce a tough, wear-resistant bainitic structure without distortion) and martempering (interrupted quench to just above M_s, hold for temperature equalisation, then cool to martensite — reducing thermal gradients and distortion). For austempering, the TTT diagram shows the temperature and time window within which the desired bainite forms completely before martensite can form: the window between the bainite start and finish curves at the target temperature.

Weld Preheat and Interpass Temperature Selection

As shown in the worked example above, the CCT diagram combined with the Rykalin t_8/5 equation provides the quantitative basis for selecting minimum preheat temperatures. By choosing a preheat that produces a t_8/5 placing the cooling curve to the right of the martensite region of the HAZ CCT diagram, the risk of hard martensitic HAZ (and hydrogen-induced cracking) is minimised. The preheat temperature calculator implements this methodology per EN 1011-2.

Normalising and Annealing Temperature Selection

For annealing operations, the CCT diagram confirms what microstructure will form at a specific furnace cooling rate. Full annealing (furnace cooling through the pearlite field) produces coarse lamellar pearlite — maximum softness and machinability. Normalising (still-air cooling) produces a finer pearlite and some ferrite — higher strength than annealed, suitable for most structural applications. The annealing and normalising guide covers the practical heat treatment cycle design for each condition.

Frequently Asked Questions

Recommended References

Further Reading