Eutectoid Composition Steel TTT Diagram 800 727 700 Coarse pearlite 600 a+Fe3C 5

Question

Eutectoid Composition Steel TTT Diagram 800 727 700 Coarse pearlite 600 a+Fe3C 500 Fine pearlite 400 Bainite 300 200 100 1 sec 1 min 1 hour 1 day 0 0.1 10 Time, seconds I. A plain-carbon 1076 steel alloy (eutectoid composition-0.76 wt% carbon) is heated to 800°C and allowed sufficient time to reach equilibrium conditions. It is then very rapidly quenched to 600°C and held at that temperature for 24 hours a. From the instant just after quenching, how much time did it take for the austenite to egin decomposing into ferrite and cementite? b. From the instant just after quenching, how much time did it take for the austenite to be completely decomposed into ferrite and cementite? c. What is the microstructure after 24 hours? Describe in terms of constituents present. 2. The sample is then reheated to 800°C and allowed sufficient time to reach equilibrium conditions, very rapidly quenched to 400°C, held for 35 seconds, then very rapidly quenched to room temperature. What is the resulting microstructure? Describe in terms of constituents present.

Explanation / Answer

a) Look at diagram. To begin the transformation from austenite to pearlite (ferrite and cementite) it takes around 1.1 s.

b) After 9 s holding at 600 C, austenite transforms completely into coarse pearlite.

c) After hiding for 24h, the microstructure will be of pearlite. It is a solid solution with alternate layers of alpha-ferrite and cementite(Fe3C). Its weight fraction can be found out using lever rule.

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