TTT diagram of eutectoid steel and martensitic transformation

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1. T-T-T Diagram of eutectoid steel and Martensitic transformation Ordered and Disordered transformation

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3. T-T-T Diagram T-T-T diagram is plotted for different length of times and temperature These curves gives us the information of transformation for different phases With the help of T-T-T diagram we can control the phase transformation by changing the conditions

4. Regions of T-T-T diagram The left side of C-shape curve is Austenitic region The right side of C-shape curve contain three regions Pearlite Pearlite+Bainite Bainite

5. Cont.. The left line of C-shape curve shows the start of phase transformation The right line shows the end of transformation The dotted line is theoretically determined line shows 50% transformat ion

6. Cont… The region below M s is martensitic region Ms shows start and M f shows end of Martensite transformation

7. Martensite Martensitic crystal structure is Body centered tetragonal (BCT) It is the intermediate structure of Fcc and Bcc It forms due to the realignment of atoms from austenitic structure to martensitic structure

8. Continue… Atoms arrange themselves to move towards stability from higher temperature to lower temperature Only realignment is occur but there is no diffusion There is no concentration change in parent and product phase

9. Continue… Due to the realignment of atoms the lattice smmetry is different from the parent phase

10. Martensitic transformation At 300 o c the bainite and martensite are formed along curve 3 As temperature decreases the bainite formation rate is lowers At 160 o C for 20min the 50% martensite and austenite are formed

11. Continue… M f line which shows the end of martensite transformation But 100% martensite can’t be formed even theoretically at absolute zero At room temperature there is 3% retaine austenite

12. Continue… The reason is the retained autenite As the little amount of austanite is remain it is difficult to transform That’s why M f lline is showed dotted

13. Order Regular arrangements of atoms in a crystal is called order. Order of atoms in crystals are mostly at room temperature

14. Disorder Irregular arrangements of atoms in a crystal is called disorder . Disorder mostly occurs at elevated temperature.

15. Order and disorder determination In most substitutional solid solutions two kinds of atoms A and B are arranged more or less random on the atomic sites of the lattice. In solution of this type of a change in temperature is to Increase or decrease of a thermal vibrations. If the ordered solution is heated above T c the atomic arrangement becomes more or less random again and the solution is said to be disordered.

16. Continue… The gold and copper atoms of AuCu 3 above a critical temperature of about 390’C are arranged more or less random on the atomistic sites of FCC lattice as shown in following figure. Below the critical temperature the gold atoms in a perfectly ordered alloy occupy only the corner position of the unit cube and the copper atoms on the face centered cubic as shown in following figure.

17. Continue..

18. Long range ordered When the periodic arrangement of A and B atoms persists over longer range distance in a crystal than it is called a long range order.

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