A TTT (Time-Temperature-Transformation) diagram maps how steel changes its internal structure when held at different temperatures over time. The vertical axis shows temperature, the horizontal axis shows time on a logarithmic scale, and the curved lines tell you when transformations begin and end. Once you understand what each region and line represents, you can predict exactly what microstructure a piece of steel will develop under a given cooling strategy.
The Two Axes and What They Tell You
Temperature runs along the vertical axis, typically in degrees Celsius or Fahrenheit. Time runs along the horizontal axis, but on a logarithmic scale, meaning each major division represents a tenfold increase (1 second, 10 seconds, 100 seconds, and so on). This is important: constant cooling rates appear as curved paths on a TTT diagram, not straight lines, because of that log scale. If you mentally picture dropping temperature at a steady rate, the line bends to the right as time stretches out.
At the very top of the diagram, you’ll see a horizontal line marking the critical temperature above which the steel exists as austenite, its high-temperature phase. For a standard eutectoid steel (1080), this line sits at 723°C. Everything on the diagram happens below this line, because transformations only begin once the steel cools past that threshold.
Start and Finish Curves
The most prominent features on any TTT diagram are two C-shaped (or sometimes S-shaped) curves. The left curve marks the “start” of transformation, meaning the moment a new microstructure begins forming from austenite. The right curve marks the “finish,” where essentially all the austenite has converted. Between these two curves, transformation is in progress, and the percentage complete depends on how far across you are. Many diagrams include a dashed line between the start and finish curves representing 50% transformation.
If you pick a temperature on the vertical axis, draw a horizontal line across, and note where it crosses the start curve, that tells you how long you’d need to hold the steel at that temperature before anything begins to change. Where it crosses the finish curve tells you how long until the transformation is complete. Any point to the left of the start curve means the steel is still fully austenite. Any point to the right of the finish curve means transformation is done.
The Nose of the Curve
The C-shaped curves bulge leftward at a particular temperature, creating a feature called the “nose.” This is the temperature where transformation begins fastest, requiring the least amount of time. The nose is critically important because it defines the minimum cooling rate you need to avoid that transformation entirely. If you’re trying to form martensite (the hardest microstructure), you need to cool the steel fast enough that your cooling path passes to the left of the nose without touching the start curve. This minimum speed is called the critical cooling rate.
For practical purposes, the nose method gives a rough estimate of the critical cooling rate by dividing the temperature drop (from the starting temperature to the nose temperature) by the incubation time at the nose. This estimate tends to be about 1.5 times higher than the true critical rate, so it builds in a safety margin.
Regions: Pearlite, Bainite, and Martensite
Different microstructures form at different temperature ranges, and the TTT diagram divides neatly into zones.
Pearlite forms at higher temperatures, closer to the critical line. It’s a layered structure of iron and iron carbide. At temperatures just below 723°C, pearlite forms slowly and the layers are coarse (called “coarse pearlite”), producing a softer material. Closer to the nose, pearlite forms faster and the layers are finer (“fine pearlite”), which is somewhat harder.
Bainite forms at temperatures below the nose. It has a different internal arrangement than pearlite and is generally harder and tougher. Like pearlite, bainite that forms at higher temperatures (upper bainite) differs from bainite that forms at lower temperatures (lower bainite), with lower bainite offering better mechanical properties.
Martensite forms differently from both pearlite and bainite. Instead of appearing as C-shaped curves, martensite is represented by horizontal lines near the bottom of the diagram. The upper line is labeled Ms (martensite start) and the lower line is labeled Mf (martensite finish). These lines are horizontal because martensite formation depends only on temperature, not on time. When austenite cools below Ms, it begins snapping into the martensite structure almost instantaneously through a shearing mechanism rather than the slow diffusion process that creates pearlite and bainite.
Why the Ms and Mf Lines Are Horizontal
Pearlite and bainite require atoms to physically migrate through the metal, which takes time. That’s why their transformation curves are C-shaped: temperature and time both matter. Martensite forms through a rapid structural rearrangement that doesn’t depend on diffusion. Once the steel reaches Ms, a certain fraction of austenite converts to martensite almost instantly. Cooling further converts more. By the time the steel reaches Mf, the transformation is essentially complete.
If cooling stops between Ms and Mf, you end up with a mixture of martensite and leftover austenite, called retained austenite. This retained austenite can cause dimensional instability and reduce hardness, which is why quenching needs to continue below Mf for a fully martensitic structure. Research on low-alloy steels confirms that even at cooling rates of 60°C per second, achieving a completely martensitic structure isn’t guaranteed in some alloys, highlighting how the specific steel composition matters.
How Alloying Elements Shift the Curves
The TTT diagram you see in a textbook typically represents a specific steel composition. Change the composition, and the curves shift. Adding carbon increases the tendency to form martensite but also lowers the Ms temperature, meaning you need to cool to a lower temperature before martensite begins forming.
Adding elements like nickel or manganese (austenite stabilizers) pushes the eutectoid transformation to lower temperatures and shifts the C-curves to the right, giving you more time before pearlite or bainite starts forming. This makes it easier to cool past the nose and achieve martensite, which is why alloy steels are more “hardenable” than plain carbon steels. Carbide-forming elements shift the curves in more complex ways, sometimes creating a distinct separation between the pearlite and bainite regions, producing two separate noses instead of one.
Reading a Cooling Path on the Diagram
To use a TTT diagram, you trace an imaginary cooling path from the starting temperature downward. For a true isothermal process, you drop vertically to a target temperature and then move horizontally to the right as time passes at that constant temperature. Where your horizontal line crosses the start and finish curves tells you when transformation begins and ends, and what microstructure you’ll get depends on the temperature zone you’re sitting in.
For example, if you quench a eutectoid steel from above 723°C down to 600°C and hold it there, you’d move horizontally at 600°C until you cross the start curve (pearlite begins forming), continue holding until you cross the finish curve (fully transformed to fine pearlite), and then cool to room temperature knowing the structure is set. If instead you quench rapidly past the nose all the way below Mf, your path stays to the left of both C-curves and you end up with martensite.
TTT vs. CCT Diagrams
TTT diagrams assume isothermal conditions: you instantly drop to a temperature and hold there. Real heat treating rarely works this way. In practice, steel cools continuously, passing through multiple temperature zones on the way down. Continuous Cooling Transformation (CCT) diagrams were developed to account for this reality.
CCT diagrams look similar to TTT diagrams but with the transformation curves shifted to the right (longer times) and downward (lower temperatures). This shift happens because continuously cooling steel spends less time at any single temperature compared to being held isothermally, so transformations need more time and lower temperatures to get going. CCT diagrams also allow you to predict hardness values along different cooling paths, which TTT diagrams cannot do directly.
TTT diagrams remain valuable because they show the fundamental transformation behavior of a steel alloy in a clean, interpretable way. They’re the best tool for understanding what’s happening metallurgically. But if you’re planning an actual quench in oil, water, or air, a CCT diagram for that specific alloy will give you more accurate predictions of the final microstructure and hardness.