Lamellate fracture in 5150 steel processed by modified ausforming
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has been shown that some m e c h a n i c a l p r o p e r t i e s of low alloy s t e e l s can be i m p r o v e d by deforming these m a t e r i a l s at t e m p e r a t u r e s at which austenite is stable before quenching and t e m p e r i n g , z'2 Such p r o c e s s i n g has been r e f e r r e d to a s " m o d i f i e d a u s f o r m i n g , ' " to distinguish it f r o m conventional ausforming which involves the deformation of m e t a s t a b l e austenite. In his s u r v e y of r e c e n t Russian work on t h e r m o m e c h a n i c a / t r e a t m e n t , Koppenaal 2 d e s c r i b e d this type of p r o c e s s i n g as High T e m p e r a t u r e T h e r m o m e c h a n i c a l T r e a t ment (HTTMT), while Irani e t a l . , 3 call it " H o t - C o l d Working." The impetus for the development of this p r o c e s s i n g method is the d e s i r e to extend the benefits of a n s f o r m ing to those a l l o y s in which m e t a s t a b l e anstenite cannot be r e t a i n e d below the t e m p e r a t u r e r e g i m e of s t a b l e austenite long enough to p e r m i t deformation by any conventional m e a n s . The c l a s s of m a t e r i a l s which might be p r o c e s s e d by modified ausforming includes plain carbon s t e e l s and a v a r i e t y of low alloy s t e e l s with h a r d e n a b i l i t y c h a r a c t e r i s t i c s typified by 5150 steel. The main limitation to the application of m o d i fied a u s f o r m i n g is the c o n t r o l needed to insure that: 1) the austenite does not r e c r y s t a l l i z e a p p r e c i a b l y p r i o r to quenching, and 2) the austenite does not d e compose because of low working t e m p e r a t u r e s . Thus, whether or not modified a u s f o r m i n g is p r a c t i c a l for a p a r t i c u l a r alloy will depend to a l a r g e d e g r e e on i t s r e c r y s t a l l i z a t i o n behavior u n d e r hot working conditions. Bock and Justusson ~ showed that substantial i m p r o v e m e n t s in bending fatigue life and charpy i m p a c t s t r e n g t h were obtained from modified a u s f o r m i n g of 5150 s t e e l . F r a c t u r e s in fatigue and i m p a c t testing w e r e d i s t i n c t l y l a m e l l a t e , that is, c r a c k s tended to be blunted by s p l i t ting o c c u r r i n g on planes roughly p a r a l l e l to the r o l l ing plane. This type of f r a c t u r e is occasionally seen in conventionally a u s f o r m e d m a t e r i a l as well. 4 The p u r p o s e of the p r e s e n t investigation was to d e t e r m i n e the m i c r o s t r u c t u r a l f e a t u r e s r e s p o n s i b l e for the iraC. M. YEN and C. A. STICKELS are with the Scientific Research Staff, Process Research Department, Ford Motor Co., Dearborn, Mich. Manuscript submitted December 29, 1969. METALLURGICAL TRANSACTIONS
proved impact s t r e n g t h and accompanying l a m e l l a t e f r a c t u r e in a u s f o r m e d 5150 steel. EXPERIMENTAL
PROCEDURE
The material used was commercially produced 5150 s t e e l obtained in the form of b a r
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