Using the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition (Figure 10.5.12), 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 760C (1400F) and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure.

(a) Cool rapidly to 700C (1290F), hold for 104 s, then quench to room temperature.

(b) Reheat the specimen in the previous part to 700C (1290F) for 20 h.

(c) Rapidly cool to 600C (1110F), hold for 4 s, rapidly cool to 450C (840F), hold for 10 s, then quench to room temperature.

(d) Cool rapidly to 400C (750F), hold for 2 s, then quench to room temperature.

(e) Cool rapidly to 400C (750F), hold for 20 s, then quench to room temperature.

(f) Cool rapidly to 400C (750F), hold for 200 s, then quench to room temperature.

(g) Rapidly cool to 575C (1065F), hold for 20 s, rapidly cool to 350C (660F), hold for 100 s, then quench to room temperature.

(h) Rapidly cool to 250C (480F), hold for 100 s, then quench to room temperature in water. Reheat to 315C (600F) for 1 h and slowly cool to room temperature.

Figure 10.5.12: The complete isothermal transformation diagram for an iron-carbon alloy of eutectoid composition: A, austenite; B, bainite; M, martensite; P, pearlite. 800 -Eutectoid temperature 1400 700 A 1200 P 600 1000 500 Temperature (C) - 800 400 Temperature (F) A 300 600 M(start) 200 50% M + A 400 M(50%) M(90%) 100 - 200 0 10-1 1 10 102 103 104 105 Time (s)