Question

Consider the same TTT diagram and phase diagram for a eutectoid steel. TTT If a specimen of this steel is solutionized (aka, annealed to equilibrium) at 800°C, rapidly cooled to 300°C and is held at 300°C for 10 seconds, and is then quenched to room temperature, what constituents will make up the final microstructure? a. Bainite and Martensite b. Pearlite c. Martensite d. Austenite and Martensite e. Austenite f. Pearlite and Martensite g. Austenite and Pearlite h. Bainite

Answer 1

As asked in the qusetion, I am assuming that you have the TTT diagram. So I am just explaining you the answer, you should refer to that diagram for understanding the explaination.

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If a specimen of eutectoid steel is solutionized at 800 degrees then 100% austenite formation takes place. Now as the steel is rapidly cooled from 800 degrees to 300 degrees then also the microstructure remains 100% austenite. This is because the cooling rate is much higher to enter into the nose area to form bainite. Holding the steel at this temperature for 10 seconds will again give 100% austenite microstructure because this time is insufficient to enter into nose region.

Again quenching this steel to room temperture leads to conversion of austenite into martensite. Hence the final microstructure consists of 100% martensite because room temperature is much lower than the martensite finish temperature for this steel.

Hence the correct option (C) martensite.