(Solved): 19 Using the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition (F ...

19 Using the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition (Figure 10.23), specify the nature of the final microstructure (in terms of microconstituents present 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 \( 760^{\circ} \mathrm{C} \) \( \left(1400^{\circ} \mathrm{F}\right) \) and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. (a) Cool rapidly to \( 700^{\circ} \mathrm{C}\left(1290^{\circ} \mathrm{F}\right) \), hold for \( 10^{4} \mathrm{~s} \), then quench to room temperature. (b) Reheat the specimen in part (a) to \( 700^{\circ} \mathrm{C} \) \( \left(1290^{\circ} \mathrm{F}\right) \) for \( 20 \mathrm{~h} \). (c) Rapidly cool to \( 600^{\circ} \mathrm{C}\left(1110^{\circ} \mathrm{F}\right) \), hold for \( 4 \mathrm{~s} \), rapidly cool to \( 450^{\circ} \mathrm{C}\left(840^{\circ} \mathrm{F}\right) \), hold for \( 10 \mathrm{~s} \), then quench to room temperature. (d) Cool rapidly to \( 400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right) \), hold for \( 2 \mathrm{~s} \), then quench to room temperature. (e) Cool rapidly to \( 400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right) \), hold for \( 20 \mathrm{~s} \), then quench to room temperature. (f) Cool rapidly to \( 400^{\circ} \mathrm{C}\left(750^{\circ} \mathrm{F}\right) \), hold for \( 200 \mathrm{~s} \), then quench to room temperature. (g) Rapidly cool to \( 575^{\circ} \mathrm{C}\left(1065^{\circ} \mathrm{F}\right) \), hold for \( 20 \mathrm{~s} \), rapidly cool to \( 350^{\circ} \mathrm{C}\left(660^{\circ} \mathrm{F}\right) \), hold for \( 100 \mathrm{~s} \), then quench to room temperature. (h) Rapidly cool to \( 250^{\circ} \mathrm{C}\left(480^{\circ} \mathrm{F}\right) \), hold for \( 100 \mathrm{~s} \), then quench to room temperature in water. Reheat to \( 315^{\circ} \mathrm{C}\left(600^{\circ} \mathrm{F}\right) \) for \( 1 \mathrm{~h} \) and slowly cool to room temperature.