Embrittling characteristics of Ni-Cr-Mo-V steels designed to exhibit low susceptibility to temper embrittlement in large
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g e n e r a l l y a c k n o w l e d g e d that t r a c e e l e m e n t s such a s p h o s p h o r u s , tin, and a n t i m o n y , c a u s e t e m p e r e m b r i t t l e m e n t in a l l o y s t e e l s in conjunction with such m a j o r a l l o y i n g e l e m e n t s a s nickel, m a n g a n e s e , or c h r o m i u m . I ~ T h e e m b r i t t l e m e n t s u s c e p t i b i l i t y of s t e e l d e p e n d s on the m a j o r a l l o y c o m p o s i t i o n a s w e l l as its trace-element contents. Therefore, composit i o n a l d e s i g n is i m p o r t a n t for s t e e l s for which a d e quate e m b r i t t l e m e n t r e s i s t a n c e is r e q u i r e d a s with l o w - p r e s s u r e (LP) t u r b i n e - r o t o r s t e e l s . T h e 3.5 p c t N i - C r - M o - V s t e e l which i s c u r r e n t l y u s e d f o r L19 t u r bine and g e n e r a t o r r o t o r s shows r e l a t i v e l y high s u s c e p t i b i l i t y to e m b r i t t l e m e n t , 4"~ although i t s high h a r d e n a b i l i t y and good o v e r a l l t o u g h n e s s p r o v i d e r e l i a b i l i t y a s a m a t e r i a l for l a r g e s e c t i o n s i z e s . Shaw 7,8 i n v e s t i g a t e d s t a t i s t i c a l l y the effect of a l l o y ing e l e m e n t s on t e m p e r e m b r i t t l e m e n t in r o t o r s t e e l s . It was found that, b e c a u s e of a l a r g e n e g a t i v e i n t e r a c t i o n t e r m f o r [Cr][Mo], t e m p e r e m b r i t t l e m e n t d e c r e a s e d on i n c r e a s i n g both c h r o m i u m and m o l y b d e n u m in the r a n g e of c h r o m i u m h i g h e r than 2.2 pct and m o l y b d e n u m h i g h e r than 0.6 pct. A s a r e s u l t , he a n t i c i p a t e d a r e g i o n in the high c h r o m i u m and high molybdenum ranges where temper-embrittlement s u s c e p t i b i l i t y is s i g n i f i c a n t l y low. F e w o t h e r i n v e s t i g a t i o n s have b e e n conducted on the t e m p e r e m b r i t t l e m e n t of s t e e l s of t h i s c o m p o s i t i o n . The p r e s e n t i n v e s t i g a t i o n was p l a n n e d to c o n f i r m and extend S h a w ' s r e s u l t s . The effect of m o l y b d e n u m on the e m b r i t t l e m e n t was d e t e r m i n e d with t e s t s t e e l s of b a s e c o m p o s i t i o n s of 0.25 pct C, 1.0 p c t Ni, 3 o r 4 p c t C r , 0.12 p c t V with 0.8 to 2.0 p c t Mo, heat t r e a t e d to 825 M19a (120 ksi) m i n i m u m 0.2 p c t offset y i e l d s t r e n g t h . B e s i d e s the v a r i a t i o n s in m o l y b d e n u m and c h r o m i u m contents, t r a c e - e l e m e n t c o n t e n t s w e r e a l s o i n v e s t i g a t e d at two l e v e l s to show t h e i r r o l e on t e m per embrittlement.
heat was c a s t a s f o u r ingots with a d d i t i o n of d i f f e r e n t l e v e l s of m o l y b d e n u m in the r a n g e f r o m 0.8 to 2 p c t . The c h e m i c a l c o m p o s i t i o n of the 1 N i - C r - M o - V s t e e l s a r e shown in T a b l e I. C h r o m i u m content was i n v e s t i gat
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