Isothermal Transformation:

Austenite is not normally converted in martensite instantly but the procedure continues for some time. Various steels take various time for full transformation and the time depends on the temperatures from, here cooling is begun. How to acquire isothermal transformation diagram was illustrated in detail in Section of Tempering. The selected test piece is austenitized and then quenched in liquid bath held at temperature to be examined. The test piece is held for a dissimilar length of time in the bath and then quenched in the water. The resulting structure can be studied beneath microscope or any other connected property like hardness might be studied. This is seen that exact times are required for the initiation and completion of the transformation and these times vary along with the temperature. The progress of transformation, as for 10%, 50% or 90% may also be found.

Figure of Isothermal Transformation Diagram for a Eutectoid Steel and Structures Present after 10⁵ Seconds are specified on the Right-Hand Side that is Figure of Tempering Diagram of Property Chart for Water Quenched 26 millimeter Diameter Bars of Eu-12 Steel as Scale for Stress only, reproduced along with more details, demonstrates complete transformation figure of eutectoid steel. Since the transformation temperature is lowered from A₁ temperature to about 550^oC, the completion and nucleation time decreases and pearlte and lamellae become finer. From about 550^oC to 250^oC, the nucleation and completion time raises. The transformation product now is called bainie such is composed of two equilibrium phases such are cementite and ferrite. The minimum time nucleation is termed as the nose or knee. Underneath concerning to 250^oC the transformation product is martensite. Such forms approximately instantly, but the amount formed depends on the temperature. The lower and upper limits of martensite transformation temperatures are known as MF and MS temperatures. These diagrams are also termed as Time-Temperature-Transformation or TTT diagrams and were discussed prior. They are concerned also to as S-curve due to their shape.

Figure: Isothermal Transformation Diagram for Eutectoid Steel. Structures present after 105 Seconds are specified on the Right-Hand Side