What is the purpose of CCT Diagram vs TTT Diagram?

CCT Diagram vs TTT Diagram is a controlled thermal process used to modify the microstructure and mechanical properties of steel. The process parameters — temperature, time, and cooling rate — are determined from the iron-carbon phase diagram and TTT/CCT diagrams for the specific steel composition.

What temperature should I use?

Temperature selection depends on the steel's A1 and A3 critical temperatures, which can be calculated from composition using the Andrews (1965) empirical equations (see our Ac1/Ac3 Calculator). For hardening: heat to 30-50°C above A3 for hypoeutectoid steels. For annealing: just above A3 for full anneal, or 690-720°C for spheroidise annealing. Always verify with the steel supplier's datasheet.

How do I verify the process was successful?

Verification methods include: (1) Hardness testing — Vickers, Rockwell, or Brinell at multiple locations to confirm uniformity; (2) Microstructural examination by optical metallography — nital-etched sections at 200-500×; (3) Mechanical testing — tensile, Charpy CVN per the applicable standard (EN ISO 6892-1, EN ISO 148-1). See our Hardness Conversion Calculator for interpreting results across scales.

CCT Diagram Explained: Continuous Cooling

The CCT diagram (Continuous Cooling Transformation) is the practical workhorse of
industrial steel heat treatment — it shows what microstructure forms when steel is cooled
continuously at different rates, as happens in every real-world quenching, normalising,
and annealing operation. While TTT diagrams describe isothermal transformations (the theoretical
foundation), CCT diagrams tell you directly what to expect from water quenching, oil quenching,
forced-air cooling, or furnace cooling of any specific steel grade. Every qualified welding
and heat treatment engineer needs to be able to read a CCT diagram.

KEY TAKEAWAYS

CCT curves are always shifted RIGHT and DOWN compared to TTT for the same steel — continuous cooling is less efficient.
The critical cooling rate (CCR) is the minimum rate producing 100% martensite — the dividing line between full hardening and mixed microstructures.
Each cooling curve on a CCT diagram ends with a label showing the final microstructure (M%, B%, P%, F%).
CCT diagrams are used to: select quench media, predict HAZ microstructure in welding, and design controlled cooling for TMCP steels.
Alloying elements (Mn, Cr, Mo, Ni, B) push CCT curves to the right, enabling thicker sections to achieve martensite with milder quenches.
The CCT diagram is steel-specific — each composition has its own diagram. Suppliers publish CCT diagrams for their grades.

Critical Cooling Rate

Water 100%M

M+B

B+P

F+P

KEY: CCT curves are shifted RIGHT and DOWN vs TTT. Continuous cooling is less efficient than isothermal hold.

Pearlite Bainite Martensite

Figure 3: Schematic CCT (Continuous Cooling Transformation) diagram. CCT curves are displaced to the right and downward compared to TTT because continuous cooling is less efficient than isothermal transformation. The red dashed line is the critical cooling rate (CCR). Cooling lines (solid arrows) with product labels: M = martensite, B = bainite, P = pearlite, F = ferrite. Created by metallurgyzone.com/

CCT vs TTT — The Critical Difference

The fundamental difference is the thermal path. TTT: the steel is quenched instantaneously to a fixed temperature and held there isothermally. CCT: the steel is cooled continuously at a constant rate from the austenitising temperature.

Because continuous cooling spends progressively less time at each temperature as the steel passes through it, the effective transformation occurs at lower temperatures and longer times than the equivalent isothermal transformation — hence the rightward and downward displacement of CCT curves relative to TTT. This displacement is larger for slower cooling rates and for steels with more sluggish transformation kinetics.

Cooling Medium	Typical Cooling Rate at 700°C (°C/s)	Microstructure in Plain C Steel	Microstructure in 4140 Alloy
Brine/ice water agitated	300–600	Martensite (may crack C>0.35%C)	Martensite
Water agitated	100–200	Martensite	Martensite
Warm oil (60–80°C)	20–60	Martensite + bainite in surface; bainite/pearlite in core	Martensite
Polymer solution (5%)	30–80 adjustable	Martensite (controlled severity)	Martensite
Forced air / fan	3–15	Bainite (alloy steel); pearlite (plain C)	Bainite + some pearlite
Still air	0.5–2	Ferrite + pearlite	Ferrite + fine pearlite
Furnace cool	0.01–0.2	Coarse ferrite + pearlite (max softness)	Coarse pearlite

The Critical Cooling Rate (CCR) and Hardenability

The CCR is the slowest cooling rate that just misses the pearlite start curve, producing 100% martensite throughout the section. It is read from the CCT diagram as the cooling line tangent to the leftmost point of the pearlite start curve. The CCR sets the maximum section size that can be through-hardened with a given quench medium.

Critical cooling rate varies enormously between grades:

  1040 plain carbon (0.4C, 0.75Mn): CCR ≈ 200°C/s  → water quench required → max 12mm dia.

  4140 Cr-Mo alloy:                  CCR ≈ 25°C/s   → oil quench sufficient → up to 50mm

  4340 Ni-Cr-Mo alloy:               CCR ≈ 3°C/s    → air hardening → up to 150mm

  H13 hot-work tool steel:           CCR < 0.5°C/s  → air hardening → very large sections

Reading a CCT Diagram: Step-by-Step

Identify the austenitising temperature — all cooling curves start here (usually printed on the diagram)
Locate the cooling curve for your quench medium and section size — either from the diagram itself or by calculating from the Grossmann H-value and section diameter
Follow the cooling curve downward and to the right — when it crosses a phase boundary line, note which phase starts forming and at what temperature
Read the microstructure label at the bottom of the cooling curve (e.g. “M 85% B 15%” = 85% martensite, 15% bainite)
Read the hardness value — most CCT diagrams include hardness (HRC or HV) for each cooling path at the end of each cooling curve

CCT Diagrams in Welding HAZ Prediction

One of the most critical engineering applications of CCT diagrams is predicting the microstructure and hardness of the weld heat-affected zone (HAZ). The weld thermal cycle imposes a specific temperature-time history on each HAZ location. By mapping the HAZ cooling curve onto the base metal CCT diagram, the engineer can predict:

Whether martensite will form in the coarse-grain HAZ (CGHAZ) — and if so, how hard (risk of hydrogen-induced cold cracking if HV >350)
Minimum preheat temperature needed to shift the cooling curve into the softer bainite region
Whether PWHT is required to temper any formed martensite
HAZ toughness (Charpy CVN) — can be correlated with microstructure from the CCT diagram

📷 IMAGE: CCT Diagram for Low-Alloy Structural Steel with HAZ Cooling Curves

CCT diagram for a C-Mn structural steel with weld HAZ cooling curves overlaid at different heat inputs. Shows martensite formation at high cooling rates (low heat input), bainite at intermediate, and ferrite+pearlite at low cooling rates (high heat input). Used to determine minimum preheat temperature for cold cracking prevention.

Search terms: CCT diagram structural steel HAZ cooling curve preheat

Source:

https://en.wikipedia.org/wiki/Continuous_cooling_transformation

Attribution: Available from steel supplier data books, ASM Handbook Vol 1, and ISO 15614 / EN 1011-2 supplementary materials.

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Frequently Asked Questions

Q: How do I get a CCT diagram for my specific steel grade?

A: The best sources are: (1) the steel manufacturer’s technical data sheet — most major steel mills publish CCT diagrams for all engineering grades; (2) ASM Handbook Vol. 1 (Properties and Selection — Irons, Steels) contains CCT diagrams for hundreds of grades; (3) the Bain/Bain-Aborn atlas and Atkins’ Atlas of Continuous Cooling Transformation Diagrams for Engineering Steels; (4) online databases such as the SGTE/Thermo-Calc TTT/CCT module (paid), or JMatPro software which can calculate CCT diagrams from composition if experimental data is unavailable.

Q: Can I use a TTT diagram to predict what happens during oil quenching?

A: A TTT diagram gives a qualitatively correct picture but quantitatively inaccurate predictions for continuous cooling. You can approximate by treating the cooling curve as passing through a series of isothermal holds, but the errors can be significant — especially near the nose of the C-curve where even small time errors change the microstructure dramatically. For serious engineering work (especially welding HAZ or heat treatment qualification), always use a CCT diagram. If only a TTT diagram is available, conservative engineering practice is to assume the CCT nose is at 30% longer times than the TTT nose — a rough but useful approximation.

Q: Why do some steels have very slow critical cooling rates that allow air hardening?

A: Certain elements — particularly Mo, Cr, Ni, Mn, and B (in small amounts) — strongly retard the nucleation and growth of ferrite and pearlite at grain boundaries, pushing the pearlite start curve to tens of minutes or hours. Steels designed for air hardening (H13, D2, A2 tool steels; also some high-alloy structural steels like 4340 in large sections) have sufficient alloy content that even still-air cooling is slower than their very low critical cooling rate. This eliminates the need for a liquid quench, dramatically reduces distortion and cracking risk, and allows large complex shapes to be through-hardened without a quench bath.

References

Atkins, M., Atlas of Continuous Cooling Transformation Diagrams for Engineering Steels. ASM International, 1980.
ASM Handbook Vol. 4A: Steel Heat Treating. ASM International, 2013.
EN 1011-2:2001 Welding — Recommendations for welding of metallic materials — Annex C (preheat calculation using CCT diagrams).