Estimate Brinell hardnesses and ductilities (%RA) for specimens of an iron–carbon alloy of...

Estimate Brinell hardnesses and ductilities (%RA) for specimens of an iron–carbon alloy of eutectoid composition that have been subjected to the heat treatments described in parts (a) through (h) of Problem 10.19.

Problem 10.19

Using the isothermal transformation diagram for an iron–carbon alloy of eutectoid composition (Figure 10.22), 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°C (1400°F) and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure.

(a) Cool rapidly to 350°C (660°F), hold for 103 s, then quench to room temperature.

(b) Rapidly cool to 625°C (1160°F), hold for 10 s, then quench to room temperature.

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

(d) Reheat the specimen in part (c) to 700°C (1290°F) for 20 h.

(e) Rapidly cool to 300°C (570°F), hold for 20 s, then quench to room temperature in water. Reheat to 425°C (800°F) for 103 s and slowly cool to room temperature.

(f) Cool rapidly to 665°C (1230°F), hold for 103 s, then quench to room temperature.

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

(h) Rapidly cool to 350°C (660°F), hold for 150 s, then quench to room temperature.

Figure 10.22 The complete isothermal transformation diagram for an iron–carbon alloy of eutectoid composition: A, austenite; B, bainite; M, martensite; P, pearlite.