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Heat Treatment of Plain Carbon Steels

Heat Treatment of Plain Carbon Steels. Isothermal Temperature-time Transformation Diagram. Let us do an experiment Choose an eutectoid steel Heat it to the austenite phase region (800˚ C) Quench to 727 ˚ C Observe transformation g a + Fe 3 C Note time when 1% pearlite has formed

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Heat Treatment of Plain Carbon Steels

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  1. Heat Treatment of Plain Carbon Steels

  2. Isothermal Temperature-time Transformation Diagram • Let us do an experiment • Choose an eutectoid steel • Heat it to the austenite phase region (800˚ C) • Quench to 727 ˚ C • Observe transformation ga + Fe3C • Note time when 1% pearlite has formed • Note time when 99% pearlite has formed • Plot on Temp.-time transformation diagram

  3. 727ºC x x x x Pearlite Temp g+P x x x x g x x Bainite Nose g+B x x x x x x Ms g+a’ Mf a’(marteniste) time

  4. 727ºC coarse Temp g+P Pearlite fine g upper g+B Bainite lower Ms g+a’ Mf a’(martensite) time

  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 (a + Fe3C) • 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 727ºC Coarse:annealing (1ºC/min) in furnace Temp g+P Pearlite g Fine:normalizing(10º C/min) I air upper g+B Bainite isothermal lower Ms g+a’ Mf a’(martensite):quenching (100º C/min) in oil or water time

  10. Tempering of martensitic steels • Martensite is too brittle to serve engineering purpose • Tempering carried out to increase toughness of martensite • Carried out by reheating martenistic steel between 150-500º C • 1st stage 150-200º C a’ a”(low C martensite) + e carbide (Fe2.3C) • 2nd stage 200-350º C g bainite • 3rd stage 350-500º C a”+ e carbide a + Fe3C(tempered martensite)

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