2. Using the isothermal transformation diagram for an iron-carbon alloy of eutec

Question

2. Using the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition, specify the nature of the final microstructure (in terms of the microconstituents present, e.g. A+B) of a small specimen that has been subjected to the following time- temperature treatment. In each case assume that the specimen begins at 760 °C and that is has been held at this temperature long enough to have achieved a complete and homogeneous austenite structure. 800 Eutectoid temperature 1400 700 1200 1000 Temperature (°C) 8 8 3 3 800 Temperature (°F) 600 M(start) 200 50% M + A 400 M(50%) M(90%) 100 200 10-1 1 10 102 103 104 105 Time (s) Figure 11.23 o John Wiley & Sons, Inc. All rights reserved. (a) Cool rapidly to 350 °C, hold for 103 s, then quench to room temperature (b) Cool rapidly to 665 °C, hold for 108 s, then quench to room temperature

Explanation / Answer

a) Cooling rapidly to 350 oC and holding it for 103 s, all of the specimen has transformed to Bainite. Hence, quenching to room temperature will have the the final microstructure consists of 100% Bainite.

b) After cooling to and holding at 665°C for 103 s, all of the specimen has transformed to Proeutectoid Ferrite. Hence, quenching to room temperature will have the the final microstructure consists of 100% Proeutectoid Ferrite.

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