Contactless measurement of recovery and recrystallization in steel
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Contactless Measurement of
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Recovery and Recrystallization
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A o 5,0 Z LO
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in Steel
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o JOHN A. DiCELLO AND B. D. CULLITY
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T H E m e c h a n i c a l and m a g n e t i c p r o p e r t i e s of i r o n a r e closely r e l a t e d . In p a r t i c u l a r , cold w o r k i n g an a n n e a l e d s a m p l e i n c r e a s e s the m e c h a n i c a l h a r d n e s s and dec r e a s e s the m a g n e t i c p e r m e a b i l i t y ; c o n v e r s e l y , r e c r y s t a l l i z a t i o n of the c o l d - w o r k e d s a m p l e will r e t u r n these p r o p e r t i e s to t h e i r o r i g i n a l l e v e l s . At any given level of applied field Ha the l a r g e r the p e r m e a b i l i t y , the l a r g e r is the m a g n e t i z a t i o n and the e x t e r n a l field Hex of the s p e c i m e n at any point e x t e r n a l to the s p e c i m e n . The c u r v e s of Fig. 1 show this effect for a v a r i e t y of applied f i e l d s . It follows that any m e t a l l u r g i c a l p r o c e s s that changes the p e r m e a b i l i t y can be continuously m o n i t o r e d by m e a s u r i n g the e x t e r n a l field of the s p e c i m e n . T h i s study i s d i r e c t e d at r e c o v e r y and r e c r y s tallization. L o w - c a r b o n a l u m i n u m - k i l l e d sheet s t e e l s p e c i m e n s , 1 by 3 by 0.03 in. (2.5 by 7.6 by 9.1 x 10 "z cm), cold w o r k e d 62 pct by r o l l i n g , w e r e a n n e a l e d in a n o n i n d u c t i v e l y wound tube f u r n a c e . A w a t e r - c o o l e d solenoid s u r r o u n d i n g the f u r n a c e supplied a dc m a g n e t i c field, Fig. 2. The e x t e r n a l field of the s p e c i m e n was m e a s u r e d with a H a l l - e f f e c t g a u s s m e t e r ; the Hall p r o b e was m o u n t e d outside the solenoid 2 in. (5 cm) above one end of the s p e c i m e n , with the plane of the probe p a r a l l e l to the p l a n e of the s p e c i m e n . The p r o b e m e a s u r e s the total field it is exposed to, n a m e l y , the s u m of the e a r t h ' s field, the field due to the solenoid, and the field due to the s p e c i m e n . I m m e d i a t e l y a f t e r the s p e c i m e n was put in the f u r n a c e , the g a u s s m e t e r was z e r o e d . The r e c o r d e d g a u s s m e t e r output i s then LXHex, which is the change in e x t e r n a l field due only to changes in the p e r m e a b i l i t y of the s p e c i m e n . During r e c r y s t a l l i z a t i o n , c o l d - w o r k e d l o w - p e r m e a b i l i t y m a t e r i a l is r e p l a c e d by s t r e s s - f r e e h i g h - p e r m e a b i l i t y g r a i n s . Mtex t h e r e f o r e i n c r e a s e s with t i m e and l e v e l s off at a c o n s t a n t value when r e c r y s t a l l i z a t i o n is c o m p l e t e , Fig. 3. Also shown t h e r e i s the Rockwell 3 0 - T h a r d n e s s of s p e c i m e n s f r o m the s a m e sheet a n n e a l e d at 650~ for v a r i o u s t i m e s . Both c u r v e s begin to l e v e
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