Ferrite formation in neutron irradiated type 316 stainless steel
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AND
K. R. GARR
Magnetic m e a s u r e m e n t s w e r e made on Type 316 s t a i n l e s s s t e e l s p e c i m e n s that had b e e n neutron irradiated to fluences of 1.8 • i 0 zz neutrons/cmz at 425~ and 3.5 • I0 zz neutrons/ cm~ (E > 0.1 MeV) at 500 and 600~ A significant increase of magnetization was observed for the irradiated specimens compared to the unirradiated specimens. The shape of the magnetizationvs field curves showed that the irradiated specimens contained many small superparamagnetic particles. The magnetic particles are assumed to be the ferrite phase although other possibilities cannot be excluded. The amount and distribution of the magnetic phase varied with pre-irradiationand post-irradiation heat treatment. The maximum value of magnetization was equivalent to 3.6 pct ferrite in a specimen annealed I00 h at 760~ before irradiation and 1 h at 500~ after irradiation at 425~
AUSTEN]TIC
s t a i n l e s s s t e e l s undergo a n u m b e r of m i c r o s t r u c t u r a l changes when i r r a d i a t e d to a r e l a t i v e l y high n e u t r o n f l u e n c e in the t e m p e r a t u r e r a n g e 400 to 600~ F o r e x a m p l e , t h e r e is the f o r m a t i o n of a l a r g e n u m b e r of s m a l l voids, the p r e c i p i t a t i o n of M2sC6 c a r b i d e s and an u n i d e n t i f i e d phase, the r e v e r s i o n of < m a r t e n s i t e , and the f o r m a t i o n of faulted and unfaulted d i s l o cation loops and d i s l o c a t i o n n e t w o r k s . 1,2 Since the m a c r o s c o p i c s w e l l i n g c a u s e d by void f o r m a t i o n c r e a t e s p r o b l e m s in the design of fast b r e e d e r r e a c t o r s , this p h e n o m e n o n has been studied e x t e n s i v e l y . A n u m b e r of l a b o r a t o r i e s have also studied void f o r m a t i o n by i r r a d i a t i o n with charged p a r t i c l e s (protons, c a r b o n ions, n i c k e l ions) to s i m u l a t e n e u t r o n i r r a d i a tions, l'e The m a i n advantage of c h a r g e d p a r t i c l e i r r a d i ation is that the t i m e needed to a c c u m u l a t e s i g n i f i c a n t damage is much l e s s than for n e u t r o n i r r a d i a t i o n . R e cently Keefer, P a r d , Rhodes and K r a m e r r e p o r t e d the d e t e c t i o n of f e r r i t e in p r o t o n - i r r a d i a t e d Types 316 and 321 s t a i n l e s s s t e e l s , a'4 In those c a s e s in which Keefer et al were able to o b s e r v e f e r r i t e b y t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , the f e r r i t e g e n e r a l l y was found to f o r m in r e l a t i v e l y l a r g e p a r t i c l e s , 2/~ d i a m and l a r g e r . In 1955 Reynolds, Low and Sullivan r e p o r t e d i n c r e a s e d m a g n e t i z a t i o n v a l u e s in n e u t r o n i r r a d i a t e d Type 347 s t a i n l e s s s t e e l s However, f e r r i t e f o r m a t i o n has not been r e p o r t e d in r e c e n t i n v e s t i g a t i o n s on n e u t r o n i r r a d i a t e d s p e c i m e n s . Also, e l e c t r o
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