8. TTT and CCT diagrams of a eutectoid steel (0.76 C) are provided below. Use the diagrams to answer the following questions. Temperature (C) Temperature (C) 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 0 10-1 0 10-1 A A A M(start) M(50%) M(90%) 1 1 M(start) Austenite M(50%) M(90%) 10 10 M+A Eutectoid temperature P B 10 10 104 Time (s) Pearlite 50% Eutectoid temperature 10 Time (s) Continuous cooling transformation 10 104 T 1400 1200 1000 800 600 400 200 105 1400 1200 1000 800 600 400 200 105 Temperature (F) Temperature (F) (a) Estimate the minimum cooling rate (approx.) required to obtain 100% martensite at room temperature? Assume continuous cooling from eutectoid temperature (727C). (b) Describe the microstructure formed by austenizing at 7600C then rapidly cooling to 625C, holding at that temperature for 4 s and rapidly cooling to 450C, holding there for 100 s, then rapidly cooling to room temperature. (c) Describe the microstructure formed by austenizing at 760C then rapidly cooling to 350C, holding at that temperature for 100 s and rapidly cooling to room temperature. (d) For (a), (b) and (c) estimate the approximate the hardness of the final product using the diagrams shown. 0 15 65 600 60 500 50 400 45 Tempered martensite 40 (tempered at 371C) 300 30 200 20 100 0 200 700 600 500 400 300 200 100 0 0 3 Percent Fe3C 0.2 6 9 Martensite 12 Fine pearlite 0.4 0.6 Composition (wt% C) 0.8 1.0 300 Bainite Pearlite- 400 Transformation temperature (C) 500 600 700 800