Question

Phase transformations and microstructural control in Fe-C alloy system (11 marks) 2. For the Fe-C alloy of eutectoid composition (shown below in the left graph): 800 1100 Austenite Eutectoid temperature 700 1000 y Fe,C 600 900 500 800 00 727 C 700 300 Mistart) 600 Fe C 50% 200 M(50 % ) M190 % ) Fe C 500 100 400 101 104 105 1.0 2.C 10- 10 1 10 Composition (wt % C) Time (s) a) Name the possible types of micro-constituents achieved by phase transformation from [1] austenite when subjected to various isothermal heat treatments. b) Mark these micro-constituen ts directly on the time-temperature-transformation (TTT) diagram (shown above in the right graph). [2] c) Rank microhard ness of these micro-constituents in increasing order. [1] d) Using the TTT diagram, specify the final microstructure (in terms of micro-constituents and approximate percentages of each) of a small specimen that has been subjected to the following time-temperature treatments. In each case, assume that the specimen begins at 760 °C and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. [5) i. Rapidly cool to 650 °C, hold for 20 s, then quench to room temperature. li, Rapidly cool to 350 °C, hold for 150 s, then quench to room temperature. ii. Rapidly cool to 600 °C, hold for 4 s, rapidly cool to 450 °C, hold for 10 s, then quench to room temperature. iv. Rapidly cool to 300 °C, hold for 10 s, then quench to room temperature v. Rapidly cool to 575 °C, hold for 20 s, rapidly cool to 350 °C, hold for 100 s, then quench to room temperature. e) Rank microhardness of the final microstructures achieved in d) in increasing order [2] ameadua Temperature ('C)

Answer 1

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