Figure 1: Time-Temperature-Transformation (TTT) Diagram for a Eutectoid Steel [1]

The phase transformations described in "Benefits of Heat Treating" Parts 2, 3, and 4 occur by nucleation and growth and are diffusion-controlled. The kinetics or rates of these transformations are a function of temperature and steel composition and can be described with a TTT diagram for the eutectoid steel as seen in Figure 1.

This particular diagram presents the kinetics for isothermal (constant-temperature) transformations of a eutectoid steel. In these diagrams the ordinate (y-axis) is temperature and the abscissa (x-axis) is time. In this case it is the logarithm of time. As seen on this diagram, at temperature above 1333°F (723°C) the austenite phase is always stable. When this steel is cooled very rapidly to 1112°F (600°C), however, the austenite becomes unstable and transforms to pearlite over time. As seen in Figure 1, after 2-3 seconds the austenite starts to transform to pearlite, after 6–7 seconds the transformation is 50% complete (i.e. 50% austenite and 50% pearlite) and after about 15 seconds the transformation is complete.

For this eutectoid steel, when rapidly cooled to a temperature below the nose of the TTT curve a slightly different type of transformation occurs. The austenite will transform to a structure called bainite. Similar to pearlite, bainite consists of a mixture of ferrite and carbide. However, the morphology is finer and can have a more feathery or acicular appearance.

As seen in Figure 1, after rapid cooling to 752°F (400°C), the austenite is unstable. After about 7 seconds the austenite starts to transform to bainite, after about 80 seconds the transformation is 50% complete and after about 200 seconds the transformation to bainite is complete. These TTT diagrams are available for a wide variety of steel compositions.

As presented in these TTT diagrams, the transformation from austenite to pearlite, bainite and primary ferrite or carbide takes time. These transformations are diffusion controlled, and diffusion takes time.