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Maraging steel can be embrittled by slowly cooling between 1850~ 1400~ from elevated temperatures (>2200~ The object of this work was to examine the effect of various refining and hardening additions on the slow cooling embrittlement response of 18Ni200 maraging steel. Small refining additions of magnesium give a partial but significant reduction of embrittlement that is caused by grain boundary precipitates. Ca, B, and Zr did not have this beneficial effect. Of the hardening elements studied, only titanium could be associated with the embrittlement. Titanium was otherwise the best hardening addition, judged from its combined effect on the yield strength and toughness of normally annealed plus aged material.

M A R A G I N G s t e e l c a n b e e m b r i t t l e d by c o o l i n g a t f a i r l y s l o w r a t e s b e t w e e n 1850 ~ and 1400~ f r o m e l e v a t e d t e m p e r a t u r e s (>2200~ This was first obs e r v e d b y N o v a k I w h o a s c r i b e d t h e p h e n o m e n o n to p r e c i p i t a t i o n at a u s t e n i t e g r a i n b o u n d a r i e s . T h e r e is now c o n s i d e r a b l e e v i d e n c e 2-5 to s h o w t h a t t h i s e m b r i t t l e m e n t i s g e n e r a l l y d u e t o g r a i n b o u n d a r y f i l m s of T i ( C , N ) . E f f o r t s to r e d u c e e m b r i t t l e m e n t by r e p l a c i n g t i t a n i u m with o t h e r h a r d e n i n g a d d i t i o n s have not been h i g h l y s u c c e s s f u l . O t h e r p r e c i p i t a t e p h a s e s ; A1N, Mo2C, V(C,N) h a v e b e e n i d e n t i f i e d in low t i t a n i u m h e a t s ; 2 h o w e v e r , Ti(C,N) i s t h e p r e d o m i n a n t e m b r i t t l i n g p h a s e in t i t a n i u m - c o n t a i n i n g s t e e l s . T h e t o u g h n e s s of e m b r i t t l e d m a t e r i a l c a n b e l a r g e l y r e s t o r e d by m e a n s of s o l u t i o n i n g h e a t t r e a t m e n t s at t e m p e r a t u r e s a b o v e a b o u t 1800~ f o l l o w i n g by r a p i d c o o l i n g . H o w e v e r , it i s d e s i r a b l e t o m i n i m i z e t h e e m b r i t t l i n g t e n d e n c y t h r o u g h c h a n g e s in c o m p o s i t i o n . In t h i s way it i s e a s i e r to o p t i m i z e m a t e r i a l t o u g h n e s s , e s p e c i a l l y in l a r g e s e c t i o n s . H e a t t r e a t m e n t i s a l s o s i m p l i f i e d . V a r i o u s e l e m e n t s m a y b e a d d e d to 18Ni200 m a r a g i n g s t e e l a s s u p p l e m e n t a l h a r d e n i n g and r e f i n i n g a g e n t s . T h e o b j e c t i v e of t h e p r e s e n t w o r k w a s to e x a m i n e t h e r e l a t i v e e f f e c t s of d i f f e r e n t h a r d e n i n g a d d i t i o n s in t h i s a l l o y and to s e e w h e t h e r m i n o r a d d i t i o n s of r e a c t i v e e l e m e n t s m i g h t b e u s e d to c o u n t e r a c t the embrittling mechanism.

EXPERIMENTAL STATISTICAL

WORK AND DESIGNS

To study compositional effects, alloys were melted in four groups of compositions based on fractional factorial exp