The isothermal diagram for an iron-carbon alloy of eutectic composition is shown

Question

The isothermal diagram for an iron-carbon alloy of eutectic composition is shown in Figure 3. In each ease assume that the specimen begins at 760 degree C and that it has been held at this temperature long enough to has c achieved a complete and homogeneous austenitic structure. 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 (i) Cool rapidly to 500 degree C, hold for 4s, then quench to room temperature (ii) Cool rapidly to 160 degree C, hold at this temperature Specify a heal treatment (temperature(s) and time(s)) that would produce the following micTostructure. (i) 50% coarse pearline and 50% bainite (ii) 50% fine pearlite, 25% bainite, and 25 % martensite

Explanation / Answer

Final Microstructure of the following will be formed

(i) Rapid cooling to 500°C will form Bainite.

(ii) Rapid Cooling to 160 °C will leads into Martensite.

2.

    (i) 50 % of Bainite and 50% pearlite is  a plate-like microstructure that forms in steels by heating it at the temperatures of 250–550 °C (depending on alloy content).

(ii) 50% of fine perlite is formed on heating the steel at 750-850°C and then cooling to 450-550 °C. Thus Martensite is formed in carbon steels by the rapid cooling.

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