The elimination of oxygen-induced intergranular brittleness in iron by addition of scavengers
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It was p r e v i o u s l y shown that v a c u u m m e l t e d i r o n can be made to exhibit i n t e r g r a n u l a r b r i t t l e n e s s at low t e s t i n g t e m p e r a t u r e s if it is quenched from t e m p e r a t u r e s where c a r b o n is in solid solution and that ductility can be r e s t o r e d by allowing c a r b o n to s e g r e g a t e to g r a i n b o u n d a r i e s (without p e r m i t t i n g the f o r m a t i o n of l a r g e c a r b i d e s ) . It was suggested that this is due to s c a v e n g i n g of r e s i d u a l g r a i n b o u n d a r y oxygen by the c a r b o n . T h i s is s u b s t a n t i a t e d by the p r e s ent e x p e r i m e n t s in which a l u m i n u m additions were used to e l i m i n a t e the i n t e r g r a n u l a r weakn e s s at all quenching t e m p e r a t u r e s . It i s shown that t i t a n i u m is not effective as an oxygen s c a v e n g e r and r e s t o r e r of g r a i n b o u n d a r y s t r e n g t h .
A L T H O U G H low t e m p e r a t u r e f r a c t u r e in i r o n gene r a l l y o c c u r s by t r a n s g r a n u l a r c l e a v a g e , it is not unc o m m o n to find an i n t e r g r a n u l a r f r a c t u r e mode. The l a t t e r is u s u a l l y a c c o m p a n i e d by b r i t t l e b e h a v i o r and is thought to be a s s o c i a t e d with the p r e s e n c e of r e sidual oxygen at grain boundaries. I-4 This oxygeninduced brittleness is accentuated if the carbon concentration of the iron is reduced,4'5 and recent work6 indicates that the grain boundary O-C ratio in an iron containing 20 ppm O and 50 ppm C can be controlled by heat treatment to give strong or weak intergranular cohesion in a predictable and reversible manner. The most recent experiments6 involved the reheating of annealed coarse-grained iron well into the solid solution region, slowly cooling to a selected temperature to establish a characteristic carbon distribution, and then quenchingand tensile testing at 77~ For high quenchingtemperatures, where carbon was dispersed in a more or less homogeneous solid solution, the specimens exhibited extreme intergranular brittleness, presumably due to the still-segregated oxygen. As the quenchingtemperature was lowered, ductility increased markedly, and the incidence of int e r g r a n u l a r f r a c t u r e s t e a d i l y d i m i n i s h e d . This was a t t r i b u t e d to a p r o p o s e d i n c r e a s e in s e g r e g a t i o n of c a r b o n to g r a i n b o u n d a r i e s as the s o l u b i l i t y l i m i t is approached. At still lower quenching t e m p e r a t u r e s the p r e c i p i t a t i o n of g r a i n b o u n d a r y c a r b i d e s r e s u l t e d in n o r m a l cleavage f r a c t u r e by the c a r b i d e - c r a c k i n g mechanism.7 The question of the m e c h a n i s m of the e n h a n c e m e n t of i n t e r g r a n u l a r cohesion by c a r b o n s e g r e g a t i o n was left open, 6 the s u g g e s t i o n s b e i n g that: a) the c a r b o n a c t s as a s c a v e n g e r by f o r m i n g C-
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