Problem 3: (25 points) The isothermal diagram for an iron-carbon alloy of eutect

Question

Problem 3: (25 points) The isothermal diagram for an iron-carbon alloy of eutectic composition is shown in Figure 3. In each case assume that the specimen begins at 760°C and that it has been held at this temperature long enough to have achieved a complete and homogenous austenitic structure 1) Specify the nature of the final microstructure (in terms of microconstituients present and approximate percentage of each), if an iron-carbon alloy of eutecoid composition has been subjected to the time-temperature treatments of Cool rapidly to 500°C, hold for 4s, then quench to room temperature (6 points) Cool rapidly to 160°C, hold at this temperature (6 points) (i) (ii)

Explanation / Answer

1)

i) When we quench austenite to 500 degree celsius and hold there for 4s, about 40% of austenite has transformed to bainite. It has both cementite and ferrite phases present. On further quenching to room temperature, austenite will not transform to martensite. Martensite formation and bainte formation is competitive to each other. So bainte in supersaturated austenite is obtained.

ii) On rapid cooling to 160 degree celsius, it us the line corresponding to 50% martensite completion line. So a body centred tetragonal shape martensite is formed.

2) Cool rapudly to 600 degree celsius for 5s, Quench to 500 degree celsius and hold for 10ns at this temperature

ii) It is impossible. Formation of martensite and Bainite is competitive to each other.

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