Chromium partitioning during the austenite-pearlite transformation

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f o r m of hot r o l l e d 12 mm d i a m b a r . The m a t e r i a l was homogenized for t h r e e days at 1200~ under a vacuum of < 10 -4 T o r r and then cut into d i s c s about 2 mm thick. These w e r e a u s t e n i t i z e d at 1000~ for 30 min in d r y argon, i s o t h e r m a l l y t r a n s f o r m e d in a lead bath for p r e d e t e r m i n e d t i m e s , water quenched, and p r e p a r e d for m e t a l l o g r a p h i c examination. P e a r l i t e growth r a t e s were m e a s u r e d using the maximum nodule r a d i u s method, while the minimum i n t e r I a m e l l a r spacing, Stain, was d e t e r m i n e d f r o m carbon r e p l i c a s in the e l e c t r o n m i c r o s c o p e . The e x p e r i m e n tal p r o c e d u r e s involved have been d e s c r i b e d p r e v i ously by Brown and Ridley. ~ Thin foils suitable for examination in the a n a l y t i c a l electron microscope were prepared from isothermally t r a n s f o r m e d s p e c i m e n s using s t a n d a r d techniques. The a n a l y t i c a l e l e c t r o n m i c r o s c o p e , EMMA-4, was used to monitor the chromium concentration in the f e r r i t e and cementite. A d e s c r i p t i o n of the i n s t r u m e n t and the techniques for c a r r y i n g out quantitative a n a l y s i s have been given by L o r i m e r and his a s s o c i a t e s . ~ The p r o c e d u r e used to study the p a r t i t i o n i n g of c h r o mium between f e r r i t e and e e m e n t i t e was s i m i l a r to that d e s c r i b e d p r e v i o u s l y by the authors for eutectoid s t e e l s containing manganese. ~ As in the previous work the i n t e n s i t i e s of X - r a y r a d i a t i o n from the alloy e l e ment p r e s e n t in the f e r r i t e and c e m e n t i t e w e r e m e a sured simultaneously, r e l a t i v e to the r e s p e c t i v e iron concentrations of the two p h a s e s using two c r y s t a l s p e c t r o m e t e r s . The p a r t i t i o n coefficient for c h r o m i um, K c e m ( c r ) , was r e l a t e d to the e x p e r i m e n t a l p r o c e d u r e used to study p a r t i t i o n i n g and was a r b i t r a r i l y defined as r~, eern ic r cem cem

K s

(Cr)=

\"C~'e ! (Ccr ~ CFe J

-

9

[11

( 'c--~r) c~ IFe

where C~r and C~ e a r e weight f r a c t i o n s of chromium in cementite and f e r r i t e r e s p e c t i v e l y , and I c ,r and I F e a r e the c o r r e s p o n d i n g K~ X - r a y i n t e n s i t i e s . - Because *Lorimerandhis associateshaveshownthat absorptionand fluorescenceof Fe andCr K~radiationcan be neglectedin a foilthinenoughto carry'out transmissionmicroscopyat 100 kV~'~. VOLUME 7A, FEBRUARY 1976-209

740i

0/

,=9*/.c~

111

/O

720

,/~

700

~ 680

0,.0''0

F i g . 1--Isothermal transformation diagram for pearlite formation in 1.29 pct Cr eutectoid steel.

/

i-

0.0.-'r

/

660

l

/

START 640

..........L

9. . . . . .

END

,.1

101

, o~

iO2

iO3

I0S

~O4

TIME I SECS.I

cementite contains ~6.67 wt pet carbon and, as a consequence less iron than does ferrite, the partiti