The effect of molybdenum on high-temperature properties of 9 pct Cr steels
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H E A T - r e s i s t a n t 9 C r - 1 Mo s t e e l (ASTM S t a n d a r d S p e c i f i c a t i o n A 213-76a, G r a d e T9) is used in the United States for r e h e a t e r and s u p e r h e a t e r tubing in s t e a m - g e n e r a t i n g u n i t s for i m p r o v e d oxidation r e s i s t a n c e c o m p a r e d with l o w e r - c h r o m i u m s t e e l s such as 2 - 1 / 4 C r - 1 Mo. The f o r m e r s t e e l does not, however, offer much advantage in r u p t u r e s t r e n g t h , which is s i m i l a r to that of 2 - 1 / 4 C r - 1 Mo s t e e l a c c o r d i n g to s t a t i s t i c a l data p u b l i s h e d by S m i t h 1 and by the I n t e r n a t i o n a l O r g a n i z a t i o n for S t a n d a r d i z a t i o n2 (Fig. 1). An i m p r o v e m e n t in r u p t u r e s t r e n g t h of the 9 C r s t e e l was a t t a i n e d by Yukitoshi e t al 3 by i n c r e a s i n g the m o l y b d e n u m content to 2 pct (Fig. 1). A f u r t h e r i n c r e a s e in r u p t u r e s t r e n g t h was shown by G e m m i l l e t al a for a s t e e l with 8.4 pct C r , 3 pct Mo, and 1 pct T i (Fig. 1). Although the i m p r o v e m e n t in r u p t u r e s t r e n g t h of G e m m i l l ' s s t e e l may not be c a u s e d only by the i n c r e a s e in m o l y b d e n u m , b e c a u s e it c o n t a i n s also t i t a n i u m , a p o s i t i v e effect of m o l y b d e n u m at c o n c e n t r a t i o n s above 2 pct on r u p t u r e s t r e n g t h is v e r y probable. The b e n e f i c i a l effect of m o l y b d e n u m a d d i t i o n s on c r e e p - r u p t u r e s t r e n g t h is not the only a t t r a c t i v e f e a t u r e of the h i g h e r - m o l y b d e n u m s t e e l s . A n i n c r e a s e in m o l y b d e n u m , e s p e c i a l l y when c o m b i n e d with a d e c r e a s e in c a r b o n , will i n t r o d u c e d e l t a f e r r i t e in the m i c r o s t r u c t u r e of the s t e e l and thus i m p r o v e w e l d a b i l i t y in c o m p a r i s o n with the fully m a r t e n s i t i c , a i r h a r d e n i n g 9 C r - 1 Mo s t e e l . B a s e d on the p r o m i s i n g p r o p e r t i e s of the h i g h e r m o l y b d e n u m s t e e l s with 9 pct Cr, the effect of 2 and 3 pct Mo on m e c h a n i c a l and p r o c e s s i n g p r o p e r t i e s was e x p l o r e d in the p r e s e n t study. The a m o u n t of d e l t a f e r r i t e was i n t e n d e d to be l i m i t e d to about 25 vol pct. F o r this p u r p o s e 1 pct Ni was added to the 9 C r - 3 Mo s t e e l . The c a r b o n content in the e x p e r i m e n t a l s t e e l s was r e d u c e d to 0.06 pct, as c o m p a r e d to 0.14 pct C in the c o m m e r c i a l 9 C r - 1 Mo s t e e l , to r e d u c e the h a r d P. J. GROBNER and W. C. HAGEL are Senior Staff Metallurgist, and Manager of Hagh Temperature MaterialsDevelopment, respectively, Climax Molybdenum Company of Miclugan, Ann Arbor, MI 48106. Manuscript submitted March 26, 1979. METALLURGICALTRANSACTIONSA
n e s s of the m a r t e n s i t i c g r a i n s and thus i m p r o v
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