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 45-55 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 150- 500º C n 1st stage 150-200º C (low C martensite) + carbide (Fe 2.3 C) n 2nd stage 200-350º C bainite n 3rd stage 350-500º C + carbide Fe 3 C(tempered martensite)