The role of nickel, copper, and columbium (Niobium) in strengthening a low-carbon ferritic steel
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T H E N i - C u - C b s t e e l d e s c r i b e d by H u r l e y and Shelton 1 is a l o w - a l l o y p r e c i p i t a t i o n h a r d e n a b l e s t e e l capable of at t ai n in g a y i e l d s t r e n g t h in e x c e s s of 90 ksi in the r o l l e d - a n d - a g e d condition without a solution t r e a t m e n t . Such a p r e c i p i t a t i o n - h a r d e n a b l e s t e e l p r o vides fabrication advantages over quenched-and-temp e r e d s t e e l s at c o m p a r a b l e s t r e n g t h l e v e l s . To gain a b e t t e r u n d e r s t a n d i n g of the r e s p e c t i v e c o n t r i b u t i o n s of the a l l o y i n g e l e m e n t s in t h i s s t e e l , Ni, Cu, and Cb w e r e added s e p a r a t e l y to a l o w - c a r b o n - s t e e l b a s e c o m p o s i t i o n . The e f f e c t s of t h e s e a d d i ti o n s on the m i c r o s t r u c t u r e and the a s - r o l l e d and the r o l l e d - a n d - a g e d t e n s i l e and i m p a c t p r o p e r t i e s of the b a s e s t e e l w e r e investigated. EXPERIMENTAL PROCEDURE T h i r t y - p o u n d h e a ts w e r e induction m e l t e d in a i r in m a g n e s i a c r u c i b l e s . The c h a r g e s c o n s i s t e d of A r m c o i r o n , e l e c t r o l y t i c n i c k e l , c o p p e r , and f e r r o a l l o y s of Cb, Mn, and Si; f e r r o p h o s p h o r u s was added to a c h i e v e a t y p i cal p h o s p h o r u s l e v e l . All m e l t s w e r e d e o x i d i z e d i n i t i a l l y with s i l i c o n - m a n g a n e s e and killed with a l u m i num p r i o r to pouring. The 30-1b, 4 by 4-in. ingots w e r e soaked and f o r g e d to 1-in. s l a b s at 2250~ A two-pass rolling operation was used c o n s i s t i n g of a ~-m.~ " r e d u c t i o n at 2250~ a s e c o n d -~-m. i . r e d u c t i o n at 1500~ to -~-in. plate, and an a i r cool to r o o m t e m p e r a t u r e . Both r o l l i n g p a s s e s w e r e in the s a m e d i r e c t i o n at 90 deg to the f o r g i n g a x i s . The final p a s s at the f i n i s h i n g t e m p e r a t u r e was a 33% pct r ed u ct i o n in e v e r y c a s e . M i n o r edge c r a c k ing was o b s e r v e d in the p l a t e s of a ll c o m p o s i t i o n s c o n taining c o p p e r and no n i c k e l . H o w e v e r , the edge c r a c k s w e r e not e x t e n s i v e enough to r e n d e r the p l a t e s unu s ab l e, p r e s u m a b l y b e c a u s e of the s h o r t soaking t i m e s i n v o l v e d with the s m a l l l a b o r a t o r y h e a t s . Duplicate 0.252 in. diam t e n s i l e s p e c i m e n s and fulls i z e Charpy V - n o t c h i m p a c t s p e c i m e n s w e r e cut f r o m the p l a t e s p a r a l l e l to the r o l l i n g d i r e c t i o n . Aging, w h ere a p p l i e d , was f o r 1 h r at 1050~
studied as 30-1b h e a t s a r e shown in T a b l e I. Individual and c o m b i n e d a l l o y additions w e r e made to the l e v e l s found in N i - C u - C b s t e e l . Fig. 1 p r e s e n t s the a s - r o l l e d m i c r o s t r u c t u r e s of the eight s t e
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