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16 Suppose that a steel of eutectoid composition is cooled to 550°C (1020°F) from 760°C (1400°F) in less than 0.5 s and held at this temperature. (a)How long will it take for the austenite-to-pearlite reaction to go to 50% completion? To 100% completion? (b)Estimate the hardness of the alloy that has completely transformed to pearlite.

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Answer:

(a) 2.5 and 6 seconds

(b) 265 HB (27 HRC)

Explanation:

(a) A horizontal line at 550°C intersects the 50% and reaction completion curves at  about 2.5 and 6 seconds, respectively; these are the times it will take for the austenite-to-pearlite reaction to go to 50% completion

(b) The pearlite formed will be fine pearlite. From Figure 10.30a, the hardness of an alloy of composition  0.76 wt% C that consists of fine pearlite is about 265 HB (27 HRC).