Partition of manganese during the proeutectoid ferrite transformation in steel
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M O S T substitutional alloying elements r e t a r d the r a t e of d i f f u s i o n - c o n t r o l l e d t r a n s f o r m a t i o n s in s t e e l . The alloying effect on growth may be attributed to three factors: 1) The effect of the alloying element on the F e - C phase d i a g r a m . 2) T e r n a r y diffusion i n t e r a c t i o n s . 3) Diffusion " d r a g " due to the slowly-diffusing a l l o y ing element. Since manganese d e p r e s s e s the Ae3 line we may expect that this addition will produce a strong r e t a r d ing effect at a given t e m p e r a t u r e due to constitutional effects alone. F u r t h e r m o r e , since the proeutectoid r e a c t i o n involves r e j e c t i o n of both carbon and m a n g a nese f r o m the f e r r i t e and since the t e r n a r y interaction between manganese and carbon is a t t r a c t i v e , we may expect the positive manganese " s p i k e " at the interface to r e t a r d the flow of carbon away from the interface and thus a l s o r e t a r d the growth at c e r t a i n s t a g e s of the transformation. 1 A c c o r d i n g to the local equilibrium theory for growth into an infinite medium, there exist two r e g i m e s which a r e defined by the r e l a t i v e s u p e r s a t u r a t i o n . F o r high s u p e r s a t u r a t i o n , as indicated in Fig. 1, 2 the r a t e equations with DC >> DMn imply that there is no p a r t i t i o n of manganese between the austenite and the f e r r i t e so f a c t o r 3 is negligible in this r e g i m e . On the other hand, in the low s u p e r s a t u r a t i o n r e g i m e , p a r t i t i o n must o c cur and diffusion control by the slowly-diffusing e l e ment takes over. T h e r e is a distinct p o s s i b i l i t y that in both r e g i m e s for e a r l y times of t r a n s f o r m a t i o n and in both the inJ. B. GILMOUR is with the Corrosion Group, Physical Metallurgy, Mines Branch, Department of Energy, Mines, and Resources, Ottawa, Ontario, Canada. G. R. PURDY and J. S. KIRI~ALDY are Professor and Stelco Professor, respectively, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada. Manuscript submitted April 17, 1972. METALLURGICAL TRANSACTIONS
finite and finite boundary conditions (the l a t t e r will always be effectively infinite for e a r l y enough times) the local e q u i l i b r i u m conditions may fail. One p o s s i b l e solution in such c a s e s is the substitution of p a r a e q u i l i b r i u m z'3 (or the a p p r o x i m a t e l y equivalent
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730 *C.
w z z
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~3
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I 2 3 4 ATOMIC PERCENT CARBON Fig. 1--730~ isotherm for iron-rich alloys in the Fe-C-Mn alloy systems. 2 The right triangles with tie-lines on the hypotenuse approximately define the boundary (. . . . ) between partition and no-partition reactions.1 VOLUME 3, DECEMBER 1972-3213
" n o - p a r t i t i o n " e q u i l i b r i u m4) for local e q u i l i b r i u m c o n d i t i o n s . U n f o r t u n a t e l y , a f i r m c r i t e r i o n for such a mode of t
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