Stress-corrosion cracking behavior of an 18 Pct Ni maraging steel
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H, W. P A X T O N
S t r e s s - c o r r o s i o n c r a c k i n g of an 18 pct Ni m a r a g i n g s t e e l in a q u e o u s s o l u t i o n s was s t u d i e d u s i n g p r e c r a c k e d c a n t i l e v e r b e a m s p e c i m e n s . By a p p r o p r i a t e h e a t t r e a t m e n t s , six d i f f e r e n t s t r u c t u r e s having the s a m e y i e l d s t r e n g t h w e r e obtained. Although s i g n i f i c a n t l y d i f f e r e n t plane s t r a i n f r a c t u r e t o u g h n e s s v a l u e s (Kic) r e s u l t e d , it was found that the t h r e s h o l d plane s t r a i n s t r e s s i n t e n s i t y (Kiscc) was the s a m e for a l l s t r u c t u r e s . Kiscc had the s a m e v a l u e in 3 pct NaC1 at v a r i o u s pH v a l u e s , in 1N H2SO4, and in d i s t i l l e d w a t e r . S p e c i m e n s t e s t e d in 3 p c t NaC1 u n d e r both anodic and c a t h o d i c a p p l i e d p o t e n t i a l s a l s o e x h i b i t e d t h i s s a m e Kiscc v a l u e . F r a c t o g r a p h i c i n s p e c t i o n of the c r a c k s u r f a c e s r e v e a l e d no a p p a r e n t d i f f e r e n c e s due to c h a n g e s in Solution, pH, o r a p p l i e d p o t e n t i a l . T h e c r a c k path was i n t e r g r a n u l a r in a l l c a s e s . H o w e v e r , s p e c i m e n s a u s t e n i t i z e d at 1500~ e x h i b i t e d c r a c k b r a n c h i n g , w h e r e a s in s p e c i m e n s a u s t e n i z e d at much h i g h e r t e m p e r a t u r e s b r a n c h i n g no l o n g e r o c c u r r e d . Aging t i m e and t e m p e r a t u r e s e e m e d to change only the t i m e to f a i l u r e . T h e m e c h a n i s m m o s t c o n s i s t e n t with a l l o b s e r v a t i o n s a p p e a r s to be h y d r o g e n c r a c k i n g .
T H E s t r e s s - c o r r o s i o n c r a c k i n g b e h a v i o r of h i g h s t r e n g t h m a r a g i n g s t e e l h a s not been i n v e s t i g a t e d e x t e n s i v e l y . F a i l u r e of m a r a g i n g o r o t h e r h i g h - s t r e n g t h m a r t e n s i t i c s t e e l s in aqueous s o l u t i o n s d o e s not s e e m to d e p e n d on the p r e s e n c e of a s p e c i f i c ion, a s is the c a s e , f o r e x a m p l e , in the a u s t e n i t i c s t a i n l e s s s t e e l s . T h e only safe g e n e r a l i z a t i o n that can be m a d e to d a t e about the c r a c k i n g b e h a v i o r of m a r a g i n g s t e e l s i s t h a t h i g h e r y i e l d s t r e n g t h tends to i n c r e a s e the s u s c e p t i b i l i t y to c r a c k i n g . Many of the p r e v i o u s i n v e s t i g a t i o n s into the s t r e s s c o r r o s i o n b e h a v i o r of m a r a g i n g s t e e l s a r e s u m m a r i z e d in the r e v i e w s by M a t t e o l i and Soyga, 1 P h e l p s , 2 and K e n n e d y and W h i t t a k e r . s E v e n though the h e a t t r e a t m e n t of m a r a g i n g s t e e l a f f e c t s i t s e l e c t r o c h e m i c a l characteristics, almost all these previous investigat i o n s w e r e c o n c e r n e d with d
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