2. Using the time-temperature-transformation (TTT) plot for an iron-carbon alloy

Question

2. Using the time-temperature-transformation (TTT) plot for an iron-carbon alloy of eutectoid composition describe the microstructure evolution and final microstructure composition if you begin with the alloy heated at 760C. The possible microstructures are: martensite, pearlite, bainite, and spheroidite. (Lecture 15, Callister 10.5) (a) Cool rapidly to 700 °C, hold for 10°s, then quench to room temperature (b) Reheat the specimen in part (a) to 700 "C for 20 h. (c) Rapidly cool to 60°c, hold for 4 s, rapidly cool to 450 %, hold for 10s,then quench to room (d) Cool rapidly to 400 °C, hold for 2 s, then quench to room temperature. (e) Cool rapidly to 400 C, hold for 200 s, then quench to room temperature. (f) Rapidly cool to 575 "C, hold for 20 s, rapidly cool to 350 "C, hold for 100 s, then quench to room 800 Eutectoid temperature 700 600 500 300 200 100 10 Time

Explanation / Answer

a) After cooling and holding at 700°C for 10^4 s, approximately 50% of the specimen has transformed to coarse pearlite. Upon cooling to room temperature, the remaining 50% transforms to martensite. Hence, the final microstructure consists of about 50% coarse pearlite and 50% martensite.

b)Heating to 700°C for 20 h the specimen in part (a) will transform the coarse pearlite and martensite to spheroidite.

c)After cooling to and holding at 600°C for 4 s, approximately 50% of the specimen has transformed to pearlite (medium). During the rapid cooling to 450°C no transformations occur. At 450°C we start timing again at zero time; while holding at 450°C for 10 s, approximately 50 percent of the remaining unreacted 50% (or 25% of the original specimen) will transform to bainite. And upon cooling to room temperature, the remaining 25% of the original specimen transforms to martensite. Hence, the final microstructure consists of about 50% pearlite (medium), 25% bainite, and 25% martensite.

d)After cooling to and holding at 400°C for 2 s, no of the transformation begin lines have been crossed, and therefore, the specimen is 100% austenite. Upon cooling rapidly to room temperature, all of the specimen transforms to martensite, such that the final microstructure is 100% martensite.

e)After cooling and holding at 400°C for 200 s, the entire specimen has transformed to bainite. Therefore, during the cooling to room temperature no additional transformations will occur. Hence, the final microstructure consists of 100% bainite.

f)After cooling and holding at 575°C for 20 s, the entire specimen has transformed to fine pearlite. Therefore, during the second heat treatment at 350°C no additional transformations will occur. Hence, the final microstructure consists of 100% fine pearlite.

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