1. Chapter 10 Phase Transformations in 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 ~ 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. Max rate of transformation
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 )
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
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.