Question

X Incorrect. Using the Animated Figure 10.40, the isothermal transformation diagram for a 0.45 wt% steel alloy, specify the nature of the final microstructure in terms of the microconstituents present) of a small specimen that has been subjected to the following temperature treatments. In each case assume that the specimen begins at 845 °C and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. a) Rapidly cool to 250 degrees C, hold for 1000 s, then quench to room temperature. all spheroidite The Choices for part a &b A) proeutectoid ferrite + pearlite B) proeutectoid ferrite + martensite C) proeutectoid ferrite + pearlite + martensite D) proeutectoid ferrite + pearlite+bainite + martensite E) all spheroidite F) all bainite G) all martensite H)bainite + martensite b) Rapidly cool to 450 degrees C, hold for 10 s, then quench to room temperatur proeutectoid ferrite + martensite

Answer 1

a) From the isothermal curve it is clear that the Martensite start temperature is around 340°C (M start in fig) and Martensite end temperature is around 225°C. So if we do fast cooling to a temperature between the above, and hold for 1000 seconds and quench to room temperature, the resultant product is all Martensite.

b)From the isothermal curve we can see that if we rapidly cool to 450°C and hold for 10seconds and quench to room temperature, the result will be in between the C-C curves. The product is A+B. A+B represents mixture of austenite and bainite.