Fracture surfaces of high-strength low-alloy (HSLA) steels

  • PDF / 825,603 Bytes
  • 4 Pages / 612 x 783.28 pts Page_size
  • 73 Downloads / 241 Views

DOWNLOAD

REPORT


Fracture Surfaces of High-Strength Low-Alloy (HSLA) Steels SIEGFRIED S. H E C K E R High s t r e n g t h - l o w a l l o y (HSLA) s t e e l s o f f e r p o t e n t i a l weight s a v i n g s in a u t o m o b i l e s t r u c t u r a l c o m p o n e n t s . H o w e v e r , t h e i r use is p r e s e n t l y r e s t r i c t e d by p o o r f o r m a b t l i t y c o m p a r e d to l o w - c a r b o n s t e e l s . P r e v i o u s w o r k by the a u t h o r 1 showed that both the t e n s i l e p r o p e r t i e s and f o r m i n g l i m i t s of the h o t - r o l l e d HSLA s t e e l s w e r e i n f e r i o r to h o t - r o l l e d , l o w - c a r b o n s t e e l s . The f r a c t u r e s u r f a c e s of t h e s e s t e e l s have b e e n e x a m i n e d in a s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) in an a t t e m p t to find what c a u s e d the r e d u c t i o n in f o r m a b i l i t y . This r e p o r t p r e s e n t s s o m e of the i n t e r e s t i n g d u c t i l e f r a c t u r e b e h a v i o r o b s e r v e d f o r HSLA s t e e l . L o w - c a r b o n and HSLA s t e e l s h e e t s p e c i m e n s w e r e f r a c t u r e d under u n i a x i a l and b i a x i a l t e n s i o n . A punch s t r e t c h i n g t e c h n i q u e d e s c r i b e d p r e v i o u s l y ~ was u s e d to t e s t s h e e t s under v a r y i n g a m o u n t s of b i a x i a l t e n s i o n . The s t r a i n r a t i o (p = e 2 / e , ) of the m e a s u r e d p r i n c i p a l s u r f a c e s t r a i n s e, and e 2 r e p r e s e n t s a m e a s u r e of b i axiality. The f r a c t u r e s u r f a c e s of t h r e e HSLA s t e e l s and one SIEGFRIED S. HECKER, formerly with the General Motors Research Laboratories, Warren, Mich., is now with the University of California, Los Alamos Scientific Laboratory, Los Alamos, NM. 87544 Manuscript submitted October 15, 1973 METALLURGICALTRANSACTIONS

Fig. 1--Biaxially fractured test specimen (200 • 200 mm). (With SEM specimen outlined.)

low-carbon steel, all fractured by punch testing at room temperature at punch rates of ~25 mm/min, were examined. The chemical compositions and alloy designations (Table I) are the same as those used previously by the a u t h o r . l C o m p l e t e t e n s i l e p r o p e r t i e s and f o r m ing l i m i t c u r v e s (which d e s i g n a t e i n s t a b i l i t y s t r a i n s a s a function of b i a x i a l i t y ) a r e a l s o g i v en in Re/. 1. F o r al l s p e c i m e n s ( r e g a r d l e s s of s t r e s s r a t i o ) f r a c t u r e was p r e c e d e d by p l a s t i c i n s t a b i l i t y . H o w e v e r , the s t r a i n paths a l l the way to f r a c t u r e did not d e v i a t e g r e a t l y f r o m p r o p o r t i o n a l l o ad i n g (constant s t r a i n r at io) . The i n s t a b i l i t y s t r a i n s v a r i e d with b i a x i a l i t y . The m i n i m u m l i m i t s t r a i n o c c u r r e d at plane s t r a i n (p = 0) and i n c r e a s e d with both p o s i t i v e and n e g a t i v e s t r a i n s . The l i m i t i n g t

Data Loading...