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Diffusion

Diffusion. Diffusion coefficient: flux J = D(T) dC/dx D(T) = D o exp (-Q/RT) . For C diffusing in Fe: D(T) = 0.21 exp (-33.8/RT) = 6.7 x 10 -11 cm 2 /sec @ 500° C = 4 x 10 -7 cm 2 /sec @ 1000° C. Diffusion. For C diffusing in Fe: D(T) = 0.21 exp (-33.8/RT)

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Diffusion

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  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 10-11 cm2/sec @ 500° C = 4 x 10-7 cm2/sec @ 1000° C

  2. Diffusion For C diffusing in Fe: D(T) = 0.21 exp (-33.8/RT) = 6.7 x 10-11 cm2/sec @ 500° C = 4 x 10-7 cm2/sec @ 1000° 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. The Fe-C system (austenite) (austenite) (ferrite) These are the solid solution regions

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

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

  9. The Fe-C system Pearlite

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

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

  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

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