An investigation of the mechanism of splitting which occurs in tensile specimens of high strength low alloy steels
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of l a m e l l a r t e a r i n g has been r e l a t e d to the elongated i n c l u s i o n s that s o m e t i m e s exist in s t r u c t u r a l s t e e l s . 1-8 It is well known that highly elongated i n c l u s i o n s can r e s u l t in a d e g r a d a t i o n in toughness and d u c t i l i t y . 7-9 T h i s d e t e r i o r a t i o n has b e e n shown to be e s p e c i a l l y p r o n o u n c e d when the t e s t i n g d i r e c t i o n lies n o r m a l to the plane of f l a t - r o l l e d m a t e r i a l s . 1~ It has also b e e n found that the d e g r e e of d e g r a d a t i o n in d u c t i l i t y and toughness is dependent upon the s e v e r i t y of i n c l u s i o n elongation; the m o r e e x t r e m e the i n c l u s i o n elongation, the lower the short t r a n s v e r s e d u c t i l i t y and toughn e s s . t3-t5 R e c e n t w o r k has s h o w n a s t r o n g c o r r e l a t i o n b e t w e e n
the s h o r t t r a n s v e r s e ductility and the s u s c e p t i b i l i t y of a s t e e l to l a m e l l a r t e a r i n g . 6 T h e s e s t u d i e s have shown that as the s h o r t t r a n s v e r s e ductility d e c r e a s e d , b e c a u s e of elongated i n c l u s i o n s , the s u s c e p t i b i l i t y of l a m e l l a r t e a r i n g that attended welding i n c r e a s e d . 6 Other f o r m s of l a m e l l a r t e a r i n g have r e c e n t l y b e e n o b s e r v e d in h i g h - s t r e n g t h , f i a t - r o l l e d s t r u c t u r a l s t e e l s . It m u s t be s t r e s s e d that t h e s e other f o r m s of l a m e l l a r t e a r i n g o c c u r r e d in s t e e l s that did not contain elongated i n c l u s i o n s , and, hence, w e r e not of the s a m e o r i g i n a t i o n as the i n c l u s i o n - r e l a t e d type d e s c r i b e d above. When this f r a c t u r e morphology is not r e l a t e d to i n c l u s i o n s , it is n o r m a l l y r e f e r r e d to as s p l i t t i n g , One f o r m of s p l i t t i n g was found in low c a r b o n b a i n i t i c s t e e l s , and has b e e n r e l a t e d to the p l a n a r b a n d i n g of the b a i n i t e - m a r t e n s i t e m i c r o s t r u c t u r e . 16 A study of the split f r a c t u r e s u r f a c e r e v e a l e d evidence of cleavage, hence, it would a p p e a r that this f o r m of s p l i t t i n g r e s u l t e d f r o m an e m b r i t t l e m e n t in the s h o r t t r a n s v e r s e d i r e c t i o n caused by the p r e s ence of p l a n a r bands of u n t e m p e r e d m a r t e n s i t e . Still a n o t h e r f o r m of s p l i t t i n g has b e e n found in highs t r e n g t h , l o w - a l l o y s t e e l s (HSLA) in the p a s t few y e a r s . 17 The o c c u r r e n c e of this f o r m of s p l i t t i n g was r e l a t e d to both the c o m p o s i t i o n of the s t e e l and the t h e r m o m e c h a n i c a l t r e a t m e n t u s e d . F o r e x a m p l e , it has b e e n r e c e n t l y shown that g r a i n shape is an VOLUME 8A, MARCH 1977-473
*-Weld *-H.A.Z. ~--Lamellar Tear -,--Plate
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