1. Diffusion
Diffusion coefficient:
flux J = D(T) dC/dx D(T) = Do exp (-Q/RT)
For C diffusing in Fe:
D(T) = 0.21 exp (-33.8/RT)
= 6.7 x ° C
= 4 x ° C

2. Diffusion
For C diffusing in Fe:
D(T) = 0.21 exp (-33.8/RT)
= 6.7 x ° C
= 4 x ° C
Ex - carburizing a steel shaft:
how long to carburize to depth of 1 mm
x2 = D t
D(1000° C) = 3 x 10-7 cm2/sec t = 10 hr
D(1200° C) = 1.8 x 10-6 cm2/sec t = 1.5 hr

3. The Fe-C system
-iron
-iron
-iron

4. The Fe-C system
-iron - dissolves considerable carbon
-iron - dissolves very little carbon

5. The Fe-C system
C forms interstitial solid solutions in Fe

6. These are the solid solution regions
The Fe-C system
These are the solid solution regions
(austenite)
(ferrite)
(austenite)

7. There is also a compound Fe3C (cementite)
The Fe-C system
There is also a compound Fe3C (cementite)
(austenite)
(ferrite)

8. The Fe-C system
The two-phase regions

9. The Fe-C system
Pearlite

10. Pearlite forms by diffusion - takes time
The Fe-C system
Pearlite is two phases
FE + CM
Pearlite forms by diffusion - takes time
See time-temperature-transformation (TTT) curves
diffusion rate high,
Grxn low
diffusion rate low,
Grxn high

11. The Fe-C system
Hypoeutectoid steels - cooling from AUS range
Proeutectoid FE
starts to form

12. The Fe-C system
Hypoeutectoid steels - cooling from AUS range
Eutectoid (P) forms

13. The Fe-C system
Hypoeutectoid steels - cooling from AUS range
Proeutectoid FE + P