Kinetic studies on surface segregation of manganese during annealing of low-carbon steel
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H. E. BIBER,
E. J. OLES,
Jr., AND
C. J, WARNING
Surface s e g r e g a t i o n of manganese, an example of a s e l e c t i v e oxidation p r o c e s s , o c c u r s at annealing t e m p e r a t u r e s when l o w - c a r b o n s t e e l is in contact with typical c o m m e r c i a l p r o tective a t m o s p h e r e s . L a b o r a t o r y kinetic studies show that the e n r i c h m e n t p r o c e s s follows a p a r a b o l i c r a t e law, that the r a t e constant in the low range of oxygen potential for the annealing a t m o s p h e r e i n c r e a s e s l i n e a r l y with oxygen potential, and that although higher r a t e s apply for r e l a t i v e l y high-manganese s t e e l s than for r e l a t i v e l y low-manganese s t e e l s , the s a m e s u r f a c e - r e a c t i o n m e c h a n i s m applies. A r a t e e x p r e s s i o n was developed that i m p l i c i t l y includes oxygen potential and f r e e manganese (uncombined, in solid solution) content in the s t e e l .
P R E V I O U S L Y r e p o r t e d work 1,2 d e m o n s t r a t e d that s u r f a c e s e g r e g a t i o n of manganese, an example of a s e l e c t i v e oxidation p r o c e s s , o c c u r s at annealing t e m p e r a t u r e s when l o w - c a r b o n s t e e l is in contact with typical c o m m e r c i a l p r o t e c t i v e a t m o s p h e r e s . D i f f e r ences between the s u r f a c e and bulk c h e m i c a l composition for s t e e l s and other alloys have been r e c o g n i z e d as important in affecting c o r r o s i o n r e s i s t a n c e , a p p e a r ance, and other p r o p e r t i e s . The effect of these changes on the p r o p e r t i e s of sheet, s t r i p , and coated s t e e l p r o duct is being investigated and will be r e p o r t e d at another time. Elimination of manganese s u r f a c e s e g r e g a t i o n by annealing at 1350~ (732~ in a vacuum (10 -5 T o r r or 1.33 • 10 -3 Pa) or in a d r y (dew point
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