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Unformatted text preview: 1 Queens University KINGSTON, ONTARIO, CANADA. MECH 270 MECH 270 Material Science and Engineering Material Science and Engineering 17. Phase Transformations 17. Phase Transformations and Properties of Steels and Properties of Steels Chapter 10 Chapter 10 Rick Holt Rick Holt Last Lecture Carbon steels exhibit a C shaped TTT curve when cooled from the range For the eutectoid composition, the upper part of the curve (where the rate increases with increased undercooling) is the austenite to pearlite transformation For hyper and hypo eutectoid alloys, transformation occurs to + or +Fe 3 C above the eutectoid temperature At higher undercooling (lower temperatures) where decreased diffusion rates slow the transformation, bainite, is formed. The Fe 3 C phase is not continuous. If the temperature is reduced to below the martensite start temperature Ms, martensite is formed, the amount depending upon the lowest temperature achieved The Fe 3 C can be spheroidized, by heating below the eutectoid temperature for long times. The resultant /Fe 3 C mixtue is spheroidite 2 Eutectoid composition, C o = 0.76 wt% C Begin at T > 727C Adapted from Fig. 10.14, Callister 7e . (Fig. 10.14 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams , American Society for Metals, 1997, p. 28.) Effect of Cooling History in Fe-C System 400 500 600 700 % p e a r l i t e 1 % 5 % Austenite (stable) T E (727 C) Austenite (unstable) T (C) 1 1 1 2 10 3 10 4 10 5 time (s) High Transformation temperature = coarse pearlite: low transformation tempearture = fine pearlite Non-Equilibrium Transformation Products: Fe-C Bainite:-- lathes (strips) with long rods of Fe 3 C-- Fe3C not continuous as in Pearlite-- diffusion controlled.-- non equilibrium (supersaturated in C forms, then the Fe 3 C precipitates Adapted from Fig. 10.18, Callister 7e ....
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