What is the purpose of TTT Diagram Explained?

TTT Diagram Explained 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.

TTT Diagram Explained: Time-Temperature-Tr

The TTT diagram (Time-Temperature-Transformation), also called the
isothermal transformation diagram or S-curve, maps the transformation
of supercooled austenite into other microstructures as a function of temperature and time at
that constant temperature. It is the foundational tool for understanding steel heat treatment
kinetics — telling the metallurgist exactly how long austenite can survive at any temperature
before transforming, and what it transforms into. Every steel has its own unique TTT diagram
determined by its composition and grain size.

KEY TAKEAWAYS

TTT diagrams show isothermal (constant-temperature) transformations — the steel is quenched instantly to a temperature and held there.
The C-shaped curves define start and finish times for pearlite and bainite transformation. The ‘nose’ is where transformation is fastest.
Martensite forms only on continuous cooling below Ms — it does NOT appear as a C-curve because it is time-independent (athermal).
The critical cooling rate (CCR) is the minimum cooling rate to avoid the pearlite/bainite nose entirely and obtain 100% martensite.
The nose of the pearlite C-curve for plain carbon eutectoid steel is approximately 550°C and ~1 second — very fast to avoid.
Alloying elements (Mn, Cr, Mo, Ni, B) shift C-curves to longer times, making hardening easier for thicker sections.
CCT diagrams (continuous cooling) are more practical for industry; TTT is used for isothermal treatment design.

Ps Pf

Bs Bf

Pearlite (+ proeutectoid ferrite if <0.77%C) Bainite Martensite fraction increases below Ms Supercooled austenite

Water quench

Oil quench

Air cool

Furnace cool

100%M M+B B+P P

Figure 2: Schematic TTT diagram for eutectoid (0.77%C) steel. Blue curves = pearlite start (Ps, solid) and finish (Pf, dashed). Green curves = bainite start (Bs) and finish (Bf). Red horizontal lines = martensite start Ms (230°C) and finish Mf (130°C). Cooling curve overlays show products: M = martensite, B = bainite, P = pearlite. Created by metallurgyzone.com/

How to Read the TTT Diagram

The C-Curves: Pearlite and Bainite

Two sets of C-curves appear on the TTT diagram — one for pearlite (upper, higher temperature) and one for bainite (lower). Each set has a start curve (Ps, Bs — conventionally 1% transformation) and a finish curve (Pf, Bf — 99% transformation). The region to the left of the start curve is untransformed supercooled austenite; between start and finish, transformation is in progress; to the right of the finish, transformation is complete.

The nose of each C-curve is the temperature of maximum transformation rate — where the competing effects of driving force (increasing with undercooling) and atomic mobility (decreasing with lower temperature) are balanced. For plain eutectoid steel: pearlite nose ≈ 550°C, ~1 second; bainite nose ≈ 400°C, ~5 seconds.

Why the Curve is C-Shaped

The shape results from two competing Arrhenius-type exponential relationships:

Driving force: ΔG increases exponentially with undercooling below A1 → favours faster transformation at lower T
Diffusivity: D decreases exponentially with temperature → slows transformation at lower T

Near A1: high diffusivity, low driving force → slow. Near Ms: high driving force, near-zero diffusivity → slow. At the nose: optimal balance → fastest. The C-curve is a natural consequence of these two opposing Arrhenius exponentials.

Temperature Region	Transformation Product	Typical HRC	Toughness	Time at Nose
650–727°C (just below A1)	Coarse pearlite (ferrite + coarse cementite lamellae)	10–20	Good	~30–300 s
550–650°C	Fine pearlite (finer lamellae)	25–35	Moderate	~1–30 s
~550°C (nose)	Very fine / troostite pearlite	35–43	Moderate	~0.5–2 s
400–550°C	Upper bainite (ferrite laths + cementite films)	38–46	Moderate	~3–100 s
250–400°C	Lower bainite (intralath carbides in ferrite)	45–58	Good	~10–1000 s
Below Ms (230°C for 0.8%C)	Martensite (BCT, athermal)	58–66	Poor as-quenched	Instantaneous

The Martensite Transformation

Martensite is unique — it forms by a diffusionless shear transformation that does not require time. Instead of a C-curve, the TTT diagram shows two horizontal lines: Ms (martensite start) and Mf (martensite finish). The fraction of martensite that forms depends only on how far below Ms the steel is cooled, described by the Koistinen-Marburger equation:

f_M = 1 − exp[−0.011 × (Ms − T)]
where:

  f_M = fraction martensite (0 = none → 1 = complete)

  Ms  = martensite start temperature (°C)

  T   = quench temperature (°C)
Example for eutectoid steel (Ms ≈ 230°C):

  At room temperature (25°C): f_M = 1 − exp[−0.011 × (230−25)] = 1 − exp(−2.255) = 0.895 = 89.5%

  The remaining ~10.5% is retained austenite

  To achieve >98% martensite, sub-zero quenching to ~−80°C is needed

Industrial Applications of TTT Diagrams

Austempering — Using the Bainite Field

Austempering exploits the TTT diagram directly: quench austenite rapidly through the pearlite nose into the bainite temperature range (typically 250–400°C) and hold until the bainite transformation is complete. Result: bainite — better toughness at equivalent hardness than tempered martensite, with minimal distortion (no martensite transformation). The steel must be read from the TTT diagram to ensure: (1) the quench is fast enough to avoid the pearlite nose, and (2) the hold time at temperature is long enough to complete bainite transformation before the austenite begins to transform to pearlite from the upper temperature.

Martempering — Uniform Martensite without Distortion

Martempering quenches steel into a bath held just above Ms, soaks until the temperature equalises through the section (without the bainite start curve being reached — must stay in the “bay” to the left of Bs), then air cools to room temperature. Martensite forms uniformly throughout the section during the final slow air cool rather than progressively from surface to core, dramatically reducing thermal stress-induced distortion.

Isothermal Annealing

From the TTT diagram: quench to the fine pearlite region (~650°C), hold until Pf (transformation complete), then air cool. Produces uniform fine pearlite without gradients — used for precision gears where dimensional consistency across a batch is critical. More reproducible than conventional full annealing.

📷 IMAGE: TTT Diagram for 1080 Eutectoid Steel (Experimental)

Experimental TTT diagram for 1080 steel (0.8%C) showing measured C-curves for pearlite and bainite transformations, Ms and Mf lines, and microstructure annotations. This is the archetypal TTT diagram used in all materials science textbooks.

Search terms: TTT diagram 1080 eutectoid steel isothermal transformation experimental

Source:

https://en.wikipedia.org/wiki/Time%E2%80%93temperature_transformation_diagram

Attribution: Widely reproduced from ASM Handbook / textbook sources. Search ‘TTT diagram eutectoid steel 1080’ and download from an open academic source.

→ Download image from the link above and upload via WordPress Media Library → Insert above

Factors Affecting TTT Diagram Shape

Factor	Effect on C-Curves	Practical Consequence
Higher carbon content (%C)	Nose shifts RIGHT (slower transformation)	Higher-C steel → easier to harden in thick sections
Alloying elements (Mn, Cr, Mo, Ni)	Nose shifts dramatically RIGHT	Alloy steels need much slower quench — oil instead of water
Boron addition (0.001–0.003%B)	Specifically retards pearlite; minimal effect on bainite	Small B addition dramatically increases hardenability cost-effectively
Finer prior austenite grain	Nose slightly LEFT (more nucleation sites)	Fine-grain steel harder to harden through thick sections
Higher austenitising temperature	Nose moves RIGHT (coarser grain, better dissolved alloys)	Higher soak temp → better hardenability but risk of grain coarsening

Frequently Asked Questions

Q: What is the fundamental difference between TTT and CCT diagrams?

A: TTT (isothermal): the steel is quenched instantaneously to a fixed temperature and held there — describes what forms during constant-temperature holding. CCT (continuous cooling): the steel is cooled continuously at different rates — describes what forms during actual heat treatment operations like quenching, normalising, and annealing. CCT curves are shifted to the right and lower temperatures compared to TTT because continuous cooling spends progressively less time at each temperature as the steel cools through it. For industrial process design, CCT diagrams are more directly applicable; TTT diagrams are essential for designing austempering and martempering cycles.

Q: Why do some steels have a ‘bay’ or gap between the pearlite and bainite noses on TTT diagrams?

A: In some alloy steels — particularly those containing molybdenum — the pearlite and bainite C-curves are separated by a temperature range where neither reaction is fast. This creates a bay or plateau on the TTT diagram between approximately 400–550°C. Molybdenum specifically retards the pearlite reaction (by reducing boundary nucleation) while having less effect on bainite. This bay is exploited commercially in martempering (quench into the bay, equalise, then air cool for uniform martensite) and in some steels to obtain bainite in thick sections without producing any pearlite.

Q: How is the TTT diagram used to select a steel for a given section size?

A: The process is: (1) determine the cooling rate at the critical location (usually the core) for your intended quench medium and section size using Grossmann charts or Jominy testing; (2) overlay this cooling rate as a curved line on the TTT diagram for the candidate steel; (3) check whether this cooling curve passes to the left of the pearlite nose (required for martensite) or intersects the bainite region (for bainitic product). If the core cooling rate is too slow, you must either choose a higher-hardenability alloy steel (with C-curves pushed further right) or use a more aggressive quench medium.

References

Bhadeshia, H.K.D.H., Bainite in Steels: Theory and Practice. 3rd ed. Maney Publishing, 2015.
ASM Handbook Vol. 4A: Steel Heat Treating Fundamentals and Processes. ASM International, 2013.
Krauss, G., Steels: Processing, Structure, and Performance. 2nd ed. ASM International, 2015.
Boyer, H.E. (ed.), Atlas of Isothermal Transformation and Cooling Transformation Diagrams. ASM International, 1977.