Heat Treatment of Plain Carbon Steels
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
Temp time P B Pearlite Bainite Nose (marteniste) 727ºC xx xx x xx x xx x xx x xx MsMs MfMf
Temp time P B Pearlite Bainite (martensite) 727ºC MsMs MfMf coarse fine upper lower
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
Microstructures Coarse pearlite Fine pearlite
Microstructures Upper Bainite Lower Bainite
Microstructure Martensite
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
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)