a) In continuous cooling curve there are no transformation lines below about  , for the austenite to pearlite transformation.
b) The start and finish transformation line is shifted to slightly longer times and to slightly lower temperatures in relation to isothermal diagrams.
Figure 36.3 shows different rates of cooling of eutectoid plain carbon steels cooled continuously from austenitic region to room temperature. It is assumed that there are no temperature gradients in the carbon steels in the austenitic region. This requires either a thin section or section has been soaked for a sufficient long time.
| Figure 36.3 |
Continuous cooling of eutectoid plain carbon steels. The cooling rates are shown with different colors |
| Cooling curve x: |
very slow cooling and will result in coarse pearlite |
| Cooling curve y: |
Slow cooling in air and fine pearlite will form. This heat treatment procedure is called normalizing. |
| Cooling curve Z: |
Steel is quenched in oil. This will result in martensite and pearlite and is called split transformation |
| Cooling curve K: |
Critical cooling rate at which a martensite is produced when steel is quenched in water. |
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What is important is to appreciate that the system possesses unique possibility to produce materials with different number, and proportion of phases.
There are other heat treatment procedures like martempering and austempering.
Austempering is an isothermal treatment aimed to produce a bainite structure in some plain carbon steels. The steel is first austenitized and then quenched in a molten salt bath kept at temperature above the  temperature. Steel is held at that temperature to allow austenite to transform to bainite. Advantages of austempering:
- Decrease in distortion
- Improved ductility and impact resistance.
Martempering is a modified quenching procedure used for steels to minimize distortion and cracking that may develop during uneven cooling of the heat treated material. The martensite process consists of austenitizing steel and then quenching in hot oil or molten salt at a temperature just slightly above (or slightly below) the MS temperature. In the hot quenchent steel is soaked to attain the uniform temperature which is then followed by cooling at a moderate rate to room temperature to prevent temperature gradient.
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