Chapter 10 Phase Transformations in Metals (2)

1. Chapter 10Phase Transformationsin Metals (2) Development of Microstructure and Alteration of Mechanical Properties

2. 10.5 Isothermal Transformation Diagrams • Bainite • Consists of ferrite and cementite phase in which elongated particles of Fe3C are in ferrite matrix. • Form only when cooling down to < 540oC, above 540oC only pearlite phase. • See Fig. 10.7 ~ Fig. 10.9 • The formation of Bainite obey kinetic equation:

3. 10.5 Isothermal Transformation Diagrams For 1.13 wt% C iron-carbon steel

4. Formation of Bainite Nose, Max rate of transformation

5. Formation of Bainite

6. Formation of Bainite • 200-300oC: lower Bainite • 300-540oC: upper Bainite • Above 540oC, transformation will be pearlite. • There is max. rate of transformation at 540oC, denoted as “N” point. • Pearlitic and bainitic transformation are competitive with each other. Once form, can not transform to the other phase.

7. 10.5 Isothermal Transformation Diagrams • Spheroidite • If pearlite or bainite is heated to temperature below eutectoid temperature but keep for a long time. • Fe3C phase become sphere-like • Diffusion of Carbon but no change in composition nor amount of phases. (ferrite, cementite) • Driving force for this transformation is the reduction in α-Fe3C phase boundary area.

8. 10.5 Isothermal Transformation Diagrams Which is the more stable microstructure ? The pearlite or the spheroidite

9. 10.5 Isothermal Transformation Diagrams • Martensite • Is a nonequilibrium single-phase structure that results from a diffusionless transformation of austenite after rapid cooling to room temperature. • F.C.C. austenite change to a B.C.T. (body-centered tetragonal) martensite. • Phase change instantaneously, time-independent process.

10. 10.5 Isothermal Transformation Diagrams • Martensite • The cooling temperature must sufficiently low in order to prevent carbon diffusion. • Martensite transformation start at certain low temperature range, the transformation is termed athermal transformation, i.e. temperature-dependent transformation, more lower temperature, more martensite forms. (M(start), M(50%), M(90%) in Fig. 10.13) • As long as the temperature is maintained, there will be no further martensite transformation.

11. B.C.T. structure

12. Martensite Microstructure

13. athermal transformation

14. Martensite Transformation • The presence of alloying elements (e.g., Cr, Ni, Mo, and W) • Shifting to longer times the nose of the austenite-to pearlite transformation) • The formation of a separate bainite nose. See Fig. 10.14 • Steel contains only carbon is called plain carbon steel. • Steel contains other elements than carbon is called alloy steel.

15. Martensite Transformation

16. Ex 10.2 • Specify the microstructure upon cooling • Rapidly cool to 350oC, hold for 104s, and quench to room temperature. • Rapidly cool to 250oC, hold for 100s, and quench to room temperature. • Rapidly cool to 650oC, hold for 20s, rapidly cool to 400oC, hold for 103 s, and quench to room temperature.

17. Ex 10.2

18. End of section (2) • You have learned: • Formation of Bainite phase. • Formation of Martensite phase. • Isothermal phase transformation • Athermal phase transformation • Cooling curve effect on phase transformation.