Stress corrosion cracking of high strength HY-180M steel in 3.5 Pct NaCl
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has been c o n s i d e r a b l e i n t e r e s t in d e v e l o p ing a high s t r e n g t h s t e e l with high opening mode f r a c t u r e toughness (KIC) and good r e s i s t a n c e to s t r e s s c o r r o s i o n c r a c k i n g (SCC) i n aqueous s a l t solutions. Little and M a c h m e i e r I i n v e s t i g a t e d the effect of m o d i fying the c o m p o s i t i o n of an e x i s t i n g s t e e l containing 1 0 N i - 2 C r - I M o - 8 C o - 0 . 1 2 C , known as HY-180, which r e l i e s upon s e c o n d a r y p r e c i p i t a t i o n h a r d e n i n g of a m a s s i v e (lath) m a r t e n s i t e m a t r i x for d e v e l o p m e n t of o p t i m u m s t r e n g t h , as shown by Speich et al. 2 Little and M a c h m e i e r developed a p r o m i s i n g m o d i f i c a t i o n , now known as e i t h e r HY-180M or USAF d e s i g n a t i o n AF-1410, which c o n t a i n s 1 0 N i - 2 C r - I M o - 1 4 C o - 0 . 1 6 C . They showed that when this alloy was s u b j e c t e d to a double a u s t e n i t i z i n g heat t r e a t m e n t , with i n t e r i m w a t e r quenching followed by a five h aging t r e a t m e n t at 510~ that a yield s t r e s s of 1550 M P a (225 ksi) and a KIC value of 160 M P a . m ~ (142 k s i ivy-n-if)could be obtained. F u r t h e r m o r e , they r e p o r t e d that p r e l i m i n a r y t e s t s indicated that the modified s t e e l was r e s i s t a n t to SCC at s t r e s s i n t e n s i t y l e v e l s in e x c e s s of 110 M P a . m ~/2 (100 k s i ~ , when exposed to an aqueous 3.5 pct NaC1 s o l u t i o n for 1000 h. Subsequent s t u d i e s by Snide 3 c o n f i r m e d the heat t r e a t m e n t p r o c e d u r e s of s t r e n g t h and f r a c t u r e toughness, but did not include any t e s t s on SCC b e h a v i o r . The p r e s e n t s t u d i e s were u n d e r t a k e n to provide q u a n t i t a t i v e SCC data on o p t i m u m heat t r e a t e d HY180M exposed to aqueous 3.5 pct NaC1 s o l u t i o n . L i n e a r e l a s t i c f r a c t u r e m e c h a n i c s t e c h n i q u e s w e r e employed, and SCC v e l o c i t y (v) was studied as a function of s t r e s s i n t e n s i t y (KI) and e l e c t r o c h e m i c a l p o t e n t i a l (E). SATHISH RAO BALA and DESMONDTROMANS are Research Associate and Associate Professor respectively, Department of Metallurgical Engineering,University of British Columbia, Vancouver, V6T 1W5, Canada. Manuscript submitted January 3, 1978. METALLURGICALTRANSACTIONSA
EXPERIMENTAL
PROCEDURE
i) Material Alloy HY-180M was prepared by vacuum induction melting and obtained from Universal-Cyclops Specialty Steel Division in the form of hot forged bars with a diameter of 38 ram. The composition in terms of wt pct of alloy elements was: 10.18Ni, 2.09Cr, 1.03Mo, 14.04Co, 0.16C, 0.13Mn, 0.03Si, 0.002S, 0.005P, 0.002AI, 0.004Ti, 0.0007N, 0.001 O, balance Fe. ii) Heat T r e a t m e n t The r e c o m m e n d e d heat t r e a t m e n t 1'3 to produce optim u m s t r e n g t h and toughness was applied to a l l s p e
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