Fracture toughness and stress corrosion characteristics of a high strength maraging steel
- PDF / 818,383 Bytes
- 6 Pages / 569 x 813.28 pts Page_size
- 14 Downloads / 206 Views
The effect of heat t r e a t m e n t on the t e n s i l e , f r a c t u r e toughness, and s t r e s s c o r r o s i o n p r o p e r t i e s of a high s t r e n g t h m a r a g i n g steel (nominal c o m p o s i t i o n 16.3 Ni-12.87 Co-4.98 Mo-0.78 Ti) is d e s c r i b e d . A m a x i m u m u l t i m a t e t e n s i l e s t r e n g t h of 323 ksi, c o m b i n e d with a f r a c t u r e toughn e s s Ktc of 62 ksi i~'~, was achieved. This s t r e n g t h l e v e l a p p e a r s to be the m a x i m u m which can be achieved i n m a r a g i u g type s t e e l s without d e c r e a s i n g the c r a c k t o l e r a n c e below that of c u r r e n t l y used high s t r e n g t h low alloy s t e e l s . R e v e r s i o n to a u s t e n i t e did not i m p r o v e either the f r a c t u r e toughness or s t r e s s c o r r o s i o n r e s i s t a n c e r e l a t i v e to c o m p l e t e l y m a r t e n s i t i c m i c r o s t r u c t u r e s with equivalent s t r e n g t h .
S I N C E t h e i r i n t r o d u c t i o n i n 1960 the 18 pct Ni ma.raging s t e e l s have been i n c r e a s i n g l y used for a v a r i e t y of a e r o s p a c e and other a p p l i c a t i o n s . The c o m m e r c i a l l y a v a i l a b l e 200, 250, and 300 grades c o n t a i n 17 to 19" Ni, *Alloy content is given as wt pct.
7 to 9 Co, 3 to 5 Mo and t i t a n i u m within the r a n g e 0.15 to 0.80 depending upon the s t r e n g t h - g r a d e r e q u i r e d . ~ The highest s t r e n g t h which can be g u a r a n t e e d for the 300 grade in heavy s e c t i o n f o r m is 280 ksi. While higher s t r e n g t h g r a d e s have been developed, z, 3 the f r a c t u r e toughness of these a l l o y s has l i m i t e d their s t r u c t u r a l application. The p u r p o s e of this study was to explore the f e a s i b i l i t y of i n c r e a s i n g the m i n i m u m t e n s i l e s t r e n g t h of 300 grade m a r a g i n g steel by m o d i fying the alloy content while m a i n t a i n i n g a n a c c e p t a b l e level of f r a c t u r e t o u g h n e s s . In addition the effect of heat t r e a t m e n t on f r a c t u r e toughness and s t r e s s c o r r o sion was investigated.
ALLOY
CONTENT
Previous studies have shown that titanium has a potent h a r d e n i n g effect on 18 pct Ni m a r a g i n g steel (approximately 90 ksi per 1 pct addition) but has an a d v e r s e effect on f r a c t u r e toughness, t T h e r e f o r e , the t i t a n i u m content i n this p r o g r a m was r e s t r i c t e d to the upper end of the 300 grade r a n g e ; i.e., 0.8 pct. To achieve the d e s i r e d strertgth the cobalt content was i n c r e a s e d f r o m 8.5 to 9.5 pct to 12 to 13 pct, t h e r e b y e n hancing the a g e - h a r d e n i n g r e s p o n s e . 4 To m a i n t a i n the Ms t e m p e r a t u r e at a s i m i l a r l e v e l to the 300 grade (300 to 350~ the nickel content was d e c r e a s e d by 2 pct. A 200 lb. heat was v a c u u m induction m e l t e d by C a r p e n t e r Technology C o r p o r a t i o n to this a n a
Data Loading...
