Using the isothermal transformation diagram for an ironcarbo

Using the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition, specify the nature of the final microstructure (in terms of micro constituents 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 degree C (1400 degree F) and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. Rapidly cool to 400 degree C (750 degree F). hold for 500 s. then quench to room temperature. (b) Reheat the specimen in part to 700 degree C (1290 degree F) for 20 h. Cool rapidly to 665 degree C (1230 degree F), hold for 103 s, then quench to room temperature. Rapidly cool to 350 degree C (660 degree F), hold for 150 s, then quench to room temperature.

Solution

A: After cooling and holding at 400°C for 500 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

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 665°C for 103s, approximately 100% of the specimen has transformed
to pearlite

D: During the rapid cooling to 350°C no transformations occur. At 350°C we start
timing again at zero time; while holding at 350°C for 150 s, approximately 100 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.