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CHE 333 Class 9

Non equilibrium conditions. Non equilibrium is when a reaction is not allowed to go to completion. An example is

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CHE 333 Class 9

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    1. CHE 333 Class 9 Non Equilibrium Heat Treatment of Steels.

    2. Non equilibrium conditions Non equilibrium is when a reaction is not allowed to go to completion. An example is “quenching” when a hot object is plunged into a bath of cool liquid, such as iced brine or oil. Under these conditions an expected phase change will be suppressed, that is, it will not happen. Several results are possible. The high temperature phase is retained at low temperature – age hardening. A new phase is produced that is not predicted by the phase diagram. If the new phase is stable at room temperature it is termed a “Metastable” phase. Example of metastable phase formation by non equilibrium heat treatment is the formation of “Martensite” in steels.

    3. Martensite Formation.

    4. Phase Changes During Quench Under equilibrium conditions, the eutectoid reaction will occur:- g > a + Fe3C FCC > BCC + Orthorhombic The quench suppresses this reaction, and a new phase is formed which is metastable:- g > Martensite FCC > Body Centered Tetragonal (BCT) For each steel composition there is a temperature threshold called the Ms temperature below which martensite is produced. If the steel is not quenched to below this temperature, no martensite will be formed. Martensite can only be formed from the g phase. Use Temperature Time Transformation Curves or TTT curves – these are Isothermal. To get 100% martensite, quench finish temperature is below 0C.

    5. TTT Curve

    6. Formation of Martensite.

    7. Martensite Structures Martensite is a strong but brittle material

    8. Martensite Amounts In the top figure, the sample was held until 1% transformation to Pearlite, 99% retained austenite, which on quenching Transformed to martensite. In the middle figure, the sample was held for 25% transformation to pearlite The 75% retained austenite transformed to Martensite upon quenching below the Mf In the bottom figure, the sample was held to 50% transformation

    9. Hypoeutectoid TTT Curve. Note the region above the eutectoid temperature of 727C, which is ferrite and austenite.

    10. TTT - CCC

    11. Homework

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