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Heat Treatment of Plain Carbon Steels. Isothermal Temperature-time Transformation Diagram n Let us do an experiment n Choose an eutectoid steel n Heat.

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Presentation on theme: "Heat Treatment of Plain Carbon Steels. Isothermal Temperature-time Transformation Diagram n Let us do an experiment n Choose an eutectoid steel n Heat."— Presentation transcript:

1 Heat Treatment of Plain Carbon Steels

2 Isothermal Temperature-time Transformation Diagram n Let us do an experiment n Choose an eutectoid steel n Heat it to the austenite phase region (800˚ C) n Quench to 727 ˚ C Observe transformation + Fe 3 C n Note time when 1% pearlite has formed n Note time when 99% pearlite has formed n Plot on Temp.-time transformation diagram

3 Temp time P B Pearlite Bainite Nose (marteniste) 727ºC xx xx x xx x xx x xx x xx MsMs MfMf

4 Temp time P B Pearlite Bainite (martensite) 727ºC MsMs MfMf coarse fine upper lower

5 Properties of phases Properties of pearlite Fairly tough Fairly ductile Depends on size of lamellae Properties of bainite Tough Structure between pearlite and marteniste ( + Fe 3 C) Properties of marteniste Very hard HRC Brittle bc tetragonal structure Supersaturated solution of C in Fe High residual stresses Metastable

6 Microstructures Coarse pearlite Fine pearlite

7 Microstructures Upper Bainite Lower Bainite

8 Microstructure Martensite

9 Continuous cooling transformation diagram Temp P B Pearlite Bainite (martensite): quenching (100º C/min) in oil or water 727ºC MsMs MfMf Coarse:annealing (1ºC/min) in furnace Fine:normalizing(10º C/min) I air upper lower time isothermal

10 Tempering of martensitic steels n Martensite is too brittle to serve engineering purpose n Tempering carried out to increase toughness of martensite n Carried out by reheating martenistic steel between º C n 1st stage º C (low C martensite) + carbide (Fe 2.3 C) n 2nd stage º C bainite n 3rd stage º C + carbide Fe 3 C(tempered martensite)


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