Using the Animated Figure 10.39, the isothermal transformation diagram for a 1.1
ID: 105892 • Letter: U
Question
Using the Animated Figure 10.39, the isothermal transformation diagram for a 1.13 wt% C steel alloy, determine 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 920 °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 700 degrees C, hold for 100,000 s, then quench to room temperature.
- Proeutectoid cementite + fine pearlite
- Proeutectoid ferrite + martensite
- Proeutectoid cementite + pearlite + martensite
- Cementite + medium pearlite + bainite + martensite
- All spherodite
- All bainite
- All martensite
- Bainite + martensite
b) Rapidly cool to 600 degrees C, hold for 7 s, rapidly cool to 450 degrees C, hold for 4 s, then quench to room temperature.
- Proeutectoid ferrite + pearlite
- Proeutectoid cementite + martensite
- Proeutectoid ferrite + pearlite + martensite
- Proeutectoid ferrite + pearlite + bainite + martensite
- All spherodite
- All bainite
- All martensite
- Bainite + martensite
900 800 600 500 400 300 200 M (start) (50%) 100 (90% 10 102 109 Time (s 50% 101 105 1600 1400 1200 1000 800 600 400 r 200 106Explanation / Answer
a) Rapidly cool to 700 degrees C, hold for 100,000 s, then quench to room temperature.
Proeutectoid cementite + pearlite + martensite Since Pearlite is formed during sufficiently slow cooling in an iron-carbon system (723 °C, eutectoid temperature) and Martensite is formed in carbon steels by the rapid cooling (quenching) of the austenite form of iron at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite
b) Rapidly cool to 600 degrees C, hold for 7 s, rapidly cool to 450 degrees C, hold for 4 s, then quench to room temperature.
Bainite + martensite Because Bainite is a plate-like microstructure that forms in steels at temperatures of 250–550 °C and some part of Martensite are present since rapid cooling is evolved.
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