Tempering of steel
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AND
W. C. LESLIE
T e m p e r i n g of m a r t e n s i t i c s t e e l s i n v o l v e s the s e g r e g a t i o n of c a r b o n , the p r e c i p i t a t i o n of c a r b i d e s , the d e c o m p o s i t i o n of r e t a i n e d a u s t e n i t e , and the r e c o v e r y and r e c r y s t a l l i z a t i o n of the m a r t e n s i t i c s t r u c t u r e . B e c a u s e t h e s e s e v e r a l r e a c t i o n s o v e r l a p and o c c u r on such a fine s c a l e , it i s only r e c e n t l y that our knowledge of the r e s u l t i n g s t r u c t u r e s h a s b e c o m e r e a s o n a b l y c o m p l e t e . O u r p r e s e n t u n d e r s t a n d i n g of the p r o c e s s e s involved in the t e m p e r ing of i r o n - c a r b o n m a r t e n s i t e s and how they a r e affected by a l l o y i n g e l e m e n t s i s r e v i e w e d .
T E M P E R I N G is the p r o c e s s of heating m a r t e n s i t i c s t e e l s to e l e v a t e d t e m p e r a t u r e s so that they b e c o m e m o r e d u c t i l e . It i n v o l v e s the s e g r e g a t i o n of c a r b o n to l a t t i c e defects and the p r e c i p i t a t i o n of c a r b i d e s , the d e c o m p o s i t i o n of r e t a i n e d a u s t e n i t e , and the r e c o v e r y and r e c r y s t a l l i z a t i o n of the m a r t e n s i t i c s t r u c t u r e . T h e s e s e v e r a l r e a c t i o n s often o v e r l a p and they o c c u r on a v e r y fine s c a l e , so it is only s i n c e the a d v e n t of t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y in 1956 that our knowledge of the s t r u c t u r e s of t e m p e r e d m a r t e n s i t e has b e c o m e r e a s o n a b l y c o m p l e t e . Each of the p r o c e s s e s involved m a y be affected by a l l o y i n g a d d i t i o n s so that the t e m p e r i n g b e h a v i o r of a l l o y s t e e l s b e c o m e s a f a s c i n a t i n g but c o m p l e x s u b j e c t . In addition, i n t e r a c t i o n of t r a c e e l e m e n t s with former austenite grain boundaries during tempering may promote embrittlement. Obviously, these subjects a r e too l a r g e to t r e a t in detail in this s h o r t r e v i e w . R a t h e r , we t r e a t the b a s i c t e m p e r i n g p r o c e s s e s in i r o n - c a r b o n m a r t e n s i t e s and then i l l u s t r a t e how t h e s e b a s i c p r o c e s s e s can b e a l t e r e d by a l l o y i n g e l e m e n t s . N. C. Bain was c l o s e l y i n t e r e s t e d in the t e m p e r i n g t e e l s d u r i n g his e n t i r e c a r e e r . He coauthored a on h i g h - s p e e d s t e e l s in 1931 with M. A. G r o s s m~ much of which was c o n c e r n e d with the s e c o n d a r y haru~ ring of h i g h - a l l o y s t e e l s . 1 In his f a m o u s book on a l l o y i n g e l e m e n t s in s t e e l s ~ he devoted a l a r g e s e c t i o n to d i s c u s s i n g the effect of a l l o y i n g e l e m e n t s on t e m p e r i n g . In h i s Eighth Hatfield M e m o r i a l L e c t u r e b e f o r e the I r o n and Steel
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